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The daily program for the May 24-26 SolFER Conference is now available online. Click on the "Scientific Program" tab to view an overview of the schedule and the "Timeline" tab for a detailed ordered list of presentations with abstracts.
The SolFER DRIVE Science Center is hosting a web-based science meeting on Solar Flare Energy Release. The meeting is open to all scientists working on the topic. The meeting will include invited talks, submitted oral talks as well as poster presentations and will provide substantial time for informal scientific discussion. We encourage paper submissions that are based on remote and in situ observational data as well as those based on theory and modeling. Extensive use will be made of Gather meeting software to facilitate interactive poster sessions as well as informal discussion between meeting participants. The meeting will be organized around SolFER’s six scientific focus areas:
What mechanisms facilitate the fast release of magnetic energy in impulsive solar flares?
What controls the onset of fast flare energy release?
Why and how do flares transfer a large fraction of the released magnetic energy into energetic electrons?
What mechanism drives the energization of ions and the measured abundance enhancements of some species during impulsive flares?
What mechanisms control energetic particle transport in flares?
How does reconnection heat plasma in flares and the small events (nanoflares) that may be responsible for heating the corona?
More information on these scientific topics can be found on the SolFER website. Submitted abstracts should specify which of the above science topics are most closely related to submission.
The current plan is to cover two of the scientific focus areas each day of the three day meeting. Two sessions each day (11:00-13:00 and 13:40-15:40) will be separated by a 13:00-13:40 break/interactive-poster-session using Gather software. All times are US EDT. Beyond the designated time for discussion after each talk, there will be 25 minutes in the morning and afternoon for discussion led by panelists who will respond to questions submitted by meeting attendees.
Connection information for the daily zoom sessions and the Gathertown poster sessions is available under the Gathertown and zoom connections tab on the conference home page.
The SolFER science team also encourages community participation in the ongoing science discussion related to flare energy release. A detailed calendar of working group meetings as well as the monthly webinar can be found on the SolFER website.
The 0.1 reconnection rate problem refers to the rate of reconnection normalized to ambient plasma parameters near where reconnection is occurring, which has been found to be remarkably similar for a wide range of plasma conditions. A rate of 0.1 is consistent with explosive energy release in solar flares and during substorms in the Earth's magnetotail. I will review some of the key simulation and observational studies regarding the 0.1 problem, and then discuss some recent work that posits that this 0.1 value may represent a theoretical maximum for the reconnection, with the implication that most systems find a way to reconnect close to this maximum value. An issue, though, is that if such a fast reconnection rate is always available then magnetic energy can never accumulate and explosive events such as solar flares would never occur. There must be a high threshold to initiate this fast reconnection, which is believed to relate to the thickness of the equilibrium current sheet. An illustrative example involves strong turbulence in solar coronal conditions and the reconnection it generates, which I will briefly review.
Energy release in solar flares is governed by fast magnetic reconnection taking place in the corona. Energy flux streams down along reconnecting field lines to the chromosphere, producing flare ribbons or kernels of impulsively enhanced optical, ultraviolet, and hard X-ray emissions. Therefore, reconnection and energy release events in the Sun's corona can be mapped, tracked, and measured with observations of the flaring lower atmosphere. Flare ribbons map topological boundaries that are dynamically evolving due to reconnection, and the tempo-spatial evolution of flare ribbons reflects the structure and dynamics of reconnection in the corona. In this talk, we will discuss the capabilities and prospects of using flare ribbon observations to infer properties of magnetic reconnection and to diagnose flare energetics. Through these exercises, we hope to determine when, where, by how much, and in what form flare energy is released, and probe how flare energetics are possibly governed by reconnection properties, based on the recent progress in observations and models.
In magnetohydrodynamics (MHD), magnetic reconnection has long been discussed by Sweet-Parker (S-P) and Petschek models. It was recently found that a laminar S-P reconnection evolves to plasmoid-dominated turbulent reconnection in a large-scale system. The reconnection rate during the plasmoid-dominated stage is known to be 0.01, regardless of other parameters. Plasma $\beta$ in the inflow region is extremely low around reconnection sites in a solar corona. However, despite its importance in a corona, many aspects of the plasmoid-dominated reconnection in the low-$\beta$ regime remain unexplored, partly because of numerical difficulties.
In this contribution, we explore basic properties of plasmoid-dominated reconnection in a low-$\beta$ background plasma. We have found that the system becomes highly complex due to repeated formation of plasmoids and vertical shocks (SZ & Miyoshi 2011, 2015). The average reconnection rate increases in the $\beta < 1$ regime, in contrast to popular results. We attribute this to compressible effects. Using a compressible S-P theory (Hesse+ 2011), we have proposed a scaling law for the reconnection rate. This prediction was verified by a numerical survey in the 2-D parameter space. We will also discuss the energy balance and the influence of the initial plasma-sheet models.
Reference: Zenitani & Miyoshi, ApJL 894, L7 (2020)
Coronal mass ejections, jets, prominence eruptions: solar eruptions are an active field with a broad range of accepted phenomena, and an even broader range of proposed mechanisms that cause the phenomena. This talk reports the observations of an event that connects the major eruption classes, and could provide a holistic explanation for all of them. The event originated in a filament channel overlying a circular polarity inversion line (PIL) and occurred on 2013 March 13 during the extended decay phase of the active region designated (sequentially) NOAA 12488/12501. This event was especially well-observed by multiple spacecraft and was seen to have the well-studied null-point topology. We analyze all aspects of the eruption using SDO AIA and HMI, STEREO-A, and SOHO LASCO imagery. One section of the filament undergoes a classic failed eruption with cool plasma subsequently draining onto the section that did not erupt, but a complex structured CME/jet is clearly observed by SOHO LASCO C2 shortly after the failed filament eruption. We describe in detail the long, slow buildup to eruption; the lack of an obvious trigger; and the immediate reappearance of the filament after the event. The unique mixture of major eruption properties that are observed in this event places severe constraints on the structure of the filament channel field and, consequently, on the possible eruption mechanism.
The physical picture of interacting magnetic islands (or flux tubes in 3D) provides a useful paradigm for certain plasma dynamics in a variety of physical environments, such as the solar corona, the heliosheath, and the Earth's magnetosphere. The successive coalescence of magnetic islands via magnetic reconnection leads to robust dissipation of magnetic energy. Meanwhile, the length scale of magnetic fields increases, giving rise to the formation of large-scale structures. We have investigated the system dynamics of a large ensemble of magnetic islands with analytical theory and direct numerical simulations (Zhou et al., 2019, 2020), and through a Boltzmann-type kinetic approach (Zhou et al. 2021). This talk will focus on the latter. We derive an island kinetic equation (IKE) to describe the evolution of the island distribution function (in area and in flux of islands) subject to a collisional integral designed to account for the role of reconnection during island mergers. We solve our IKE numerically for three different types of initial distribution that are relevant to space and astrophysical environments: delta-distribution, Gaussian and power-law distribution. The time evolution of several key quantities is found to agree well with our analytical predictions: magnetic energy decays as $\tilde t^{-1}$, the number of islands decreases as $\tilde t^{-1}$, and the averaged area of islands grows as $\tilde t$, where $\tilde t$ is the time normalized to the characteristic reconnection time scale of islands. General properties of the distribution function and the magnetic energy spectrum are also discussed. This study provides the statistics of island mergers and is a building block to studying the statistics of particle energisation and solar flares.
Panel members: Sophie Musset, Noriyuki Narukage and Amitava Bhattacharjee
Solar flares are the most energetic phenomena in the solar atmosphere with consequences for space weather through the generation of solar energetic particles and/or CMEs. Despite tremendous advances in understanding their characteristics, the complete physics of their origin and response to plasma in various layers of the solar atmosphere is not developed. Here, we study the characteristics of the spectral line profiles during different stages of flares as a function of photospheric magnetic flux density and compare those with the characteristics observed in quiescent active regions and quiet Sun. For this purpose, we use archival observations from the Interface Region Imaging Spectrograph (IRIS). For context purposes, we used the full disk observations from Atmospheric Imaging Assembly (AIA). We use the line-of-sight (LOS) magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). We compare the flare results with those obtained for active regions (ARs) as well as quiet Sun (QS). Some preliminary results will be presented.
We study the structure and evolution of solar flare ribbons in the chromosphere to infer properties of magnetic reconnection that occurs in the corona. We analyze the imaging observations of the M7.3 SOL2014-04-18T13 flare obtained by IRIS in both the near and far ultraviolet passbands and by SDO/AIA in the 1600A passband. Two flare ribbons are observed to spread away from the magnetic polarity inversion line as the flare progresses. Using the high-resolution IRIS observations, we measure the width of the newly brightened ribbon front along the extension of the ribbon, which maps the feet of magnetic field lines reconnecting in the current sheet in the corona. We find that the width of the ribbon front is highly structured, possibly reflecting corresponding structure in the coronal current layer. Further the ribbon grows most rapidly with time in regions where non-thermal hard X-ray (HXR) emission is concentrated (Brosius, et al. 2015). The light curve of the ultraviolet emissions in this region, as measured by IRIS and AIA, also well matches the HXR light curve at photon energies above 25 keV. In contrast, the ribbon-width evolution and light curves in other regions along the ribbons do not correlate well with the HXR emission. These results suggest that there is a strong connection between the production of non-thermal electrons and locally enhanced perpendicular extent of flare ribbon fronts, which reflect the inhomogeneous structure and/or reconnection dynamics of the flare current sheet in the corona.
A spatio-temporal analysis of IRIS spectra of Mg II, C II, and Si IV ions allows us to study the dynamics and the stratification of the flare atmosphere along the line of sight during magnetic reconnection at the jet base.
Strong asymmetric Mg II and C II line profiles with extended blue wings observed at the reconnection site are interpreted by the presence of two chromospheric temperature clouds: one explosive cloud with blueshifts at 290 km/s and one cloud with smaller Doppler shift (around 36 km/s).
