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We report high-resolution observations of the\udsouthward-IMF cusp/cleft ionosphere made on December 16th 1998 by the EISCAT\ud(European incoherent scatter) Svalbard radar (ESR), and compare them with\udobservations of dayside auroral luminosity, as seen at a wavelength of 630 nm by\uda meridian scanning photometer at Ny Ålesund, and of plasma flows, as seen by\udthe CUTLASS (co-operative UK twin location auroral sounding system) Finland HF\udradar. The optical data reveal a series of poleward-moving transient red-line\ud(630 nm) enhancements, events that have been associated with bursts in the rate\udof magnetopause reconnection generating new open flux. The combined observations\udat this time have strong similarities to predictions of the effects of soft\udelectron precipitation modulated by pulsed reconnection, as made by Davis and\udLockwood (1996); however, the effects of rapid zonal flow in the ionosphere,\udcaused by the magnetic curvature force on the newly opened field lines, are\udfound to be a significant additional factor. In particular, it is shown how\udenhanced plasma loss rates induced by the rapid convection can explain two\udoutstanding anomalies of the 630 nm transients, namely how minima in luminosity\udform between the poleward-moving events and how events can re-brighten as they\udmove poleward. The observations show how cusp/cleft aurora and transient\udpoleward-moving auroral forms appear in the ESR data and the conditions which\udcause enhanced 630 nm emission in the transients: they are an important first\udstep in enabling the ESR to identify these features away from the winter\udsolstice when supporting auroral observations are not available.  <b>Key words: </b>Ionosphere (polar ionosphere) -\udMagnetospheric physics (magnetopause; cusp and boundary layers; solar\udwind-magnetosphere interactions

annales geophysicae

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