Ion Pressure at the Auroral Precipitation Boundaries and Its Relationship with the Solar Wind Dynamic Pressure

Abstract

The data from the DMSP F7 satellite for 1986 are used to study the behavior of ion pressure at the boundaries of auroral precipitation. The study is based on 7489 satellite passages in the nightside sector of the auroral zone, including over 5000 passages in the 2100–2400 MLT sector. Ion pressure is determined as the average for 5 s of observations, which corresponds to a distance of ~40 km in the satellite trajectory portions adjacent to the precipitation boundaries. It is shown that the plasma pressure at the boundaries of auroral precipitation almost linearly increases at all levels of magnetic activity with an increase in the solar wind dynamic pressure (Psw). The pressure distribution as a function of MLT indicates that the precipitation boundaries, including the isotropy boundary, are not isobars, even at a low level of geomagnetic activity. The plasma pressure is maximal in the 22–24 MLT sector and decreases both in the morning and evening sides. The latitudinal position of the precipitation boundaries and the plasma pressure at the boundaries are found during all the phases of a typical substorm with an intensity of AL = –410 nT at its maximum. The latitudinal profile of ion pressure is constructed with respect to the isotropy boundary (IB) at the beginning of substorm. It is shown that, with an increase in dynamic pressure, there is not only a substantial increase in plasma pressure at the auroral precipitation boundaries but also a change in the latitudinal position of the boundaries themselves. As Psw grows, the latitude of the polarward boundary of the oval increases, while that of the equatorward boundary decreases. Despite the considerable expansion of the precipitation region, the latitudinal pressure gradient between the oval boundaries increases, even during quiet periods (average AL = –18 nT, IMF Bz = +1.4 nT) without any disturbances in the auroral zone, by approximately a factor of 2, from 0.06 to 0.12 nPa/deg.