Solar Physics Laboratory

The NASA High Energy Solar Physics (HESP) mission offers the opportunity for major breakthroughs in ourunderstanding of the fundamental energy release and particle acceleration processes at the core of the solar flare problem. HESP's primary strawman instrument, the High Energy Imaging Spectrometer (HEISPEC). will provide X-ray and y-ray imaging speciroscopy. i.e.. high-resolution spectroscopy at each spatial point in the image. Ithas the following unique capabilities: (1) high-resolution (-keV) spectroscopy from 2 keV-20MeV to resolve flare gamma-ray lines and sharp features in the continuum; (2) hard X-ray imaging with 2" angular resolution and tens of millisecond temporal resolution, commensurate with the travel and stopping distances and timesfor the accelerated electrons; (3) gamma-ray imaging with 4"-8"resolution withthe capability ofimaging in specificlines or continuum regions; (4) moderate resolution imaging of energetic (20 MeV to-i GeV) gamma-rays and neutrons. Additional strawman instruments include a Bragg crystal spectrometer for diagnostic infonnation and a soft X-ray/XUV/UV imager to map the flare coronal magnetic field and plasma structure. The HESP mission also includes extensive ground-based observational and supporting theory programs. Presently HESP is planned for aFY 1995 new start and late 1999 launch, in time for the next solar activity maximum. INTRODUCrION The overarching scientific objective of the High Energy Solar Physics (HESP) mission is to explore the processes of impulsive energy release and particle acceleration inmagnetized plasmas. Thefundamental importance ofthese high-energy processestranscends their significance in solar physics since they are found to play a major role throughout the universe at sites ranging from planetary and neutron star magnetospheres to active galaxies. The detailed understanding of these processes is one of the majorgoals of space physics and astrophysics, but inessentially all cases, we are only just beginning toperceive the relevant basic physics. Nowhere can one pursue the study ofthis basic physics better than in the active Sun. where solar flares are the direct insult ofimpulsive energy release and particle acceleration. The accelerated particles, notably theelectrons with energiesoftens ofkeV, appear tocontain a major fraction ofthe total flareenergy, thus indicating the fundamental role of the high-energy processes. The acceleration of electrons is revealed by hard X-ray and gamma-ray bremsstrahlung the acceleration ofprotons andnuclei isrevealed by nuclear gamma-rays, pion-decayradiation, and neutrons. The proximity of the Sun means that these high-energy emissions appear orders of magnitude more intense than from any other cosmic source, plus they can be better resolved, both spatially and temporally. Consequently, the Sun is the only astrophysical object where the phenomena can be studied with thedetail necessary tounderstand the fundamental processes. HESP is designed to study the high-energy processes in solar flares. These involve the rapid release ofenergy stored in unstable magnetic configurations, the equally rapidconversion ofthis energy into kinetic energy of accelerated particles and hot plasma,the transport ofthese particles, and the subsequent heating of the ambient solar atmosphere. Observations ofhard X-rays, gamma rays, and neutrons serve as the best diagnostic of these processesby providing direct evidence for the interaction ofaccelerated particles in solar flares. The necessary spatial andtemporal resolving powers mustmatch the spatial and temporal scales that characterize the processes ofenergy release, acceleration, and transport. The sensitivity should be high enough to detect the initial energy release and particle acceleration, and also to provide observations over a wide range of intensities from microflares to large flares. Equally important, the spectral resolving power must be high enough to allow the deciphering ofthe rich infonnaticai encoded in both the gamma-ray lines and the highly-structured photon continuum. The observations should provide imaging with spectroscopy and should cover the entire photon energy range fran soft thermal X-rays to high-energy gamma rays, as well as energetic neutrons. It is the primary goal of IIESP to provide, for the first time, such comprehensive observations. Additional objectives of IIESP include the study of thecomposition ofthe solar atmosphere,using gamma-ray spectroecopy, and the investigation ofmicroflares and their contribution tothe heatingof the corona.

