HIGH-LATITUDE HALL AND PEDERSEN CONDUCTANCES DURING SUBSTORM ACTIVITYIN THE SUNDIAL-ATLAS CAMPAIGN

Simultaneous high time resolution observations of the Hall and Pederse n conductances, Sigma(H) and Sigma(P); respectively, the ionospheric e lectric field and the ground magnetic field during a magnetospheric su bstorm are reported. The measurements discussed here were taken during the SUNDIAL/ATLAS 1 campaign of March 24 to April 2, 1992. The Europe an Incoherent Scatter (EISCAT) UHF special programme SP-UK-ATLAS, whic h operated on March 27, 1992, provided continuous measurements of the electron density and the ion vector velocity from which Sigma(H) and S igma(P) and the ionospheric electric field were calculated. During the substorm growth phase, Sigma(P) and Sigma(H) were less than 10 S and the ratio, R = Sigma(H) /Sigma(P), was less than 1. Although both Sigm a(H) and Sigma(P) increased at the onset of the expansion phase, R rem ained close to 1. This ratio provides information on the mean energy O f the precipitating particles responsible for the enhanced conductance s. A ratio of 1 implies a mean energy of the particles of 2.56 keV. Tw o distinct expansion phases were identified, the second of which inclu ded a number of intensifications. Both Sigma(H) and Sigma(P) 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 exceedin g 2, equivalent td mean energies of more than 5.78 keV. The largest va lue of R was 3.25, which occurred during the substorm recovery phase a nd 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 maxim um 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 cur rent. were observed, for which the electric field was responsible in m ost 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 westwar d current during the initial part of the expansion phase is dominated by the electric field, in contrast to a previously published model whi ch suggested that this current would be conductivity dominated. It is suggested that, further spatial structure of the current is necessary to explain these observations.