Magnetospheric Particle Injections and their Relation to Impulsive, Localized Electric Fields
|Department:||Geophysics & Space Physics|
|Keywords:||Plasma physics; Planetology; injections; magnetosphere; modeling; particle energization; particle transport; substorms|
|Full text PDF:||http://www.escholarship.org/uc/item/4fm9d1wt|
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 RE wide) bursty bulk flows and to propagate from the mid-tail regions (at geocentric radial distances R > 25RE) towards Earth, up to and at times inside of geosynchronous orbit (GEO, R=6.6RE). Motivated by these observations, I 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. Additionally, I perform a statistical analysis of the electron and ion injections to better understand their properties observationally. I find a good correlation between injections and azimuthally localized fast flows, dipolarization fronts and impulsive, dawn-dusk electric field increases. This correlation is present regardless of distance, from inside GEO out to 30 RE. The findings are inconsistent with the classical concept of injections forming from an azimuthally wide injection boundary moving earthward from ~9-12 RE to GEO under an enhanced, large-scale, duskward electric field. Modeling of electron injections assuming a localized, impulsive, potential electric field transported from mid-tail to near-Earth at bursty flow speeds of ~400 km/s successfully reproduces the observations at multiple spacecraft. Addition of a small, inductive electric field component, related to the dipolarizing magnetic field consistent with observations, further improves the agreement between modeled and observed electron spectra. The impulsive, localized, and vortical nature of the earthward-propagating electromagnetic pulse 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.