Rapid enhancement of radiation belt electron fluxes due to substorm dipolarization of the geomagnetic field

Abstract

Abstract. The classical pure radial diffusion mechanism appears not to fully explain the frequently observed rapid enhancement in the timescales of minutes to hours in the radiation belt electron fluxes in the Earth's magnetosphere. We here consider other physical mechanisms, such as energization mechanisms associated with substorm processes, to account for these sudden increases. A three-dimensional electron kinetic model is used to simulate the dynamics of the geomagnetically trapped population of radiation belt electrons during a substorm injection event.In the past this model has been extensively used to study dynamics of energetic ions in the ring current. This work, for the first time, constitutes the development of a combined convection and diffusion model to radiation belt electrons in the 0.04-4 MeV kinetic energy range. The Tsyganenko 89 geomagnetic field model is used to simulate the time-varying terrestrial magnetosphere during the growth phase elongation and the expansion phase contraction. We find that inductive electric field associated with the magnetic reconfiguration process is needed in order to transport substorm electrons into the trapped particle region of the magnetosphere. The maximum enhancement in energetic electron fluxes is found to be located around the geosynchronous orbit location (L = 6.6), with up to 2 orders of magnitude enhancement in the total fluxes (0.04-4 MeV). Although this enhancement in the inner magnetosphere is very sensitive to the temperature and, to a less extent, density of the source population in the plasma sheet, we suggest that the substorm-associated energization in the magnetotail and the subsequent adiabatic acceleration in the earthward region account for the enhanced MeV electrons (killer electrons) seen at the geosynchronous orbit during storms and substorms.

Evidence for acceleration of outer zone electrons to relativistic energies by whistler mode chorus

“…Beutier and Boscher (1995) have developed an extensive computer code (Salammbô) to solve a threedimensional phase space diffusion equation for the electron radiation belt population. Kim et al (2000) and Fok et al (2001) modelled the inward radial diffusion of plasma sheet electrons and their injection into the outer zone during a dipolarisation event.…”

Impact of Solar Wind on the Earth Magnetosphere: Recent Progress in the Modeling of Ring Current and Radiation Belts

“…As the dipolarization proceeds in the magnetic field, field lines convect earthward. Electrons which are gyrating along these collapsing field lines can be accelerated significantly on a time scale of minutes (Fok, Moore & Spjeldvik, 2001;Glocer et al, 2009), much faster than the time scale for energization by whistler mode chorus waves, which is typically of the order of 1-2 days (Summers & Ma, 2000). The T04 model, which is driven by Dst and solar wind parameters, does not contain clear substorm signatures.…”

“…The T04 model, which is driven by Dst and solar wind parameters, does not contain clear substorm signatures. Empirical magnetic field models of this kind cannot directly simulate substorm reconfiguration unless special tricks are applied (Delcourt et al, 1990;Fok, Moore & Spjeldvik, 2001;Pulkkinen et al, 1991). For moderate storms such as this one on 3-5 September 2008, convection is weak and the dominant energization and transport mechanism is sub-storm reconfiguration and the resulting dipolarization electric field.…”

“…The Radiation Belt Environment (RBE) model has been rigorously developed over past 10 years (Fok et al, 2005;Fok et al, 2011;Fok, Moore & Spjeldvik, 2001;Fok et al, 2008;Glocer et al, 2011;Zheng et al, 2011;. It models both the radiation belts and ring current electrons.…”

“…In this review paper, we will describe recent progress in the modeling of the ring current/radiation belt plasma with the Comprehensive Ring Current Model (CRCM) (Fok, Wolf, Spiro & Moore, 2001) and Radiation Belt Environment model (RBE) (Fok et al, 2005;Fok, Moore & Spjeldvik, 2001;Fok et al, 2008). Both models are based on a solution of the bounce-averaged transport equation for evolution of phase space density.…”