Electron versus Proton Timing Delays in Solar Flares
  Both electrons and ions are accelerated in solar flares and carry nonthermal energy from the acceleration site to the chromospheric energy loss site, but the relative amount of energy carried by electrons versus ions is subject of debate. In this {\sl Letter} we test whether the observed energy-dependent timing delays of 20-200 keV HXR emission can be explained in terms of propagating electrons versus protons. For a typical flare, we show that the timing delays of fast ($\lapprox 1$ s) {\sl HXR pulses} is consistent with time-of-flight differences of directly precipitating electrons, while the timing delays of the {\sl smooth HXR} flux is consistent with collisional deflection times of trapped electrons. We show that these HXR timing delays cannot be explained either by $\le 1$ MeV protons (as proposed in a model by Simnett \& Haines 1990), because of their longer propagation and trapping times, or by $\approx 40$ MeV protons (which have the same velocity as $\approx 20$ keV electrons), because of their longer trapping times and the excessive fluxes required to generate the HXRs. Thus, the HXR timing results clearly rule out protons as the primary generators of $\ge 20$ keV HXR emission. 
Markus J. Aschwanden (Univ. Maryland)