Energetic Particle Increases Associated with Stream Interaction Regions

TL;DR

This study reports on proton and helium energetic particle increases associated with stream interaction regions observed by Parker Solar Probe, revealing their spectral characteristics, composition ratios, and connection to solar wind structures at 1 au.

Contribution

First detailed analysis of energetic particle increases in SIRs close to the Sun, including spectral properties and corotation-based event associations.

Findings

Proton spectra extend to a few MeV with power-law indices from -4.3 to -6.5.

Helium spectra have similar indices, with He/H ratios of 0.016-0.031.

Some events observed at Parker Solar Probe correlate with those at 1 au.

Abstract

The Parker Solar Probe was launched on 2018 August 12 and completed its second orbit on 2019 June 19 with perihelion of 35.7 solar radii. During this time, the Energetic particle Instrument-Hi (EPI-Hi, one of the two energetic particle instruments comprising the Integrated Science Investigation of the Sun, ISOIS) measured seven proton intensity increases associated with stream interaction regions (SIRs), two of which appear to be occurring in the same region corotating with the Sun. The events are relatively weak, with observed proton spectra extending to only a few MeV and lasting for a few days. The proton spectra are best characterized by power laws with indices ranging from -4.3 to -6.5, generally softer than events associated with SIRs observed at 1 au and beyond. Helium spectra were also obtained with similar indices, allowing He/H abundance ratios to be calculated for each event. We find values of 0.016-0.031, which are consistent with ratios obtained previously for corotating interaction region events with fast solar wind < 600 km s-1. Using the observed solar wind data combined with solar wind simulations, we study the solar wind structures associated with these events and identify additional spacecraft near 1 au appropriately positioned to observe the same structures after some corotation. Examination of the energetic particle observations from these spacecraft yields two events that may correspond to the energetic particle increases seen by EPI-Hi earlier.