Plasma structure within poleward-moving cusp/cleft auroral transients: EISCAT Svalbard radar observations and an explanation in terms of large local time extent of events

摘要

We report high-resolution observations of thesouthward-IMF cusp/cleft ionosphere made on December 16th 1998 by the EISCAT(European incoherent scatter) Svalbard radar (ESR), and compare them withobservations of dayside auroral luminosity, as seen at a wavelength of 630 nm bya meridian scanning photometer at Ny ?lesund, and of plasma flows, as seen bythe CUTLASS (co-operative UK twin location auroral sounding system) Finland HFradar. The optical data reveal a series of poleward-moving transient red-line(630 nm) enhancements, events that have been associated with bursts in the rateof magnetopause reconnection generating new open flux. The combined observationsat this time have strong similarities to predictions of the effects of softelectron precipitation modulated by pulsed reconnection, as made by Davis andLockwood (1996); however, the effects of rapid zonal flow in the ionosphere,caused by the magnetic curvature force on the newly opened field lines, arefound to be a significant additional factor. In particular, it is shown howenhanced plasma loss rates induced by the rapid convection can explain twooutstanding anomalies of the 630 nm transients, namely how minima in luminosityform between the poleward-moving events and how events can re-brighten as theymove poleward. The observations show how cusp/cleft aurora and transientpoleward-moving auroral forms appear in the ESR data and the conditions whichcause enhanced 630 nm emission in the transients: they are an important firststep in enabling the ESR to identify these features away from the wintersolstice when supporting auroral observations are not available.Key words: Ionosphere (polar ionosphere) -Magnetospheric physics (magnetopause; cusp and boundary layers; solarwind-magnetosphere interactions)