Stationary magnetospheric convection on November 24, 1981. 2. Small-scale structures in the dayside cusp/cleft
A case study of the dayside cusp/cleft region during an interval of stationary magnetospheric convection (SMC) on November, 24, 1981 is presented, based on detailed measurements made by the AUR-EOL-3 satellite. Layered small-scale ®eld-aligned current sheets, or loops, superimposed to a narrow V-shaped ion dispersion structure, were observed just equatorward from the region of the"cusp proper". The equatorward sheet was accompanied by a very intense and short (less than 1 s) ion intensity spike
... ion intensity spike at 100 eV. No major dierences were noted of the characteristics of the LLBL, or"boundary cusp", and plasma mantle precipitation during this SMC period from those typical of the cusp/cleft region for similar IMF conditions. Simultaneous NOAA-6 and NOAA-7 measurements described in Despirak et al. were used to estimate the average extent of the"cusp proper" (de®ned by dispersed precipitating ions with the energy¯ux exceeding 10 A3 erg cm A2 s A1 ) during the SMC period, as $0.73°ILAT width, 2.6±3.4 h in MLT, and thus the recently merged magnetic¯ux, 0.54±0.70´10 7 Wb. This, together with the average drift velocity across the cusp at the convection throat, $0.5 km s A1 , allowed to evaluate the cusp merging contribution to the total cross-polar cap potential dierence, $33.8±43.8 kV. It amounts to a quite signi®cant part of the total crosspolar cap potential dierence evaluated from other data. A"shutter" scenario is suggested for the ion beam injection/penetration through the stagnant plasma region in the outer cusp to explain the pulsating nature of the particle injections in the low-and medium-altitude cusp region.