Cause of Proton Depletion in Solar Impulsive Particle Events

It is found from our reanalysis of the relevant data that the high electron-to-proton ratio (E/P) observed in large impulsive gammaray line (GRL) flares can be alternatively explained by proton depletion rather than electron richness as formerly suggested. The term "impulsive events" in this paper is exclusively used as the large impulsive events associated with some energetic flare phenomena like GRL emissions and meter-wave type II emissions. Consequently, other weaker impulsive events that occur much more frequently, and in association with 3He-rich particle events, are outside the scope of this study. We propose that the proton depletion can be explained by either intrinsic E/Ps of escaping particles from the flare itself or a selection effect in the acceleration process at the lower coronal shock. In the latter case, we suggest that shock waves in the lower corona accelerate only a very few protons because there are no such seed particles as are usually supplied by non-Maxwellian suprathermal high-energy tails in the case of upper collisionless atmosphere. Such suprathermal seed particles are actually observed even in the steady solar wind ubiquitously. As for the former case of the explanation, the flare origin hypothesis for large impulsive particle events seems unlikely, because of the difference in the emission cone angles between the large and small impulsive events. For gradual events, on the other hand, shocks are thought to be driven by coronal mass ejections and can survive even in the upper atmosphere, accompanied by seed particles, resulting in acceleration of plenty of protons.