Spatial distribution of conductances and currents associated with a north-south auroral form during a multiple-substorm period

Abstract

Abstract. Using the method of characteristics to invert ground-based data of the ground magnetic ®eld disturbance and of the ionospheric electric ®eld, we obtain spatial distributions of ionospheric conductances, currents, and ®eld-aligned currents (FACs) associated with a north-south auroral form that drifts westwards over northern Scandinavia around 2200 UT on December 2, 1977. This auroral form is one in a sequence of such north-south structures observed by all-sky cameras, and appears 14 min after the last of several breakups during that extremely disturbed night. Our analysis shows that the ionospheric Hall conductance reaches values above 200 S in the center of the form, and upward¯owing FACs of up to 25 lA/m 2 are concentrated near its westward and equatorward edge. The strong upward owing FACs are fed by an area of more distributed, but still very strong downward-¯owing FACs northeastward of the auroral form. In contrast to the conductances, the electric ®eld is only slightly a ected by the passage of the form. We point out similarities and di erences of our observations and results to previously reported observations and models of`auroral ®ngers', north-south aurora', and`auroral streamers' which are suggested to be ionospheric manifestations of bursty bulk¯ows in the plasma sheet.

Bursty bulk flow intrusion to the inner plasma sheet as inferred from auroral observations

“…Our observations are consistent with the previous findings associating auroral streamers (or north‐south auroras) with R1‐type current systems [ Amm et al , 1999]. The CCW vortex in EC vectors at the western flank of the streamer supports this conclusion as well as the spatial distribution of electron and proton auroras, former (latter) concentrating to the western (eastern) flank of the streamer (cf.…”

“…In the region of auroras ECs can be assumed to be large as the ionospheric conductivity is high there. Observing equally large ECs in the dark dawnside flank implies strong electric field, i.e., high‐speed plasma flows in this region, which is consistent with the result of Nakamura et al, although in some cases conductances can be significant also outside the electron arcs [ Amm et al , 1999].…”

“…In our case the R1 current system persisted still during the duskward flow diversion. The EC pattern drifted as a stable configuration westward similarly as in the case of a substorm time streamer analyzed by Amm et al [1999]. Around the time when the H β band was brightest and at its most poleward position (1958 UT) the region of upward FAC already left the MIRACLE field of view.…”

“…In meridional scanning photometer data streamers appear first as poleward boundary intensifications which later drift equatorward [ Lyons et al , 1999; Zesta et al , 2000]. Observational evidence for the wedge‐like R1‐type current system is presented in the studies by Amm et al [1999] and Kauristie et al [2000]. In the former paper the real three dimensional FAC current system related with an auroral streamer is deduced from ionospheric observations, while in the latter study, the connection between this type of current system (as monitored with equivalent current vectors) with bursty plasma sheet flows is established.…”

High‐latitude irregularities of the magnetospheric electric field and their relation to solar wind and geomagnetic conditions

“…Hereinafter, the transient flows we consider will be termed “rapid magnetospheric streams” rather than “bursty bulk flows” (BBFs), for we admit some difference between the two phenomena in spite of great similarity. Thus the authors who discovered the BBFs typically relate them to high auroral electrojet ( AE ) activity, impulsive magnetic reconnection as a basic mechanism, and substorm‐associated, north‐south‐aligned auroral structures as an ionospheric manifestation [ Baumjohann and Paschmann , 1990; Angelopoulos et al , 1992, 1993, 1994; Henderson et al , 1998; Lyons et al , 1999; Amm et al , 1999; Zesta et al , 2000]. We find the rapid magnetospheric streams to be most common under northward B z interplanetary magnetic field (IMF) and quiet geomagnetic conditions.…”