Observing the magnetosphere through auroral imaging.

Abstract:

Although the terrestrial aurora is often regarded as 2 dimensional projection of the 3 dimensional magnetosphere there are fundamental limitations in observing magnetospheric processes through their auroral footprints. It has been shown that most electron auroras are produced in the auroral acceleration region at lower altitudes (<2Re) in the last steps of processing the auroral particles. From FAST, IMAGE , Cluster and THEMIS data we can distinguish between four fundamentally different types of auroral acceleration regions. A primary task is to distinguish (1) the upward current, (2) downward current, (3) diffuse aurora and (4) Alfven wave accelerated types of auroral acceleration regions. Type (1) contains the “inverted V” type electron precipitation distinguishable by several keV mono-energetic electron spectra, and low number flux consistent with the source population in the plasma sheet. Our understanding of how these auroras relate to magnetospheric processes is still vague, probably associated with convection sheer. Alfven wave electron auroras (4) are of low average energy (<2 keV) high electron flux consistent with ionospheric electron source predominantly occurring during substorms, and they are generated by wave energy carried from the magnetosphere into the ionosphere, where it is converted into electron energy. These are most promising candidates for observing the footprints of source regions associated with reconnection sites or magnetospheric dB/dt events. Optical measuring techniques of electron energy use the atmosphere as a spectrometer, obtaining the penetration altitude as a proxy for energy, that can be obtained from atmospheric composition, quenching lifetime of the emitters, UV absorption pass-length of O₂ to the source or the local atmospheric temperature. Precipitating protons are usually an order of magnitude more energetic and less affected by fields in the low altitude auroral acceleration region. Energetic proton precipitation is a more direct mapping tool of highly stretched closed field lines, the high latitude open-closed-field-line and the low latitude isotropic boundary. Ground and satellite based spectral imaging of the aurora will remain a significant tool in understanding space plasma physics in the magnetosphere.