Anomalous Cosmic Ray Oxygen Observations in to 0.1 au

TL;DR

This study reports the first measurements of cosmic ray oxygen near 0.1 au by the Parker Solar Probe, revealing an anomalous cosmic ray profile, a strong radial intensity gradient, and insights into solar modulation effects on cosmic rays.

Contribution

It provides new observations of cosmic rays at unprecedented proximity to the Sun, highlighting the influence of magnetic field variations on cosmic ray behavior.

Findings

Cosmic ray oxygen spectra at 0.1 au show an anomalous profile similar to 1 au.

Radial intensity gradient of 49.4%/au from 0.1 to 0.94 au.

Elevated galactic cosmic ray component compared to previous solar minima.

Abstract

The Integrated Science Investigation of the Sun instrument suite onboard NASA's Parker Solar Probe mission continues to measure solar energetic particles and cosmic rays closer to the Sun than ever before. Here, we present the first observations of cosmic rays into 0.1 au (21.5 solar radii), focusing specifically on oxygen from ~2018.7 to ~2021.2. Our energy spectra reveal an anomalous cosmic ray-dominated profile that is comparable to that at 1 au, across multiple solar cycle minima. The galactic cosmic ray-dominated component is similar to that of the previous solar minimum (Solar Cycle 24/25 compared to 23/24) but elevated compared to the past (Solar Cycle 20/21). The findings are generally consistent with the current trend of unusually weak solar modulation that originated during the previous solar minimum and continues today. We also find a strong radial intensity gradient: 49.4 +/- 8.0 %/au from 0.1 to 0.94 au, for energies of 6.9 to 27 MeV/nuc. This value agrees with that measured by Helios nearly 45 years ago from 0.3 to 1.0 au (48 +/- 12 %/au; 9 to 29 MeV/nuc) and is larger than predicted by models. The large ACR gradients observed close to the Sun by the Parker Solar Probe Integrated Science Investigation of the Sun instrument suite found here suggest that intermediate-scale variations in the magnetic field's structure strongly influences cosmic ray drifts, well inside 1 au.