Non-Observation of the He II 304 A Charge-Exchange Continuum in Major Solar Flares

We report on a search for flare emission via charge-exchange continuum radiation in the wings of the Lyman-alpha line of He ii at 304 A, as originally suggested for hydrogen by Orrall and Zirker (1976). Via this mechanism a primary alpha particle that penetrates into the neutral chromosphere can pick up an atomic electron and radiate recombination continuum 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 four EVE events: the gamma-ray events SOL2010-06-12 (M2.0) and SOL20 11-02-24 (M3.5), the latter a limb flare, and the X-class flares SOL2010-02-15 (X2.2) and SOL2011-03-09 (X1.2). No clear signature of the charge-exchange continuum appears, but SOL2010-02-15 (X2.2) does reveal a gradual broad-band signature that we tentatively interpret as due to unresolved emission lines or instrumental scattering.