Stationary magnetospheric convection on November 24, 1981. 1. A case study of "pressure gradient/minimum-B" auroral arc generation

[摘要] We present two case studies in the night andevening sides of the auroral oval, based on plasma and field measurements madeat low altitudes by the AUREOL-3 satellite, during a long period of stationarymagnetospheric convection (SMC) on November 24, 1981. The basic feature of bothoval crossings was an evident double oval pattern, including (1) a weak arc-typestructure at the equatorial edge of the oval/polar edge of the diffuse auroralband, collocated with an upward field-aligned current (FAC) sheet of \sim1.0 µAm^-2, (2) an intermediate region of weaker precipitation within theoval, (3) a more intense auroral band at the polar oval boundary, and (4) polardiffuse auroral zone near the polar cap boundary. These measurements arecompared with the published magnetospheric data during this SMC period,accumulated by Yahnin et al. and Sergeev et al., including asemi-empirical radial magnetic field profile B_Z in thenear-Earth neutral sheet, with a minimum at about 10-14 R_E.Such a radial B_Z profile appears to be very similar to thatassumed in the "minimum- B/cross-tail line current" model byGalperin et al. (GVZ92) as the "root of the arc", or the arcgeneric region. This model considers a FAC generator mechanism byGrad-Vasyliunas-Boström-Tverskoy operating in the region of a narrow magneticfield minimum in the near-Earth neutral sheet, together with the concept of ionnon-adiabatic scattering in the "wall region". The generated upwardFAC branch of the double sheet current structure feeds the steady auroralarc/inverted-V at the equatorial border of the oval. When the semi-empirical B_Zprofile is introduced in the GVZ92 model, a good agreement is found between themodelled current and the measured characteristics of the FACs associated withthe equatorial arc. Thus the main predictions of the GVZ92 model concerning the"minimum-B" region are consistent with these data, while somesmall-scale features are not reproduced. Implications of the GVZ92 model arediscussed, particularly concerning the necessary conditions for a substorm onsetthat were not fulfilled during the SMC period.

Key words. Magnetospheric physics (auroral phenomena;magnetospheric configuration and dynamics; plasma sheet).

[学科分类] 地球科学(综合)