Abstract
Abstract. We investigate the dayside auroral dynamics and ionospheric convection during an interval when the interplanetary magnetic field (IMF) had predominantly a positive Bz component (northward IMF) but varying By. Polar UVI observations of the Northern Hemisphere auroral emission indicate the existence of a region of luminosity near local noon at latitudes poleward of the dayside auroral oval, which we interpret as the ionospheric footprint of a high-latitude reconnection site. The large field-of-view afforded by the satellite-borne imager allows an unprecedented determination of the dynamics of this region, which has not previously been possible with ground-based observations. The location of the emission in latitude and magnetic local time varies in response to changes in the orientation of the IMF; the cusp MLT and the IMF By component are especially well correlated, the emission being located in the pre- or post-noon sectors for By < 0 nT or By > 0 nT, respectively. Simultaneous ground-based observations of the ionospheric plasma drift are provided by the CUTLASS Finland HF coherent radar. For an interval of IMF By \\approx 0 nT, these convection flow measurements suggest the presence of a clockwise-rotating lobe cell contained within the pre-noon dayside polar cap, with a flow reversal closely co-located with the high-latitude luminosity region. This pattern is largely consistent with recent theoretical predictions of the convection flow during northward IMF. We believe that this represents the first direct measurement of the convection flow at the imaged location of the footprint of the high-latitude reconnection site.Key words: Magnetospheric physics (auroral phenomena; magnetopause · cusp · and boundary layers; plasma convection)
Highlights
The dynamics of the dayside aurora and their relationship to the orientation of the interplanetary magnetic ®eld (IMF) have received much attention recently
No new openux is created in this process, but thelobe stirring'' which results is thought to produce sunwardow in the dayside polar cap, asux tubes are forced equatorward at the ionospheric footprint of the reconnection site, with a largely clockwise or anticlockwise circulatory pattern resulting if IMF By is positive or negative
This study has presented coordinated observations from a spacecraft imager, the Polar ultra violet imager (UVI) instrument, and the CUTLASS Finland HF coherent radar, during an interval of predominantly northward IMF
Summary
The dynamics of the dayside aurora and their relationship to the orientation of the interplanetary magnetic ®eld (IMF) have received much attention recently. Milan et al.: Dayside convection and auroral morphology reconnection occurs between the IMF and pre-existing openux in the magnetotail lobe These ®eld lines map to the high-latitude side of the ionospheric footprint of the cusp, and are no longer constrained to be situated on the open/closedux boundary, but can be located at higher latitude within the polar cap. Type 1 and type 2 cusp aurorae are observed to co-exist under the conditions that the IMF is northward and has a signi®cant By component (for`clock angles'' in the range 45° < jhj < 90°, see Sandholt et al, 1998d), an orientation of the IMF for which it is postulated reconnection can occur at the high- and low-latitude reconnection sites simultaneously (Rei and Burch, 1985). The present observations allow the IMF By inuence on the location of the cusp emission to be determined for the ®rst time, and demonstrate that the associatedow pattern is largely consistent with previously proposed theoretical models (éieroset et al, 1997; Sandholt et al, 1998a, b, c)
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