Determining the boundaries of the auroral oval from CHAMP field-aligned current signatures – Part 1

C Xiong,H Lühr,M G Johnsen,H Wang

doi:10.5194/angeo-32-609-2014

Abstract

Abstract. In this paper we present the first statistical study on auroral oval boundaries derived from small- and medium-scale field-aligned currents (FACs, < 150 km). The dynamics of both the equatorward and poleward boundaries is deduced from 10 years of CHAMP (CHAllenging Minisatellite Payload) magnetic field data (August 2000–August 2010). The approach for detecting the boundaries from FACs works well under dark conditions. For a given activity level the boundaries form well-defined ellipses around the magnetic pole. The latitudes of the equatorward and poleward boundaries both depend, but in different ways, on magnetic activity. With increasing magnetic activity the equatorward boundary expands everywhere, while the poleward boundary shows on average no dependence on activity around midnight, which seems to be stationary at a value of about 72° Mlat. Functional relations between the latitudes of the boundaries and different magnetic activity indices have been tested. Best results for a linear dependence are derived for both boundaries with the dayside merging electric field. The other indices, like the auroral electrojet (AE) and disturbance storm time (Dst) index, also provide good linear relations but with some caveats. Toward high activity a saturation of equatorwards expansion seems to set in. The locations of the auroral boundaries are practically independent of the level of the solar EUV flux and show no dependence on season.

Highlights

The best-known aurora features are the northern and southern auroral ovals (Feldstein, 1963; Feldstein et al, 1969; Akasofu, 1966)
The auroral oval often consists of two different parts: the discrete oval, which lies embedded in the continuous, permanent diffuse oval
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Summary

Introduction

The best-known aurora features are the northern and southern auroral ovals (Feldstein, 1963; Feldstein et al, 1969; Akasofu, 1966). They are the regions in the ionosphere which receive the main part of the magnetospheric particle precipitation. Viewed from space with a global imager, the aurora appears as diffuse, continuous, luminous bands that surround both geomagnetic poles at ionospheric altitudes (Frey, 2007). The auroral oval often consists of two different parts: the discrete oval (visual luminosity, e.g. rays and arcs), which lies embedded in the continuous, permanent diffuse oval. Compared to the diffuse oval the discrete arcs are more dynamic in size, shape, and location within seconds or minutes, with smallscale structures ranging from a few hundred metres to hundreds of kilometres (Borovsky, 1993)

Objectives

Discussion

Conclusion