Unraveling the drivers of the storm time radiation belt response

Abstract

We present a new framework to study the time evolution and dynamics of the outer Van Allen belt electron fluxes. The framework is entirely based on the large‐scale solar wind storm drivers and their substructures. The Van Allen Probe observations, revealing the electron flux behavior throughout the outer belt, are combined with continuous, long‐term (over 1.5 solar cycles) geosynchronous orbit data set from GOES and solar wind measurements A superposed epoch analysis, where we normalize the timescales for each substructure (sheath, ejecta, and interface region) allows us to avoid smearing effects and to distinguish the electron flux evolution during various driver structures. We show that the radiation belt response is not random: The electron flux variations are determined by the combined effect of the structured solar wind driver and prestorm electron flux levels. In particular, we find that loss mechanisms dominate during stream interface regions, coronal mass ejection (CME) ejecta, and sheaths while enhancements occur during fast streams trailing the stream interface or the CME.

Coronal mass ejections and their sheath regions in interplanetary space

“…On the other hand, the large-scale convection generates a seed particle population in the inner parts of the magnetotail, which can later be accelerated by a multitude of processes to relativistic energies. As shown by Kilpua et al (2015a) the electron fluxes start typically rise during the end part of the fast ICMEs that have strongly southward magnetic fields. If a fast solar wind stream follows the ICME, the main enhancements, however, occur typically during this fast stream.…”

“…The figure illustrates that the magnetic field variations are considerably larger, and the temperature and density much higher in the sheath than in the MC. Figure 18 gives probability distributions for IMF intensity, its southward component, and several solar wind parameters calculated using the event list of Kilpua et al (2015a). Only the magnetic field and velocity distributions (panels 18a, b, d) are rather similar for sheaths and ICMEs, but all other parameters show significant differences.…”

“…If the flux rope has northward field, it can add to the depleting effect of the sheath, causing particularly long-lasting drop-outs of the high-energy electrons and magnetospheric quiescence (e.g., Alves et al 2016). Sustained southward field in the MC will in turn erode the magnetosphere, enhancing the electron losses at the magnetopause, but as the MCs have generally lower dynamic pressure and lower ULF wave power, the losses are not as pronounced as during the sheaths (Kilpua et al 2015a). On the other hand, the large-scale convection generates a seed particle population in the inner parts of the magnetotail, which can later be accelerated by a multitude of processes to relativistic energies.…”

“…This strong depleting tendency stems from the enhanced ULF wave power and dynamic pressure causing pitch angle scattering and radial diffusion, which combined with the highly compressed dayside magnetopause efficiently removes electrons from the belts. Although depleting processes clearly dominate during sheath passages, the sheaths are typically associated with strong substorm activity that can lead to effective injection of seed electrons into the radiation belt region (e.g., Li et al 1998), as discussed in Kilpua et al (2015a). These seed electrons may be energized by the later processes.…”

“…In particular, if the drift velocity of an electron matches the speed of the magnetosonic wave, the electron can "surf" on the wave and attain very high energies. The sheath regions in turn cause typically significant depletions of electron fluxes (e.g., Hietala et al 2014;Kilpua et al 2015a). This strong depleting tendency stems from the enhanced ULF wave power and dynamic pressure causing pitch angle scattering and radial diffusion, which combined with the highly compressed dayside magnetopause efficiently removes electrons from the belts.…”