Radial Dependence of Ion Fluences in the 2023 July 17 Solar Energetic Particle Event from Parker Solar Probe to STEREO and ACE

In the latter moments of 2023 July 17, the solar active region (AR) 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 (ISeIS) 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 ∼1400 km s⁻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 AR for a 537 km s⁻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