RHESSI-20 Workshop: Preparing for the Next Decade in High-Energy Solar Physics Research

Europe/Rome
Virtual

Virtual

Connection details will be available soon
Description

Launched in 2002, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was a NASA Small Explorer Mission designed to explore the basic physics of particle acceleration and explosive energy release in solar flares by making pioneering imaging spectroscopy observations of the X-ray and gamma-ray emissions of these energetic events. Decommissioned in October 2018, RHESSI covered more than a complete solar cycle in its 16-year lifetime and tremendously advanced the understanding of high energy phenomena at the Sun.

This workshop is the 20th in a series of workshops to explore current topics in high-energy in solar flare physics. It will have a format similar to previous workshops, with a balance of plenary sessions, invited and contributed talks, and working groups focused on specific topics.  However, due to COVID related restrictions the meeting will be 100% virtual. The workshop will feature results from ongoing analysis and interpretation of archival RHESSI data, and will also emphasize coordinated observations of high-energy solar eruptive events using instruments/facilities such as EOVSA, NuSTAR, PSP, Fermi, and MinXSS.  It will also include a session dedicated to EOVSA and STIX tutorials.

Participants
  • Aabha Monga
  • Aatiya Ali
  • Ajesh Asokan Pillai
  • Albert Shih
  • Alena Zemanova
  • Alexander Hillaris
  • Alexander Kosovichev
  • Alexander MacKinnon
  • Alexander Warmuth
  • Alexandra Lysenko
  • Amir Caspi
  • Amy Winebarger
  • Andrea Francesco Battaglia
  • Andrew Inglis
  • André Csillaghy
  • Anna Kępa
  • Anna Massone
  • Anne Tolbert
  • Anwesha Maharana
  • Arnold Benz
  • Arun Kumar Awasthi
  • Astrid Veronig
  • Barbara Sylwester
  • Bin Chen
  • Brandon Panos
  • Brendan Clarke
  • Brian Dennis
  • BRIANS AMADI
  • Camille Lorfing
  • Cheng-Ying Chiang
  • Christian Vocks
  • Christopher Moore
  • Ciara Maguire
  • Costas Bouratzis
  • Crisel Suarez
  • Dale Gary
  • Daniel Ryan
  • Diana Morosan
  • Dipali Burud
  • Eduard Kontar
  • Emma Perracchione
  • Erica Lastufka
  • Ewan Dickson
  • FAN GUO
  • Fanxiaoyu Xia
  • Fernando Marcelo Lopez
  • Frantisek Farnik
  • Frederic Schuller
  • Funda Bostanci
  • Galina Motorina
  • Gelu Nita
  • GERALD SHARE
  • Girjesh Gupta
  • Gordon Emslie
  • Gordon Holman
  • Gordon Hurford
  • Graham Kerr
  • Gregory Fleishman
  • Guillermo Giménez de Castro
  • Haihong Che
  • Hamish Reid
  • Hariharan Krishnan
  • Hasret ATASOY
  • Hualin Xiao
  • Huei-Wen Siao
  • Hugh Hudson
  • Iain Hannah
  • Ivan Zimovets
  • James McTiernan
  • Jana Kasparova
  • Jane Pratt
  • Janusz Sylwester
  • Jeffrey Reep
  • Jiale Zhang
  • Jiong Qiu
  • Joel Dahlin
  • Jonas Saqri
  • Juan Camilo Buitrago-Casas
  • Juan Carlos Martinez Oliveos
  • Juliana Vievering
  • Kathy Reeves
  • Komal Choraghe
  • Kristopher Cooper
  • Lakshmi Pradeep Chitta
  • Larisa Kashapova
  • Laura Hayes
  • Lindsay Glesener
  • Malte Bröse
  • Marek Siarkowski
  • Marina Battaglia
  • Mary Geer Dethero
  • Maya Prabhakar
  • Meiqi Wang
  • Melissa Pesce-Rollins
  • Meredith Wieber
  • Meriem Alaoui
  • Michele Piana
  • Morgan Stores
  • Nat Gopalswamy
  • Nataliia Meshalkina
  • Natasha Jeffrey
  • Noriyuki Narukage
  • Osanyin Taiwo
  • Pankaj Kumar
  • Paolo Massa
  • Pascal Saint-Hilaire
  • Paulo Simões
  • Peter Gallagher
  • Petr Heinzel
  • Phil Chamberlin
  • Priyank Srivastava
  • Rajat Verma
  • Rohit Sharma
  • Ronald Murphy
  • Rositsa Miteva
  • Rositsa Miteva
  • Ross Pallister
  • Ryan Milligan
  • Sabrina Savage
  • Sam Krucker
  • Sarah Paterson
  • Savannah Perez-Piel
  • Sean Sellers
  • Sebastian Castellanos Duran
  • Sergio Szpigel
  • Shaheda Begum Shaik
  • Shane Maloney
  • Shaun McLaughlin
  • Sherry Chhabra
  • Shilpi Bhunia
  • shirsh soni
  • Shivangi Bhardwaj
  • Shunsaku Nagasawa
  • Sijie Yu
  • Silvina Guidoni
  • Smriti Srivastava
  • Stephen Bradshaw
  • Stephen White
  • Su-Chan Bong
  • Susan Samwel
  • Sylwester Kołomański
  • Teddy Cheung
  • Thangasamy Velusamy
  • Tomasz Mrozek
  • Tomin James
  • Trevor Knuth
  • Tri Than
  • Vahe Petrosian
  • Viacheslav Sadykov
  • Victor Melnikov
  • VILMER Nicole
  • Weiqun Gan
  • Wen wang
  • William Setterberg
  • Xin Cheng
  • Xudong Sun
  • Ya-Hui Yang
  • Yang Su
  • Yi Chai
  • Yihong Wu
  • Yingjie Luo
  • Yixian Zhang
  • Yuankun Kou
  • Yuqian Wei
  • Zhentong Li
  • Żaneta Szaforz
    • Tutorial session
      • 1
        STIX Tutorial
        • a) STIX website demo

          • Where is Solar Orbiter?
          • STIX operation event calendar
          • Quicklooks/housekeeping
          ◦ lightcurves
          ◦ flare flag
          ◦ housekeeping
          • Calibration data
          • Science data
          ◦ L4 spectrograms: time bin sizes, short time bins, spectrum
          ◦ L1 pixel data: CFL, spectra, imaging
          • STIX Flare list
          • Fits file status

          Speakers: Dr E.C.M. Dickson (Institute of Physics, University of Graz, University of Applied Sciences and Arts Northwestern Switzerland) , Dr H. Xiao (University of Applied Sciences and Arts Northwestern Switzerland) , Prof. S. Krucker (University of Applied Sciences and Arts Northwestern Switzerland, Space Sciences Laboratory, University of California) , Dr Shane Maloney
        • b) OSPEX (IDL) demo
          Speaker: Dr E.C.M. Dickson (Institute of Physics, University of Graz, University of Applied Sciences and Arts Northwestern Switzerland)
        • c) Python tools
          Speakers: Daniel Ryan (FHNW) , Dr Shane Maloney
        • d) Aspect
          Speakers: Dr A. Warmuth (Leibniz-Institut für Astrophysik Potsdam (AIP)) , Frederic Schuller (Leibniz-Institut für Astrophysik Potsdam (AIP), Germany)
        • e) Amplitude imaging
          Speaker: Paolo Massa (Dipartimento di Matematica, Università degli Studi di Genova, Via Dodecaneso 35, 16146 Genova, Italy)
      • 2
        EOVSA Tutorial
        • a) EOVSA Imaging
          • Introduction to EOVSA
          • Introduction to SunCASA
          • Imaging with Sample Data (2021 May 07 M4 flare)
          Speaker: Sijie Yu (New Jersey Institute of Technology)
        • b) Spectral FItting
          • Microwave spectral fitting
          • Introduction to GSfit
          Speakers: Dale Gary, Dr Gelu Nita (Center for Solar-Terrestrial Research, New Jersey Institute of Technology)
        • c) Future Directions
          • GUI for EOVSA imaging
          • Python-based spectral fitting
          Speakers: Bin Chen (New Jersey Institute of Technology) , Dale Gary
    • Plenary session
      Convener: Dale Gary
      • 5:00 PM
        Workshop welcome
      • 3
        The RHESSI Archiving Plan
        Speakers: Dr Brian Dennis (Goddard Space Flight Center) , Dr Kim Tolbert (America University)
      • 4
        STIX Update
        Speaker: Prof. Sam Krucker (FHNW)
      • 5
        EOVSA Update: Science Highlights and Future Outlook

        The Expanded Owens Valley Solar Array (EOVSA) continues to provide daily observations of the Sun in the 1–18 GHz microwave band. The daily multi-frequency full-disk images from EOVSA's pipeline have provided a unique context for supporting other missions including the Solar Orbiter and Parker Solar Probe. The full-resolution science data have offered spatially-resolved spectral diagnostics of accelerated electrons in solar flares, coronal mass ejections, jets, as well as thermal diagnostics of coronal plasma and coronal magnetic field in active regions. In this talk, I will highlight some recent science results that have utilized EOVSA data. I will also briefly describe the improvements in EOVSA's data products and analysis software. Finally, I will discuss future plans and the status of the next-generation EOVSA: the Frequency Agile Solar Radiotelescope.