Simultaneously at the same location a mini flare was observed with strong emission in multi temperatures (AIA), in several spectral IRIS lines (e.g. O IV and Si IV, Mg II), with absorption of identified chromospheric lines in Si IV broad, with enhancement of the Balmer continuum and X-ray emission (FERMI/GBM). With the standard thick-target flare model we calculate the energy of non thermal electrons observed by FERMI/GBM and compare it to the energy radiated by the Balmer continuum emission. We show that the low energy input by non thermal electrons above 20 keV was still efficient to produce the excess of Balmer continuum.
We present the first results of the SOL2020-May-29T07:13 event study based on simultaneous observations within the 4-8 GHz range by Siberian Radio Heliograph 48 and the spectropolarimeter 4-8 GHz. The microwave (MW) time profiles of the flare demonstrated at least three quasi-periodic bursts. We obtained the spectra for the bursts and defined the position of the MW sources at different frequencies. We found that the first burst and the consequence bursts occurred in distinct locations. The relation of the burst locations and their MW spectral properties are discussed.
The GOES/XRS data show low-level soft X-ray emissions prior to a
flare in a "hot onset" precursor of the main flare development
(2021MNRAS.501.1273H). This phenomenon provides clear evidence for
a heating process not identifiable with the impulsive-phase energy
release. The hot onset phase may last for tens of seconds to minutes,
characterized by steady growth of emission measure at characteristic
isothermal temperatures of 10-15 MK and no clear pattern of temperature
increase. The newer GOES-R data also show this effect, providing
higher time resolution (1 s) and better noise properties, although
with higher background levels. I characterize these new data and
discuss interpretations in terms of physical processes in the context
of the AIA imagery.
What role magnetic reconnection plays in the initiation and evolution of the CME eruptions is still not clear. In a recent work by Zhu et al. (2020), we conducted a statistical study of 42 CME-flare events. We found a significant correlation between CME acceleration and flare reconnection in various aspects, suggesting that flare reconnection is key to acceleration of both fast and slow CMEs and may dominate the acceleration of fast CMEs. We also analyzed time lags of the peak CME acceleration relative to flare reconnection rate, and found that, on average, acceleration-led events have a smaller reconnection rate, and are likely driven by ideal instabilities. To further probe what mechanism triggers the eruption, in this study, we focus on the early-stage evolution of CMEs, flare reconnection, as well as hard X-ray bursts, using a subset of CME-flare events well observed with high temporal and spatial resolutions by SDO, STEREO, and RHESSI. We examine the tempo-spatial relationship between the CME acceleration and flare signatures in the low corona, and compare onsets of CME acceleration and flare reconnection in these events.
This work examines eleven solar microflares observed in hard x-rays (HXRs) by the Nuclear Spectroscopic Telescope ARray (NuSTAR). HXR emission in solar flares originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, both of which are diagnostic of flare energy release. NuSTAR’s direct focusing optics give it a dramatic increase in sensitivity over indirect imagers in the HXR range, allowing for unique insight into the energetics of faint microflares. We discuss the temporal, spatial, and energetic properties of all eleven microflares in context with other published HXR brightenings. They are seen to display several `large-flare' properties, such as impulsive time profiles and earlier peaktimes in higher energy HXRs. For two events where active region background could be removed, microflare emission did not display spatial complexity: differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high energy excess over a single thermal model in all events. This excess was found to most likely originate from additional higher-temperature plasma volumes in 10/11 microflares, and from a nonthermal accelerated particle distribution in the last. These spectral results motivate a more general discussion of the incidence of nonthermal emission across these and other similar-magnitude microflares observed by NuSTAR and other HXR instruments.
Observations of solar flare ribbons show significant fine structure in the form of wave-like perturbations and spirals. The origin of this structure is not well understood, but one possibility is that it is related to the tearing instability in the flare current sheet. Here we study this connection by constructing an analytical three-dimensional magnetic field representative of an erupting flux rope with a flare current sheet below it. We introduce small-scale flux ropes representative of those formed during a tearing instability in the current layer, and use the squashing factor on the solar surface to identify the shape of the presumed flare ribbons. Our analysis suggests there is a direct link between flare ribbon fine structure and flare current sheet tearing, with the majority of the ribbon fine structure related to oblique tearing modes. We discuss how the nature and relative location of the tearing modes is related to spirals/waves in particular parts of the flare ribbon and conclude that fine structure in flare ribbons could potentially be used to indirectly analyse the bursty nature of flare reconnection.
Magnetic flux ropes are the centerpiece of solar eruptions. Direct measurements for the magnetic field of flux ropes are crucial for understanding the triggering and energy release processes, yet they remain heretofore elusive. Here we report microwave imaging spectroscopy observations of an M1.4-class solar flare occurred on 2017 September 6, using data obtained by the Expanded Owens Valley Solar Array. This flare event is associated with a failed eruption of a twisted filament observed in H$\alpha$ by the Goode Solar Telescope at the Big Bear Solar Observatory. The filament, initially located along the magnetic polarity inversion line prior to the event, undergoes a failed eruption during the course of the flare. This partially erupting filament has a counterpart in microwaves, whose spectral properties indicate gyrosynchrotron radiation from flare-accelerated nonthermal electrons. Using spatially resolved microwave spectral analysis, we derive the magnetic field strength along the filament spine, which ranges from 600--1400 Gauss from its apex to the legs. The results agree well with the non-linear force-free magnetic model extrapolated from the pre-flare photospheric magnetogram. The multi-wavelength signatures of the event are consistent with the standard scenario of eruptive flares, except that the eruption failed to fully develop and escape as a coronal mass ejection. We conclude that the failed eruption is likely due to the strong strapping coronal magnetic field above the filament.
In March 2024, sounding rockets will be launched in response to a solar flare for the first time. The Hi-C Flare mission will be among the first to take advantage of this new observing campaign, which was instituted by the NASA Sounding Rocket Program Office in response to a 2019 white paper submitted by Winebarger, Glesener, and Reeves. A soft X-Ray radiometer in development at Montana State University is the smallest of three instruments that will fly on Hi-C Flare. We describe the motivation, prospects, and instrumentation for high speed (1 kHz) measurement of soft X-ray (SXR) variability in solar flares.
Fast magnetic reconnection is expected to occur in the current sheet region during the solar eruptions, where outflows near the Alfven speed are predicted from the classic flare models. In observations, the dark, finger-shaped plasma downflows (also referred to as SADs) moving toward the flare arcade are believed as the principal observational evidence of such reconnection-driven outflows. However, they are often much slower than those expected in theories. Here, we report a three-dimensional magneto-hydrodynamics model, and conclude that the SADs are not the reconnection outflows themselves. Instead, they are self-organized structures formed in a turbulent interface region below the flare termination shock where the outflows meet the flare arcade.
The X1.6 flare observed on 22 October 2014 (SOL2014-10-22T14:28) was among the strongest flares occurred in the magnetically complex, great active region NOAA 12192. It was a confined flare, without an accompanying CME,despite the large amount of released energy.
In our work we attempt to deepen our understanding of the magnetic field configuration of the AR 12192. We analyzed the polarization signatures during the flare using spectro-polarimetric data acquired by the IBIS/DST instrument along the photospheric Fe I 617.3 nm and the chromospheric Ca II 854.2 nm lines in a time interval immediately following the peak of the X1.6 flare. The results obtained provided evidence of significant changes in the magnetic field configuration during the analyzed time interval.
In the standard eruptive flare model, magnetic reconnection originates in a thin current sheet; created by the inflow of oppositely orientated magnetic fields under a rising magnetic flux rope. The current sheet is notoriously difficult to observe directly, primarily due to the small size of the region. However, insights into current sheet dynamics can be revealed by the behaviour of flare ribbon substructure, as magnetic reconnection accelerates particles down reconnected field lines to the chromosphere to mark the flare footpoints. Behaviour in the ribbons can therefore potentially be used to probe processes occurring in the current sheet. Motivated by a similar study on the Earth's magnetosphere (Kalmoni et al 2015, 2018), we exploit this magnetic connectivity between the current sheet and magnetic footpoints to probe for signatures of waves and instabilities at the current sheet during the pre-impulsive phase of a small B-class solar flare (Jeffrey et al 2018). We use Fast Fourier Transforms of high-cadence (1.7 s) IRIS slit-jaw observations, back mapping observed spatial frequency scales and growth rates to the reconnection site. In this work-in-progress, we aim to compare these parameters to those expected from theoretical current sheet processes, such as the tearing mode or Kelvin-Helmholtz instabilities. This provides an observational constraint (for a single flare case-study) on the current sheet processes contributing towards the small-scale breakdown of ideal MHD needed to trigger the onset of fast magnetic reconnection on the Sun.
Quasi-periodic pulsations (QPPs), characterized by periodic variations in flux, are universally observed during solar flares. QPPs are believed to be intimately related to the modulations of the flare energy release or transport processes. However, up to date, there is no conclusive interpretation of their physical nature. Here, we report a C1.8 confined flare on 2016 February 18. During its impulsive phase, radio and X-ray QPPs are observed. Utilizing the radio spectroscopic imaging technique provided by Karl G. Jansky Very Large Array (VLA), we found four distinct periodic radio sources in 1.0-2.0 GHz. One is apparently right-hand-polarized. It locates close to a large sunspot with a relatively high brightness temperature reaching 20 MK. It covers nearly the whole band in 1.0-2.0 GHz with a period of ~6 seconds. The other three relatively weak radio sources are located along a corona loop, two at different footpoints (with different polarization) and one at the loop top region. Their periods differ from 25 to 47 seconds. Concurrent X-ray QPPs are also observed from Fermi/GBM with a period of ~43 seconds. We present a detailed study of all the sources and their correlation, and discuss their physical nature and the energy release, transport, and modulation processes in the event.
