We summarize Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray (HXR) and gray imaging and spectroscopy observations of the intense (X4.8) g-ray line flare of 2002 July 23. In the initial rise, a new type of coronal HXR source dominates that has a steep double-power-law X-ray spectrum and no evidence of thermal emission above 10 keV, indicating substantial electron acceleration to tens of keV early in the flare. In the subsequent impulsive phase, three footpoint sources with much flatter double-power-law HXR spectra appear, together with a coronal superhot ( MK) thermal source. The north footpoint and the coronal T ∼ 40 source both move systematically to the north-northeast at speeds up to ∼50 km s Ϫ1 . This footpoint's HXR flux varies approximately with its speed, consistent with magnetic reconnection models, provided the rate of electron acceleration varies with the reconnection rate. The other footpoints show similar temporal variations but do not move systematically, contrary to simple reconnection models. The g-ray line and continuum emissions show that ions and electrons are accelerated to tens of MeV during the impulsive phase. The prompt de-excitation g-ray lines of Fe, Mg, Si, Ne, C, and O-resolved here for the first time-show mass-dependent redshifts of 0.1%-0.8%, implying a downward motion of accelerated protons and a-particles along magnetic field lines that are tilted toward the Earth by ∼40Њ. For the first time, the positron annihilation line is resolved, and the detailed high-resolution measurements are obtained for the neutron-capture line. The first ever solar g-ray line and continuum imaging shows that the source locations for the relativistic electron bremsstrahlung overlap the 50-100 keV HXR sources, implying that electrons of all energies are accelerated in the same region. The centroid of the ion-produced 2.223 MeV neutron-capture line emission, however, is located ∼ away, implying 20 ‫ע‬ 6 that the acceleration and/or propagation of the ions must differ from that of the electrons. Assuming that Coulomb collisions dominate the energetic electron and ion energy losses (thick target), we estimate that a minimum of ∼ ergs is released in accelerated 1∼20 keV electrons during the rise phase, with ∼10 31 ergs in ions above 31 2 # 10 2.5 MeV nucleon Ϫ1 and about the same in electrons above 30 keV released in the impulsive phase. Much more energy could be in accelerated particles if their spectra extend to lower energies.

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We characterize and catalog 30 solar eruptive events observed by the Fermi Large Area Telescope (LAT) having late-phase >100 MeV γ-ray emission (LPGRE), identified 30 yr ago in what were called long-duration gamma-ray flares. We show that LPGRE is temporally and spectrally distinct from impulsive phase emission in these events. The spectra are consistent with the decay of pions produced by >300 MeV protons and are not consistent with primary electron bremsstrahlung. Impulsive >100 keV X-ray emission was observed in all 27 LPGRE events where observations were made. All but two of the LPGRE events were accompanied by a fast and broad coronal mass ejection (CME). The LPGRE start times range from CME onset to 2 hr later. Their durations range from ∼0.1 to 20 hr and appear to be correlated with durations of >100 MeV solar energetic particle (SEP) proton events. The power-law spectral indices of the >300 MeV protons producing LPGRE range from ∼2.5 to 6.5 and vary during some events. Combined γ-ray line and LAT measurements indicate that LPGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of LPGRE protons >500 MeV is typically about 10× the number in the impulsive phase of the solar eruptive event and ranges in nine events from ∼0.01× to 0.5× the number in the accompanying SEP event, with large systematic uncertainty. What appears to be late-phase electron bremsstrahlung with energies up to ∼10 MeV was observed in one LPGRE event. We discuss how current models of LPGRE may explain these characteristics.

Total cross sections for the production of gamma-ray lines from nuclear deexcitation as a function of the projectile energy are evaluated and presented. Included are proton and α reactions with He, C, N, O, Ne, Mg, Al, Si, S, Ca and Fe. Such functions are essential for interpretation of gamma-ray line observations of astrophysical sites which contain large fluxes of energetic particles such as solar flares, the Earth's atmosphere, planetary atmospheres and surfaces, the interstellar medium and galactic nebulae.

Analyses of gamma-ray line emission in solar flares have provided information about conditions in flaring magnetic loops, the abundances of the chromosphere where the gamma rays are produced, and the composition and spectrum of the flare-accelerated ions. While laboratory measurements of the cross sections for production of the strongest lines seen in flare spectra are available, these measurements often only cover a limited range of projectile energies. In addition, the bulk of the gamma-ray emission arises from the numerous weaker lines for which there are no measurements. The gamma-ray de-excitation-line production code, developed originally by Ramaty, Kozlovsky, and Lingenfelter, has been and continues to be the primary theoretical tool used for analyses of solar-flare gamma-ray data. The code uses both measured cross sections and estimated cross sections where measurements are inadequate. We have improved the completeness and accuracy of this code in three ways. (1) We use recent cross section measurements to improve cross sections for those lines already explicitly included in the code and to provide cross sections for new explicit lines. (2) For the first time, we give a detailed evaluation of the unresolved-line "continuum" (i.e., all line emission not accounted for by the explicit lines in the code). Because adequate laboratory measurements for this emission are not available, the primary tool for this evaluation was the theoretical nuclear program TALYS. We explore how this unresolved-line continuum depends on parameters relevant for solar flares. (3) We use TALYS to improve those line cross sections where available laboratory measurements are inadequate and to provide cross sections for new explicit lines for which no measurements exist. Numerical cross section values for all lines explicitly addressed by the code and for the unresolved-line continua are given in the Appendix.