High‐latitude Hall and Pedersen conductances during substorm activity in the SUNDIAL‐ATLAS campaign

Abstract

Simultaneous high time resolution observations of the Hall and Pedersen conductances, ΣH and ΣP, respectively, the ionospheric electric field and the ground magnetic field during a magnetospheric substorm are reported. The measurements discussed here were taken during the SUNDIAL/ATLAS 1 campaign of March 24 to April 2, 1992. The European Incoherent Scatter (EISCAT) UHF special programme SP‐UK‐ATLAS, which operated on March 27, 1992, provided continuous measurements of the electron density and the ion vector velocity from which ΣH and ΣP and the ionospheric electric field were calculated. During the substorm growth phase, ΣP and ΣH were less than 10 S and the ratio, R = ΣH/ΣP, was less than 1. Although both ΣH and ΣP increased at the onset of the expansion phase, R remained close to 1. This ratio provides information on the mean energy of the precipitating particles responsible for the enhanced conductances. A ratio of 1 implies a mean energy of the particles of 2.56 keV. Two distinct expansion phases were identified, the second of which included a number of intensifications. Both ΣH and ΣP increased toward the end of the second expansion phase with peak values of 71 S and 34 S, respectively. The ratio R also increased to values exceeding 2, equivalent to mean energies of more than 5.78 keV. The largest value of R was 3.25, which occurred during the substorm recovery phase and is equivalent to mean energies of more than 10 keV. The increase in mean energy as the substorm progresses may be interpreted in terms of changes in the acceleration processes in the magnetosphere. The maximum zonal (east‐west) current during this interval was 2.20 A m−1 and occurred toward the end of the second substorm expansion phase. At a number of intensifications, reversals or enhancements in the zonal current were observed, for which the electric field was responsible in most cases. While the peak value of the Pedersen conductance of 40 S is similar to recent published results, the Hall conductance peak of 75 S is less than recent published measurements. Furthermore, the westward current during the initial part of the expansion phase is dominated by the electric field, in contrast to a previously published model which suggested that this current would be conductivity‐dominated. It is suggested that further spatial structure of the current is necessary to explain these observations.