A Statistical Study on the Acceleration Conditions of Ultrarelativistic Electrons in the Earth's Outer Radiation Belt During Geomagnetic Storms

Abstract

Using the data from Van Allen Probes, we statistically study the acceleration conditions of ultrarelativistic electrons (Ek > 3 MeV) in the Earth's outer radiation belt. Based on the different high energy cutoffs of ultrarelativistic electrons, the acceleration events of ultrarelativistic electrons are divided into the four groups of “3.4 MeV events,” “5.2 MeV events,” “6.3 MeV events,” and “7.7 MeV events.” According to different solar wind and substorm conditions during the recovery stage, we classify solar wind into three categories: no high‐speed flows, short‐duration high‐speed flows, and long‐duration high‐speed flows. Substorms are divided into continuous substorm activity and non‐continuous substorm activity. Our statistical results show that (a) The different solar wind speeds can cause enhancements of ultrarelativistic electrons with different energies. The high solar wind speed is more important in the acceleration of ultrarelativistic electrons with higher energies (7.7 MeV) than lower energies (3.4, 5.2, and 6.3 MeV). (b) The different substorm activities during the recovery stage can cause enhancements of ultrarelativistic electrons with different energies. Continuous intense substorms in the early recovery stage can contribute to the rapid recovery and enhancements of ultrarelativistic electrons. The timing, intensity, and duration of substorms during the recovery stage are important to the acceleration of ultrarelativistic electrons. (c) Relativistic (1 MeV) electrons are the direct “source” of ultrarelativistic electrons (3.4, 5.2, and 6.3 MeV) in the outer radiation belt. These can help us further understand the electron dynamics of the Earth’s outer radiation belt.

The effect of continuous geomagnetic storms on enhancements of ultrarelativistic electrons in the Earth’s outer radiation belt

“…Previous studies have shown that high solar wind speed (V sw > 500 km/s) and continuous substorms play an important role in the acceleration of ultrarelativistic electrons in the Earth's outer radiation belt (e.g., Baker et al, 1997;Baker et al, 2019;Hajra et al, 2015;Kim et al, 2015;Li et al, 2015;Zhao et al, 2019a). Using the data from Van Allen Probes, Chen et al (2023) statistically studied 82 acceleration events of ultrarelativistic electrons in the outer radiation belt. They found that the acceleration of ultrarelativistic electrons at different energies requires the different solar wind speeds and different substorm activities.…”

“…In this event, the flux of 5.2 MeV electrons did not enhance (Figure 1C). According to the definition of Chen et al (2023), this storm event was defined as an event with a cutoff energy of 3.4 MeV event (Figures 1A−F).…”

“…Compared to F 1 during the main phase of the first geomagnetic storm, F 1 during the main phase of the second geomagnetic storm increased by nearly one order of magnitude. These enhanced relativistic electrons could be used as the source of ultrarelativistic electrons and contribute to ultrarelativistic electron acceleration during the second geomagnetic storm (e.g., Allison et al, 2021;Chen et al, 2023;Li et al, 2014;Shprits et al, 2022;Thorne et al, 2013). The fluxes of 3.4 MeV electrons (F 2 ) decreased to 6.2 cm −2 /s/sr/MeV during the main phase of the first geomagnetic storm and increased to 754 cm −2 /s/sr/MeV during the late recovery phase of the first geomagnetic storm.…”

“…The Earth's outer radiation belt is occupied by electrons of a broad energy range (10 s keV-a few MeV). The extreme and rapid variations of the electrons of different energies during geomagnetic storms are observed due to different loss and acceleration mechanisms (e.g., Reeves et al, 2003;Chen et al, 2023;Wang et al, 2023). During the storm main phase, large depletions of the outer radiation belt electrons are usually observed.…”

“…They suggested that geomagnetic storms with higher solar wind speed, prolonged southward IMF B z , higher solar wind E y , lower solar wind number density, and continuous and intense substorm activities could contribute to enhancements of ultrarelativistic electrons in the outer radiation belt. Using the Van Allen Probes data, Chen et al (2023) statistically studied the acceleration conditions of ultrarelativistic electrons with different cutoff energies in the outer radiation belt during geomagnetic storms. They found that different solar wind speeds and substorm activities during the recovery stage could cause enhancements of ultrarelativistic electrons with different energies.…”