Contribution of ULF Wave Activity to the Global Recovery of the Outer Radiation Belt During the Passage of a High‐Speed Solar Wind Stream Observed in September 2014

Abstract

Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high-speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground-and space-based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L-shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.

Dynamic Mechanisms Associated With High‐Energy Electron Flux Dropout in the Earth's Outer Radiation Belt Under the Influence of a Coronal Mass Ejection Sheath Region

“…A global MHD simulation of the Earth's magnetosphere interaction with the ICME on July 19, 2016, has been performed to confirm again the likelihood of magnetopause shadowing occurrence. The simulation setup is similar to that done by Alves et al (2016) and Da Silva et al (2019). The whole simulation domain is set with the following dimensions −224 ≤ x ≤ 32 RE, and −64 ≤ y, z ≤ 64 RE, where x, y, and z are in the DA SILVA ET AL.…”

“…It also has been widely used to address the low-frequency waves generation (in a few to tens of mHz range) like the ULF waves in the inner magnetosphere, specifically in the radiation belts (e.g., Alves et al, 2016;Claudepierre et al, 2010Claudepierre et al, , 2008Da Silva et al, 2019;Jauer et al, 2019;Komar et al, 2017;Souza et al, 2017). Da Silva et al (2019) and Souza et al (2017) used the SWMF/BATS-R-US model to study the ULF waves in the equatorial and nightside magnetosphere during the influence of the Alfvénic fluctuations associated with HSS. Claudepierre et al (2008) used the Lyon-Fedder-Mobarry (LFM) model to study the production of ULF waves in the magnetosphere to the HSS.…”

“…D LL , which is the sum of is estimated from the Kp index (OMNI database) according to Equations 8 and 9 of Da Silva et al. (2019), and intensifies when the shock reaches the Earth's magnetosphere. This intensity remains for approximately 4 h during the influence of the ICME's sheath, which is concomitant with the beginning of ULF wave activities recorded in ground‐based data.…”

“…This violation can occur due to the presence of several electrodynamic and magnetohydrodynamic processes in the magnetosphere, causing variations in the outer radiation belt electron flux, such as dropouts (e.g., Alves et al., 2017; Turner et al. 2012; Turner & Ukhorskiy, 2019), and enhancements (e.g., Baker et al., 1994, 2014; Baker & Kanekal, 2008; Boyd et al., 2018; Da Silva et al., 2019). Usually, it is hard to distinguish the dominant process leading to outer belt electron flux variability when several of them are acting at the same time.…”

“…Figure 7shows (top) the ground-based ULF PSD in the 1-8.33 mHz frequency range, and (bottom) the radial diffusion coefficient D LL from July 16-22, 2016. D LL , which is the sum of EB LL LL D D is estimated from the Kp index (OMNI database) according to Equations 8 and 9 of DaSilva et al (2019), and intensifies when the shock reaches the Earth's magnetosphere. This intensity remains for approximately 4 h…”