Simulation of Energetic Neutral Atom Imaging of Solar Energetic Particles

Energetic Neutral Atoms (ENAs) provide the only way to observe solar energetic particles (SEPs) where they are accelerated. In intense SEP events, the high-energy protons appear to be accelerated by the fast CME driven-shock mostly at altitudes of ~1.5 to tens of Rs, while the low-energy protons are continuously produced as the shock propagates outward, even near 1 AU. These protons can charge exchange with the ambient solar wind or interstellar neutrals to become ENAs. Assuming a CME-driven shock propagates from 1.5 to 40 Rs with a constant speed of 1800 km/s and compression ratio of 3.0, we simulate the accelerated SEPs and the corresponding ENAs originating from these SEPs via various charge-exchange interactions. We also calculate their survival probabilities to reach 1 AU. To reproduce the STEREO ENA fluence observations at ~2-5 MeV/nuc (Mewaldt et al., 2009) requires that the accelerated protons are effectively trapped downstream of the shock. We find that ENAs above ~100 keV arriving at 1 AU mainly originate from the source below 5 Rs, whereas most of the ENAs below ~20 keV originate from above 10 Rs. We also estimate the flux of ENAs coming from the charge exchange of the energetic storm protons (ESPs), accelerated by the fast CME-driven shock near 1 AU, with interstellar hydrogen and helium, using the WIND/3DP ESP measurements. These simulations/estimates suggest that suitable instrumentation would be able to detect ENAs from SEPs, and to even make images of where ENAs and correspondingly SEPs, are being produced as a function of energy.