Diffuse Shock-Aurora: the Characteristics, Evolution and Cause (Invited)

The magnetospheric compression and magnetic reconnection are enhanced in the local noon sector when interplanetary shocks impinge on the subsolar magnetopause. Auroras in the dayside high latitude ionosphere light up within seconds to few minutes indicating causes of the aurora in the outer magnetosphere and/or on the magnetopause ~10 RE away from the ionosphere. Consistent observation results from space and the ground revealed that the equatorward expanding and anti-sunward propagating shock aurora first seen from space is primarily diffuse aurora. The aurora is generated by hard electrons with energies near or greater than 1 keV at FAST. The electrons have an isotropic pitch angle distribution and an evident enhancement in the energy and number fluxes during the shock compression. These precipitated electrons penetrate to lower ionosphere at ~150 km altitude than the soft electrons and excite green auroral emissions of 557.7 nm that have been identified by the ground all-sky imager (ASI) and the meridian-scanning photometer (MSP). The diffuse auroral intensity can exceed the red arcs (which is a rare situation in auroral emissions) and can be brighter than 5 kR. Theoretically, the speculated cause in the outer magnetosphere is the temperature anisotropy near equator where the most intense compression develops. The temperature anisotropy provides a possible source of free energy for different plasma instabilities that lead to the wave-particle scattering enhancement. Consequently, isotropic electrons flourish, so do those in the loss cone, which explains what have been measured at FAST and the ground. Although near-equator in situ measurements of the temperature anisotropy have not been widely reported, relevant observations are documented and will be reviewed in this paper to examine the theory.