Energetic particle injections are critical for supplying particles and energy to the inner magnetosphere. Recent case studies have demonstrated a good correlation between injections and transient, narrow, fast flow channels as well as earthward reconnection (dipolarization) fronts in the magnetotail, but statistical observations beyond geosynchronous orbit (GEO) to verify the findings were lacking. By surveying trans-geosynchronous injections using Time History of Events and Macroscale Interactions during Substorms (THEMIS), we show that their likely origin is the earthward traveling, dipolarizing flux bundles following near-Earth reconnection. The good correlation between injections and fast flows, reconnection fronts and impulsive, dawn-dusk electric field increases is not limited to within 12 R E but extends out to 30 R E . Like near-Earth reconnection, both ion and electron injections are most probable in the premidnight sector. Similar to bursty bulk flows (BBFs), injection-time flow speeds are faster farther from Earth. With faster flows, injection intensity generally increases and extends to higher energy channels. With increased geomagnetic activity, injection occurrence rate increases (akin to that of BBFs) and spectral hardening occurs (κ decreases). The 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 to 12 R E to GEO under an enhanced cross-tail electric field. Rather, particle injection 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.

Recent observations have suggested that spatially localized flows of high‐temperature, low‐density plasma carrying a dipolarized magnetic field (dipolarizing flux bundles, DFBs) play a key role in hot plasma transport toward the inner magnetosphere. What controls plasma heating in DFBs and how do thermodynamic parameters (such as density, temperature, pressure, and specific entropy) and spectral properties of the DFB population depend on ambient plasma sheet properties and geocentric distance R remains unknown. By statistical analysis of 271 DFB events detected by the Time History of Events and Macroscale Interactions during Substorms mission during the 2008–2009 tail seasons, we find that on average, plasma inside DFBs is a factor of 0.6 less dense and a factor of 1.5 to 2 hotter than ambient tail plasma. The radial profiles of average thermodynamic parameters inside and outside DFBs are similar; when fitted by the κ‐function, their energy spectra have similar κ‐exponents, but a factor of 2 larger peak energies inside DFBs. Our analysis suggests that average DFB plasma properties are closely linked to those of the ambient plasma sheet population. Estimations show that on average, adiabatic heating of the ambient plasma in the increased magnetic field is the major factor in DFB plasma heating.

[1] Motivated by recent observations of intense electric fields and elevated energetic particle fluxes within flow bursts beyond geosynchronous altitude , we apply modeling of particle guiding centers in prescribed but realistic electric fields to improve our understanding of energetic particle acceleration and transport toward the inner magnetosphere through model-data comparisons. Representing the vortical nature of an earthward traveling flow burst, a localized, westward-directed transient electric field flanked on either side by eastward fields related to tailward flow is superimposed on a nominal steady state electric field. We simulate particle spectra observed at multiple THEMIS spacecraft located throughout the magnetotail and fit the modeled spectra to observations, thus constraining properties of the electric field model. We find that a simple potential electric field model is capable of explaining the presence and spectral properties of both geosynchronous altitude and "trans-geosynchronous" injections at higher L-shells (L > 6.6 R E ) in a manner self-consistent with the injections' inward penetration. In particular, despite the neglect of the magnetic field changes imparted by dipolarization and the inductive electric field associated with them, such a model can adequately describe the physics of both dispersed injections and depletions ("

We study energetic electron injections by using an analytical model that self‐consistently describes electric and magnetic field perturbations of a transient, localized dipolarizing flux bundle (DFB). This simple model reproduces most injection signatures at multiple locations simultaneously, reaffirming earlier findings that an earthward‐traveling DFB can both transport and accelerate electrons to suprathermal energies, and can thus be considered an important driver of short‐lived (~ < 10 min) injections. We find that energetic electron drift paths are greatly influenced by the sharp magnetic field gradients around a localized DFB. Because a DFB is so localized (only a few RE wide across the tail), there are strong duskward magnetic field gradients on the DFB's dawn flank and strong dawnward magnetic field gradients on its dusk flank. Electrons on the DFB's dawnside therefore ∇B drift farther earthward from the reconnection site, whereas electrons on its duskside can potentially evacuate the inner magnetosphere by ∇B drifting tailward. This results in flux decrease at the front's duskside. As a result, the source of electrons observed during injection depends sensitively on the spacecraft location relative to the DFB and on the DFB's properties. We similarly find that the process of electron energization depends on how the electrons interact with the DFB. The initial injection signature is from electrons that interact with the front and gain the majority of their energy from the increasing magnetic field (∂B/∂t), whereas populations that arrive later gain most of their energy from ∇B drifting across the flow channel and against the DFB's electric fields.