The Modulation of Anomalous and Galactic Cosmic-Ray Oxygen over Successive Solar Cycle Minima

Creators: Strauss, R. D.; Leske, R. A.; Rankin, J. S.

Abstract

Both the recent 2009 and 2020 solar minima were classified as unusually quiet and characterized with unusually high galactic cosmic-ray (GCR) levels. However, unlike the trends from previous decades, in which anomalous cosmic-ray (ACR) and GCR levels strongly agreed, the ACR intensities did not reach such high, record-setting levels. This discrepancy between the behavior of GCRs and ACRs is investigated in this work by simulating the acceleration and transport of GCR and ACR oxygen under different transport conditions. After using recent observations to constrain any remaining free parameters present in the model, we show that less turbulent conditions are characterized by higher GCR fluxes and lower ACR fluxes, due to less efficient ACR acceleration at the solar wind termination shock. We offer this as an explanation for the ACR/GCR discrepancy observed during 2009 and 2020, when compared to previous solar cycles.

Additional Information

© 2023. 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. This work is based on research supported in part by the National Research Foundation of South Africa (NRF grant Nos. 119424, 120345, and 120847). Opinions expressed and conclusions arrived at are those of the authors and are not necessarily to be attributed to the NRF. The responsibility of the contents of this work is with the authors. R.A.L. acknowledges support from NASA grant 80NSSC18K0223. This work was also supported as a part of the Integrated Science Investigation of the Sun on NASA's Parker Solar Probe mission, under contract NNN06AA01C. Figures were prepared with Matplotlib (Hunter 2007) and certain calculations were done with NumPy (Harris et al. 2020). Sunspot number measurements were provided courtesy of the Royal Observatory of Belgium, Brussels. HCS tilt angle and solar polar magnetic field measurements were provided courtesy of the Wilcox Solar Observatory (WSO). WSO data used in this study were obtained via the website http://wso.stanford.edu at 2022:11:04_18:40:53 PDT, courtesy of J. T. Hoeksema. The WSO is currently supported by NASA.

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