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 Letter, we test whether the observed energy-dependent timing delays of 20-200 keV hard X-ray (HXR) emission can be explained in terms of propagating electrons versus protons. For a typical flare, we show that the timing delays of fast (less than or similar to 1s) HXR pulses is consistent with time-of-flight differences of directly precipitating electrons, while the timing delays of the smooth HXR flux is consistent with collisional deflection times of trapped electrons. We show that these HXR timing delays cannot be explained either by less than or equal to 1 MeV protons (as proposed in a model by Simnett & Haines), because of their longer propagation and trapping times, or by approximate to 40 MeV protons (which have the same velocity as approximate to 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 greater than or equal to 20 keV HXR emission.