        Speaker: Prof. Bin Chen (New Jersey Institute of Technology)
      • 6
        Artificial Intelligence and the analysis of solar flares data

        This talk focuses on the exploitation of multi-energy and multi-modal space observations of solar flares by means of computational methods relying on artificial intelligence. Specifically, I will show that SDO/HMI magnetograms and machine learning can identify the magnetic properties that mostly impact the occurrence of extreme flaring events; that SDO/AIA EUV maps and image processing can determine in details the flare morphology even in the case of complex and highly energetic events; and, finally, that RHESSI and STIX visibilities together with inverse problems methods can infer information on electron acceleration in the flaring loops

        Speaker: Prof. Michele Piana (Università di Genova)
      • 7
        Using machine learning to calculate the information in flare ribbons

        Shannon entropy is one of the fundamental concepts of information theory and allows us to quantify the uncertainty of a random variable. A strongly related quantity is called mutual information (MI), which is a measure of the shared information between two random variables, or equivalently, the decrease in uncertainty/entropy of one random variable based on the knowledge of another. We use machine learning techniques to calculate the rate of flow of information between the different layers of the atmosphere, both in the quiet Sun and during solar flares. The analysis is performed on several IRIS spectral lines that have different formation heights. We find that the MI is maximized over the flare ribbons and at the peak of the GOES X-ray flux, indicating a positive relationship between MI and energy deposition.

        Speaker: Dr Brandon Panos (FHNW)
      • 8
        The SolFER DRIVE Center: Exploring Solar Flare Energy Release

        SolFER is a NASA funded DRIVE Science Center whose goal is to explore all
        aspects of Solar Flare Energy release. The Center includes 10
        institutions within the US as well as international collaborators. Our
        goal is a transformative advance in the understanding of explosive
        magnetic energy release and energetic particle production in the solar
        corona across a range of scales, from major eruptions that
        significantly impact the Earth space environment to small events that
        may play a role in coronal heating. To accomplish this goal we are
        bringing together observers across a wide range of platforms and
        theorists and modelers for discussion and collaboration on the key
        science issues. Key to the success of such cross-collaboration is the
        development of a common language so that direct comparisons between
        data and models are possible. I will give an overview of some of the
        significant advances that have already been made in the SolFER
        collaboration. This progress points to a future in which direct
        simulation of energy release and particle acceleration in macro-scale
        flare events will be possible. We welcome participation in SolFER by
        the international community. Details and meeting times of the key
        science groups can be found onthe SolFER website (solfer.umd.edu).

        Speaker: Prof. James Drake (University of Maryland)
    • Working Group 1: Flare thermal response

      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.

      Conveners: Jeffrey Reep, Laura Hayes (DIAS/TCD)
      • 9
        A new database for flare activities and coronal structures

        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 studies of flares. In particular, the flaring activities detected in quiet regions may reveal new clues for solving the coronal heating problem. (2) an improved sparse method for differential emission measure (DEM) calculations of full corona. This is used to obtain the long-term evolution of the (EM-weighted) temperature maps of both quiet and eruptive corona. The resulted dataset allows us to quantitatively study the multi-thermal nature of corona and long-term evolution of large-scale structures.
        I will also briefly report on the recent progress of the Hard X-ray Imager (HXI payload) onboard the ASO-S.

        Speaker: Yang Su (Purple Mountain Observatory, CAS)
      • 10
        Imaging Spectroscopy of Extended Hard X-ray Sources

        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 large as 40 arcsec containing up to 25% of the total emission. We used the following new and improved tools to better characterize real extended sources and to eliminate false ones: (1) establish the relative sensitivity of the different detector segments averaged over both a full 4 s rotation and over the short (ms) time scales of the rapid modulation, (2) account for pulse pile-up that affects each detector output differently depending on its sensitivity, (3) use the visibility forward-fit (Vis_FwdFit) image reconstruction algorithm to obtain spectra of a combination of individual sources with circular and elliptical Gaussian shapes. Analysis of the nine events using these new techniques shows that the claimed halo sources are most probably not real and that other examples of extended sources must be re-examined using these new analysis tools.
        Schmahl, E. J., & Hurford, G. J. 2002, Solar Physics, 210, 273.
        Schmahl, E. J., & Hurford, G. J. 2003, Advances in Space Research, 32, 2477.

        Speaker: Brian Dennis (NASA)
      • 11
        Response of ionospheric foF2 to the solar flare at mid latitude

        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 frequency of F2 layer (foF2) obtained by using the ground based Ionosonde observations. To quantify the effect of solar flares we have considered the X-ray flux (0.1–0.8 nm) and EUV flux (26–34 nm). An increase of 3.5 MHz to 7.5 MHz was recorded in the value of foF2 during the flares. Therefore, it can be well concluded that solar flares have positive effect on ionosphere foF2 at mid latitudes.

        Key Words: Solar flare, Ionosphere, Critical frequency, foF2

        Speaker: Dr Shivangi Bhardwaj (Sri Sai University, Palampur, Himachal Pradesh, India)
      • 12
        The STIX Aspect System: instrument design, operations, and first results

        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 change the relative direction of the STIX optical axis in the spacecraft reference frame, so that relying on the spacecraft aspect solution alone does not provide the required accuracy to place STIX images in the context of data acquired at other wavelengths. Therefore, a dedicated optical system, the STIX Aspect System (SAS), was specifically designed to measure the pointing direction of STIX with respect to the Sun. Here we provide a description of the SAS, its limitation, and an overview of the results obtained during the first year of operations. We conclude by showing how the SAS measurements can help improving the pointing stability of Solar Orbiter over the course of the mission.

        Speaker: Frederic Schuller (Leibniz-Institut für Astrophysik Potsdam (AIP), Germany)
      • 13
        Analysis of Sunquakes of Solar Cycle 24

        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.

        Speaker: Alexander Kosovichev (New Jersey Institute of Technology)
      • 14
        Current progress on MinXSS-RHESSI joint DEMs

        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 contained in the nonthermal electrons, as the low-energy extent ("cutoff") of the non-thermal population is difficult to characterize while overshadowed by the thermal population at those energies.

        An accurate characterization of the full thermal electron population is critical both to understanding plasma heating processes and to accurately measuring the "residual" nonthermal electron population to determine its energetics. Hard X-rays (HXRs; 5-100 keV) probe both the nonthermal population and the hot, ~10-50 MK plasma, but are most sensitive to the hottest portion of the temperature distribution. Soft X-rays (SXRs; 1-10 keV) offer deeper insight into cooler, ~2-25 MK temperatures, as well as elemental abundances that further probe the coronal or chromospheric plasma origins. Complete 1-100 keV spectral coverage thus also provides the comprehensive 2-50 MK temperature coverage necessary for full characterization of both thermal and nonthermal electron populations.

        From May 2016 to May 2017, over 10 M-class flare and over 100 C-class flares were simultaneously observed in SXRs by the MinXSS-1 CubeSat and in HXRs by RHESSI. We present current progress in joint analysis of simultaneous MinXSS and RHESSI X-ray spectra to determine flare temperature distributions (differential emission measure, DEM) and abundances of key low-FIP elements, as well as the residual nonthermal emission and associated low-energy cutoff. We discuss the details of the joint analysis technique, and its application to a number of jointly-observed flares to determine thermal and nonthermal parameters and their evolution.

