Conveners
Working Group 1: Flare thermal response
- Jeffrey Reep
- Laura Hayes (DIAS/TCD)
Description
Understanding the deposition and transport of energy across the solar atmosphere is a critical problem in solar physics. In flares, the time scale for energy release is short, so that the plasma evolves rapidly, necessitating high cadence observations and modeling to understand the dynamics. Furthermore, the energy transport occurs across the entirety of the atmosphere – from the corona through the deep chromosphere and perhaps to the photosphere. The observational coverage of X-rays, EUV, and radio waves have all shed light on the non-thermal and thermodynamic processes occurring in flares, as well as allowing for model tests and validation. In this session, we will address the thermal response of solar flares to the impulsive energy release that drives them. We invite contributions that discuss the modeling, observations, and the inter-comparison between the two that can improve our understanding of flares. We encourage discussions of potential capabilities of Solar Orbiter or DKIST, additionally.
NOTE: Each contribution will be discussed within the WG but there will NOT be any specific talks and the duration assigned in the Timetable is NOT the true time allocated to the topic presented in the abstract.
We present a new database of both quiet and eruptive corona over a full solar cycle (2010-2021). Using the multi-narrow-band EUV images observed by SDO/AIA, we developed two data mining methods. (1) a new code (RFD) for automatic detection of flares from AIA 94 images. The database includes a more complete list of flares and provides us with essential info for both statistical studies and case...
We have used improved imaging spectroscopy techniques to re-analyze the nine events in early 2002 for which Schmahl and Hurford (2002, 2003) found evidence for an extended “halo” X-ray source around a single compact source. They used two different innovative techniques to show that these relatively simple events featured an extended source in the 12-25 keV energy range with a FWHM width as...
We investigate the response of critical frequency of F2 layer (foF2) of ionosphere to the solar flare on the mid latitude during the high solar activity period of solar cycle 23 i.e. 2003 and 2004. A mid latitude station, Guangzhou (23.1N, 113.4E) was selected to carry out the investigation. The ionospheric behaviour at the selected station is characterized by considering the critical...
The Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter has been acquiring data since April 2020. Because only flaring regions are visible in hard X-rays, no other solar features that are conventionally used for co-alignment (e.g. the solar limb) can be used to assess the pointing. Moreover, thermoelastic deformation of the spacecraft or STIX mechanical structures can...
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...
Solar flares efficiently heat coronal plasma to temperatures of 10-50 MK, and accelerate electrons to energies of hundreds of keV up to hundreds of MeV. It is still poorly understood how much of the plasma heating is from collisions by the accelerated electrons versus direct heating from the reconnection process that powers the flare. It is also not well determined how much energy is...
Flare X-ray emission prior to the impulsive phase typically corresponds to a GOES isothermal temperature in the range 10-15 MK, regardless of flare type. [Hudson et al.][1] demonstrate this in a small but representative sample of flare events, and confirm the GOES temperatures with RHESSI. This "hot onset" phase appears commonly in many if not all solar flares. We show here that it is...
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’...
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...
Hard X-rays (HXR) contain the most direct information of non-thermal electron population in solar flares. The HXR emission mechanism, known as the thick-target model, is well developed. It gives an opportunity to diagnose the physical conditions within a flaring structure. The thick-target model predicts that in flare foot points we should observe lowering of HXR sources altitude with...
Spatially unresolved data shows that the cooling phase in solar flares can be much longer than theoretical models predict. It was not yet determined whether this is also the case for different sub-regions within the flare structure.
Two questions are in the focus of this case study: 1. Are the cooling times, which are observed separately in coronal loops and the supra-arcade fan (SAF), in...
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...
Solar eruptive events are characterized by a complex interplay of energy release, transport, and conversion processes. Over the past two decades, RHESSI has been instrumental for quantitatively characterizing the energetics of both the thermal plasma and the accelerated nonthermal electrons. We will review the relevant results obtained from RHESSI observations (supported by EUV and bolometric...
Solar flares, explosions caused by a rapid release of magnetic energy through reconnection, impact the entire solar atmosphere from the photosphere to the corona. While flux ropes and post flare arcades associated with flares are observed at coronal altitudes of several 10 Mm, their footpoints, rooted in the lower atmosphere, are thought to be impacted by energy transported from the site of...
While solar flares are predominantly characterised by an intense broadband enhancement to the solar radiative output, certain spectral lines and continua will, in theory, exhibit flare-induced dimmings. Observations of ortho-helium spectral transitions (He I 10830Å and the He I D3 lines near 5876Å) have shown evidence of such dimming in some weak flares, usually followed by enhanced emission....
As energetic electrons propagate from the corona toward the lower atmosphere during a solar flare, a co-spatial counter-streaming return current is induced, thereby balancing the current density of the nonthermal flare-accelerated electron beam. In response to the return current electric field, a fraction of the ambient electrons are accelerated into the runaway regime. The background return...
The Nuclear Spectroscopic Telescope Array (NuSTAR) is an astrophysical X-ray telescope capable of observing the Sun with direct imaging spectroscopy providing a unique sensitivity >2.5 keV. We use NuSTAR to investigate highly frequent and weak flares (microflares) thought to contribute to heating the Sun's atmosphere particularly in active regions. I will present several X-ray microflares from...
Using data from SDO/HMI, Hinode/SOT, and LYRA instruments we study the white-light continuum emission during the X9.3 solar flare (SOL2017-09-06T11:53). Assuming that the emission is due to hydrogen Balmer and Paschen continua, we estimate the temperature evolution during that solar flare.
In simulations of flaring loops, it is almost universally assumed that the loop is semi-circular with constant cross-section. Observations with many imagers, however, clearly indicate that loops are often elliptical. Furthermore, the decreasing magnetic field strength from photosphere to corona requires that loops expand in cross-section. In this work, we conduct a series of simulations...
Quasi-periodic pulsations (QPPs) are found in solar flares of various magnetic morphologies, e.g. in two-ribbon or circular-ribbon flares, and mechanisms of their generation are not yet clear. Here we present the analysis of QPPs with a period P = 54±13 s found in RHESSI observations of a three-ribbon M1.1 class flare SOL2012-07-05T06:49. QPPs are manifested in the time profiles of temperature...
A common feature of electromagnetic emission from solar flares is the presence of intensity pulsations that vary as a function of time. Known as quasi-periodic pulsations (QPPs), these variations in flux appear to include periodic components and characteristic time-scales. Here, we analyse a GOES M3.7 class flare exhibiting pronounced QPPs across a broad band of wavelengths using imaging and...
Quasi-periodic pulsations (QPPs) have been observed in nearly all wavelength regimes with different periods ranging from sub-seconds to several minutes during solar flares. It has been argued that flare-associated QPPs can be attributed to the quasi-periodic modulations of the flare energy release, loop oscillations, or emission processes. However, their exact physical nature and relation to...
We analyze a C-class solar flare with the microwave (MW) data from Expanded Owens Valley Solar Array (EOVSA) and the SDO/AIA data. We find that the flare is led by an erupting filament. The interesting result is that the flare produces two MW sources, which lay at loop-top region and the region above that, respectively. Moreover, the brightness temperatures of the two sources, as well as...
