The Source of Energetic Storm Particles for the 2023 March 13 Event Observed by Parker Solar Probe

On 2023 March 13, Parker Solar Probe was located about 0.23 au from the Sun when it was crossed by a very fast interplanetary shock. The intensities of ∼0.08–10 MeV energetic protons were markedly enhanced at the shock crossing. At energies from ∼0.1 to 1 MeV, the intensity at the shock was 3–4 orders of magnitude larger than it was ∼4 hr prior. In this study, we investigate the possible source of particles that led to this enhancement. Pre-existing high-energy particles that were present during the few-hour period prior to the shock could possibly serve as a source, assuming they are accelerated to even higher energies by the shock. We apply a “seeds test”—which determines the expected enhancement at the shock—to test this. We find that none of the pre-existing particle populations identified can account for the observed enhancement at the shock. The solar wind itself provides a very abundant source. To test whether shock-heated thermal solar wind could account for the source, we perform a series of self-consistent hybrid plasma simulations of this event and make direct comparisons with the observed particle spectra and magnetic field. We conclude that shock-heated solar wind protons are the source of energetic particles in this event. The direct comparisons of our simulations and observations also suggest that the shock was moving about 1350 km s⁻¹ and had an Alfvén Mach number of about 4, both of which are considerably lower than previously estimated.

This project was supported by the IS⊙IS instrument suite on NASA’s Parker Solar Probe Mission, contract NNN06AA01C. Parker Solar Probe was designed and built, and is now operated, by the Johns Hopkins Applied Physics Laboratory as part of NASA’s Living with a Star program. Support from the LWS management and technical team has played a critical role in the success of the Parker Solar Probe mission. J.G. also acknowledges support from the NSF/AGS under grant 1931252, and NASA under grants 80NSSC20K1283, 80NSSC18K1213, and 80NSSC21K0119. All data shown in this paper came from the Space Physics Data Facility at Goddard Space Flight Center using the Coordinated Data Analysis Web.