The Sun 93
FIGURE 20 Top: The geometry of the acceleration region inferred from direct detections of
above-the-looptop hard X-ray sources withYohkoh/HXT (Hard X-Ray Telescope) (contours) and
simultaneous modeling of electron time-of-flight distances based on energy-dependent time delays of
20–200 keV hard X-ray emission measured with Burst and Transient Source Experiment,
BATSE/CGRO(crosses marked with ACC). Soft X-rays detected withYohkoh/SXT or thermal hard
X-ray emission from the low-energy channel ofYohkoh/HXT/Lo are shown in colors, outlining the
flare loops. Bottom: The observations in the left panel show aYohkoh/HXT 23–33 keV image (thick
contours) and Be119SXTimage (thin contours) of the Masuda flare, January 13, 1992, 17:28 UT.
The interpretation of the above-the-looptop source is that temporary trapping occurs in the
acceleration region in the cusp region below the reconnection point (bottom right).in the chromosphere, as well as of the directivity and pitch
angle distribution of the precipitating protons and ions that
have been accelerated in coronal flare sites, presumably
in magnetic reconnection regions. Critical issues that have
been addressed in studies of gamma-ray data are the max-
imum energies of coronal acceleration mechanisms, the
ion/electron ratios (because selective acceleration of ions
indicate gyroresonant interactions), the ion/electron tim-
ing (to distinguish between simultaneous or second-step
acceleration), differences in ion/electron transport (e.g.,
neutron sources were recently found to be displaced from
electron sources), and the first ionization potential (FIP)
effect of chromospheric abundances (indicating enhanced
abundances of certain ions that could be preferentially ac-
celerated by gyroresonant interactions). Although detailed
modeling of gamma-ray line profiles provides significant
constraints on elemental abundances and physical proper-
ties of the ambient chromospheric plasma, as well as on the
energy and pitch angle distribution of accelerated particles,
little information or constraints could be retrieved about the