Radial dependence of ion fluences in the 2023 July 17 SEP event from Parker Solar Probe to STEREO and ACE

TL;DR

This study investigates how solar energetic particle (SEP) intensities and compositions vary with distance from the Sun during the 2023 July 17 event, revealing a strong radial dependence along a shared magnetic field line.

Contribution

It provides the first detailed analysis of SEP radial dependence from 0.65 to 1 au using Parker Solar Probe and near-Earth data for a specific solar event.

Findings

SEP fluences decrease with radial distance, following a power law.

Radial dependence is stronger for heavier ions like O and Fe.

The observed gradients help understand particle acceleration and transport mechanisms.

Abstract

In the latter moments of 17 July 2023, the solar active region 13363, near the southwestern face of the Sun, was undergoing considerable evolution, which resulted in a significant solar energetic particle (SEP) event measured by Parker Solar Probe's Integrated Science Investigation of the Sun (ISOIS) and near-Earth spacecraft. Remote observations from GOES and CHASE captured two M5.0+ solar flares that peaked at 23:34 and 00:06 UT from the source region. In tandem, STEREO COR2 first recorded a small, narrow coronal mass ejection (CME) emerging at 22:54 UT and then saw a major halo CME emerge at 23:43 UT with a bright, rapidly expanding core and CME-driven magnetic shock with an estimated speed of $\sim$1400 $kms^{-1}$. Parker Solar Probe was positioned at 0.65 au, near-perfectly on the nominal Parker spiral magnetic field line which connected Earth and the active region for a 537 $kms^{-1}$ ambient solar wind speed at L1. This fortuitous alignment provided the opportunity to examine how the SEP velocity dispersion, energy spectra, elemental composition, and fluence varied from 0.65 to 1 au along a shared magnetic connection to the Sun. We find a strong radial gradient, which is best characterized for H and He as $r^{-4.0}$ and most surprisingly is stronger for O and Fe which is better described by $r^{-5.7}$.