        Speaker: Dr Jim McTiernan (Space Sciences Laboratory, UC Berkeley)
      • 15
        Flare hot onsets: linear emission-measure growth

        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. 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 characterized by a linear growth of emission measure with time, which may last for more than a minute. We have found no evidence for simple heating of any fixed plasma, in the sense of dT/dt > 0, and have extended the characterization to the 1-s sampling of the GOES-R/XRS.

        Speaker: Hugh Hudson (UC Berkeley and University of Glasgow)
      • 16
        Hot X-ray Onsets of Solar Flares

        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 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.

        Speaker: Paulo Simões (CRAAM/Mackenzie)
      • 17
        Plasma Diagnostics of Microflares observed by STIX and AIA

        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 68 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 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.

        Speaker: Jonas Saqri (Institute of Physics, University of Graz)
      • 18
        Plasma dynamics in the flaring loop observedby RHESSI

        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 increasing energy. The foot point HXR sources result from direct interaction of non-thermal electron beams with plasma in the lower part of the solar atmosphere, where the density increases rapidly. Therefore, we can estimate the plasma density distribution along the non-thermal electron beam directly from the observations of the altitude-energy relation obtained for the HXR foot point sources. However, the relation is not only density dependent. Its shape is determined also by the power-law distribution of non-thermal electrons. Additionally, during the impulsive phase, the plasma density and a degree of ionization within foot points may change dramatically due to heating and chromospheric evaporation. For this reason the interpretation of observed HXR foot point sources' altitudes is not straightforward and needs a detailed numerical modelling of the electron precipitation process. We present the results of a detailed analysis of one well observed solar flare. We used HXR observations obtained by RHESSI. The numerical model was calculated using the hydrodynamic 1D model with an application of the Fokker-Planck formalism for non-thermal beam precipitation. We found that HXR data may be used to trace details of chromospheric density changes. The estimated amount of mass evaporated from the chromosphere is of the order of the amount of additional mass that was observed to occur in the loop-top source.

        Speaker: Tomasz Mrozek (Space Research Centre Polish Academy of Sciences)
      • 19
        Temperature and Differential Emission Measure Evolution of a Limb Flare on 13 January 2015

        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 accordance with the existing cooling models? 2. Do the supra-arcade downflows (SADs) have different temperature and emission measure than their surrounding? An M5.6 limb flare on 13 January 2015 is analysed by using SDO/AIA data. A differential emission measure (DEM) reconstruction code derives spatially resolved temperature and emission measure maps. This output is used to investigate the thermal evolution of coronal loops, the supra-arcade fan (SAF), and the supra-arcade downflows (SADs).

        Speaker: Malte Bröse (AIP)
      • 20
        Dynamic evolution of a solar flare current sheet

        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.

        Speaker: Dr Lakshmi Pradeep Chitta (Max Planck Institute for Solar System Research)
      • 21
        Energy partition in solar flares: the results from RHESSI and the prospects with STIX

        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 data) and try to understand why several of these studies have come to differing conclusions. Finally, we will discuss the prospects for better understanding energy partition that will be provided by a new generation of hard X-ray instruments, in particular STIX on Solar Orbiter.

        Speaker: Alexander Warmuth (Leibniz Institute for Astrophysics Potsdam (AIP))
      • 22
        Hard X-ray emission from an unusual flare in the solar chromosphere

        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 reconnection that is typically situated in the corona. Flares are rarely observed to be confined to the lower solar atmosphere. Here we present observations of such an unusual compact C-class flare (SOL2013-10-12T00:31) confined to altitudes below the chromospheric canopy. The event is triggered during the merging of opposite-polarity magnetic elements in the photosphere. The event exhibited cusp-shaped flare arcade (a spatial morphological trait that is typical to some major flares), in the ultraviolet (UV) diagnostics, that is largely obscured by the chromospheric canopy. One of its footpoints displayed hard X-ray emission to energies of up to 25 keV as observed with RHESSI. By performing spectral fits to the hard X-ray emission, we found that the flare reached temperatures in excess of 20 MK. Our observations provide a clear evidence for plasma heating to high temperatures from the magnetic energy that is released directly in the lower atmosphere. The implications of such a process in major flares will be discussed.

        Speaker: Lakshmi Pradeep Chitta (Max Planck Institute for Solar System Research)
      • 23
        He I 10830Å Dimming During Solar Flares: The Crucial Role of Non-Thermal Collisional Ionisations

        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. It has been suggested that the presence of non-thermal collisional ionisation of helium by the electron beam, followed by recombinations to ortho-helium, is responsible for overpopulating the ortho-helium levels leading to stronger absorption. However it has not been possible observationally to preclude the possibility of overpopulating ortho-helium via enhanced photoionisation of He I by EUV irradiance from the flaring corona followed by recombinations. Here we present radiation hydrodynamics simulations of non-thermal electron beam-driven flares where (1) both non-thermal collisional ionisation of Helium and coronal irradiance are included, and (2) only coronal irradiance is included. A grid of simulations covering a range of total energies deposited by the electron beam, and a range of non-thermal electron beam low-energy cutoff values, were simulated. For each simulation the He I 10830Å line was forward modelled. In order to obtain flare-induced dimming of the He I 10830Å line it was necessary for non-thermal collisional ionisations to be present. Further, the effect was more prominent in flares with harder non-thermal electron spectrum (larger low-energy cutoff values) and longer lived in weaker flares and flares with a more gradual energy deposition timescale. These results demonstrate the usefulness of ortho-helium line emission as a diagnostic of flare energy transport.

        Speaker: Dr Graham Kerr (The Catholic University of America / NASA GSFC)
      • 24
        How do supra-thermal runaway electrons affect the return current/nonthermal beam dynamics in solar flares?

        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 current plasma is therefore not simply a drifting Maxwellian but a distribution with a high energy tail of freely accelerated suprathermal electrons. I will show in this talk the results of 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 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 find that (1) a significant number of suprathermal electrons can reach the acceleration region in the weak-field sub-Dreicer regime, where they can be further accelerated to higher energies, (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.

        Speaker: Meriem Alaoui (CUA & NASA/GSFC)
      • 25
        NuSTAR observations of a repeatedly microflaring active region

        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 a recently emerged active region, AR12721, that were observed on 2018 September 9-10 with NuSTAR. In combination with SDO/AIA, I describe the temporal, spatial, and spectral evolution of these GOES sub-A class microflares that reach temperatures above those of the surrounding active region (>5 MK). One of the microflares presented is the faintest non-thermal microflare so far observed with NuSTAR with an equivalent GOES class of A0.1. Using SDO/HMI, I also present evidence of photospheric magnetic flux cancellation/emergence at the footpoints in 8 of the NuSTAR microflares.

        Speaker: Kristopher Cooper (University of Glasgow)
      • 26
        Study of the White-light Emission During the X9.3 Flare on September 06, 2017

        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.

        Speaker: Jana Kasparova (Astronomical Institute of the CAS)
      • 27
        The Role of Magnetic Geometry on Hydrodynamic Response in Flaring Loops

        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 examining the plasma response to beam heating under the effects of elliptical geometry and area expansion. We find that, for strong heating events, the eccentricity of a loop is relatively unimportant, while area expansion, including the magnitude of increase, the rate of increase, and the location of increase, can drastically impact the resultant densities, temperatures, and cooling times of the loop.