We present high-resolution and multiline spectropolarimetric observations of a C2-class solar flare (SOL2019-05-06T08:47). The rise, peak, and decay phases of the flare were recorded continuously and quasi-simultaneously in the Ca II K line with the CHROMIS instrument and in the Ca II 8542 Å and Fe I 6173 Å lines with the CRISP instrument at the Swedish 1 m Solar Telescope. At the flare footpoints, a non-LTE inversion code (STiC) was employed to infer the temperature, magnetic field, line-of-sight (LOS) velocity, and microturbulent velocity. All the observed lines are inverted simultaneously in order to infer the stratification of the inferred parameters in the flaring and non-flaring atmosphere. The temporal analysis of the inferred temperature at the flare footpoints shows that the flaring atmosphere is heated up to ∼11 kK in the chromosphere. During the flare peak time, the LOS velocity shows both upflows and downflows around the flare footpoints in the chromosphere. Moreover, the temporal analysis of the LOS magnetic field at the flare points exhibits a maximum change of ∼600 G in the chromosphere. After the flare, the LOS magnetic field decreases to the non-flaring value, exhibiting no permanent or step-wise change. We also notice that the Ca ii lines exhibit enhanced sensitivity to the deeper layers of the flaring atmosphere compared to the non-flaring atmosphere. We suggest that a fraction of the apparent increase in the LOS magnetic field at the flare footpoints may be due to the increase in the sensitivity of the Ca II 8542 Å line in the deeper layers, where the field strength is relatively strong. The rest may be due to magnetic field reconfiguration during the flare. In the photosphere, we do not notice significant changes in the physical parameters during the flare or non-flare times. Our observations illustrate that even a less intense C-class flare can heat the deeper layers of the solar chromosphere, mainly at the flare footpoints, without affecting the photosphere.
We report on identification of new three-dimensional reconnection geometries in eruptive solar flares. These involve reconnection of the erupting flux rope either with the surrounding corona (ar-rf reconnection) or with itself (rr-rf), leading to creation of new flux rope field lines and flare loops. In addition, the CME legs are found to drift across the solar surface. The new reconnection geometries were observationally verified in multiple flares, including the discovery of the general saddle-shape of solar flare arcades, and observations of direct conversion of filament strands to flare loops.
The onset of eruptive flare energy release requires both a buildup of stored energy and a trigger for the release of that energy. This talk will review key models of how this storage and release occurs in solar eruptions, in particular for breakout eruptions and for torus instability eruptions. In both cases, the eruptions require the buildup of free magnetic energy in the form of sheared field. For the breakout mechanism the energy is built up as sheared magnetic fields in coronal arcades, while for the torus instability the energy is built up as a combination of axial and twist field in coronal flux ropes. We will review recent work on the buildup of this energy to eruptive states, both via velocity shearing at the photosphere and via the emergence of sheared flux from the convection zone into the corona. Then we will review recent work exploring how the emergence of new magnetic flux into the corona can act as a trigger for these eruptive events. Much of the recent work to be discussed here is being carried out within the framework of NASA’s Living with a Star focused science team on Understanding the Onset of Major Solar Eruptions.
This work is supported by the NASA Living with a Star program.
What happens in the solar atmosphere in the hours, minutes and seconds before a flare starts, and what is the relationship, spatial or otherwise, to the impulsive phase? Observational studies of flare onsets are relatively few compared to the later phases of a flare, but clearly we need to understand conditions both before and after the flare 'trigger' interval to be able to progress on particle acceleration, heating, instability and all of the other flare phenomena. I will review what is known observationally about flare onset, referring to CME onset where appropriate. Topics will include pre-flare heating, spectral line broadening, and the early appearance of faint flare ribbons.
The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray ‘onset’ interval of enhanced isothermal plasma temperatures in the range of 10-15 MK up to tens of seconds prior to the flare’s impulsive phase. This ‘hot onset’ interval occurs during the pre-flare time during which elevated GOES soft X-ray flux is detected, but prior to detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite (GOES) X-ray sensor, and confirmed with data from RHESSI, show no signs of gradual increase and occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we identify this early hot onset soft X-ray emission mainly within footpoint and low-lying loops, rather than with coronal structures, based on images from the Atmospheric Imaging Assembly (AIA) and the use of limb occultation. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenges the standard flare heating modelling techniques.
Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. For eruptive flares, the free energy source is ultimately the highly sheared magnetic field of a filament channel above a polarity inversion line. During the flare, the shear field becomes the reconnection guide field, the strength of which has recently been demonstrated via kglobal hybrid-MHD modeling to control the efficiency of reconnection-driven particle acceleration. We present new high-resolution 3D MHD simulations that demonstrate the critical role of the magnetic shear/guide field throughout an eruptive flare. The magnetic shear evolves in three distinct phases: shear first builds up in a narrow region about the PIL, expands outward to drive the formation of a thin current sheet, and is finally transferred by the flare reconnection into sheared post-flare loops and erupting flux rope. We show that the guide field weakens more than an order of magnitude over the course of the flare, and instantaneously varies over a similar range along the three-dimensional current sheet. We demonstrate how the guide field may be inferred from observations of sheared post-flare loops. Interestingly, we find that the number of plasmoids increases with weakening guide field, underscoring the important role of the guide field in particle acceleration. We discuss implications for observations by IRIS, SDO/AIA, and DKIST. This work was supported in part by the SolFER DRIVE center.
Current sheets play a key role in solar flares as they are the locations where magnetic energy is liberated through reconnection and is converted to other forms. Yet, their formation and evolution during the impulsive phase of a flare remain elusive. In this talk, we will report new observations of a current sheet formation and subsequent evolution in the early stages of a solar flare. In particular, we will present multiphase evolution of a dynamic current sheet from its formation to quasi-stable evolution and disruption. Implications for the onset and evolution of reconnection will be discussed.
Panel members: Angelos Vourlidas, Xudong Sun and Peter Wyper
Solar flares are efficient particle accelerators and prime laboratories for studying astrophysical acceleration and transport processes. Our understanding of electron acceleration and transport in flares has been enhanced by observationally-driven kinetic modelling and multi-wavelength observations from X-rays to (E)UV to radio. However, many questions remain about how and where energetic electrons are accelerated, and how different plasma environments (e.g., collisions, turbulence) affect their transport and importantly, our interpretation of their accelerated properties from observation. In this talk, I will review electron transport processes in the flaring corona. Moreover, I will discuss how transport modelling will help to constrain the electron angular distribution from upcoming joint observations with Solar Orbiter/STIX and X-ray missions at Earth. Finally, I will briefly discuss how the properties of the acceleration region might be constrained by combining observations and modelling of flare energetic electrons transported at the Sun and in the heliosphere.
I will discuss the transport of charged particles in energy- and position-space inside a convection zone-driven 3D turbulent solar corona. It is important to address the formation of the small- and large-scale 3D Current Sheets, the 3D turbulence, and the 3D large scale magnetic disturbances (let us call this environment “3D turbulent reconnection”) in the solar corona before addressing the transport of charged particles in energy (heating and acceleration) and space. The transport in space is an important parameter since the volume where the energy is released during turbulent reconnection is finite and defines the time particles remain trapped (escape time). The energization time (or acceleration time) of the particles is limited by the escape of particles from the fragmented energy release volume towards the heliosphere and/or the solar chromosphere. Numerous observations, especially in radio (groups of type III bursts, millisecond radio spikes, fast drifting radio bursts) and X-rays suggest that the energy dissipation in the low solar corona is fragmented.
Fermi proposed two mechanisms for particle acceleration (1949,1952). One is the stochastic scattering of charged particles off magnetic clouds and the other the systematic scattering off converging turbulent flows (as it appears in turbulent shocks). Adopting the methodology proposed by Fermi for the transport (scattering) of particles in a 3D turbulent reconnection environment, we have discovered that both mechanisms operate simultaneously. One controls the heating of the plasma (stochastic) and the other the acceleration of the nonthermal tail (systematic). The 3D turbulent reconnection environment inside a finite volume is a relatively new concept and many of the assumptions adopted by Fermi in his original analysis need revisions. We encounter turbulent reconnection environments in many places in the heliosphere (solar corona, solar wind, downstream of shocks, magnetosheath, magnetotail, termination shock) and laboratory plasmas (Edge Localized Modes). The transport of particles in energy and space are anomalous for the mildly relativistic particles in 3D finite volumes the high energy particles execute complicated trajectories (Levy walks) in a large-scale fragmented energy release volume, suggesting that in both, energy and space, respectively, Fractional Transport Equations are needed to analyze their behavior. On the other hand, the low energy particles are executing classical random walks in energy and remain still an open question for their transport in space, leading to their heating, which can be analyzed by the well-known Fokker Planck equation.
In summary, three fundamental questions are open, and we hope to settle them with the help of modern observational and modeling tools over the next decade: (1) How the complex magnetic topologies in the solar corona set up and maintain a turbulent reconnection environment? Several suggestions have been proposed, e.g. random shuffling of the magnetic filaments (loops) by the photospheric motions, emerging of new magnetic flux, 3D magnetic eruptions of unstable filaments, etc. (2) How the charged particles are transported in energy and space inside a finite turbulent reconnection environment? (3) What are the radiation signatures of the heated and accelerated particles inside the 3D turbulent reconnection and what are the observational signatures needed to test the above ideas?
The analysis proposed in this talk suggests a new vision for our models and observations for the next decade. We should find ways to approach the complex 3D global time-dependent connection of the photosphere with corona using an interacting systems method. The convection zone drives the emerged complex extrapolated fields forming millions of current sheets and other large-scale disturbances. The emerging magnetic flux interacting with the complex emerged magnetic fields leads the solar eruptions. Global eruption starts on large scales but quickly forms complex multi-scale environments where turbulent reconnection sets in. Large research consortia are needed to integrate the processes of turbulence, reconnection, radiation, and observations to understand the coupling of the convection zone, solar atmosphere, and the base of the solar wind.
We have carried out the first comprehensive investigation of enhanced line emission from molecular hydrogen, H$_{2}$ at 1333.79 Å, observed at flare ribbons in SOL2014-04-18T13:03. The cool H$_{2}$ emission is known to be fluorescently excited by Si IV 1402.77 Å UV radiation and provides a unique view of the temperature minimum region (TMR). Strong H$_{2}$ emission was observed when the Si IV 1402.77 Å emission was bright during the flare impulsive phase and gradual decay phase, but it dimmed during the GOES peak. H$_{2}$ line broadening showed non-thermal speeds in the range 7-18 km/s, possibly corresponding to turbulent plasma flows. Small red (blue) shifts, up to 1.8 (4.9) km/s were measured. The intensity ratio of Si IV 1393.76 Å and Si IV 1402.77 Å confirmed that plasma was optically thin to Si IV (where the ratio = 2) during the impulsive phase of the flare in locations where strong H$_{2}$ emission was observed. In contrast, the ratio differs from the optically thin value of 2 in parts of ribbons, indicating a role for opacity effects. A strong spatial and temporal correlation between H$_{2}$ and Si IV emission was evident supporting the notion that fluorescent excitation is responsible.
