Mesoscale ionospheric electrodynamics of omega bands determined from ground-based electromagnetic and satellite optical observations

O Amm,R.R Vondrak,G Lu,G.A Germany,A Viljanen,N Østgaard,J Stadsnes,A Aksnes

doi:10.5194/angeo-23-325-2005

Abstract

Abstract. We present ground-based electromagnetic data from the MIRACLE and BEAR networks and satellite optical observations from the UVI and PIXIE instruments on the Polar satellite of an omega band event over Northern Scandinavia on 26 June 1998, which occured close to the morning side edge of a substorm auroral bulge. Our analysis of the data concentrates on one omega band period from 03:18-03:27 UT, for which we use the method of characteristics combined with an analysis of the UVI and PIXIE data to derive a time series of instantaneous, solely data-based distributions of the mesoscale ionospheric electrodynamic parameters with a 1-min time resolution. In addition, the AMIE method is used to derive global Hall conductance patterns. Our results show that zonally alternating regions of enhanced ionospheric conductances ("tongues") up to ~60S and low conductance regions are associated with the omega bands. The tongues have a poleward extension of ~400km from their base and a zonal extension of ~380km. While they are moving coherently eastward with a velocity of ~770ms-1, the structures are not strictly stationary. The current system of the omega band can be described as a superposition of two parts: one consists of anticlockwise rotating Hall currents around the tongues, along with Pedersen currents, with a negative divergence in their centers. The sign of this system is reversing in the low conductance areas. It causes the characteristic ground magnetic signature. The second part consists of zonally aligned current wedges of westward flowing Hall currents and is mostly magnetically invisible below the ionosphere. This system dominates the field-aligned current (FAC) pattern and causes alternating upward and downward FAC at the flanks of the tongues with maximum upward FAC of ~25µA m-2. The total FAC of ~2MA are comparable to the ones diverted inside a westward traveling surge. Throughout the event, the overwhelming part of the FAC are associated with gradients of the ionospheric conductances, and 66-84% of the FAC are connected with ionospheric Hall currents.

Highlights

Auroral omega bands are periodic, wave-like undulations of the poleward boundary of the morning side diffuse aurora occurring in the recovery phase of substorms
Since for uniform ionospheric conductances the combined magnetic effect of Pedersen currents and FACs is invisible below the ionosphere (e.g. Fukushima, 1976), the ground magnetic effect is completely caused by the Hall currents which are circulating around the centers of the electric field divergences, anticlockwise inside the auroral tongues and clockwise in the dark auroral holes. Both current systems described are assumed to be comoving with the auroral form and have to be regarded as being superposed on the background westward electrojet. While these initial models were composed from data of single latitudinal chains of magnetometers, later studies using combined two-dimensional data of the Scandinavian Magnetometer Array (SMA; Kuppers et al, 1979) and the STARE radar (Greenwald et al, 1978) showed that the conductances are clearly enhanced inside the omega bands with respect to the surrounding regions, and after subtraction of the background field, there is a radial electric field component pointing towards the tongues, indicating upward field-aligned currents there (e.g. Andreand Baumjohann, 1982; Opgenoorth et al, 1983a)
Two techniques to derive ionospheric conductances are used for our analysis: the two-dimensional (2-D) method of characteristics and a technique to derive the conductances from the Ultraviolet Imager (UVI) and Polar Ionospheric X-ray Imaging Experiment (PIXIE) data

Summary

Introduction

Auroral omega bands are periodic, wave-like undulations of the poleward boundary of the morning side diffuse aurora occurring in the recovery phase of substorms. Both current systems described are assumed to be comoving with the auroral form and have to be regarded as being superposed on the background westward electrojet While these initial models were composed from data of single latitudinal chains of magnetometers, later studies using combined two-dimensional data of the Scandinavian Magnetometer Array (SMA; Kuppers et al, 1979) and the STARE radar (Greenwald et al, 1978) showed that the conductances are clearly enhanced inside the omega bands with respect to the surrounding regions, and after subtraction of the background field, there is a radial electric field component pointing towards the tongues, indicating upward field-aligned currents there (e.g. Andreand Baumjohann, 1982; Opgenoorth et al, 1983a). For a recent summary of research related to omega bands which covers these topics, see Paschmann et al (2003, their Sect. 6.3)

Instrumentation and observations

Analysis techniques

Analysis results and discussion

Summary and conclusions

Findings

The current system of the omega bands can be decomposed into two parts