Abstract
Abstract. We present ground-based optical, riometer and magnetometer recordings together with Polar UVI and GOES magnetic field observations of a substorm that occurred over Canada on 24 November 1997. This event involved a clear optical onset followed by poleward motion of the aurora as a signature of an expanding auroral bulge. During the expansion phase, there were three distinct types of meso-scale (10–1000 km) auroral structures embedded in the bulge: at first a series of equatorward moving auroral arcs, followed by a well-defined spiral pair, and finally north-south directed aurora (a streamer). The spirals occurred several minutes after the onset, and indicate a shear in the field-aligned current. The north-south aligned aurora that formed about 10 min after the onset suggest bursty bulk flow type flows taking place in the central plasma sheet. Polar UVI observations of the polar cap location indicate that the southward drifting arcs were associated with magnetospheric activity within closed field lines, while the auroral streamer was launched by the bulge reaching the polar cap boundary, i.e. the mid-tail reconnection starting on the open field lines. The riometer data imply high energy electron precipitation in the vicinity of the the poleward moving edge of the auroral bulge, starting at the onset and continuing until the formation of the north-south structure. In this paper, we examine this evolving auroral morphology within the context of substorm theories.
Highlights
1.1 The substorm concept and modelsThe reconnection of the interplanetary and the terrestrial magnetic fields on the dayside magnetopause provides the energy that powers the substorm cycle
The current wedge is recognised as the self-consistent corollary to the current disruption in the nearEarth Central Plasma Sheet (CPS)
A major expansion phase related dipolarisation was recorded by GOES 8 and a simultaneous All-Sky Imager (ASI) image from GIL showed a pair of auroral spirals close to the poleward boundary of the proton emission band
Summary
The reconnection of the interplanetary and the terrestrial magnetic fields on the dayside magnetopause provides the energy that powers the substorm cycle. According to the Current Disruption (CD) model (Lui, 1996, 2001), instabilities generate fluctuations in the magnetic and electric fields increasing the resistivity in the crosstail current region This leads to the diversion of the current to the ionosphere and the corresponding dipolarisation. The instability in the earthward end of the plasma sheet propagates tailward as a rarefaction wave thinning the plasma sheet and resulting in conditions favourable for reconnection In both models, the current wedge is recognised as the self-consistent corollary to the current disruption in the nearEarth Central Plasma Sheet (CPS). In the NENL model, the poleward expansion is an ongoing signature of the mid-tail reconnection which launches fast Earthward flows (Bursty Bulk Flows or BBF’s, Angelopoulos et al, 1992), which in turn cause the flux pile-up in the earthward end of the plasma sheet, the disruption of the cross-tail current and the dipolarisation
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