During solar flares, magnetic reconnection unleashes magnetic energy and drives strong electron acceleration and emission within minutes or shorter. Recent multi-wavelength observations (e.g., by EOVSA, RHESSI, and STIX) show that non-thermal radio and hard X-ray emissions could fill up a significant portion of the solar flare region. The electrons responsible for these emissions are thought to contain a substantial fraction of the released magnetic energy and often develop power-law energy tails with various spectral indices. In this study, we model the large-scale acceleration by solving the energetic particle transport equations using background MHD simulations with realistic boundary conditions to provide accurate magnetic field configuration and flow dynamics. Due to flow compression and shear effects, electrons can be accelerated to hundreds of keV and develop nonthermal power-law distributions, both of which are consistent with the observations. We quantify the relative importance of reconnection exhaust, magnetic islands, and flare looptop regions in accelerating nonthermal electrons and discuss the potential roles of second-order acceleration. The model-generated spatially and temporally dependent electron distributions can be used for producing synthetic radio or hard X-ray emission maps, which can be directly compared with radio and hard X-ray observations. These results have important implications for understanding large-scale electron acceleration during impulsive flares.
The Sun frequently accelerates near-relativistic electron beams that travel out through the solar corona and interplanetary space, producing type III radio bursts. The formation and motion of type III fine frequency structures is a puzzle but is commonly believed to be related to plasma turbulence in the solar corona and solar wind. Combining a theoretical framework with kinetic simulations and high-resolution radio type III observations using the Low Frequency Array, we quantitatively show that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. Our results highlight how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures in astrophysical plasma, both significantly expanding the current diagnostic potential of solar radio emission.
Panel members: Stuart Bale, Victor Melnikov, and Meriem Alaoui
Recent Parker Solar Probe (PSP) observations of several small SEP events show highly variable helium to hydrogen ratios over a factor of ~50 and evidence of variable ion compositions in other species. We use numerical simulations for calculating SEP propagation in a turbulent interplanetary magnetic field with a Kolmogorov power spectrum from large scale down to the gyration scale of energetic particles. We show that when the source regions for different species are offset by a distance comparable to the size of the source region, the observed energetic particle composition He/H can be strongly variable over more than one order of magnitude, even if the source region ratio is at the nominal value. Assuming a nominal 3He/4He ratio of 10% in small impulsive 3He-rich events and same source offsets, the 3He/4He ratio can also be quite variable. The variability of the ion composition ratios depends on the source offset, source size, and additional background source. We will discuss the implication of these results to the variability of ion composition of impulsive events and on further PSP and solar orbiter observations close to the Sun.
We used H$\alpha$, Ca II 8542, Ca II K line and Ca II K continuum point observations of an X9.3 flare on Sept 6th 2017 from the Swedish Solar Telescope and, where possible, hard X-ray maps from RHESSI to describe the morphology and evolution of a flare ribbon. This highlighted systematic variations of the line profiles over photospheric features including granulation, light bridges, penumbral, and umbral features such as umbral bright points, as well as a set of curious elongated features over Sunspot umbrae that may be the flare equivalents of umbral fibrils.
Moreover, small kernels of extremely broad, highly Doppler-shifted flare profiles have been reported in chromospheric line profiles since the mid-1900s. In the flare presented there were many emission profiles showing monotonically increasing intensity against wavelength, throughout the entire Fabry Perot etalon spectral windows. These occurred in both Calcium and Hydrogen lines. When faced head-on, these profiles imply something uncomfortable about the standard conceptions of chromospheric physics in flares. Modern instruments have also been designed in ways that coincidentally conceal this omission.
Plausible explanations of the broadening of these profiles include that there could be parts of the flare ribbons emitting in lines such as H$\alpha$ and Ca II 8542 that are much hotter than is being discussed. There could also be an exacerbation of the persistent underestimation of chromospheric line widths from forward modeling of non-flaring atmosphere. There may also be other explanations, such as alternative methods of energy transfer. We present warts-and-all observational evidence from chromospheric spectral lines during flares in order to force the evaluation of plausible explanations, to highlight future observation possibilities that could resolve these issues, and to invite insights from the wider solar community.
Solar flares are very efficient particle accelerators on a short timescale. The X-ray and type III radio emission emitted during a flare are direct signatures of the accelerated electrons. Hard X-rays are emitted from the accelerated electrons through bremsstrahlung radiation primarily in the dense atmosphere, while type III emissions are caused by the accelerated electrons propagating through the upper corona where they produce Langmuir waves which then converts to radio emission near the local plasma frequency or its harmonics. The aim of the present study is to understand the link between these two different electron populations. The analysis is based on a list of HXR/ type III radio bursts observed by RHESSI or FERMI/GBM, the Nançay Radioheliograph and the PHOENIX/ORFEES/NDA spectrographs in the 1000 GHz-10 MHz range in the 2002-2015 time period. For the list of almost 200 events, we analysed the relationship between the energetic electrons producing the HXR emissions and the type III radio fluxes at different frequencies. We shall present and discuss here some of the results: the correlation between the number of HXR emitting electrons and the peak flux of the type III emissions decreases with increasing frequency and the correlation is better for electrons above 20 keV than for electrons above 10keV. We also find that the peak radio flux is anti-correlated with the electron power-law index deduced from low electron energies from the HXR spectral analysis. These results will be briefly discussed in the context of the numerical simulations or models describing the production of type III bursts in the corona.
Solar type III radio bursts are generated by beams of energetic electrons that travel outward along open magnetic field lines through the corona and the interplanetary space. Here we report a type III burst event observed jointly by the Expanded Owens Valley Solar Array (EOVSA) and the Parker Solar Probe (PSP) shortly after its second perihelion in April 2019. This type III burst event is associated with a solar jet near the western boundary of a solar active region, which manifests in EOVSA 1–18 GHz dynamic spectrum as a group of impulsive microwave bursts. The type III burst event continues to the interplanetary space in the decameter–kilometer wavelength range (300 kHz–30 MHz) observed by multiple spacecraft including PSP/FIELDS, Wind/WAVEs, and STEREO/WAVES, and appears to reach the local plasma frequency at the PSP spacecraft. The multi-point spacecraft measurements allow us to constrain the source location of the bursts and their directivity in the interplanetary space. In addition, the type III burst event coincides with an enhanced suprathermal electron population with an anti-sunward beam-like component as measured by PSP/SWEAP. We discuss the source region of the type-III-burst-emitting energetic electrons and their transport from near the solar surface to the interplanetary space.
Helioseismic response to solar flares ("sunquakes") occurs due to localized force or/and momentum impacts observed during the flare impulsive phase in the lower atmosphere. Such impacts may be caused by precipitation of high-energy particles, downward shocks, or magnetic Lorentz force. However, the current theories of solar flares are unable to explain the origin of sunquakes. Our statistical analysis of M-X class flares observed by the Solar Dynamics Observatory during Solar Cycle 24 has shown that contrary to expectations, many relatively weak M-class flares produced strong sunquakes, while for some powerful X-class flares, helioseismic waves were not observed or were weak. The analysis also revealed that some active regions were characterized by the most efficient generation of sunquakes during the solar cycle. We found that the sunquake power correlates with maximal values of the X-ray flux derivative better than with the X-ray class, indicating that the sunquakes are associated with energetic particles. The impulsive nature of seismic flares hints that they are compact with a fast energy release rate, suggesting that low-lying short magnetic loops are involved in the flare-energy release process.
Type III radio bursts are usually associated with energetic electrons that are accelerated by solar flares and propagate out from the corona. The standard theoretical paradigm links these emissions to a conversion of electrostatic Langmuir oscillations excited by the bump-on-tail instability into electromagnetic waves. Since the electron beams are observed to propagate to large heliospheric distances where they continue to generate Langmuir and radio waves, the instability must be finely balanced so as not to completely disrupt the beam propagation. In this study, we perform 2D PIC simulations in a large system and without imposing periodic boundary conditions to more accurately model the beam propagation, its interaction with background plasma and the evolution of the waves. The results demonstrate that the beam decouples from electrostatic oscillations it excites in the injection region and propagates freely through the background plasma. Only 15% of the beam energy density is lost during the initial relaxation process. The instability continues to operate only at the front of the beam, where velocity gradients are maintained by the time-of-flight effects. The main body of the beam reaches a quasi-steady state, where it no longer loses energy to wave generation. This stable beam can then propagate to larger heliospheric distances. Radio emissions at plasma frequency and its second harmonic are observed in the simulation. They are generated primarily near the injection region, where classical signatures of the three-wave conversion processes are detected. The main body of the beam may become unstable again and generate radio emissions only if it propagates into regions where background plasma is colder or less dense. The simulation results are consistent with satellite data that show that electron beams often continue to generate Type III radio bursts even beyond 1 AU.
We report on the Atmospheric Imaging Assembly observations of plasma outflows from a coronal dimming during the 2015 April 28 filament eruption. After the filament started to erupt, two flare ribbons formed, one of which developed a well-visible hook enclosing a core dimming region. Along multiple funnels located inside this dimming region, outward-directed motions of plasma started to be visible in the 171 and 193 filter channels of the instrument. Time-distance diagrams constructed along the funnels revealed a rib-like pattern indicating periodic outflows of plasma with velocities between 70 and 140 km/s, persisting for more than five hours. We briefly discuss the processes which can lead to the formation of outflows from dimming regions at such long timescales. The characteristics of the outflows were similar to those we observed in an ordinary coronal hole located in the vicinity of the dimming region. This indicates that the outflows were possibly signatures of solar wind flowing along the field lines extending from the dimming region which `opened’ during the eruption. To our knowledge, our observations present the first imaging evidence for plasma outflows from a dimming region.
