Widespread 1-2 MeV Energetic Particles Associated with Slow and Narrow Coronal Mass Ejections: Parker Solar Probe and STEREO Measurements

TL;DR

This study shows that slow, narrow CMEs can generate widespread 1-2 MeV energetic particles that persist in the inner heliosphere, with measurements from Parker Solar Probe revealing their continuous generation and spread.

Contribution

It provides new insights into how slow, narrow CMEs contribute to SEP events and seed particle populations closer to the Sun, expanding understanding beyond fast, wide CMEs.

Findings

SEP particles spread over large heliospheric regions

Seed particles are continuously generated by slow, narrow CMEs

Particles remain for tens of hours even with minimal near-Earth enhancements

Abstract

Suprathermal ions in the corona are thought to serve as seed particles for large gradual solar energetic particle (SEP) events associated with fast and wide coronal mass ejections (CMEs). A better understanding of the role of suprathermal particles as seed populations for SEP events can be made by using observations close to the Sun. We study a series of SEP events observed by the Integrated Science Investigation of the Sun (IS$\odot$IS) suite on board the Parker Solar Probe (PSP) from 2020 May 27 to June 2, during which PSP was at heliocentric distances between $\sim$0.4 and $\sim$0.2 au. These events were also observed by the Ahead Solar TErrestrial RElations Observatory (STEREO-A) near 1 au, but the particle intensity magnitude was much lower than that at PSP. We find that the SEPs should have spread in space as their source regions were distant from the nominal magnetic footpoints of both spacecraft, and the parent CMEs were slow and narrow. We study the decay phase of the SEP events in the $\sim$1-2 MeV proton energy range at PSP and STEREO-A, and discuss their properties in terms of both continuous injections by successive solar eruptions and the distances where the measurements were made. This study indicates that seed particles can be continuously generated by eruptions associated with slow and narrow CMEs, spread over a large part of the inner heliosphere, and remain there for tens of hours, even if minimal particle intensity enhancements were measured near 1 au.