Anomalous Cosmic-Ray Oxygen Observations into 0.1 au

Creators: Rankin, J. S.; McComas, D. J.; Leske, R. A.; Christian, E. R.; Cohen, C. M. S.; Cummings, A. C.; Joyce, C. J.; Labrador, A. W.; Mewaldt, R. A.; Schwadron, N. A.; Stone, E. C.; Strauss, R. D.; Wiedenbeck, M. E.

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⁻¹ from 0.1 to 0.94 au, for energies of 6.9–27 MeV nuc⁻¹. This value agrees with that measured by Helios nearly 45 yr ago from 0.3 to 1.0 au (48% ± 12% au⁻¹; 9–29 MeV nuc⁻¹) and is larger than predicted by models. The large anomalous cosmic-ray 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 influence cosmic-ray drifts, well inside 1 au.

Additional Information

© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 September 16; revised 2021 October 23; accepted 2021 October 26; published 2022 January 20. This work was supported as a part of the Integrated Science Investigation of the Sun on NASA's Parker Solar Probe mission, under contract NNN06AA01C. The IS⊙IS data and visualization tools are available to the community at https://spacephysics.princeton.edu/missions-instruments/isois; data are also available via the NASA Space Physics Data Facility (https://spdf.gsfc.nasa.gov/). Parker Solar Probe was designed, built, and is now operated by the Johns Hopkins Applied Physics Laboratory as part of NASA's Living with a Star (LWS) program. Support from the LWS management and technical team has played a critical role in the success of the Parker Solar Probe mission. We thank all the scientists and engineers who have worked hard to make PSP a successful mission. We also acknowledge the support provided by Michigan State University's National Superconducting Cyclotron Laboratory, Texas A&M University's Cyclotron Institute, and the Lawrence Berkeley National Laboratory's 88-inch Cyclotron Laboratory during EPI-Hi's calibration and testing.

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