The computational model kglobal was developed to explore energetic
particle production via magnetic reconnection in macroscale systems.
It is based on the observation that the production of energetic
particles during reconnection is controlled by Fermi reflection in
large-scale magnetic fields and not by parallel electric fields
localized in kinetic scale boundary layers. Earlier work with kglobal
has produced the first self-consistent simulations of non-thermal
electron acceleration during reconnection in a macroscale system.
Although the original formulation of kglobal only treated the
nonthermal energization of electrons, the equations can be extended to
include ions as long as they remain magnetized so that a guiding
center description remains valid. In this poster we discuss the
appropriate extension of the kglobal equations and preliminary results
from the simulations.
Observations of impulsive solar energetic particle events occasionally show enhancements of helium-3 up to ~104 greater than coronal abundances. Fisk (1978) and Temerin and Roth (1992) proposed that these enhancements could be caused by ion cyclotron waves and that helium-3 would be preferentially heated due to its unique cyclotron frequency. In order to test this theory, we have run kinetic simulations of magnetic reconnection in the corona using the particle-in-cell code p3d. Initial parameters were set to coronal values: 5% helium-4 number density, β equal to 0.1, and the guide field equal to 0.5 times the reconnecting field. Initial results showed instabilities develop along the separatrices of the reconnection outflows, though temperature anisotropies associated with ion cyclotron waves were not present. Fourier transforms in time of the parallel electric field at multiple locations along the separatrices showed frequencies between 1.2 and 1.6 times the proton cyclotron frequency. A second simulation with the same parameters including helium-3 as test particles is being performed to investigate the effects on helium-3.
In this talk I will demonstrate with time-dependent Fokker-Planck calculations that steady injection of energetic electron beams into loop legs is compensated by returning electrons scattered the precipitating beams combined with the accelerated electrons from the ambient plasma. We evaluate the time for establishing the electric circuit of precipitating and returning electrons. We also evaluate a proportion of the precipitating an ambient electrons contributing to the return current and their resulting HXR emission in loop legs affected by collisions, Ohmic losses, pitch-angle scattering in converging magnetic field. By evaluating velocities of precipitating and returning electrons it was shown that the particle density numbers producing the observed HXR emission cannot exceed 1-10% of the ambient coronal density.
During solar flares, a large flux of energetic electrons propagate from the tops of reconnecting magnetic flux tubes toward the lower atmosphere. Over the course of the electrons' transport, a co-spatial counter-streaming return current is induced, thereby balancing the current density. In response to the return current electric field, a fraction of the ambient electrons will be accelerated into the runaway regime. We develop a model in which an accelerated electron beam drives a steady-state, sub-Dreicer co-spatial return-current electric field, which locally balances the direct beam current and freely accelerates a fraction of background (return-current) electrons. The model is self-consistent, i.e., the electric field induced by the co-evolution of the direct beam and the runaway current is considered. We derive the range of injected beam fluxes for which the collisional (Ohmic) treatment of the return current is no longer acceptable. We find that (1) the return current electric field can return a significant number of suprathermal electrons to the acceleration region, where they can be further accelerated to higher energies, runaway electrons can be a few tens of percent of the return current flux returning to the nonthermal beam's acceleration region, (2) the energy gain of the suprathermal electrons can be up to 10-35 keV, (3) the heating rate in the corona can be reduced by a factor three for medium range injected fluxes in comparison to models which neglect the runaway component. The results depend on the injected beam flux density, the temperature and density of the background plasma.
The role of whistler waves in scattering energetic electrons as they undergo acceleration during magnetic energy release in solar flares is explored with particle-in-cell (PIC) simulations and in an analytic model. Energetic electrons accelerated in flares reach relativistic velocities. The transit time of these energetic electrons across the energy release region in flares (< 0.1s) is much shorter than the energy gain time of these electrons (~10s). This disparity of time scales is associated with the fact that the Alfvén speed in the corona is much smaller than the velocity of light. Further, there is evidence from observations that energetic electron fluxes into the chromosphere are significantly below those measured higher in the corona, suggesting that electrons are self-confined. PIC simulations have established that oblique whistler waves can resonate with and scatter energetic electrons through high-order cyclotron resonances (Roberg-Clark et al., 2019). However, these simulations did not provide sufficient information to establish a complete model for electron scattering in flares. It remains unclear if the whistlers are able to scatter field-aligned, high-energy electrons past 90 degrees in pitch angle. Further, because the energy release regions in flares are enormous compared with all kinetic scales, PIC models are typically only able to explore systems with free energy that greatly exceeds that expected in real flares with the result that scattering time scales are far shorter than expected in real systems. We have begun a modeling effort based on both PIC simulations and analytic analysis to explore how gradients in distributions of energetic electrons self-consistently drive whistlers and are scattered. The goal is establish the physics basis for describing whistler growth and saturation along with electron scattering rates for realistic flare energy release domains.
Radio zebras are detected in radio observations from Sun, Jupiter, and also from the Crab pulsar. They are Type IV radio fine structures and can diagnose the local density, magnetic field, and velocity distribution of the particles released into the magnetic loops during the solar flares. The double plasma resonance model of solar radio zebra assumes the dense and cold background isotropic plasma and rare hot component with a loss-cone type of distribution function for $\omega_\mathrm{pe} > \omega_\mathrm{ce}$. This constitution generates the upper-hybrid waves that transform into electromagnetic radiation and emit into a narrow cone towards the observer.
We used analytical theory and 3D electromagnetic relativistic Particle-in-Cell simulations for analyses of this instability. For DGH velocity distribution function of hot electrons, we found that increasing the temperature the growth-rate maxima shift to lower values of ratio $\omega_\mathrm{pe} / \omega_\mathrm{ce}$ and the maxima are not distinguishable for temperature $v_\mathrm{t} \geq 0.3\, c$. We estimated the brightness temperatures, physical size, the energy density in the double plasma resonance region, and the conversion rates to electromagnetic waves from zebra observations. We also calculated the growth rates for loss-cone power-law and loss-cone Kappa distribution functions and different loss-cone angles.
$^3$He-rich solar energetic particles (SEPs) are believed to be accelerated in solar flares or jets by a mechanism that depends on the ion charge-to-mass ($Q/M$) ratio. It implies that the flare plasma characteristics (e.g., temperature) may be effective in determining the elemental abundances of $^3$He-rich SEPs. This study examines the relationship between the suprathermal ($<$0.2 MeV nucleon$^{-1}$) abundances of the He$-$Fe ions measured on the Advanced Composition Explorer and temperature in the solar sources for 24 $^3$He-rich SEP events in the period 2010$-$2015. The differential emission measure technique is applied to derive the temperature of the source regions from the extreme ultraviolet imaging observations on the Solar Dynamics Observatory. The obtained temperature distribution peaks at 2.0$-$2.5 MK that is surprisingly consistent with earlier findings based on in-situ elemental abundance or charge state measurements. We have found a significant anti-correlation between the $^3$He/$^4$He ratio and solar source temperature with a coefficient of $-$0.6. It is most likely caused by non-charge-stripping processes, as both isotopes would be fully ionized in the inferred temperature range. This study shows that the elemental ratios $^4$He/O, N/O, Ne/O, Si/O, S/O, Ca/O, Fe/O generally behave with temperature as expected from abundance enhancement calculations at ionization equilibrium. The C and Mg, the two species with small changes in the $Q$/$M$ ratio in the obtained temperature range, show no such behavior with temperature and could be influenced by similar processes as for the $^3$He/$^4$He ratio.
Active Region 12673 produced an X8.2 flare on September 10, 2017 at around 16:00 UT when it was rotating to the West limb of the Sun. The flare was partially occult to ground telescopes, therefore a significant fraction of the photospheric and part of the chromospheric emission may have not been observed from Earth. The Solar Submillimeter Telescope (SST) registered intense radiation at 212 and 405 GHz: the submillimeter emission during flares is often attributed to synchrotron radiation of relativistic electrons of sources at chromospheric heights. In this work we present flux density time profiles and source centroid positions determined with the SST unique multibeam method and comparisons with hard X- and gamma-rays from RHESSI and FERMI satellites.
Solar Orbiter, a joint ESA/NASA mission, is to study the Sun and inner heliosphere in greater detail than ever before. Launched in February 2000, Solar Orbiter already completed its first orbit in reaching perihelion of 0.5 au from the Sun in June 2000.
Understanding the physical processes operating in Solar Energetic Particle (SEP) events is a major goal of the Solar Orbiter mission because of the importance of acceleration processes in solar system and astrophysical sites, and because of the potential impact of these events on space hardware. The Energetic Particle Detector (EPD) investigation on Solar Orbiter is a suite of four different sensors plus the instrument control unit to measure the energetic particles from slightly above solar wind energies to hundreds of MeV/nucleon. We report here data from Suprathermal Ion Spectrograph (SIS) sensor of the EPD that covers the energy range of 0.1 – 10 MeV/nucleon for H-Fe with high mass resolution during the first orbit. SIS observed several 3He-rich SEP events inside of 1 au. Even though these events were small, their spectral forms, 3He content, and association with type III bursts convincingly identifies them as 3He-rich SEP events with properties similar to those previously observed at 1 au.
Solar flare and magnetotail observations show simultaneous acceleration of ions and electrons into power-law energy distributions extending to high energy. This suggests a common reconnection acceleration process but the underlying physics is not well understood. During magnetic reconnection, energetic particles undergo a universal Fermi acceleration process involving the curvature drift of particles. However, the efficiency of this mechanism is limited by the trapping of energetic particles within flux ropes. Using 3D fully kinetic simulations, we demonstrate that the flux-rope kink instability leads to field-line chaos in weak-guide-field regimes where the Fermi mechanism is most efficient, thus allowing particles to transport out of flux ropes and undergo further acceleration. As a consequence, both ions and electrons form clear power-laws which contain a significant fraction of the released energy. The low-energy bounds, which control the nonthermal energy contents, are determined by the injection physics, while the high-energy cutoffs are limited only by the system size. These results have strong relevance to observations of nonthermal particle acceleration in both the solar corona and magnetotail.
