Search

Here are analyzed 3D velocity distribution functions (VDFs) of protons in the suprathermal energy-per-charge ( E / Q ) domain of 6.2 to 223.1 keV/e, as observed by the WIND Suprathermal Ion Composition Spectrometer (WIND/STICS) between 1995 and 2019, upon passage of interplanetary (IP) shocks. “Suprathermal” designates energies above the bulk, “thermal” solar wind, including inner-source pickup ions, cometary ions, solar energetic particles, and suprathermal tail particles. Within the WIND/STICS measurements, here treated as a standalone data set, only 3D VDFs within 9 hr prior to and following the shock passage are selected, as identified by the Center for Astrophysics (CfA) Harvard Interplanetary Shock Database. These are subsequently averaged, first over the STICS field of view, then over 3 hr intervals in the 9 hr about the shock. The averages, with errors assuming Poisson statistics, are then fitted using the Levenburg–Marquardt nonlinear least squares technique to two simple, observationally suggested functions arising from diffusive shock acceleration (DSA) formalism: a power law in E / Q with spectral index (model I), and a power-law with exponential rollover having a cutoff in E / Q (model II). The first result is a comparison of upstream spectral indices, e-folding energies, and normalization constants with corresponding downstream values. The second is a comparison of fitted spectral indices against those given by measured CfA shock compression ratios and a comparison of fitted e-folding energies with those given by measured ratios and shock normal angles, per DSA-derived predictions. There is additionally a comparison between fitted parameters given by the two functional forms. Little agreement is found with values given by DSA.

Many aspects of solar energetic particles are not well understood, including their acceleration mechanism. There has been recent interest in the potential of energetic neutral atoms (ENAs) as remote probes of solar energetic particles (SEPs) and their acceleration. The single accidental observation (in physical units) has been modeled as accelerated by a coronal mass ejection (CME)-driven shock by several authors, all of whom have assumed that the upstream component of the shock can be ignored. In this article, we relax this assumption and model the flux of ENAs at 1 au due to a CME-driven shock with an upstream component. We show the effect of varying parameters of the shock acceleration model, specifically α , the exponent of the power law in momentum of the mean free path, and η , a measure of the relative turbulence level. The main result is that including the upstream component significantly increases the flux at 1 au for typically assumed parameters in the energy range of the STEREO observation. We also derive the form of the ENA transport equation that we used in this study. These results enable a better understanding of potential observations of ENAs due to SEPs.