Electron time-of-flight distances and flare loop geometries compared from CGRO and Yohkoh observations

The distance between the coronal acceleration site and the chromospheric hard X-ray (HXR) emission site can be determined from velocity-dependent electron time-of-flight (TOF) differences in the framework of the thick-target model. We determine these electron TOF distances l with relative time delay measurements in the 30-300 keV energy range, using 16 channel data from BATSE/CGRO for the eight largest flares simultaneously observed with Yohkoh. We filter the HXR fine structure from the smoothly varying HXR flux with a Fourier filter in order to separate competing time delays. In the Yohkoh/HXT images we identify the corresponding flare loops that show greater than or equal to 30 keV HXR footpoint emission and project the electron TOF distances into the loop plane, assuming a semicircular shape (with radius r). The flare loop radii vary in the range of r = 5600-17,000 km. In all eight flares we find that the projected electron TOF distance l' exceeds the loop half-length s = r(pi/2), with a scale-invariant ratio of l'/s = 1.3 +/- 0.2. Projecting the electron TOF distances onto an open field line that extends to the cusp region above the flare loop, we find an average ratio of h/r = 1.7 +/- 0.4 for the height h of the acceleration site. This geometry is compatible with acceleration mechanisms operating in the cusp region, perhaps associated with magnetic reconnection processes above the flare loop. Alternatively, acceleration sites inside the flare loop cannot be ruled out (since l'/s < 2), but they do not provide a natural explanation for the observed length ratio l'/s. Large-scale electric DC held acceleration mechanisms are found to be less suitable to explain the observed HXR timing and pulse durations.