Group Summaries

Working Group 1
Leaders: Eoin Carley, Bin Chen

Radio diagnostics for accelerated electrons

Radio and X-ray instruments provide complementary diagnostics of accelerated electrons in a wide variety of energy release events throughout the solar atmosphere, including flares and transient brightenings, as well as associated eruptive activities such as jets and CMEs. In this working group we invite contributions of radio observations that cover all aspects of flares and other eruptive activities, from the low solar atmosphere into the heliosphere. We particularly welcome contributions that utilize joint radio and X-ray data from, but certainly not limited to, radio facilities including ALMA, EOVSA, LOFAR, LWA, MUSER, MWA, NoRH, NRH, SSRT, and VLA, as well as X-ray instruments including RHESSI, FERMI, FOXSI, MinXSS, and NuSTAR. We also welcome any contributions discussing future possibilities with upcoming or proposed radio facilities such as FASR, ngVLA, SKA, SunRISE, and their synergy with the current and future X-ray missions beyond the RHESSI era.

Working Group 2
Leaders: Sophie Musset, Matej Kuhar

Observations of the Sun with focusing HXR instruments

RHESSI has proven to be an enduring and successful source of observations for the field of high energy solar physics. However, the next great strides in this field will benefit from focusing X-ray imaging, which allows better sensitivity and provides opportunity to search for faint non thermal signatures on the Sun. Recent technological developments have enabled direct HXR imaging with grazing incidence optics, already used in the astrophysical NASA telescope NuSTAR. With much success in NuSTAR solar observations and recent advances in the development of FOXSI, the focus now shifts to this new approach in high energy solar physics. This session invites presentations on new instruments and observations with astronomical and solar focusing HXR telescopes, to investigate high energy processes in the solar atmosphere.

Working Group 3
Leaders: Natasha Jeffrey, Meriem Alaoui

Reconnection and related acceleration/transport mechanisms

Title:  “Particle acceleration and transport at the Sun and beyond”

Many astrophysical plasmas are efficient particle accelerators, often converting magnetic energy released as the result of magnetic reconnection into the kinetic energy of accelerated particles. At the Sun, X-ray observations with RHESSI greatly improved our understanding of solar flare particle acceleration and transport, particularly when combined with radio, soft X-ray and (E)UV observations. However, there are still many questions about how and where energetic particles are accelerated and how different plasma conditions (e.g., collisions, turbulence, return currents) affect the transport and observed properties of energetic particles. Proposed missions with direct X-ray imaging capabilities (e.g., FOXSI) will probe high energy processes on the Sun in unprecedented detail, and data from new observatories (Solar Probe, Solar Orbiter) will help to understand the connection between energetic particle populations at the Sun and those measured in situ in the heliosphere. This working group welcomes all contributions that aim to address the common challenges involved in understanding particle acceleration and transport in solar flares, coronal mass ejections, jets, and beyond in other astrophysical objects, using theory, numerical simulations and observations (e.g. hard X-ray, soft X-ray, radio, (E)UV, in situ).

Working Group 4
Leaders: Graham Kerr, Chris Moore

Flare thermal response

Diagnostics related to the heating and ionisation of the solar atmosphere during flares, transient events, and perceived “steady states”, offer the chance to understand the transport of energy, mass, and radiation in these events. These are key aspects of the flare and coronal heating phenomena; that models must be able to reproduce. Additionally, the physical properties through the various layers of the atmosphere can highlight new insights, particularly as the spatial, temporal, and spectral resolution, and temperature coverage, of observatories continues to improve. A wide scope for discovery exists by studying multi-wavelength (multi-temperature) observations. This allows the stratification of plasma motions, plasma density, temperature gradients, and differential emission measures, or other properties, to be derived from observables.

This session will address all layers of the solar atmosphere with the goal of reaching a comprehensive description from corona to photosphere: for example, super-hot coronal sources (are return currents necessary?), the transition region and chromosphere (do we understand the transport and deposition of energy into these layers well enough to reproduce observations?), and the temperature minimum region & photosphere (how does flare energy reach these depths, are more exotic forms of energy transport required, and to what extent is the photosphere disturbed?). We invite abstracts that discuss observations, models, or model-data comparisons of the thermodynamic response during flares, with the aim of facilitating discussions.

Working Group 5
Leaders: Erica Lastufka

Directivity / Stereoscopic HXR observations

During the next solar maximum, Solar Orbiter's STIX, traveling along its highly elliptical orbit, combined with present and proposed HXR imagers in low Earth orbit, will make stereoscopic HXR observations of flares possible for the first time.This new type of observation can yield the directivity of HXR emission from flare electrons, which itself is a signature of electron pitch angle distributions. Linear polarization of the observed HXRs is another consequence of the electron pitch angle distribution. This session invites authors to discuss relevant theory, anticipated new discoveries, instrumental or analytical requirements, or even present new instrument concepts.

Working Group 6
Leaders: Linghua Wang, Fan Guo

Connections between accelerated electrons at the Sun and in interplanetary space

The Sun is capable of accelerating electrons from ~ tens of eV up to hundreds of MeVs. After acceleration, some of the energized electrons travel upwards along the open magnetic field lines into the interplanetary space, to be detected in situ by various spacecraft, while the rest propagate downwards into the lower atmosphere, to generate HXR emissions via non-thermal bremsstrahlung processes. Thus, these upward-traveling and downward-traveling electrons, as well as their relations, carry crucial information on the particle acceleration and transport processes at the Sun and/or in interplanetary space. However, many details of these processes still remain unknown.This working group invites contributions that discuss space-borne and ground-based observations, and theory/modeling of the processes connecting the accelerated electrons at the Sun and in the interplanetary space.

Working Group 7
Leaders: Melissa Pesce-Rollins

Ion studies and Fermi/LAT

Recent years have seen our understanding of electron acceleration in flares reach a new level of refinement, primarily through RHESSI, but the same cannot be said regarding ions interacting at the Sun. Gamma-rays and SEPs from flares are the primary sources of information on accelerated ions. Observational data in the MeV range can reveal critically important information on ion distributions in the 1-30 MeV range, whereas in the GeV energy range, observations from Fermi-LAT, PAMELA and AMS02 are providing new clues on the essentially unexplored region of >300 MeV ion acceleration during solar flares. This group will discuss such observations in the context of acceleration processes associated with flares and CME shocks. We will also discuss what information the UV/optical/IR/radio counterparts to these gamma-ray sources can provide on the acceleration processes. Finally, we will discuss what new observations are needed and requirements for potential future instrumental capabilities.

Working Group 8
Leaders: Julie Vievering, Amir Caspi

Future instrumentation

RHESSI provided over 16 years of groundbreaking measurements to revolutionize the field of high-energy solar physics. New soft and hard X-ray and gamma-ray instrumentation has been and is being developed, notably on the FOXSI rockets and pending SMEX, the PhoENiX mission concept, the MinXSS CubeSats and CubIXSS mission concept, the MaGIXS rocket, the GRIPS balloon, and others that will continue and expand upon RHESSI’s outstanding legacy. This session invites presentations on additional new technologies, instruments, and orbital,  or sub-orbital and ground-based concepts that can provide new and deeper insights into high-energy processes in the solar corona. This is not limited to X-ray/gamma-ray observations, but includes radio, UV/EUV, energetic particles, and other measurements that can shed light on solar eruptive processes and high-energy coronal physics, particularly in combination with upcoming and/or proposed X-ray missions such as FOXSI SMEX, Solar Orbiter / STIX, PhoENiX, and others.