Observing the magnetosphere through global auroral imaging: 1. Observables

Abstract

Over the years, it has become clear that there are fundamental limitations in observing magnetospheric processes through their auroral footprints. Most electron auroras are formed in the auroral acceleration region relatively close to the Earth at altitudes (<2 RE). There are four distinct auroral types: (1) downward field-aligned current (FAC) regions where ion precipitation is dominant, (2) pitch angle diffusion aurora (or briefly “diffusion aurora”) region without significant FAC, (3) upward FAC regions of precipitating electrons and monoenergetic auroral arc formations, and (4) Alfvénic auroral regions, where low-energy electrons from the ionosphere are accelerated by incoming Alfvén waves. Alfvénic auroras are the footprints of magnetospheric regions where waves are produced by dynamic events such as reconnection, substorm onset initiation, and magnetic field dipolarization. Based on the mean energy and density of the precipitating electrons, ground-based and spacecraft-based optical observations can be used to distinguish between auroras where the source is the plasma sheet (types 1, 2, and 3) and Alfvénic auroras, where the source is the ionosphere (type 4). Imaging of the Alfvénic auroral region could be used to map the dynamically active regions of the magnetosphere. The energy distribution of the most significant precipitating ions, protons, can be measured from the Doppler profile of the hydrogen emission lines. Mapping of the time dependent global energy distribution of proton precipitation would allow the observation of the associated magnetospheric boundaries.