f"Compression aurora": Particle precipitation driven by long-duration high solar wind ram pressure

The transient behavior of shock-induced auroras reported by previous workers suggests a cause-and-effect relationship between the shock front and the auroral transient. However, it is not known if the high solar wind dynamic pressure downstream of a shock can have significant effects on the global auroral morphology. In this brief report we present results from an observational study on global auroral response to large solar wind dynamic pressure, using global auroral images acquired by the ultraviolet imager on board the Polar spacecraft. It is found that the luminosity of the aurora showed a general prompt and lasting increase in the dawn and dusk flanks of the oval. Such an auroral activity is closely related to the compression of the magnetosphere and is thus christened "compression aurora." The luminosity increase, though predominantly in the dawn sector of the oval, may persist as long as the solar wind dynamic pressure stays high and may disappear ~10 min after the dynamic pressure drops. A detailed examination of Defense Meteorological Satellite Program particle data on one fortuitous event on 26 September 1999 indicates that the compression aurora is produced mainly by precipitating electrons originating from the central plasma sheet (CPS). Plasma wave observations made from Geotail in the dawn sector of the near-Earth plasma sheet indicates no expected plasma wave activity that may have been otherwise considered a primary cause of enhanced CPS particle precipitation through a pitch angle loss by wave-particle interactions. We propose that reductions of the Earth's magnetic field mirror ratio by magnetospheric compression can be a prime mechanism for the compression aurora as long as there is a continuing supply of particle source.