Simulation of Energetic Neutral Atoms From Solar Energetic Particles

Abstract

Energetic neutral atoms (ENAs) provide the only way to observe the acceleration site of coronal-mass-ejectiondriven (CME-driven) shock-accelerated solar energetic particles (SEPs). In gradual SEP events, energetic protons can charge exchange with the ambient solar wind or interstellar neutrals to become ENAs. Assuming a CME-driven shock with a constant speed of 1800 km s −1 and compression ratio of 3.5, propagating from 1.5 to 40 R S , we calculate the accelerated SEPs at 5-5000 keV and the resulting ENAs via various charge-exchange interactions. Taking into account the ENA losses in the interplanetary medium, we obtain the flux-time profiles of these solar ENAs reaching 1 AU. We find that the arriving ENAs at energies above ∼100 keV show a sharply peaked flux-time profile, mainly originating from the shock source below 5 R S , whereas the ENAs below ∼20 keV have a flat-top time profile, mostly originating from the source beyond 10 R S . Assuming the accelerated protons are effectively trapped downstream of the shock, we can reproduce the STEREO ENA fluence observations at ∼2-5 MeV/nucleon. We also estimate the flux of ENAs coming from the charge exchange of energetic storm protons, accelerated by the fast CME-driven shock near 1 AU, with interstellar hydrogen and helium. Our results suggest that appropriate instrumentation would be able to detect ENAs from SEPs and to even make ENA images of SEPs at energies above ∼10-20 keV.

Neutralized Solar Energetic Particles for SEP Forecasting: Feasibility Study of an Innovative Technique for Space Weather Applications

“…We reproduce the HENA production and propagation model from Wang et al (2014) as the baseline model for this study, and further change some parameters to investigate variations of the expected HENA properties that can be observed. No cross-variation relations (two or more parameters deviating from the baseline values at the same time) are evaluated.…”

“…Nevertheless, the changed parameters have in many cases independent effects from each other, and therefore the effects of combined parameter changes are straightforward and understandable. The baseline model (Wang et al, 2014) consists of a partially spherical-shaped shock front formed at a heliocentric distance of 1.5RS with a width angle of 60° at t = 0, and it then propagates in the direction perpendicular to the Sun-observer line with a speed of 1800 km/s (Figure 1). The coordinate system is centered on the Sun, with the x-axis pointing to the observer at the Earth, the x-y plane containing the symmetry axis of the cone that defines the CME.…”

“…To verify the conclusion by Mewaldt et al (2009), Wang et al (2014) proposed a simplified model to predict the properties of solar HENAs. The model consists of a sphericalcap-shaped shock driven by a CME erupting at t = 0.…”

“…We follow the method used by Wang et al (2014), considering the photoionization, and proton and electron impact ionization losses for propagating HENAs. Wang et al (2014) did not specify how the electron velocity distribution was considered to apply the impact ionization cross section. We thus compute the ionization rate by integration over a Maxwellian velocity distribution of solar wind electrons.…”

“…This investigation is done by modelling the solar HENA properties under different upstream conditions and comparing them with the performance of a typical HENA instrument. For the modelling part, we expand the ranges of several parameters describing the production and propagation of HENAs in addition to Wang et al (2014), and investigate the variability of solar HENA properties depending on these parameters. For the detection feasibility part, although the STEREO/LETs observed the possible solar HENA signals, they are not ENA instruments, and there has been no dedicated solar HENA instrument in space.…”