Charge transfer X‐rays in solar and stellar flares

Abstract

Most of the energy in a solar flare, and presumably a stellar flare as well, takes the form of a power law of energetic particles. The energetic electrons produce a bremsstrahlung continuum, while the most energetic nuclei produce gammarays. Nuclei around 1 MeV/AMU can produce X-rays during and after charge transfer with neutrals. This paper predicts the fluxes for some prominent X-ray lines and compares them to existing spectra.

Remote sensing of low-energy SEPs via charge exchange

“…Unfortunately, suitable solar observations have not been available, and the best reported non-solar case [10] was only marginally significant. The analogous process for primary alpha particles and the He II Ly-α line at 304 Å was discussed in detail by Peter et al [11], and for X-rays by Raymond [12]. The range of particle energies from 0.1-1.0 MeV/nucleon maps to shifts ∆λ ≈ 4 − 14 Å relative to 304 Å.…”

Charge‐exchange X‐ray emission at hot/cool gas interfaces

“…In all specific cases, multi-fluid models must be built that allow for different orders of magnitude for the mean free paths of charged particles and of neutrals, and all consequences of the neutral-hot gas reactions considered in parallel. In particular, collisional excitation of the neutrals by hot electrons prior to their ionization and the subsequent emissions, as well as their spectral characteristics, must be found to be consistent with the CTX emissivities (see Raymond 2012).…”

“…For a few years there have been suspicions of charge-transfer X-ray detections in other astrophysical objects, most of them being described in this issue. Still, as described by Raymond (2012), outside the solar system there is no definite proof that what is observed is CTE. One difficulty is the need for detailed multi-fluid models able to reproduce the CT reactions and the associated emission.…”