Lockheed Martin Advanced Technology Center, Solar and Astrophysics Laboratory, Dept L941, Bldg.252, 3251 Hanover St.,Palo Alto, CA 94304, USA

The energy-dependence of the timing of hard X-ray radiation produced in solar flares provides important diagnostics on the mean free path length of energized particles, the Coulomb collisional deflection time, the collisional loss time, and free-streaming propagation times. Combining this timing information with spatial scales measured from Yohkoh and future HESSI hard X-ray images we can reconstruct electron propagation trajectories and pinpoint their acceleration site. New timing results from the analysis of multiple interacting flare loops reveal the configuration of multi-path trajectories in quadrupolar magnetic geometries. Additionally, the relative timing between hard X-ray and gyrosynchrotron-producing electrons as observed now with the Nobeyama radioheliograph reveals further clues on the spatial topology of particle acceleration sites. We discuss some timing experiments that can be anticipated with HESSI.