        Speaker: Dr Jeffrey Reep (US Naval Research Laboratory)
      • 28
        Quasi-periodic energy release in a three-ribbon solar flare

        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 (T) and emission measure (EM) of superhot (Ts ~ 30-50 MK) plasma, but are almost invisible in the profiles of hot (Th ~ 15-20 MK) plasma parameters when approximating the X-ray spectrum of the flare with the bremsstrahlung spectrum of a two-temperature thermal plasma (the one-temperature approximation gives a poor approximation with a much higher residual). In addition, QPPs with a similar period are found in the time profiles of the flux and spectral index of nonthermal electrons if the observed X-ray spectrum is approximated by a combination of the bremsstrahlung spectra of a single-temperature Maxwellian plasma and nonthermal electrons. In this case, the power-law spectrum of nonthermal electrons is very soft, with an index from -7 to -10, and shows the known soft-hard-soft dynamics for each pulsation. QPPs are not expressed in the X-ray flux according to RHESSI and GOES data, as well as in radio data. Remarkable, QPPs are accompanied by apparent systematic movement of a “single” X-ray source at a speed below 120 km / s (average speed ~ 35 km / s) along the central flare ribbon over a narrow (<5 Mm) “tongue” of negative magnetic polarity, elongated (~ 20 Mm) between two areas of positive polarity. The results of magnetic extrapolation in the nonlinear force-free field (NLFFF) approximation show that the X-ray source could “move” along a separator and/or magnetic flux-rope in the corona. It is worth noting that in the homologous three-ribbon M6.1 flare SOL2012-07-05T11:39 that occurred in the same region about five hours later, X-ray sources “moved” much less systematically and did not produce similar QPPs. We interpret the observed QPPs as a result of successive episodes of energy release in different magnetic tubes (loops) of the flare region. In each pulsation approximately (4-7) x 10 ^ 29 erg is released in the form of thermal energy of hot and superhot plasmas (or accelerated electrons). The total energy release during all pulsations is ~ (3-6) x 10 ^ 30 erg, which is less than the value of the magnetic energy (~ 8 x 10 ^ 30 erg) released in the flare region. We discuss possible triggers of the “propagating” front of energy release (slow and fast magnetoacoustic waves, flapping oscillations and thermal instability of a reconnecting current sheet with strong guide-field formed around the separator, asymmetric eruption of a magnetic flux-rope, etc.), and argue that the detected QPPs are not an instrumental effect.

        Speaker: Ivan Zimovets (Space Research Institute of the Russian Academy of Sciences (IKI RAS))
      • 29
        Quasi-Periodic Particle Acceleration in a Solar Flare

        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 timeseries analysis. We identify QPPs in the timeseries of X-ray, low frequency radio and EUV wavelengths using wavelet analysis, and localise the region of the flare site from which the QPPs originate via X-ray and EUV imaging. It was found that the pulsations within the 171 A, 1600 A, soft X-ray (SXR), and hard X-ray (HXR) light curves yielded similar periods of ~122 s, ~131 s, ~123 s, and ~137 s, respectively, indicating a common progenitor. The low frequency radio emission at 2.5 MHz contained a longer period of ∼231 s. Imaging analysis indicates that the location of the X-ray and EUV pulsations originates from a HXR footpoint linked to a system of nearby open magnetic field lines. Our results suggest that intermittent particle acceleration, likely due to ‘bursty’ magnetic reconnection, is responsible for the QPPs. The precipitating electrons accelerated towards the chromosphere produce the X-ray and EUV pulsations, while the escaping electrons result in low frequency radio pulses in the form of type III radio bursts. The modulation of the reconnection process, resulting in episodic particle acceleration, explains the presence of these QPPs across the entire spatial range of flaring emission.

        Speaker: Mr Brendan Clarke (Dublin Institute for Advanced Studies/Trinity College Dublin)
      • 30
        Radio and X-Ray Quasi-Periodic Pulsations during the Impulsive Phase of a Confined Solar Flare

        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 the flare energy release are not well understood. Here, we report QPPs observed in radio and X-rays during the impulsive phase of a C1.8-class confined flare. Utilizing the radio spectroscopic imaging technique provided by Karl G. Jansky Very Large Array (VLA), we found that the radio QPPs, observed in the 1--2 GHz L band, consist of four spatially distinct radio sources with different periodicities. The brightest QPP, whose brightness temperature reaches 20 MK, is located close to the main sunspot. The emission is right-hand-circular-polarized and covers nearly the entire 1.0--2.0 GHz band with a period of ~6 s. The other three relatively weak radio QPP sources are located near the brightened flare arcade. One weakly polarized radio QPP source coincides spatially with a looptop X-ray source with a period of ~43 s. The other two radio QPP sources are located at the conjugate footpoints with the opposite sense of circular polarization. Their periods range from 25 to 47 seconds. We discuss the emission mechanism and the possible source of the energetic electrons responsible for the QPPs.

        Speaker: Yingjie Luo (New Jersey Institute of Technology)
      • 31
        Radio imaging of quasi-periodic magnetic reconnection and electron acceleration during a solar flare

        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 derivative curve of GOES soft X-ray flux, change synchronously with a quasi-period of about 40 seconds. At the peak moments, the spectra of both sources show non-thermal gyrosynchrotron characteristics. The results suggest that quasi-periodic magnetic reconnection accelerates electrons, which then produce quasi-periodic MW radiation near the reconnection region and at the loop-top region.

        Speaker: Yuankun Kou (Nanjing University)
    • Working Group 2: Particle acceleration

      X-ray and radio imaging and spectroscopy observations yield essential information for the understanding of plasma heating and the acceleration and transport of high-energy particles in solar eruptive events. Radio data provides plasma, magnetic field, and particle diagnostics in the corona and complements hard X-ray diagnostics in the chromosphere and dense coronal structures. The next generation of missions (e.g., Solar Orbiter/STIX) and advanced radio telescopes will bring light to currently unanswered questions, but also introduce new challenges and questions. It is likely that substantial progress in the field can only be achieved by collaborative work among observers and modelers. Therefore, this working group welcomes contributions on recent results in X-ray and radio observations, especially those addressing the complementary nature of radio and X-ray observations, as well as advances in associated particle acceleration models. We are also interested in studies that explore the limitations of high-energy observations and models, provide diagnostic tools or describe needed observations to constrain theories, and aim to develop a common language for scientists working in these areas or research. 

      NOTE: The times and duration of each talk are the true times and duration of the talks for this session.

      Conveners: Dr Diana E. Morosan (Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland.) , Silvina Guidoni (American University)
      • 5:00 PM
        Introduction
      • 32
        EOVSA and STIX: jointly observed flares in 2021

        Radio and hard X-ray diagnostics provide very strong complementary tools to probe electron acceleration in solar flares. With the Expanded Owens Valley Solar Array (EOVSA) and the Spectrometer/Telescope for Imaging X-rays (STIX) we have currently two solar dedicated observatories jointly observing on a daily basis. To date, we already have more than 10 jointly observed flares ranging from small microflares up to M-class flares. We will present initial results in the hope of sparking discussions, and finding interest for future collaborations.

        Speaker: Sam Krucker
      • 33
        Early results on STIX imaging

        The Spectrometer/Telescope for Imaging X-rays (STIX) is the remote sensing instrument onboard Solar Orbiter dedicated to the observation of the X-ray emission during solar flares. The goal of the instrument is to provide information about the electron acceleration at the Sun through the measurement of the photons emitted by bremsstrahlung or by thermal mechanisms. The telescope consists of 30 subcollimators, i.e. couples of tungsten grids mounted in front of a detector. The photon flux incident on the instrument is modulated by the grids and it gives rise to Moire patterns on the detector surfaces. The measurements of these patterns can be interpreted as Fourier components of the photon flux, named visibilities. Therefore, the imaging problem for STIX is the one of reconstructing the emitted radiation from a very sparse sampling of its angular Fourier transform. In this talk we present the first results concerning the image reconstruction problem from semi-calibrated STIX data consisting of visibility amplitudes only. We addressed this problem by using parameterized source shapes and by retrieving their parameters with forward fitting methods. Specifically, we present the results obtained via Particle Swarm Optimization (PSO) and Sequential Monte Carlo (SMC) and we also show how these algorithms give quantitative estimates of the reconstructed parameters. Moreover, we overview the problem of the visibility phase calibration and we give some insights about imaging methods used during the calibration process. We validate the reliability of our results by comparing them to other maps of the same events obtained with instruments such as the Extreme Ultraviolet Imager (EUI) within Solar Orbiter, and the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA).

        Speaker: Paolo Massa (Dipartimento di Matematica, Università degli Studi di Genova, Via Dodecaneso 35, 16146 Genova, Italy)
      • 34
        Radio and X-ray Observations of Electron Acceleration in a Series of Solar Microflares

        Like their larger counterparts, solar microflares release magnetic energy and accelerate particles to relativistic speeds. Even though they are generally shorter and often more compact than larger flares, they can display a surprising complexity and provide new insights into where and when particles are accelerated.

