Observations of the 2019 April 4 Solar Energetic Particle Event at the Parker Solar Probe

TL;DR

This study reports on a small solar energetic particle event detected by Parker Solar Probe close to the Sun, revealing details about particle acceleration, anisotropy, and magnetic field expansion during the event.

Contribution

First detailed observation of a small solar energetic particle event near the Sun, highlighting particle anisotropy and magnetic field expansion effects.

Findings

Event was very small with peak 1 MeV proton intensities of ~0.3 particles (cm^2 sr s MeV)^-1.

Particles showed strong anisotropy, flowing outward from the Sun.

Source active region was nearly 80° east of PSP's magnetic footpoint.

Abstract

A solar energetic particle event was detected by the Integrated Science Investigation of the Sun (ISOIS) instrument suite on Parker Solar Probe (PSP) on 2019 April 4 when the spacecraft was inside of 0.17 au and less than 1 day before its second perihelion, providing an opportunity to study solar particle acceleration and transport unprecedentedly close to the source. The event was very small, with peak 1 MeV proton intensities of ~0.3 particles (cm^2 sr s MeV)^-1, and was undetectable above background levels at energies above 10 MeV or in particle detectors at 1 au. It was strongly anisotropic, with intensities flowing outward from the Sun up to 30 times greater than those flowing inward persisting throughout the event. Temporal association between particle increases and small brightness surges in the extreme-ultraviolet observed by the Solar TErrestrial RElations Observatory, which were also accompanied by type III radio emission seen by the Electromagnetic Fields Investigation on PSP, indicates that the source of this event was an active region nearly 80 degrees east of the nominal PSP magnetic footpoint. This suggests that the field lines expanded over a wide longitudinal range between the active region in the photosphere and the corona.