Statistical Characteristics of Particle Injections throughout the Equatorial Magnetotail

Abstract:

Energetic particle injections are critical for supplying particles and energy to the inner magnetosphere. Recent case studies have demonstrated a good correlation between injection signatures (sudden increase in energy flux of 10s-100s keV particles) and transient, narrow, fast flow channels and dipolarization fronts in the magnetotail, but statistical observations outside of geosynchronous orbit (GEO) to verify the findings were lacking. By surveying trans-geosynchronous injections using THEMIS, we show that the earthward-traveling dipolarizing flux bundles (DFBs) generated by reconnection are the likely source of particle acceleration and transport resulting in injection signatures throughout the tail. Like near-Earth reconnection, both ion and electron injections are most probable in the pre-midnight sector. Similar to bursty bulk flows (BBFs), flow speeds observed with injection signatures are faster with increasing distance from Earth. Also similar to BBFs, injection occurrence rate increases with increased geomagnetic activity. With increased activity, spectral hardening occurs, implying the increased number of injections is heating the plasma sheet. The injection occurrence rate increase within the inner magnetosphere suggests that injections populate the radiation belts more effectively under enhanced activity. Our results are inconsistent with the classical concept of an azimuthally wide “injection boundary” moving earthward from ~9-12 R_E to GEO under an enhanced cross-tail electric field. Rather, particle acceleration and transport occur along a large range of radial distances due to effects from earthward-penetrating, azimuthally localized, transient, strong electric fields of recently reconnected, dipolarizing flux bundles. Thus, the statistical superposition of individual DFBs is an explanation of the classical “injection boundary”.