        We present observations of multiple electron acceleration sites and times during a series of solar microflares observed with RHESSI and the Karl G. Jansky Very Large Array (VLA).
        Microflares were observed from the same active region over a period of 45 minutes. The VLA was observing the Sun during the same period at a time resolution of 50 ms at frequencies between 1 to 2 GHz. The radio dynamic spectra show a variety of features, like extremely short lived periodic spikes, drifting bursts, and broad band emission. While some of these emissions were temporally associated with X-ray sources, they originated from a different location. The observations suggest that, even in short, compact flares, acceleration of electrons can take place at multiple locations, either co-temporally or at multiple instances during the course of the flare.

        Speaker: Dr Marina Battaglia
      • 35
        Radio and X-ray Observations of Short-lived Episodes of Electron Acceleration in a Solar Microflare

        Solar flares release enormous magnetic energy into the corona, producing the heating of ambient plasma and the acceleration of particles. The flaring process is complex and often shows multiple spatially separated temporal individual episodes of energy releases, which can be hard to resolve based on the instrument capability. We analysed the multi-wavelength imaging and spectroscopy observations of multiple electron acceleration episodes during a GOES B1.7-class two-ribbon flare observed simultaneously with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, the Reuven Ramatay High Energy Solar Spectroscopic Imager (RHESSI) in X-rays, and the Solar Dynamics Observatory in extreme ultraviolet (EUV).
        We observed a total of six radio bursts. The first three bursts were co-temporal but not co-spatial nonthermal X-ray source and represented multiple electron acceleration episodes. We model the radio spectra by optically thick gyrosynchrotron emission and estimate the magnetic field strength and nonthermal electron spectral parameters in each acceleration episode.
        We note that the nonthermal parameters derived from X-rays differ considerably from the nonthermal parameters inferred from the radio and originates in the lower corona. Although co-temporal, the multi-wavelength analysis shows that different electron populations produce multiple acceleration episodes in radio and X-rays.

        Speaker: Rohit Sharma (FHNW, Windisch)
      • 36
        Magnetic Field Measurements of a Twisted Flux Rope in a Partial Solar Eruption

        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 partial eruption of a twisted filament observed in Hɑ 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 partial 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 partial eruption is likely due to the strong strapping coronal magnetic field above the filament.

        Speaker: Mr Yuqian Wei (New Jersey Institute of Tech)
      • 5:25 PM
        Block 1 Discussion
      • 37
        Solar Energetic Electron Events Associated with Hard X-Ray Flares

        We investigate 16 solar energetic electron (SEE) events measured by WIND/3DP with a double-power-law spectrum and the associated western hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. In all the 16 cases, the presence of an SEE power-law spectrum extending down to ~5 keV at 1 au implies that the SEE source would be high in the corona, at a heliocentric distance of >1.3 solar radii, while the footpoint or footpoint-like emissions shown in HXR images suggest that the observed HXRs are likely produced mainly by HXR-producing electrons via thick-target bremsstrahlung processes very low in the corona. We find that for all the 16 cases, the estimated power-law spectral index of HXR-producing electrons is no less than the observed high-energy spectral index of SEEs, and it shows a positive correlation with the high-energy spectral index of SEEs. In addition, the estimated number of SEEs is only ∼10−4–10−2 of the estimated number of HXR-producing electrons at energies above 30 keV, but with a positive correlation between the two numbers. These results suggest that in these cases, SEEs are likely formed by upward-traveling electrons from an acceleration source high in the corona, while their downward-traveling counterparts may undergo a secondary acceleration before producing HXRs via thick-target bremsstrahlung processes. In addition, the associated 3He/4He ratio is positively correlated with the observed high-energy spectral index of SEEs, indicating a possible relation of the 3He ion acceleration with high-energy SEEs.

        Speaker: wen wang
      • 38
        Electron Acceleration and the Development of Power-Law Energy Spectra in Magnetic Reconnection with A Force-free Current Sheet

        Extensive observations have discovered that a huge number of energetic electrons with energy up to MeV (~0.9c and Lorentz factor ~2) are produced during solar flares. These very mild relativistic energetic electrons demonstrate two-stage power-law spectral evolutions. What mechanism efficiently accelerates non-relativistic particles to a power-law has been a long-standing “ injection problem” in particle acceleration theory since Fermi first proposed his famous Fermi-acceleration model in 1949. In this talk, I will discuss why particle acceleration in solar flares is an “injection problem” and what problems are with the previous and current widely invoked models. I will present a new acceleration mechanism in magnetic reconnection. I will show how the velocity shear stored naturally in force-free currents drives an electron Kevin-helmholtz instability (EKHI) during magnetic reconnection and efficiently acceleration electrons to a power-law energy spectrum via a two-stage soft-hard-hard evolution. Finally, I will discuss the potentially broad application of this mechanism in solar physics and how the complexity of solar flares may impact the further development of this model.

        Speaker: Haihong Che (University of Alabama in Huntsville)
      • 39
        Evidence of energy cutoffs in flare-accelerated electrons

        Flares are violent explosions and natural particle accelerators in solar atmosphere. The accelerated particles play an essential role in flare energy release and distribution. High and low energy cutoffs define the upper and lower limits of accelerated electrons. They are important parameters in understanding particle acceleration and energy distribution. However, the existence of acceleration-related low-energy cutoff is still a question, and the high-energy cutoff has been rarely studied and discussed. We present a recent study using X-ray and SEP (solar energetic particles) observations and report on the evidence of low and high energy cutoffs that are related to acceleration process in flares. The result provides new clues and constraint for understanding high energy spectra, electron acceleration, and transportation.

        Speakers: Fanxiaoyu Xia (Purple Mountain Observatory) , Yang Su (Purple Mountain Observatory, CAS)
      • 40
        On the Origin of Hard X-Ray Emissions from the Behind-the-limb Flare on 2014 September 1

        The origin of hard X-rays and γ-rays emitted from the solar atmosphere during occulted solar flares is still debated. The hard X-ray emissions could come from flaring loop tops rising above the limb or coronal mass ejection shock waves, two by-products of energetic solar storms. For the shock scenario to work, accelerated particles must be released on magnetic field lines rooted on the visible disk and precipitate. We present a new Monte Carlo code that computes particle acceleration at shocks propagating along large coronal magnetic loops. A first implementation of the model is carried out for the 2014 September 1 event, and the modeled electron spectra are compared with those inferred from Fermi Gamma-ray Burst Monitor (GBM) measurements. When particle diffusion processes are invoked, our model can reproduce the hard electron spectra measured by GBM nearly 10 minutes after the estimated on-disk hard X-rays appear to have ceased from the flare site.

        Speaker: Yihong Wu
      • 41
        The Extreme-ultraviolet late phase of a solar flare triggered by injection of nonthermal electrons to the footpoints of coronal loops

        Extreme-ultraviolet late phase (ELP) refers to the second extreme-ultraviolet (EUV) radiation enhancement in some solar flares, minutes or hours after the X-ray emission peak at the flare impulsive phase. ELP loops often have distinct configurations from the main flaring loops and the enhanced EUV emission may imply an additional heating process. Here we analyzed a C1.4 flare which has a typical EUV late phase. The ELP appeared right after a ~6 minute-long enhancement of microwave emission in the radio dynamic spectrum. Microwave imaging reveals a radio-emitting nonthermal loop structure that connects the flaring region to a remote footpoint of the ELP loop. Our analysis suggests that nonthermal electrons propagated from the energy release site in the corona to this loop footpoint, leading to evaporation of the heated plasma to fill the ELP loop.

        Speaker: Jiale Zhang (Peking University)
      • 42
        Time Evolution of Thermal and Non-thermal Emission from M7.6 Class Flare Observed with MinXSS and RHESSI

        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 with 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 multi-thermal and non-thermal emissions is clearly resolved with a resolution of 10 seconds cadence, which corresponds to the Alfven 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) are detected and the emission measure of cool and hot plasma emissions are drastically increased more than hundreds of times as the non-thermal emission becomes harder. In addition, we also quantify the time variation in Fe, Ca and Si abundances with the help of high energy resolution of MinXSS. 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) 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.