The major results from SMM are presented as they relate to our understanding of the energy release and particle transportation processes that lead to the high-energy X-ray aspects of solar flares. Evidence is reviewed for a 152-158 day periodicity in various aspects of solar activity including the rate of occurrence of hard X-ray and gamma-ray flares. The statistical properties of over 7000 hard X-ray flares detected with the Hard X-Ray Burst Spectrometer are presented including the spectrum of peak rates and the distribution of the photon number spectrum. A flare classification scheme introduced by Tanaka is used to divide flares into three different types. Type A flares have purely thermal, compact sources with very steep hard X-ray spectra. Type B flares are impulsive bursts which show double footpoints in hard X-rays, and soft-hard-soft spectral evolution. Type C flares have gradually varying hard X-ray and microwave fluxes from high altitudes and show hardening of the X-ray spectrum through the peak and on the decay. SMM data are presented for examples of type B and type C events. New results are presented showing coincident hard X-rays, O v, and UV continuum observations in type B events with a time resolution of < 128 ms. The subsecond variations in the hard X-ray flux during < 10% of the stronger events are discussed and the fastest observed variation in a time of 20 ms is presented. The properties of type C flares are presented as determined primarily from the non-imaged hard X-ray and microwave spectral data. A model based on the association of type C flares and coronal mass ejections is presented to explain many of the characteristics of these gradual flares.

The major results from SMM are presented as they relate to our understanding of the energy release and particle transportation processes that lead to the high-energy X-ray aspects of solar flares. Evidence is reviewed for a 152-158 day periodicity in various aspects of solar activity including the rate of occurrence of hard X-ray and gamma-ray flares. The statistical properties of over 7000 hard X-ray flares detected with the Hard X-Ray Burst Spectrometer are presented including the spectrum of peak rates and the distribution of the photon number spectrum. A flare classification scheme introduced by Tanaka is used to divide flares into three different types. Type A flares have purely thermal, compact sources with very steep hard X-ray spectra. Type B flares are impulsive bursts which show double footpoints in hard X-rays, and soft-hard-soft spectral evolution. Type C flares have gradually varying hard X-ray and microwave fluxes from high altitudes and show hardening of the X-ray spectrum through the peak and on the decay. SMM data are presented for examples of type B and type C events. New results are presented showing coincident hard X-rays, O v, and UV continuum observations in type B events with a time resolution of < 128 ms. The subsecond variations in the hard X-ray flux during < 10% of the stronger events are discussed and the fastest observed variation in a time of 20 ms is presented. The properties of type C flares are presented as determined primarily from the non-imaged hard X-ray and microwave spectral data. A model based on the association of type C flares and coronal mass ejections is presented to explain many of the characteristics of these gradual flares.

Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations of 18 double hard X-ray sources seen at energies above 25 keV are analyzed to determine the spatial extent of the most compact structures evident in each case. The following four image reconstruction algorithms were used: Clean, Pixon, and two routines using visibilities-maximum entropy and forward fit (VFF). All have been adapted for this study to optimize their ability to provide reliable estimates of the sizes of the more compact sources. The source fluxes, sizes, and morphologies obtained with each method are cross-correlated and the similarities and disagreements are discussed. The full width at half-maximum (FWHM) of the major axes of the sources with assumed elliptical Gaussian shapes are generally well correlated between the four image reconstruction routines and vary between the RHESSI resolution limit of ∼ 2 up to ∼ 20 with most below 10. The FWHM of the minor axes are generally at or just above the RHESSI limit and hence should be considered as unresolved in most cases. The orientation angles of the elliptical sources are also well correlated. These results suggest that the elongated sources are generally aligned along a flare ribbon with the minor axis perpendicular to the ribbon. This is verified for the one flare in our list with coincident Transition Region and Coronal Explorer (TRACE) images. There is evidence for significant extra flux in many of the flares in addition to the two identified compact sources, thus rendering the VFF assumption of just two Gaussians inadequate. A more realistic approximation in many cases would be of two line sources with unresolved widths. Recommendations are given for optimizing the RHESSI imaging reconstruction process to ensure that the finest possible details of the source morphology become evident and that reliable estimates can be made of the source dimensions.