Fine structure and pitch angle dependence of synchronous orbit electron injections

Abstract

For time resolution on the order of seconds, electron substorm‐related injections at synchronous orbit still show energy dispersion and have considerable structure. The structure is at Pi 2 periods and can be characterized as the successive heating and cooling of a Maxwellian distribution. This study lends support to the concept that synchronous orbit injections are due to the passage of hot tail plasma over the spacecraft on an injection front. In this injection front model one might understand Pi 2 periodicities as an inherent periodicity in the flow velocity of the driving gas of the injection front causing the front to oscillate as it envelops the spacecraft. One might view the injection process as a series of mini‐injections and relaxations which continually move the spacecraft into hotter plasma. The pitch angle dependence of the plasma on the injection front either is isotropic or has a loss cone at high energies (several keV and greater) and is field aligned at the very low energies (less than few keV), suggesting an ionospheric source for this population. The arrival of electrons of selected energies after substorm onset but before passage of the injection front over the spacecraft indicates that the injection front is of limited spatial size. Even after an analysis of the most rapid (least energy dispersed) electron injection measured over a span of 3 years of data it is not certain that a synchronous orbit spacecraft is ever observing within the acceleration region. If electrons less than a few hundred electron volts are depleted in the newly accelerated electron spectrum, then their field‐aligned enhancement as ionospheric secondaries might identify magnetic lines of force with energetic electron precipitation and perhaps the acceleration region.