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

O Amm,A Pajunpää,U Brandström

doi:10.1007/s00585-999-1385-6

Abstract

Abstract. Using the method of characteristics to invert ground-based data of the ground magnetic field disturbance and of the ionospheric electric field, we obtain spatial distributions of ionospheric conductances, currents, and field-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 flowing FACs of up to 25 µA/m2 are concentrated near its westward and equatorward edge. The strong upward flowing FACs are fed by an area of more distributed, but still very strong downward-flowing FACs northeastward of the auroral form. In contrast to the conductances, the electric field is only slightly affected by the passage of the form. We point out similarities and differences of our observations and results to previously reported observations and models of 'auroral fingers', 'north-south aurora', and 'auroral streamers' which are suggested to be ionospheric manifestations of bursty bulk flows in the plasma sheet.Key words. Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (magnetosphere · ionosphere interactions)

Highlights

A number of studies has been carried out to investigate the relation between auroral forms and ionospheric electrodynamic quantities, such as conductances, currents, and ®eld-aligned currents (FACs) during substorms
The event studied here started at 2155 UT, 14 min after one of several auroral breakups observed during that night and a corresponding renewed intensi®cation of the ground magnetic disturbance, when a north-south (N-S) auroral form showed up over the zenith at Muonio, i.e., at the eastern edge of the STARE ®eld of view
Since the pictures of the cameras at Muonio and Kiruna were partly disturbed by clouds, in Fig. 3 we show an undisturbed example of such a north-south aurora from Ivalo at 2201 UT, which is taken from the one in the sequence of westward-moving north-south auroral forms during the period of interest

Summary

Introduction

A number of studies has been carried out to investigate the relation between auroral forms and ionospheric electrodynamic quantities, such as conductances, currents, and ®eld-aligned currents (FACs) during substorms. The event studied here started at 2155 UT, 14 min after one of several auroral breakups observed during that night and a corresponding renewed intensi®cation of the ground magnetic disturbance, when a north-south (N-S) auroral form showed up over the zenith at Muonio, i.e., at the eastern edge of the STARE ®eld of view (compare Fig. 2). In the part current system ~tdf , the typical ground magnetic signatures are represented which are added on the westward electrojet by the event (compare Fig. 5a): The north deection of the by 90° rotated horizontal magnetic ®eld between 20 and 24° longitude is clearly visible, and the curl-like structure in the southwestern part of the analysis area is represented

Discussion

Summary and conclusions