Electron time-of-flight measurements during the Masuda flare, 1992 January 13

The solar flare of 1992 January 13, 1729 UT, has become famous because Masuda's discovery of a hard X-ray loop-top source (Masuda 1994). Here we analyze energy-dependent time delays occurring in 30-120 keV hard X-ray (HXR) emission during this flare, observed by BATSE on board the Compton Gamma Ray Observatory with a time resolution of 64 ms. The purpose of this study is to reconstruct the kinematics of HXR-producing electrons from energy-dependent HXR delays, and to relate the inferred time-of-flight distance to the spatial geometry of the flare loop, as observed by SXT and HXT on board Yohkoh. The findings are the following: 1. The HXR flux can be decomposed into a sequence of pulses with approximate to 2-3 s duration and into a smoothly varying envelope that accounts for 90% of the greater than or equal to 30 keV flux. Cross-correlating the pulses between five different energy channels in the 30-120 keV range, we find that the HXR pulses are delayed (tau(p) = 40-220 ms) at the lower energies with respect to the higher energies. For the HXR envelopes, we find much larger delays (-tau(E) = 2.1-6.6 s) of opposite sign. 2. We fit kinematic models that quantify electron acceleration and propagation times to the observed HXR timing, for small-scale and large-scale accelerating fields, in semicircular and cusplike flare loop geometries. We find that the acceleration site is most likely located in an altitude of h = 44,000 +/- 6000 km, in the cusp region above the SXR-emitting flare loop (h = 12,500 km), and also significantly above Masuda's coronal HXR source (h = 22,100 km). This finding offers an interpretation of Masuda's HXR source in terms of nonthermal bremsstrahlung by electrons partially confined in the cusp region either by magnetic mirroring or by wave turbulence in the reconnection outflow. 3. The delay of the smoothly varying HXR flux is found to be consistent with trapping time differences in terms of collisional deflection, based on estimates of the electron density (n(e) less than or similar to 2 x 10(11) cm(-3)) from SXT emission measure maps. This study provides the first quantitative localization of the electron acceleration site in a solar hare, and demonstrates that energy-dependent HXR delays offer a sensitive diagnostic for electron acceleration, propagation, and trapping in solar flares.