        Speaker: Mr Shunsaku Nagasawa (Department of Physics, The University of Tokyo)
      • 6:08 PM
        Block 2 Discussion
      • 6:28 PM
        Break
      • 43
        A systematic study of HXR flares and Metric type III radio bursts between 2002 and 2015

        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 at 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.

        Speaker: Dr Tomin James (Observatoire de Paris)
      • 44
        Constraints on the acceleration region of type III radio bursts from radio bursts and X-ray bursts

        During the gradual phase of flare SOL2017-09-09T10:50, a faint increase of non-thermal X-ray emission has been detected by Fermi/GBM, and is associated with radio decimetric spikes and type III radio bursts detected by the ORFEES radio-spectrograph, the LOFAR radio-telescope, and the Wind/WAVE instrument. These signatures indicate that a faint acceleration event was the source of electron beams propagating to the heliosphere. We show that the non-thermal X-ray bursts and radio decimetric spikes correlate on short time scales, which support the idea that the fragmentation of the radio emission into spikes is linked to the fragmentation nature of the acceleration process itself. The combination of HXR and radio diagnostics in the corona is used to provide strong constraints on the density of the acceleration site. Using spectroscopic imaging of the radio emission at lower frequencies using the LOFAR observations, and the constraints on the acceleration site derived from the X-ray and higher frequency radio emission, we show that the observed radio source sizes are much larger than the expected size of the electron beam in the high corona, confirming that radio source properties are strongly affected by radio-wave scattering due to turbulent density fluctuation of the ambient plasma.

        Speaker: Sophie Musset (ESA/ESTEC)
      • 45
        LOFAR observations of fundamental and harmonic type III emission during an M class flare

        We present observations of an M class flare with the LOw Frequency ARray (LOFAR) in the morning hours of 7 September 2017. The flare was accompanied by strong type III radio bursts. LOFAR interferometric images in the low band frequency range of 20 - 80 MHz show distinct sources that show variations in their positions, and intermittent dual source structures. We identify these as fundamental and harmonic emission, with the one or other being dominant at times. These distinct sources and their evolution allow for obtaining separate lightcurves for both fundamental and harmonic emission. The data show that transport effects due to refraction and scattering play a significant role, both in source separation and drift of their apparent positions.
        Comparing the light curves of fundamental and harmonic pairs, e.g. 35 MHz fundamental and 70 MHz harmonic, enables studies of radio wave propagation in the solar corona. Observations of harmonic emission can provide information on source locations high in the corona, where fundamental emission would be near or below the ionospheric cutoff at 10 MHz. These are relevant for the transition into the solar wind, and for joint observing campaigns with Parker Solar Probe and Solar Orbiter that are currently investigating the inner heliosphere.

        Speaker: Christian Vocks (Leibniz-Institute for Astrophysics Potsdam)
      • 46
        Energy release processes during the partially occluded flare on 29th May 2020, according to microwave observations.

        We present the preliminary results of the SOL 29-05-2020T07:20 flare study based on observations within the 4-8 GHz range by Siberian Radioheliograph 48, spectropolarimeter 4-8 GHz and the other available microwave (MW) data. The MW time profiles of the flare demonstrated at least three quasi-periodic bursts. The current study aims to find mechanisms generating the emission of the different bursts and suggest the preliminary scenario of the event. We analysed the spectra of the MW bursts and their source position at different frequencies. The analysis revealed that the first burst and the consequence bursts occurred in the two distant places. The mechanisms of MW emission generation differ from burst to burst. The relation of the burst locations and their MW spectral properties are discussed and compared to valuable X-ray observations.

        Speaker: Larisa Kashapova (Institute of Solar Terrestrial Physics SB RAS, Irkutsk, Russia)
      • 47
        Analysis of Flare Events Associated with Slowly Positively Drifting Bursts Observed at 800-2000 MHz Range

        In this contribution we present a study of 2 flare events (of M and C class) which were associated with Slowly Positively Drifting Burst (SPDBs) observed at radio frequencies in the range of 800-2000 MHz. These burst are similar to reverse type III burst but their drift is much less than < 1GHz/s and they are rarely observed. Both flare events started within an active region but later involved also supergranular field nearby the active region. The results of the study show that SPDBs can be linked with initial phase of magnetic reconection and very likely with beams of accelerated particles.

        Speaker: Alena Zemanova (Astronomical Institute, Czech Academy of Sciences)
      • 7:00 PM
        Block 3 Discussion
      • 48
        Flare Ribbon Signatures of Plasmoids

        Flare Ribbon Signatures of Plasmoids
        J. T. Dahlin

        Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration. Understanding the structure and dynamics of the magnetic reconnection that powers flares is critical for predicting the energy release. In particular, the amount of energy transferred to energetic particles is thought to be highly dependent on whether the reconnection is primarily turbulent (e.g., plasmoid dominated) or instead laminar. We present new high-resolution MHD simulations of three-dimensional reconnection in an eruptive flare and compare the results to recent data. Although flare reconnection is challenging to observe directly in the corona, highly detailed constraints on its dynamics can be obtained from observations of flare ribbons that track the chromospheric footpoints of newly reconnected field lines. The analogues of flare ribbons in our simulations are identified by tracking discontinuous changes in field-line magnetic connectivity due to the reconnection. In our highest-resolution calculations, we find that these ribbon analogues are highly structured and exhibit many ‘whorl’ patterns that are linked to turbulent plasmoids in the reconnecting current sheet. Such flare ribbon fine structure therefore reveals crucial information about the fundamental turbulent vs. laminar nature of the reconnection. Plasmoid-driven particle acceleration models would therefore predict co-location of energetic particle signatures with plasmoid-associated ribbon fine structure.

        Speaker: Joel Dahlin (NASA GSFC)
      • 49
        Investigating Energy Release during Solar Eruptive Events with RHESSI, STEREO, and SDO

        In this work, we seek to understand the relationships among magnetic reconnection, flare energy release and initiation/acceleration of coronal mass ejections (CMEs) for solar eruptive events. RHESSI, STEREO, and SDO data are utilized to study the relative timing between the HXR, CME acceleration, and reconnection rate profiles for 12 CME-flares. This analysis expands upon previous CME-flare timing studies by examining the fast-varying features, or “bursts,” in the HXR and reconnection rate profiles. These bursts represent episodes of energy release which can provide insight on reconnection and acceleration mechanisms during the eruptive event. We find, qualitatively, that the analyzed events fall into two categories: events with a single dominant HXR burst and events with a train of multiple HXR bursts. Through this work, we explore how intermittent energy release impacts the evolution of CME-flares.

        Speaker: Juliana Vievering (JHU/APL)
      • 50
        LOFAR observations of a jet-driven piston shock in the low solar corona

        The Sun produces highly dynamic and eruptive events that can drive shocks through the corona. These shocks can accelerate electrons, which result in plasma emission in the form of a type II radio burst. Despite the large number of type II radio bursts observations, the precise origin of coronal shocks is still subject to investigation. Here we present a well observed solar eruptive event that occurred on 16 October 2015, focusing on a jet observed in the extreme ultraviolet (EUV) by the Atmospheric Imaging Assembly (SDO/AIA), a streamer observed in white-light by the Large Angle and Spectrometric Coronagraph (SOHO/LASCO), and a metric type II radio burst observed by the LOw Frequency Array (LOFAR). LOFAR interferometrically imaged the fundamental and harmonic sources of a type II radio burst and revealed that the sources did not appear to be co-spatial, as would be expected from the plasma emission mechanism. We correct for the separation between the fundamental and harmonic using a model which accounts for scattering of radio waves by electron density fluctuations in a turbulent plasma. This allows us to show the type II radio sources were located $\sim$0.5 R$_\odot$ above the jet and propagated at a speed of $\sim$1000\,km\,s$^{-1}$, which was significantly faster than the jet speed of $\sim$200\,km\,s$^{-1}$. This suggests that the type II burst was generated by a piston shock driven by the jet in the low corona.

