Motion of the dayside polar cap boundary during substorm cycles: II. Generation of poleward-moving events and polar cap patches by pulses in the magnetopause reconnection rate

摘要

Using data from the EISCAT (European Incoherent Scatter)VHF and CUTLASS (Co-operative UK Twin-Located Auroral Sounding System) HFradars, we study the formation of ionospheric polar cap patches and theirrelationship to the magnetopause reconnection pulses identified in thecompanion paper by Lockwood et al. (2005). It is shown that the poleward-moving,high-concentration plasma patches observed in the ionosphere by EISCAT on23 November 1999, as reported by Davies et al. (2002), were often associated withcorresponding reconnection rate pulses. However, not all such pulsesgenerated a patch and only within a limited MLT range (11:00-12:00 MLT) did apatch result from a reconnection pulse. Three proposed mechanisms for theproduction of patches, and of the concentration minima that separate them,are analysed and evaluated: (1) concentration enhancement within the patchesby cusp/cleft precipitation; (2) plasma depletion in the minima between thepatches by fast plasma flows; and (3) intermittent injection ofphotoionisation-enhanced plasma into the polar cap. We devise a test todistinguish between the effects of these mechanisms. Some of the eventsrepeat too frequently to apply the test. Others have sufficiently longrepeat periods and mechanism (3) is shown to be the only explanation ofthree of the longer-lived patches seen on this day. However, effect (2) alsoappears to contribute to some events. We conclude that plasma concentrationgradients on the edges of the larger patches arise mainly from local timevariations in the subauroral plasma, via the mechanism proposed by Lockwood et al. (2000).