We create combined data-based models of magnetic reconnection, and particle acceleration and transport in actual solar flares. This is done by combining 3D MHD simulations initialised using non-linear force-free reconstruction of the coronal magnetic field, with relativistic guiding-centre test-particle simulations of particle acceleration and transport. Using the obtained trajectories for a large number of electrons and protons, we evaluate the locations where the energetic particles precipitate and escape, and calculate bremsstrahlung hard X-ray emission.
Comparison of the results with observations shows that our approach is able to predict some key characteristics of energetic particles in the considered flares, including the locations and relative intensities of hard X-ray and radio emission, as well as locations of helioseismic sources. It can also be used for interpreting in-situ observations of energetic particles in the inner heliosphere by Solar Orbiter and Parker Solar Probe. Hence, the adopted approach can be used for comprehensive observationally-driven modelling of magnetic fields, thermal plasma and non-thermal particles in individual solar flares and their observables.
We analyze the coronal elemental abundances during a small flare using Hinode/EIS observations. Compared to the preflare elemental abundances, we observed a strong increase in coronal abundance of Ca xiv 193.84 Å, an emission line with low first ionization potential (FIP < 10 eV), as quantified by the ratio Ca/Ar during the flare. This is in contrast to the unchanged abundance ratio observed using Si x 258.38 Å/S x 264.23 Å. We propose two different mechanisms to explain the different composition results. First, the small flare-induced heating could have ionized S, but not the noble gas Ar, so that the flare-driven Alfvén waves brought up Si, S, and Ca in tandem via the ponderomotive force which acts on ions. Second, the location of the flare in strong magnetic fields between two sunspots may suggest fractionation occurred in the low chromosphere, where the background gas is neutral H. In this region, high-FIP S could behave more like a low-FIP than a high-FIP element. The physical interpretations proposed generate new insights into the evolution of plasma abundances in the solar atmosphere during flaring, and suggests that current models must be updated to reflect dynamic rather than just static scenarios.
We have developed new common analysis tools to study archival solar-flare gamma-ray data from three satellite spectrometers. These tools are based on a revised set of spectral templates incorporating the latest nuclear line cross sections that are used to fit flare spectra in combination with electron-produced continua. We have applied these toolsto study emission from 19 SMM, 1 RHESSI, and 1 Fermi/GBM flares with the commonly used OSPEX Solar SoftWare. Here we discuss the methods used in a representative flare from each of the instruments. These studies can provide power-law spectral indices of the accelerated ions, the accelerated alpha/proton ratios, heavy-ion and 3He compositions, the abundances of the ambient material in which the ions interact, and the heavy ion/(proton+alpha) ratios in flares. Ion acceleration onto magnetic loops with turbulence is consistent with the observations. We discuss how well the three different detectors provided information on these target parameters. Significant advances in our understanding of solar-flare particle acceleration and interaction are possible with instruments having larger effective area and volume.
Panel members: Vahe Petrosian, Glenn Mason and Melissa Pesce-Rollins
This talk will provide a review of how electron energization in solar flares can be investigated through multi-wavelength observations.
Signatures of flare accelerated electrons and plasma heating are most readily observed at X-ray, radio, and extreme ultra-violet wavelengths. Observations at these wavelengths provide information on electron energization and transport, such as, where electrons are accelerated, how much energy they contain or where they deposit this energy. In addition, properties of the surrounding plasma, like temperature and density, and the coronal magnetic field strength, can be inferred.
Of particular interest is the acceleration region itself and its surroundings. I will describe the nature of observations at different wavelengths and what we can learn from them about electron energization and the acceleration region. The most important tools we have for multi-wavelength exploitation of the data will be presented along with highlights of the past years from multi-wavelength studies. I will conclude with a brief outlook onto anticipated future progress with observatories such as Solar Orbiter.
We conduct the wide-band X-ray spectral analysis in the energy range of 1.5 keV-100 keV and study the time evolution of the thermal and non-thermal emission in the July 23, 2016 M7.6 Class solar flare observed by the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). As a result, the time evolution of the non-thermal and multi-thermal components can be obtained with a resolution of 10 seconds cadence, which corresponds to the Alfv?en time scale in the solar corona and it makes possible to track the detailed spectral phases as the flare progresses. A maximum of three temperature components: a "cool" plasma (T~3 MK), a "hot" plasma (T~15 MK), and a "super-hot" plasma (T~30 MK) have been detected and the emission measure of cool and hot thermal components is drastically increasing more than hundreds of times as the non-thermal emission becomes harder. This detailed time evolution information is a key to estimate each emission mechanism even though MinXSS has no spatial information. By comparing the 17 GHz radiowave flux observed by the Nobeyama Radio Polarimeters (NoRP) satellite and the spatial information obtained by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), we find that a cool and a hot plasma thermal emission are related to chromospheric evaporation and a super-hot thermal emission may come from the thermalization of the non-thermal electrons trapped in the flaring loop.
The details of electron acceleration in solar flares is still an open question. I will be presenting a brief overview on the observations and challenges before describing results from a new model. Electron distributions in solar flares typically take the form of a thermal core with a power law tail. These nonthermal electrons contain more energy than the thermal electrons in 85% of solar flares and have spectral indices ranging from ~3-6. Additionally, the nonthermal electrons produce hard x-rays through bremsstrahlung radiation when they thermalize in the chromosphere. This strong signal produced by nonthermal electrons makes understanding the energization process important for characterizing solar flares. Unfortunately, the huge separation between kinetic and macro scales makes simulating solar flares challenging. Recently, results from particle-in-cell simulations suggest that Fermi reflection is the dominant mechanism for electron energy gain. I will present results from a new model that includes this relevant kinetic physics but is valid in a macroscale system. Consistent with solar flare observations the spectra of energetic electrons take the form of power-laws that extend more than two decades in energy. The drive mechanism for these nonthermal electrons is Fermi reflection in growing and merging magnetic flux ropes. A strong guide field suppresses the production of nonthermal electrons by weakening the Fermi drive mechanism. For a weak guide field the total energy content of nonthermal electrons dominates that of the hot thermal electrons even though their number density remains small. Our results are benchmarked with the hard x-ray, radio and extreme ultra-violet (EUV) observations of the X8.2-class solar flare on September 10, 2017.
Quasi-periodic pulsations in flare emission may provide important information about the underlying energy release process. Here we investigate how reconnection, in the absence of external oscillating driving, may naturally generate oscillations, and we forward model the observable emission. Firstly, we consider 3D MHD simulations of a flaring twisted coronal loop with multiple reconnection sites, allowing for inclusion of a population of non-thermal particles produced at current sheets. We predict oscillations in the intensity of microwave emission in the range 1-15 GHz Two types of oscillations are identified: (a) slowly decaying oscillations with period about 70-75s and relative amplitude 5-10%, seen both in loops with and without energetic particles, and (b) a more transient burst of shorter period and larger amplitude oscillations, only in the loop with energetic particles. We interpret the longer period oscillations as the result of a global sausage mode modulating the average magnetic field strength in the loop, while fast intermittent oscillations associated with energetic particles are likely to be produced by fast variations of the electric field, responsible for energy release particle acceleration in this scenario. Secondly, a more generic MHD model is explored, with reconnection at a single current sheet formed as two twisted flux ropes merge, in order better to understand how reconnection can drive waves. We find the reconnection is oscillatory, and we investigate how this generates MHD waves propagating away from the reconnection site.
Non-thermal electrons accelerated by solar eruptive events can excite Langmuir waves which can convert into radio radiation through the plasma emission mechanism. These radio emissions are directly related to the local plasma frequency, and thus used as a remote-sensing diagnostic of the local coronal conditions. Recent high-resolution LOFAR observations conducted at sub-second scales have allowed for unprecedented imaging of the sources of Type II solar radio bursts, emissions driven by shock waves in the corona. A Type II burst that transitions between a stationary state (where no frequency-drift is observed) to a drifting state was observed for the first time. The emissions were temporally and spatially related to an erupting jet and a streamer-puff CME. Unlike previous observations that relate stationary Type II bursts to termination shocks in solar flares, this observation has been related to the interaction between the streamer and the CME-driven shock that followed the jet eruption, accelerating electrons near the flanks of the CME where the shock is believed to be quasi-perpendicular to the local magnetic field. The location, however, of Type II radio sources with respect to the shock front has been debated. Turbulence in the solar corona affects the propagation of photons, distorting the true properties of radio sources and subsequently affecting the interpretation of the coronal properties. For example, scattering causes radio sources to appear farther from the Sun that their true emission location. Using a LOFAR observation of a Type II burst that experiences band splitting, it was shown that sources which appear to be spatially separated in images could in fact be co-spatial, once the frequency-dependent scattering shift was taken into account. These findings demonstrate the importance of considering radio-wave propagation effects when attempting to localise radio sources to characterise the origins of the emissions and hence the electron acceleration mechanism.
Panel members: Joel Dahlin, Mitsuo Oka and Philippa Browning
The corona of the Sun is built up by loops defined through the magnetic field. With high resolution observations made possible by new instruments, coronal structures can be increasingly well resolved. Observations show individual strands with diameters down to a few 100 km, and so far it remains open what defines these strands, in particular their width, and which processes lead to their heating.
The aim of our study is to understand how the magnetic field couples the different layers of the solar atmosphere, how energy generated by magnetoconvection is transported into the upper atmosphere and dissipated, and how this process determines the scales of observed bright strands in the loop.
To this end, we conduct high resolution 3D resistive MHD simulations with the MURaM code.
We study an isolated coronal loop that is rooted in a shallow convection zone layer. To properly resolve the internal structure of the loop, the coronal loop is modelled as a straightened magnetic flux tube.
We study the spatial scales and time scales of energy transport and find that motions on small spatial scales play an important role for the energy transport in the loop model.
The energy injected into the loop is generated by internal coherent motions, including persistent vortex motions, within strong flux tubes with near-kilogauss magnetic field strengths. While the loop shows signs of twisting and braiding by those internal motions, leading to small-scale reconnection events, there is little evidence of heating by braiding of magnetic field from different magnetic concentrations at a footpoint.
Turbulent behaviour develops in the upper atmospheric layers in response to the photospheric driving.
