Spectral Properties of Suprathermal Ions Associated with Passage of Interplanetary Shocks at 1 AU

Here are analyzed three-dimensional velocity distribution functions (3D VDFs) of protons (H+) and alpha particles (He2+), in the suprathermal energy-per-charge domain of 6.2 to 223.1 keV/e, as observed by the WIND/STICS instrument at 1 AU between 1995 and 2019, and associated with the passage of interplanetary shocks observed aboard. "Suprathermal'' designates energies above the bulk, "thermal" solar wind, at which energies may be found inner-source and interstellar pick-up ions, cometary ions, solar energetic particles (SEPs), heated solar wind ions, and particles entrained in co-rotating interaction regions (CIRs) or stream interaction regions (SIRs). Within the Level 2 dataset, 3D VDFs on days of shock passage are averaged over instrument field of view, and spans a viewing angle of nearly 4π Sr across three polar sectors and sixteen azimuthal sectors. The native cadence is 180s, and the VDFs were also averaged over a time window of one day. The averages, with associated errors propagated assuming Poisson statistics, are then fitted using the Levenburg-Marquardt least squares technique. First, they are fit to a simple, observationally suggested power-law relationship (f(E) ~ Eγ), and then to a power-law with an exponential rollover term (f(E) ~ Eγ * exp(E/E0)) based on a model for acceleration by shocks bounding a CIR (Fisk & Lee, 1980). Our main result is a comparison of the fitted spectral indices with those predicted by the quasi-linear, diffusive shock acceleration theory, using the measured shock compression ratio given by the CfA Interplanetary Shock Database. Implications on the acceleration of suprathermals at 1AU, in the context of shock acceleration formalism, may then be inferred. Generally, predicted spectral indices were found to be harder than for a pure power-law fit. For a power-law fit with exponential rollover, there is better correlation yet once more with departure from the model of Fisk & Lee. Also discussed will be dependence of γ on time and on other relevant parameters. Differences may also arise on days for which the spacecraft was located primarily upstream of the shock.