Charge-exchange limits on low-energy α-particle fluxes in solar flares

This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyman-α line of He ii at 304 Å, as originally suggested for hydrogen by Orrall & Zirker (1976). Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and radiate recombination radiation before it stops. The Extreme-ultraviolet Variability Experiment (EVE) on board the Solar Dynamics Observatory (SDO) gives us our first chance to search for this effect systematically. The Orrall-Zirker charge-exchange mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon. We study ten events in total, including the γ-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles (SEPs). The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broad-band signatures, which could correspond to continua from other origins. Subject headings: Sun: flares — Sun: photosphere