The coronal loop responds to the heating with the formation of bright transient strands.
The energy needed to heat these strands is supplied internal motions within a magnetic concentration.
With this model we can build a coherent picture of how energy and matter are transported into the upper solar atmosphere and how these processes structure the interior of coronal loops.
Observations of solar flare reconnection at very high resolution can be indirectly made at the footpoints of reconnected loops into which flare energy is deposited. The response of the lower atmosphere to this energy input includes a downward-propagating shock called chromospheric condensation, which can be observed at wavelengths including UV and visible. In order to characterize reconnection using high resolution observations of this shock, one must develop a quantitative relationship between the two. Such a relation was recently developed in previous work and here we test it on observations of chromospheric condensation in a single footpoint in the flare ribbon of the X1.0 flare SOL2014-10-25T16:56:36. Measurements taken of Si\,\textsc{iv}\,1402.77\,\AA\ emission spectra with the Interface Region Imaging Spectrograph (IRIS) using a 5\,s cadence show a red-shifted component undergoing typical condensation evolution, with a peak downward velocity of 35\,km\,s$^{-1}$ and a half-life of 16\,s. Simultaneous observations taken with the Atmospheric Imaging Assembly (AIA) reveal a temporally and spatially correlated increase in UV emission in the 1600\,\AA\ band. We apply a technique called the Ultraviolet Footpoint Calorimeter (UFC) to the 1600\,\AA\ lightcurve to infer the energy deposition into the footpoint. We then input this energy into a one-dimensional, hydrodynamic simulation to compute the chromospheric response, including condensation. From this simulation we synthesize Si\,\textsc{iv} spectra and compute the time-evolving Doppler velocity. This is found to compare reasonably well with the IRIS observation, thus corroborating our reconnection-condensation relationship.
Particles are accelerated to very high, non-thermal energies during explosive energy-release phenomena in the solar corona and Earth’s magnetotail. While some similarities and differences of the particle acceleration in these environments have been discussed in the literature, it remains unclear exactly how the particle energy spectrum evolves during the energy-release process. Here we show that the electron energy spectrum can be mostly represented by the kappa distribution throughout an event in which NASA’s MMS spacecraft observed the entire sequence of a substorm (i.e., the explosive energy-release phenomena in Earth’s magnetotail). We present electron spectra from the different phases of magnetotail reconnection for this event, including energy build-up, slow energy release, and explosive energy release. We demonstrate that the power-law index δ (~3.5) is strikingly constant throughout these phases and is similar to the power-law indices derived from the hard X-ray measurements of solar flares (both coronal and chromospheric sources). We envision that such a detailed analysis of Earth’s magnetotail data will be helpful for a better understanding of possible scaling laws in particle acceleration as well as how universal they are.
A 'proof of principle' is presented, whereby the Ohmic and viscous heating determined by a three-dimensional (3D) MHD model of a coronal avalanche are used as the coronal heating input for field-aligned, one-dimensional (1D) hydrodynamic modelling.
Three-dimensional MHD models cannot afford the computational resources to follow the magnetic field and the thermodynamic transport along field lines with realistic parameters.
From a 3D MHD simulation, we extract the heating along single field lines and use these heating functions for 1D simulations that follow transport of energy.
Proceeding from simple, ordered photospheric motions, this heating is spatially localized, dispersed, and impulsive, occurring in discrete, reconnection-facilitated bursts.
MHD heating is shown to sustain coronal temperatures and densities, around $1\,\mathrm{MK}$ and $10^{14}\text{-}10^{15}\,\mathrm{m}^{-3}$ respectively, in a $90\,\mathrm{Mm}$ loop.
Thermodynamic feedback on the plasma dynamics is limited, and the MHD evolution is largely robust to the field-aligned thermodynamic response.
Advantages and drawbacks of the 3D and 1D models, within their respective spheres, are discussed and compared.
Both models report similar temperature and density, but velocities diverge.
Heating causes strongly asymmetric plasma flows, which differ significantly between 3D and 1D models and may have observable signatures.
Velocities in the 1D model are comparable with 3D reconnection jets in the MHD model.
Solar flare observations at mid infrared (MID-IR) wavelengths is a relatively new tool that allows to understand the dynamics of flares at chromospheric heights. We present the analysis of a C2.0 class flare, which was observed at the frequency of 30 THz (7.5-13 $\mu$m) with a commercial thermal camera attached to the focus of a Newtonian 20-cm telescope. In order to characterize the temporal evolution of the flare emission, we compare our Mid-IR observations with data from multiple wavelengths and instruments including ultraviolet emission, soft X-rays, microwaves and H$\alpha$ images. The 30 THz emission shows an excellent temporal and spatial agreement with the other chromospheric passbands, such as 1600 and 1700 \AA\ observed by AIA. No white light emission was detected. The event presents two bright sources in the Mid-IR, which can be spatially associated with several bright kernels observed in UV. At microwaves the event can be characterized as an optically thin thermal source. We use magnetograms and UV images to reconstruct the magnetic field configuration and evolution. Based on all this information, we discuss possible mechanisms of energy transport related to the origin of the chromospheric emission.
Solar flares are generally thought to be the impulsive release of magnetic energy giving rise to a wide range of solar phenomena that influence the heliosphere and in some cases even conditions of earth. Part of this liberated energy is used for particle acceleration and to heat up the solar plasma. The Spectrometer/Telescope for Imaging X-rays (STIX) instrument onboard the Solar Orbiter mission launched on February 10th 2020 promises advances in the study of solar flares of various sizes. It is capable of measuring X-ray spectra from 4 to 150 keV with 1 keV resolution binned into 32 energy bins before downlinking. With this energy range and sensitivity, STIX is capable of sampling thermal plasma with temperatures of ≳ 10 MK, and to diagnose the nonthermal bremsstrahlung emission of flare accelerated electrons. During the spacecraft commissioning phase in the first half of the year 2020, STIX observed 69 microflares. Of this set, 26 events could clearly be identified in at least two energy channels, all of which originated in an active region that was also visible from earth. These events provided a great opportunity to combine the STIX observations with the multi-band EUV imagery from the Atmospheric Imaging Assembly (AIA) instrument on board the earth orbiting Solar Dynamics Observatory (SDO). For two of the largest events (GOES class B6 and B2), it was possible to perform a spectroscopic analysis and to fit the spectra assuming thermal and nonthermal sources. These results are combined with plasma diagnostics derived from AIA EUV images. To this aim, the Differential Emission Measure (DEM) was reconstructed from AIA observations to infer maps of plasma temperatures and EM in the flaring regions. This allows us to supplement the information gained with STIX with high spatial resolution diagnostics of thermal plasma from the DEM reconstructed from AIA data.
Time delays between sub-THz (> 100 GHz) and soft X-ray emission from solar flares with the positive spectral slope at sub-THz frequencies are considered. For 11 solar events we did the cross correlation analysis of light curves obtained with KOSMA (230 GHz), SST (212 GHz), RT-7.5 (93 GHz), and GOES satellites in the 1-8 Å channel in order to detect the Neupert effect. All flares were divided into two types. Type I includes four X -class flares with well pronounced Neupert effect. Type II includes seven М-class flares where Neupert effect was not clearly defined, and time delays do not exceed 30 s.
To explain the obtained results within the framework of the thermal chromospheric model we consider hard X-ray and radio emission from the SOL2012-07-04T09:55 solar flare with the positive spectral slope at sub-THz frequencies. The temporal evolution of thermal bremsstrahlung of chromospheric plasma observed at sub-THz frequency range from this flare has been studied. For that reason we employ F-CHROMA models based on RADYN code which describes the response of chromospheric plasma to the flux of accelerated electrons in the triangular pulse. The region of low temperature and high plasma density in a flare chromospheric source, which moves to the higher altitudes and absorbs the sub-THz emission has been revealed. The calculated time profile of sub-THz emission suggests that we cannot eхplain Type II events in terms of the proposed model. The plasma of solar chromosphere heated by accelerated coronal electrons produces the sub-THz emission several times less than the observed sub-THz emission fluxes during the SOL2012-07-04T09:55 flare. The interpretation of the results is proposed.
Ambient particles passing through reconnecting current sheets gain substantial energy and pitch angle distributions imposed by magnetic field topology. By applying particle-in-cell (PIC) approach in 3D current sheets with single and multiple X and O-nullpoints, or magnetic islands, we explore energy and pitch angle distributions (PADs) of accelerated particles. We show that particles of opposite charges are separated at ejection from the current sheet into opposite semiplanes from the current sheet midplane. Particles of the same charge are also divided onto transit and bounced particles which are shown to gain different energy and pitch angle distributions. We present a few examples of virtual satellite crossings of the X and O-nullpoints and compare their PADs with available observations of electron PADS from WIND, STEREO and Parker Probe payload.
We are planning a new solar satellite mission, "PhoENiX", for understanding of particle acceleration during magnetic reconnection, which are ubiquitous features exhibited by a wide range of plasmas in the universe. The main observation targets of this mission are solar flares, which are generated by magnetic reconnection and accelerate plasma particles. The sun is a unique target in the sense that it can be investigated in great detail with good spatial, temporal and energy resolutions.
The scientific objectives of this mission are (1) to identify particle acceleration sites, (2) to investigate temporal evolution of particle acceleration, and (3) to characterize properties of accelerated particles, during solar flares. In order to achieve these science objectives, the PhoENiX satellite is planned to be equipped with three instruments of (1) Photon-counting type focusing-imaging spectrometer in soft X-rays (up to ~10 keV) to observe the contexts of particle accelerations (e.g., shocks, plasmoids, flows, etc.), (2) Photon-counting type focusing-imaging spectrometer in hard X-rays (up to ~30 keV) to identify the accelerated particles, and (3) Spectropolarimeter in soft gamma-rays (spectroscopy is available in the energy range of from > 20 keV to < 600 keV; spectropolarimetry is available from > 60 keV to < 600 keV) to detect the anisotropy of accelerated particles. We plan to realize PhoENiX satellite mission in Solar Cycle 26 (in 2030').
In this presentation, we explain the details of science goal and objectives, and instruments of PhoENiX mission.
