Radar observations of auroral zone flows during a multiple-onset substorm

Abstract

Abstract. We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every ~12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the pre-existing extended current system driven by the large-scale flow. These filaments are ~1 h MLT wide (~600 km), and initially expand poleward to a width of ~300 km before contracting equatorward and coalescing with the pre-existing current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s–1 or less during the first few minutes, before surging equatorward at 0.5–1.0 km s–1 during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed ~100 mho, before decaying over a few minutes. This value compares with Hall conductivities of ~20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.

Observations of two complete substorm cycles during the Cassini Earth swing‐by: Cassini magnetometer data in a global context

“…In effect, the foci of the flow cells are shown as having been displaced toward the dayside. In common with some previous studies, therefore, we find that the flow within the high-conductivity bulge region became semi-stagnant, with the surrounding flow being diverted around it [e.g., Kirkwood et al, 1988; Morelli et al, 1995;Yeoman et al, 2000]. We note, however, that the flow in the vicinity of the Cassini footprint, which lay well to the east of the disturbed region, had also become small in this interval, a fact which will be commented upon in the next section below.…”

Defining and resolving current systems in geospace

“…It was shown that the location of its upward and downward current roughly coincide with the west and east terminations of the auroral bulge, respectively (e.g., Untiedt et al, 1978;Baumjohann et al, 1981;Gelpi et al, 1987;Sergeev et al, 1996). Like auroral bulge, the SCW is not a static structure; once it is formed it broadens azimuthally and radially (Nagai, 1982;Lopez and Lui, 1990;Jacquey et al, 1993;Ohtani et al, 1998). Although the sense of the SCW system for a typical substorm which initiates at the midnight-premidnight local time sector is the same as for the large-scale region 1 system, there are a few important differences.…”

The ionospheric response to interplanetary magnetic field variations: Evidence for rapid global change and the role of preconditioning in the magnetosphere

“…Strong electron precipitation, such as that associated with the onset of the substorm expansion phase, ionizes the E region and enhances the ionospheric conductivity. There are several reports in the literature which suggest that the ionospheric conductivity modifies the electric field pattern and produces flow bursts [e.g., Morelli et al, 1995]. A high-conductivity region creates an obstacle which diverts the background flow around it, and the diverted flow is accelerated.…”