        Speaker: Ciara Maguire (Trinity College Dublin)
      • 51
        LOFAR observations of a jet-driven piston shock in the low solar corona

        The Sun produces highly dynamic and eruptive events that can drive shocks through the corona.
        These shocks can accelerate electrons, which result in plasma emission in the form of a type II radio
        burst. Despite a large number of type II radio bursts observations, the precise origin of coronal
        shocks is still subject to investigation. Here we present a well-observed solar eruptive event that
        occurred on 16 October 2015, focusing on a jet observed in the extreme ultraviolet by the SDO Atmospheric Imaging Assembly, a streamer observed in white-light by the Large Angle and Spectrometric Coronagraph, and a metric type II radio burst observed by the LOw-Frequency Array (LOFAR) radio telescope. For the first time, LOFAR has interferometrically imaged the fundamental and harmonic sources of a type II radio burst and revealed that the sources did not appear to be co-spatial, as would be expected from the plasma emission mechanism. We correct for the separation between the fundamental and harmonic using a model which accounts for the scattering of radio waves by electron density fluctuations in a turbulent plasma. This allows us to show the type II radio sources were located ∼0.5 R$_{\odot}$ above the jet and propagated at a speed of ∼1000 km s$^{−1}$, which was significantly faster than the jet speed of ∼200 km s $^{−1}$. This suggests that the type II burst was generated by a piston shock driven by the jet in the low corona.

        Speaker: Ciara Maguire (Trinity College Dublin)
      • 52
        Diagnosing temperature and density turbulence properties from type III fine structure

        The Sun frequently accelerates near-relativistic electron beams that travel out through the solar corona and interplanetary space. Undergoing wave-particle interactions with Langmuir waves, these beams are the driver for type III radio bursts, the brightest radio bursts produced by the Sun. 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, wave-particle simulations and high-resolution radio type III observations, we quantitatively show that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. Our results show how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures. This new plasma diagnostic for the solar corona and solar wind is at distances from the Sun where these properties normally cannot be measured, and significantly expands the current potential of solar radio emission.

        Speaker: Dr Hamish Reid (University College London)
      • 53
        LOFAR Observations of Radio Burst Source Sizes and Scattering in the Solar Corona

        Low frequency radio wave scattering and refraction can have a dramatic effect on the observed size and position of radio sources in the solar corona.
        The scattering and refraction is thought to be due to fluctuations of electron density caused by turbulence. Hence, determining the true radio source size can provide information on the turbulence in coronal plasma.
        However, the lack of high spatial resolution radio interferometric observations at low frequencies such as with the LOw Frequency ARray (LOFAR) have made it difficult to determine the true radio source size and level of radio wave scattering.
        Here we directly fit the visibilities of a LOFAR observation of a Type IIIb radio burst with an elliptical Gaussian to determine its source size and position. This circumvents the need for imaging of the source followed by deconvolution, which can introduce spurious effects on source size and shape.
        For a burst at 34.76~MHz, we find a full width at half maximum height (FWHM) along the major and minor axes to be $18.8^\prime$~$\pm~0.1^\prime$ and $10.2^\prime$~$\pm~0.1^\prime$ respectively at a plane of sky heliocentric distance of 1.75~R$_\odot$.
        Our results suggest that the level of density fluctuations in the solar corona is the major cause of the scattering of radio waves, resulting in large source sizes. However, the magnitude of $\varepsilon$ may be smaller than previously derived in comparison to observations of radio wave scattering in tied-array images.

        Speaker: Pearse Murphy (Trinity College Dublin/Dublin Institute for Advanced Studies)
      • 54
        LOFAR imaging of the solar corona during the 2015 March 20 solar eclipse

        The solar corona is a highly-structured plasma which can reach temperatures of more than ?~2 MK. At low frequencies (decimetric and metric wavelengths), scattering and refraction of electromagnetic waves are thought to considerably increase the imaged radio source sizes (up to a few arcminutes). However, exactly how source size relates to scattering due to turbulence is still subject to investigation. The theoretical predictions relating source broadening to propagation e?ffects have not been fully confirmed by observations due to the rarity of high spatial resolution observations of the solar corona at low frequencies. Here, the LOw Frequency ARray (LOFAR) was used to observe the solar corona at 120–180 MHz using baselines of up to ?3.5 km (corresponding to a resolution of ?~1–2') during the partial solar eclipse of 2015 March 20. A lunar de-occultation technique was used to achieve higher spatial resolution (~?0.6') than that attainable via standard interferometric imaging (?~2.4'). This provides a means of studying the contribution of scattering to apparent source size broadening. It was found that the de-occultation technique reveals a more structured quiet corona that is not resolved from standard imaging, implying scattering may be overestimated in this region when using standard imaging techniques. However, an active region source was measured to be ?~4' using both de-occultation and standard imaging. This may be explained by the increased scattering of radio waves by turbulent density fluctuations in active regions, which is more severe than in the quiet Sun.

        Speaker: Aoife Maria Ryan (Trinity College Dublin/Dublin Institute for Advanced Studies)
      • 55
        Assessing quiet Sun hard X-rays using observations from the FOXSI-2 Sounding Rocket

        In solar and helio-physics, the coronal heating problem relates to the question of identifying and explaining the mechanism(s) causing the corona's temperatures to be a few hundred times hotter than the solar surface. Among the various plausible hypotheses proposed to explain this problem, one of the strongest candidates relates to copious low energy magnetic reconnections (nanoflares) occurring throughout the solar corona. When examined thoroughly, this mechanism implies heating that happens impulsively on individual flux tubes (strands). Emission of hard X-rays (HXRs) should be a consequence of such non-thermal phenomena, or even of purely thermal transients, if hot enough. In quiescent solar corona areas, nanoflares should manifest in HXRs via very faint signatures covering vast regions. Observing feeble HXRs demands an instrument with high sensitivity and dynamic range for energies between 4 and 15 keV. FOXSI (which stands for the Focusing Optics X-ray Solar Imager) is such an instrument. As a payload of a NASA/LCAS (low-cost access to space program) sounding rocket, FOXSI has successfully completed three launches. The two most recent flights (FOXSI-2 and -3) included quiescent areas of the Sun as part of the targets. For this presentation, we will show a full assessment of the HXR flux from the quiet Sun observed with FOXSI. We begin by presenting a thorough characterization of the stray light (ghost rays) impinging into FOXSI's detectors caused by sources outside of the field of view. We then identify areas free of ghost rays where the instrument sensitivity reaches a maximum to quiet Sun HXR detections. Finally, we implement a Bayesian (known as ON/OFF analysis) to estimate an upper detectability threshold of quiet Sun HXRs and a probability distribution for quiet-Sun HXR fluxes when sources are supposed to exist.

        Speaker: Milo Buitrago-Casas (SSL - UC Berkeley)
      • 7:43 PM
        Block 4 Discussion
    • Working Group 3: Ion studies and Fermi/LAT

      Over the past 12 years, Fermi-LAT observations of high-energy solar flares have revealed an extremely rich and diverse sample of events with a wide variety of characteristics providing valuable information on accelerated ions. In order to fully understand the acceleration mechanisms at work during these flares it is imperative to combine gamma-ray observations with the observations of the UV/optical/IR/radio counterparts as well as the frequently accompanying CMEs and SEPs. Although there are space-based observatories in high energy range operating at present, every instrument has its own limitations, thus it is also important to understand what new observations are needed and to develop requirements for potential future instrumental capabilities. This working group invites contributions relating to solar flares observed in gamma-ray range by Fermi-LAT and other instruments and their connection with multiwavelength counterparts, theory/modeling of the acceleration processes associated with these flares. Also contributions on new instrumentation requirements in gamma-ray range are welcomed.

      NOTE: The times and duration of each talk are the true times and duration of the talks for this session.

      Conveners: Alexandra Lysenko (Ioffe Institute, St. Petersburg, Russian Federation) , Melissa Pesce-Rollins (PI)
      • 56
        Hard X-rays from flare ions

        As well as their gamma-ray signatures, energetic flare ions at the Sun also produce hard X-rays (HXRs) by various mechanisms. Compton scattering of gamma-ray photons was recently studied by Murphy and Share and proton bremsstrahlung by Heristchi and others. Further bremsstrahlung contributions come from secondary electrons (and positrons) produced via pion decay and also as knock-on products of primary ion collisions. We use the Monte Carlo code FLUKA to study the resulting HXRs. We note that the HXR spectrum from knock-on electrons would directly reflect the primary ion energy distribution. Ion contributions to the HXR spectrum will not be competitive with bremsstrahlung from primary electrons. However we discuss the possibility of diagnosing HXR’s from ions alone in “over-the-limb” Fermi LAT events and comment on the additional information offered by microwave observations.

