We present and analyze the first high-resolution hard X-ray spectra from a solar flare observed in both X-ray/ g-ray continuum and g-ray lines. Spatially integrated photon flux spectra obtained by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are well fitted between 10 and 300 keV by the combination of an isothermal component and a double power law. The flare plasma temperature peaks at 40 MK around the time of peak hard X-ray emission and remains above 20 MK 37 minutes later. We derive the nonthermal mean electron flux distribution in one time interval by directly fitting the RHESSI X-ray spectrum with the thin-target bremsstrahlung from a double-power-law electron distribution with a low-energy cutoff. We find that relativistic effects significantly impact the bremsstrahlung spectrum above 100 keV and, therefore, the deduced mean electron flux distribution. We derive the evolution of the injected electron flux distribution on the assumption that the emission is thick-target bremsstrahlung. The injected nonthermal electrons are well described throughout the flare by a double-power-law distribution with a low-energy cutoff that is typically between 20 and 40 keV. We find that the power in nonthermal electrons peaks before the impulsive rise of the hard X-ray and g-ray emissions. We compare the energy contained in the nonthermal electrons with the energy content of the thermal flare plasma observed by RHESSI and GOES. The minimum total energy deposited into the flare plasma by nonthermal electrons, ergs, is on the order of the energy in the thermal plasma. 31 2.6 # 10 Subject headings: Sun: flares -Sun: X-rays, gamma raysThe time history of the flare emission in three energy bands is shown in Figure 1a. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) uses two sets of aluminum attenuators, known as thin shutters and thick shutters, to avoid saturating the detectors during large flares. The July 23 flare was observed in two attenuator states. The instrument was primarily in the A3 state, with both sets of attenuators in place. Early in the flare, before 00:26:08 UT, and late in the flare, after 00:59:21 UT, the instrument was in the A1 state, with only the thin shutters in place. There were also four brief periods during which the instrument switched from A3 to A1 and back to A3. These transitions in attenuator state are apparent in the time history of the lowest energy band in Figure 1a. The flux calibration is currently uncertain during these four brief periods, so these time periods appear as gaps in subsequent results derived from the data.We corrected the observed counts for pulse pileup and decimation (see Smith et al. 2002). Pulse pileup occurs at high count rates, with multiple photons recorded as a single photon with an energy equal to the sum of the energies of the individual photons. Decimation conserves onboard memory by recording only a fraction of the incident photons. Background counts were determined from the data by linearly interpolating between the background level...

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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.