Remote sensing of low-energy SEPs via charge exchange

Charge-exchange reactions at high energies provide new channels for the remote sensing of solar high-energy particles, as demonstrated by the recent detection of 1.8-5 MeV hydrogen atoms from a solar flare [1]. Orrall and Zirker [2] had earlier proposed the detection of low-energy protons via charge-exchange atomic reactions in the solar atmosphere, leading in the simplest case to extended red-wing emission in the Lyman-alpha line. We discuss the analogous process for the He II 304 Å line (for alpha particles) and also assess the feasibility of the analogous process in the solar wind, whereby ambient He and (C, N, O) ions allow low-energy alpha particles to undergo resonant charge exchange in the ambient corona and thereby produce 304 Å wing emission close to the acceleration region.