        Speaker: Alexander MacKinnon (University of Glasgow)
      • 57
        Gamma-ray Line Studies in Solar Flares

        We have updated the nuclear templates used in our fits to gamma-ray spectra and have corrected a systematic that we found in SMM/GRS data. We have now fit spectra from 19 SMM flare and 1 RHESSI flare clearly separating nuclear lines and continua from electron continua. We discuss these fits and information derived on the ambient composition of the solar atmosphere where the flare-accelerated particles interact and abundances and spectra of these particles.

        Speaker: GERALD SHARE (University of Maryland)
      • 58
        Late Phase Gamma-Ray Emission in the 2005 January 20 Flare

        We discuss RHESSI observations of the 2005 January 20 flare that revealed Late Phase Gamma-Ray Emission extending for several hours after the impulsive flare.

        Speaker: GERALD SHARE (University of Maryland)
      • 59
        Cosmic Ray Electron Transport and Radiation in the Inner Heliophysics and RHESSI Observations of Hard X-ray emission from the Quiet Sun

        Hanna et al. (2010) show the 3-200 keV spectrum of the total Sun observed by RHESSI during the
        quiet phase of the Sun from 2005 to 2009. A possible source of this radiation could be synchrotron
        emission by Cosmic Ray Electrons (CRes). I will present results from a preliminary exploration of
        this model. Several near Earth instruments have observed CRe spectra at 1 AU during quiet and active
        phase of the Sun. The spectrum of the CRes at the Sun are not observed but can be evaluated by a
        detailed study of the transport of CRes from 1 AU to the solar photosphere. The transport is
        affected by three physical process; energy loss due to synchrotron and inverse Compton (IC), and the
        magnetic field convergence and scattering by the MHD turbulence in the solar wind. There are many
        observations and subsequent models for the structure of the magnetic field in the inner heliosphere
        which allow to address the first two processes fairly accurately. However, the third requires a
        knowledge of the energy density and spectrum of turbulence from 1 AU to the Sun. Up to recently
        these characteristics of the turbulence were measured around 1 AU, but Parker Solar Probe (PSP) has
        extended this knowledge to 0.17 AU. Extrapolating these observations to the Sun I will present
        result on transport of CRes using a novel and simple version of Fokker Planck equation. This will
        give us the spectral evolution of the CRes from 1 AU to the photosphere. The spectra at the
        photosphere can then be used to calculate the synchrotron spectrum and determine how well it
        reproduces the RHESSI observations.

        Speaker: Prof. Vahe' Petrosian (Stanford University)
      • 60
        Gamma-ray Observations of Impulsive Flare-Accelerated Electrons and Ions on 2017 September 10

        We use background-subtracted spectra from Fermi GBM to separate electron and ion components of the impulsive phase of the 2017 September 10 solar flare. This phase is distinct from the Late Phase Gamma Ray Emission (LPGRE) that peaks at 16:00 UT that is explained by CME shock acceleration of protons onto field lines returning to the Sun (Kouloumvakos,et al. 2020). We show evidence for hardening of the electron spectrum between 15:56 and 16:00 UT. We also discuss estimates of the power-law spectral indices of accelerated ions from 5 to 300 MeV in both the impulsive phase and LPGRE.
        .

        Speaker: GERALD SHARE (University of Maryland)
      • 61
        High Energy Particle Transport in a Solar Magnetised Medium with Geant4

        Monte Carlo codes are used in many fields to study energetic particle propagation, secondary production and radiation. They can thus be useful tools for interpreting flare gamma-rays and drawing conclusions about energetic ions at the Sun. In magnetised plasmas such as those found in solar active regions. The enormous disparity between particle gyroradii and system scales proves to be a major computational obstacle. To address this problem we have written a new module in Geant4 using the Guiding Center (GC) approach in which the particle motion is averaged over a gyrofrequency. We describe the formulation and implementation of this method in particular dealing with the uncertainty in gyrophase so that particle velocities are well-defined for input to the modules handling reactions. We compare the propagation and slowing down of primary protons, secondary particle production and runtimes in the GC limit with the Newton Lorentz (NL) approach, finding very good agreement between the two methods and orders of magnitude improvement in run times in the GC case. We show illustrative results for neutron and gamma-ray emission from ions in a dipole loop.

        Speaker: Dr Jordi Tuneu (Mackenzie Presbyterian University)
      • 62
        Development of FLUKA Templates for the Modelling of Nuclear De-Excitation Gamma-Ray Line Spectra from Solar Flares

        The modelling of gamma-ray emission spectra observed in solar flares is
        generally carried out via the best-fit of data using a set of independent
        templates and functions for the several spectral components produced by the
        relevant physical processes (bremsstrahlung of electrons and positrons, nuclear
        de-excitation, neutron capture, positron annihilation and decay of pions). In
        recent works (Tusnski et al., 2019; MacKinnon et al., 2020), we have
        demonstrated the potential of the Monte Carlo package FLUKA as an effective
        tool for the simulation of nuclear processes in the context of solar flares, as well
        as its capability to implement a self-consistent treatment of the several spectral
        components in the energy range from 100’s keV to 100’s MeV. In this work we
        use FLUKA to calculate nuclear de-excitation gamma-ray line spectra expected
        from solar flare accelerated ion distributions. We implement a simulation
        strategy which allows to synthesize photon spectra for primary accelerated ions
        with arbitrary energy distributions and chemical abundances. We show model
        spectra obtained from a range of assumed primary accelerated ion distributions
        which exhibit reliable statistics and energy resolution and are in good
        agreement with those obtained using the code developed by Murphy et al.
        (2009). From these model spectra we build templates which can be
        incorporated into the software package Objective Spectral Executive (OSPEX)
        and used in the analysis of solar flare gamma-ray data.

        Speaker: Mx Raphael Malagoli Thereza (Centro de Rádio-Astronomia e Astrofísica Mackenzie (CRAAM), Escola de Engenharia, Universidade Presbiteriana Mackenzie, São Paulo, Brazil)
      • 63
        The COMPLETE mission concept for the Heliophysics Decadal Survey

        It is commonly accepted that impulsive solar eruptions (flares/CMEs) and more gradually-evolving energetic processes (coronal heating, solar wind outflows) are powered by the Sun's complex coronal magnetic field. However, despite many decades of research, it is still poorly understood exactly how magnetic energy is stored and impulsively released to power plasma heating, particle acceleration, and bulk flows in these phenomena. One of the largest gaps preventing a deeper understanding of these processes is the lack of knowledge of the 3D coronal magnetic field and its evolution.

        We present the Co-Optimized Multi-Perspective Loop/Eruption Tracing and Energetics (COMPLETE) mission concept, currently under study for the upcoming Heliophysics Decadal Survey. COMPLETE would provide the first comprehensive measurements of the 3D low-coronal magnetic field and simultaneous 3D energy release diagnostics, from large eruptions (flares and CMEs) down to small-scale processes (coronal heating and solar wind outflows). COMPLETE's measurements of the 3D field evolution and corresponding energy release diagnostics will finally allow closure on the long-standing question of exactly how energy is stored, released, and transported in impulsive events at all scales.

        COMPLETE comprises an instrument suite with hard and soft X-ray spectral imagers, gamma-ray and energetic neutral atom spectral imagers, high-resolution wide-field EUV filtergram imagers, photospheric Doppler vector magnetographs, and Hanle-effect UV (Ly-a) coronal magnetographs. Distributed across three spacecraft at the L1, L4, and L5 Earth-Sun Lagrange points, the suite on each spacecraft is optimized for the measurements from that vantage point and for the mission as a whole. Data from all instruments will be processed to enable systems-level analysis from the entire observatory.

        Speaker: Dr Amir Caspi (Southwest Research Institute)
      • 7:40 PM
        Discussion and Questions
    • Closing Plenary session
      Convener: Prof. Sam Krucker
      • 64
        New Instrumentation and Medium-Term/Long-Term Planning
        Speaker: Dr Albert Shih (Goddard Space Flight Center)
      • 65
        In memoriam
        Speakers: Prof. Alec MacKinnon, Prof. Anna Maria Massone, Dr Brian Dennis (Goddard Space Flight Center)
      • 66
        Closing Remarks