Contrasting Storm‐Time Radiation Belt Events With and Without Dropouts—The Importance of CME Shocks

• Published in: Journal of geophysical research. Space physics Vol. 128; no. 10
• Main Authors: Nnadih, Stanislaus, Blum, Lauren, Xiang, Zheng, Tu, Weichao, Lyu, Xingzhi, Usanova, Maria E., Rodger, Craig J.
• Format: Journal Article
• Language: English
• Published: 01.10.2023
• Subjects: CME; dropouts; geomagnetic storms; IP shocks; non‐dropouts; radiation belt
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2023JA031293

This study compares three different geomagnetic storms (designated as storms 1, 2, and 3) observed by NASA's Van Allen Probes within the spacecraft's first 50 days in orbit. These storms were Coronal Mass Ejection (CME)‐driven with minimum DST around −138, −100, and −106 nT, respectively. Storms 1 and 2 occurred following the arrival of fast‐forward shocks, which compressed the magnetopause inward to about 6.5 and 7.5 RE, respectively, as a result of the increase in solar wind dynamic pressure and density. The inward magnetopause motion helped contribute to a rapid depletion of MeV electrons across the entire outer belt. For the third storm, however, there was little or no dropout of MeV electrons in the heart of the outer belt during the storm main phase. This third storm was generated by a CME without an associated shock, and the magnetopause actually moved outward at the start of the storm, suppressing loss of electrons through the outer boundary. The study reveals that under certain solar wind driving conditions radiation belt electron dropouts may not occur, even during large geomagnetic storms (Dst_min < −100 nT). Plain Language Summary In this study, we evaluated the response of near‐Earth space to three different geomagnetic storms between 25 September and 25 October 2012. Storms 1 and 2 led to the rapid loss of high‐energy electrons across the whole of the outer Van Allen radiation belt, while little or no electron loss was observed during the third storm. We investigate the solar wind drivers of these three storms to better understand the different responses of the radiation belt. The study shows that, contrary to the typical pattern observed, under certain solar wind conditions radiation belt electron loss may not occur even for large storms. Key Points The response of the outer radiation belt is compared during three intense (Dst_min < −100 nT) Coronal Mass Ejection (CME)‐driven geomagnetic storms The first two storms occurred following the arrival of shocks and exhibit characteristic “dropouts” of MeV electrons during the main phase The third CME was not preceded by a shock, resulting in different solar wind and magnetospheric conditions and an unusual “non‐dropout”