The relationship between electric fields, conductances and currents in the high-latitude ionosphere: a statistical study using EISCAT data

J A Davies,M Lester

doi:10.1007/s00585-999-0043-3

Abstract

Abstract. The relationship between electric fields, height-integrated conductivities and electric currents in the high-latitude nightside electrojet region is known to be complex. The tristatic nature of the EISCAT UHF radar facility provides an excellent means of exploring this interrelationship as it enables simultaneous estimates to be made of the full electric field vector and the ionospheric Hall and Pedersen conductances, further allowing the determination of both field-perpendicular electric current components. Over 1300 h of common programme observations by the UHF radar system provide the basis of a statistical study of electric fields, conductances and currents in the high-latitude ionosphere, from which preliminary results are presented. Times at which there is significant solar contribution to the ionospheric conductances have been excluded by limiting the observations according to solar zenith angle. Initial results indicate that, in general, the times of peak conductance, identified from the entire set of EISCAT observations, do not correspond to the times of the largest electric field values; the relative contribution of ionospheric conductance and electric field to the electrojet currents therefore depends critically on local time, a conclusion which corroborates work by previous authors. Simultaneous measurements confirm a tendency for a decrease in both Hall and Pedersen conductances to be accompanied by an increase in the electric field, at least for moderate and large electric field value, a tendency which is also identified to some extent in the ratio of the conductances, which acts as an indicator of the energy of precipitating particles.Key words. Ionosphere (auroral ionosphere; electric fields and currents)

Highlights

Electric currents in the lower ionosphere, a manifestation of magnetosphere-ionosphere coupling, can be intensi®ed through enhancements in the conductivity, enhancements in the electric ®eld, or some combination of both
Previous authors have suggested that the eastward electrojet is a signature of magnetospheric convection whereas the westward electrojet, at least in the midnight sector, represents current intensi®cation during substorms (e.g. Baumjohann, 1983)
Radar observations support this view in that, while the eastward electrojet is controlled by northward electric ®elds, the westward electrojet current appears to be dominated by the Hall conductivity associated with precipitating auroral electrons (e.g. Brekke et al, 1974; Horwitz et al, 1978)

Summary

Introduction

Electric currents in the lower ionosphere, a manifestation of magnetosphere-ionosphere coupling, can be intensi®ed through enhancements in the conductivity, enhancements in the electric ®eld, or some combination of both. A less widely used method for studying the currents whichow in the ionosphere uses incoherent scatter radar observations from which estimates of the electric ®eld and height pro®les of both the Hall and Pedersen conductivity may be made (e.g. Kamide and Vickrey, 1983; Brekke et al, 1990; Kirkwood et al, 1988; Aikio and Kaila, 1996; Lester et al, 1996, amongst others). The majority of these studies consider only speci®c intervals whereas in the present study more than 1300 h of EISCAT observations, spanning the years 1992 to early 1997, provide the basis for a statistical analysis of conductances, electric ®elds, ionospheric currents and their interrelationship. No assumptions regarding the spatial scales of ionospheric features are, necessary

Experimental arrangement

Results and discussion

Summary and conclusion