Abstract
Abstract. We present two case studies in the night and evening sides of the auroral oval, based on plasma and field measurements made at low altitudes by the AUREOL-3 satellite, during a long period of stationary magnetospheric convection (SMC) on November 24, 1981. The basic feature of both oval crossings was an evident double oval pattern, including (1) a weak arc-type structure at the equatorial edge of the oval/polar edge of the diffuse auroral band, collocated with an upward field-aligned current (FAC) sheet of \\sim1.0 µA m-2, (2) an intermediate region of weaker precipitation within the oval, (3) a more intense auroral band at the polar oval boundary, and (4) polar diffuse auroral zone near the polar cap boundary. These measurements are compared with the published magnetospheric data during this SMC period, accumulated by Yahnin et al. and Sergeev et al., including a semi-empirical radial magnetic field profile BZ in the near-Earth neutral sheet, with a minimum at about 10-14 RE. Such a radial BZ profile appears to be very similar to that assumed in the "minimum- B/cross-tail line current" model by Galperin et al. (GVZ92) as the "root of the arc", or the arc generic region. This model considers a FAC generator mechanism by Grad-Vasyliunas-Boström-Tverskoy operating in the region of a narrow magnetic field minimum in the near-Earth neutral sheet, together with the concept of ion non-adiabatic scattering in the "wall region". The generated upward FAC branch of the double sheet current structure feeds the steady auroral arc/inverted-V at the equatorial border of the oval. When the semi-empirical BZ profile is introduced in the GVZ92 model, a good agreement is found between the modelled current and the measured characteristics of the FACs associated with the equatorial arc. Thus the main predictions of the GVZ92 model concerning the "minimum-B" region are consistent with these data, while some small-scale features are not reproduced. Implications of the GVZ92 model are discussed, particularly concerning the necessary conditions for a substorm onset that were not fulfilled during the SMC period.Key words. Magnetospheric physics (auroral phenomena; magnetospheric configuration and dynamics; plasma sheet).
Highlights
Auroral homogeneous arcs are the typical stationary features of the auroral oval during steady conditions
We present two case studies in the night and evening sides of the auroral oval, based on plasma and ®eld measurements made at low altitudes by the AUREOL-3 satellite, during a long period of stationary magnetospheric convection (SMC) on November 24, 1981
The data collected on the consecutive eveningside and nightside auroral passes are presented in Sect. 3; we shall show that the electron intensity distribution across the auroral oval as measured by A3 resembles that of the double oval observed by satellite UV imagers (Elphinstone et al, 1995a, b) and, for the SMC period under study, by the DE-1 satellite
Summary
Auroral homogeneous arcs are the typical stationary features of the auroral oval during steady conditions (see Oguti, 1981; Galperin, 1992, 1994). Under certain conditions (increase of the CTC, and/or sharpening of its earthward edge or, its radial gradient), the magnetic ®eld BTOT within the MFM could reach low values sucient for the plasma sheet electron demagnetization, or even reach zero, or reverse its BZ direction This kind of evolution of the MFM was suggested in GVZ92 as the probable cause for the substorm onset. 3; we shall show that the electron intensity distribution across the auroral oval as measured by A3 resembles that of the double oval observed by satellite UV imagers (Elphinstone et al, 1995a, b) and, for the SMC period under study, by the DE-1 satellite This set of A3 data will be used in the following parts for comparisons with the results of the arc/inverted-V model GVZ92 summarized in Sect. We shall try to answer to the question: why substorms onsets did not occur during the IMF-BZ < 0 conditions prevailing during that prolonged period?
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