Simulation of Multi-Spacecraft Observed Energetic Electron Injection By the Electromagnetic Field of a Transient, Localized Dipolarizing Flux Bundle

Abstract:

Energetic particle injections in the near-Earth plasma sheet are critical for supplying particles and energy to the radiation belts and ring current. Their origin, however, has been elusive due to the lack of equatorial, multi-point observations. After the launch of NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission in 2007, intense electric fields and elevated energetic particle fluxes have been observed to accompany localized (1-4 R_E wide) bursty bulk flows and to propagate from the mid-tail regions (at geocentric radial distances R > 25R_E) towards Earth, up to and at times inside of geosynchronous orbit (GEO, R=6.6R_E). Motivated by these observations, we model simultaneous multi-point observations of electron injections using guiding center approximation in prescribed but realistic electric and magnetic fields to better understand the nature of their acceleration. Modeling of electron injections assuming a localized, impulsive, dipolarizing flux bundle and its accompanying electric field transported from mid-tail to near-Earth at bursty flow speeds successfully reproduces the observations at multiple spacecraft. The impulsive, localized nature of the earthward-propagating electromagnetic pulse with attending vortical/tailward flow is what makes this model particularly effective in reproducing both the injection and the dispersed decrease in energy flux often observed simultaneously with the injection but at lower energies (~10-30 keV). The results suggest that particle acceleration and transport towards the inner magnetosphere can be thought of as a superposition of individual bursts of varying intensity and cadence depending on global geomagnetic activity levels.