A mechanism for the proliferation of large numbers of slow shocks upstream of
reconnecting current layers during impulsive flares is explored. A surprising result of recent macro-scale kinetic simulations of electron energy gain during reconnection was the discovery of the generation of large numbers of slow shocks that extend far upstream of the reconnecting current layer (Arnold et al., 2021). The formation mechanism of these shocks and their role in producing the hot thermal electrons in impulsive flares are being explored with the macroscale simulations (kglobal and ARMS codes) and supporting PIC simulations. The formation of these slow shocks upstream is a consequence of the rapid growth, merging, and ejection of plasmoids in reconnecting current layers. The Alfvénic motion of plasmoids in current layers produces fast flows in the upstream region as plasma moves to fill in the low-pressure regions created by plasmoid motion. These high-speed flows steepen into slow shocks that heat the plasma upstream of reconnecting current layers. Global simulations of CMEs with ARMS also reveals the proliferation of these slow shocks as CME launch leads to Alfvénic downflows and the resulting formation of slow shocks. The mechanism of electron and ion heating of these slow shocks is being explored with local PIC simulations with parameters relevant to flare energy release: high sonic Mach number but with low Alfvénic Mach number. The goal is to fully understand the driver of hot thermal electrons and ions beyond those produced by evaporation in impulsive flares.
Abstract
The radio continuum burst from Sun can be classified into Type I and Type IV. Among Type IV, it can be moving or stationary. The emission process changes as this has a complex radio emission mechanisms. We present the results from the unusual complex type IV bursts. The main component of the radio burst will be a frequently occurring type III burst that is used for flux calibration. The multiple drifting lanes of slowly drifting bursts ( Type II) that are superimposed within the continuum emission in the frequency range from 180 MHz to 18 MHz represent the shock signatures of CME. The coronal magnetic field is estimated using Rankine - Hugoniot (RH) equations from the up/downstream of these shocks. The propagation of the shocks superimposed in the background of continuum emission will be presented.
The discovery of large-amplitude narrowband whistler-mode waves at frequencies of tenths of the electron cyclotron frequency in large numbers both at ~1 AU by STEREO S/WAVES and inside ~.3 AU by the Parker Solar Probe Fields Suite provides an answer to longstanding questions about scattering, energization and of solar wind electrons, and regulation of the heat flux. Simultaneous observations of whistler waves by the Fields Suite and of electrons by the Solar Wind Electrons Alphas and Protons (SWEAP) Investigation provide strong evidence for pitch angle scattering of strahl-energy electrons by narrowband whistler-mode waves at radial distances less than ~0.3 AU. These narrowband large amplitude whistler-mode waves are, therefore, the most likely candidates for regulating the electron heat flux and scattering of strahl electrons into the halo. Using a full 3d particle tracing code, we have examined interactions of electrons with energies from 0 eV to 2 keV with whistler-mode waves with amplitudes of 20 mV/m and propagation angles from 0 to 180 degrees to the background magnetic field. Interactions with wave packets and single waves are both modeled based on observations at ~0.3 AU and 1 AU. The simulations demonstrate the key role played by these waves in rapid scattering and energization of electrons. In addition to modeling the interactions with core and strahl energy electrons, we also investigated the interaction with more energetic electrons. Results for both energy ranges provide evidence for nonlinear effects, indicating that quasi-linear methods are not adequate for modeling the role of whistlers in the evolution and transport of solar wind electrons.
Particle heating in reconnection is essential to understand the heating in the solar corona, solar flares and the magnetotail. It plays an important role distributing magnetic energy into different species and between thermal and nonthermal components. Previous observational and theoretical studies on electron heating in reconnection exhausts within the beta~1 regime suggest a simple linear scaling where the electron heating is proportional to the magnetic energy per particle. Using kinetic reconnection simulations in the low-beta regime (with beta down to 0.005), we demonstrate that electron heating follows a sub-linear scaling below beta~0.01, with or without guide fields. As a result, the maximum heating is limited to only ~5 times of upstream electron temperature. This electron heating scaling may be testable by MMS observations at the magnetotail. This new finding has strong implications for the efficiency of electron heating in reconnection at low-beta environments throughout heliosphysics.
We conduct detailed thermal analysis of the plasma sheet region during the post-eruption phase of a flare that occurred on September 10, 2017. The plasma sheet that develops is observed using the 131A and 193A filters of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamic Observatory. Intensity data is used to distinguish between loops and the plasma sheet. We utilize the differential emission measure in order to calculate the emission measure, emission measure average weighted temperature, density, and thermal energy inside the plasma sheet region. These quantities are observed over time to assess how the plasma conditions of the plasma sheet region change throughout the duration of the event. Initial measurements show minimal change in temperature and thermal energy as the current sheet region evolves. From examining the conductive loss rate and radiative loss rate, rapid cooling is expected, contradicting the temperature results. These preliminary findings give some indication that there are underlying thermal processes that may be contributing to sustained plasma conditions.
The outer atmosphere of the Sun, the corona is comprised of tenuous, highly ionized plasma, that is governed by magnetic fields and is heated to more than a million Kelvin. Such hot coronal plasma is thought to be powered by numerous impulsive heating events called nanoflares. What drives these impulsive nanoflares? What role does magnetic field play in coronal heating? We address these long-standing questions through multi-wavelength observations of the Sun that span from the photosphere through the corona. In this talk, we will present new results that reveal an intricate link between the impulsive coronal heating and the evolution of magnetic fields at the solar surface. In particular, we will discuss the role of magnetic reconnection, a process through which magnetic energy is liberated, in the heating of the solar corona.
According to our current understanding, solar flares occur when magnetic energy stored in the solar corona is rapidly converted into other forms. This process appears to be initiated by, if not fully explained by, fast magnetic reconnection occurring somewhere in the corona. I will describe some physical constraints on the process of energy release and conversion. First, magnetic energy stored throughout a large coronal volume must be released by a process, magnetic reconnection, currently believed to operate on very small length scales. This length-scale discrepancy suggest energy conversion occurring outside the reconnection site proper. In his seminal work, Petschek invoked slow-mode shocks as a means of converting energy mostly into heat and bulk kinetic energy. These shocks extended well away from the reconnection site. I will show that an updated generalization of this model is capable of generating temperatures and densities consistent with those observed in solar flare plasmas. Those observations show density enhancement, by up to an order of magnitude or more, within a high-temperature plasma sheet surrounding the most likely site of reconnection. This large density enhancement, coinciding with substantial heating, would be a natural consequence of slow mode shocks and would be hard to explain in any other way. I show how a simple model of magnetic energy release and conversion, initiated by reconnection at the observed rate, produces temperatures, densities, emission measures, and even hard X-ray spectra matching those actually observed in particular flares.
Solar eruptive events are characterized by a complex interplay of energy release, transport, and conversion processes. A quantitative characterization of the different forms of energy therefore represents a crucial observational constraint for models of solar eruptions in general, as well as for magnetic reconnection, heating, and particle acceleration processes in particular. This talk will focus on the energy partition between the thermal plasma and the nonthermal particles and review recent studies that have tried to constrain this partition using X-ray, EUV, and bolometric
observations. Theses studies have come to dissimilar conclusions, and an effort will be made to identify the reasons for this. Finally, the first results on energy partition from the STIX instrument on Solar Orbiter will be presented.
One of the striking observations from the NASA Parker Solar Probe (PSP) spacecraft is the prevalence in the inner heliosphere of large amplitude, Alfvenic magnetic field reversals termed 'switchbacks'. These $\delta B_R/B \sim \mathcal{O}(1$) fluctuations occur on a range of timescales, are spherically polarized, and occur in "patches" separated by intervals of more quiet, radial solar wind magnetic field. Neither the source region, generation mechanism, nor the role in solar wind evolution are well-understood, with some models suggesting a fundamental role in solar wind heating and energization. We use measurements from the FIELDS and SWEAP instrument suites on PSP to demonstrate that patches of magnetic field switchbacks are localized within stable solar wind extensions of structures originating at the base of the corona. These structures are characterized by an increase in alpha particle abundance, Mach number, plasma $\beta$ and pressure and by depletions in the magnetic field magnitude and electron core and strahl temperature. These intervals are in local pressure-balance, which implies stationary spatial structure, and the central magnetic field depressions are consistent with overexpanded flux tubes. The structures are asymmetric in longitude with the leading edge being steeper and with a small ($\sim$1$^\circ$) edge of hotter plasma and enhanced magnetic field fluctuations. The structures are separated in Carrington longitude by angular scales associated with supergranulation and chromospheric network magnetic field. This implies both an origin of the streams and suggests that these switchbacks originate within and near the leading edge of the diverging magnetic field funnels associated with the photospheric network magnetic field.
The large-scale magnetic configuration and plasma beta of solar flares are similar to those of the magnetotail during reconnection. Studies of suprathermal electrons in the magnetotail may thus shed light on suprathermal electron production during flares. We will discuss statistical analysis and case studies of MMS magnetotail measurements to test out the following: (1) whether the primary electron energization occurs at the reconnection X-line or downstream, and (2) if magnetic islands are the dominant accelerators. Among the implications learned, one lesson is that the large-scale magnetic-field configuration plays a critical role in energizing electrons to suprathermal energies.
Solar flares release tremendous amounts of energy which is transported through the various layers of the Sun's atmosphere, resulting in heating, ionisation, mass flows, and non-thermal effects. The response of the plasma is detected by the radiation it produces. Often the radiation emitted during flares is formed under complex conditions, requiring forward modelling to guide the interpretation. This synthetic radiation can also be used to attack flare model assumptions. Here I discuss recent work that aimed to determine what is responsible for observations of flare-induced dimming of the He I 10830A line, and what potential diagnostics it might reveal of flare energy transport into the chromosphere. Many electron beam driven flares were performed, with the result that non-thermal collisional ionisation of helium (e.g. with the beam itself) is required to reproduce this dimming, and that the properties of the the dimming depend on the non-thermal electron distribution. Simulations that did not include non-thermal collisional ionisation failed to reproduce the observed dimming.
Panel members: Jim Drake, Jeff Reep, Paola Testa