Abstract
Observations are presented of the response of the dayside cusp/cleft aurora to changes in both the clock and elevation angles of the interplanetary magnetic field (IMF) vector, as monitored by the WIND spacecraft. The auroral observations are made in 630 nm light at the winter solstice near magnetic noon, using an all-sky camera and a meridian-scanning photometer on the island of Spitsbergen. The dominant change was the response to a northward turning of the IMF which caused a poleward retreat of the dayside aurora. A second, higher-latitude band of aurora was seen to form following the northward turning, which is interpreted as the effect of lobe reconnection which reconfigures open flux. We suggest that this was made possible in the winter hemisphere, despite the effect of the Earth's dipole tilt, by a relatively large negative X component of the IMF. A series of five events then formed in the poleward band and these propagated in a southwestward direction and faded at the equatorward edge of the lower-latitude band as it migrated poleward. It is shown that the auroral observations are consistent with overdraped lobe flux being generated by lobe reconnection in the winter hemisphere and subsequently being re-closed by lobe reconnection in the summer hemisphere. We propose that the balance between the reconnection rates at these two sites is modulated by the IMF elevation angle, such that when the IMF points more directly northward, the summer lobe reconnection site dominates, re-closing all overdraped lobe flux and eventually becoming disconnected from the Northern Hemisphere.Key words. Magnetospheric physics (magnetopause · cusp and boundary layers; solar-wind-magnetosphere interactions) · Space plasma physics (magnetic reconnection)
Highlights
The possibility that reconnection would take place between northward-pointing interplanetary magnetic ®eld (IMF) and the geomagnetic ®eld in the tail lobes, poleward of the magnetic cusps, was ®rst suggested by Dungey (1963)
The ®rst observational evidence that this northward-IMF lobe reconnection does take place was sunwardow in the polar cap ionosphere, ®rst deduced from geomagnetic observations (Maezawa, 1976) and con®rmed by direct observations of theow (e.g. Burke et al, 1979; Cumnock, 1992) and of the associated ®eldaligned current pattern (Zanetti et al, 1984)
We suggest that event J is similar to event E and that this brief swing was sucient to cause a brief burst of low-latitude reconnection whilst marking the end of the lobe reconnection in the Northern Hemisphere
Summary
The possibility that reconnection would take place between northward-pointing interplanetary magnetic ®eld (IMF) and the geomagnetic ®eld in the tail lobes, poleward of the magnetic cusps, was ®rst suggested by Dungey (1963).
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