Analyses of ∼0.05-2 Mev Ions Associated With the 2022 February 16 Esp Event Observed by Parker Solar Probe

We present analyses of 0.05–2 MeV ions from the 2022 February 16 energetic storm particle event observed by Parker Solar Probe's (PSP) IS⊙IS/EPI-Lo instrument at 0.35 au from the Sun. This event was characterized by an enhancement in ion fluxes from a quiet background, increasing gradually with time with a nearly flat spectrum, rising sharply near the arrival of the coronal mass ejection (CME) driven shock, becoming nearly a power-law spectrum, then decaying exponentially afterwards, with a rate that was independent of energy. From the observed fluxes, we determine diffusion coefficients, finding that far upstream of the shock the diffusion coefficients are nearly independent of energy with a value of 1020 cm2/s. Near the shock, the diffusion coefficients are more than one order of magnitude smaller and increase nearly linearly with energy. We also determine the source of energetic particles, by comparing ratios of the intensities at the shock to estimates of the quiet time intensity to predictions from diffusive shock acceleration theory. We conclude that the source of energetic ions is mostly the solar wind for this event. We also present potential interpretations of the near-exponential decay of the intensity behind the shock. One is that we suggest the shock was over-expanding when it crossed PSP and the energetic particle intensity decreased behind the shock to fill the expanding volume. Over-expanding CMEs could well be more common closer to the Sun, and this is an example of such a case.