Chapter 14 - Magnetospheric “Killer” Relativistic Electron Dropouts (REDs) and Repopulation: A Cyclical Process

Abstract

The variation of the relativistic “killer” electrons trapped in the Earth's outer radiation belt is an extreme space weather phenomenon. We exemplify the processes by first studying intervals of high-speed solar wind streams (HSSs) in the declining phase of solar cycle 23. This is the portion of the solar cycle where the relativistic electron fluxes are maximum. Particle losses and repopulation are two separate processes. Both are controlled by solar wind/interplanetary variability. The electrons are depleted by a slow solar wind high-density interplanetary heliospheric plasma sheet (HPS) impingement onto the magnetosphere. The loss processes are magnetopause shadowing and electromagnetic ion cyclotron (EMIC) wave-particle interactions. Losses by chorus wave-particle interactions (relativistic microbursts) are argued to not be important. Next, an HSS with an embedded Alfvén wave train characterized by substantial and frequent southward interplanetary magnetic fields causes intense auroral activity, known as a HILDCAA (high-intensity long-duration continuous AE activity) event. The HILDCAA event leads to intense electromagnetic chorus that, in turn, accelerates the relativistic (>0.6 to >4.0MeV) electrons. Electron acceleration associated with HILDCAAs occurs irrespective of whether geomagnetic storms precede or not. It is possible that HILDCAAs lasting longer than 4 days may cause the acceleration of electrons to energies much greater than 4.0MeV. Although solar cycle maximum is less important for relativistic electron losses and acceleration, this is reviewed for completeness. During solar maximum, upstream shocks and fast interplanetary coronal mass ejections (ICMEs) are associated with relativistic electron decreases and acceleration. It is speculated that the upstream shocks cause the particle losses by the same processes that the HPSs cause for the solar cycle declining phase. Relativistic electron repopulation is caused by the storm electric field induced particle injection, chorus wave-particle interactions, and PC5 radial diffusion. Other plasma waves may be involved in the acceleration process as well. In the one example that was chosen at random for this paper, there are an HSS and an HILDCAA following the ICME, and the main acceleration process was the same as for the declining phase (chorus acceleration of ~100–300keV electrons).