Freja's contribution to the ISTP event of October 27, 1992. A distorted magnetosphere

Abstract

A unique arrangement of satellites and ground‐based magnetometers has provided the opportunity to study the distortion of the magnetosphere by conditions of elevated solar wind dynamic pressure and high geomagnetic activity on October 27, 1992. Data were acquired by the near‐Earth satellites Freja, UARS, DMSP‐F8 and DMSP‐F11, and by IMP‐8. This study will focus on patterns of large‐scale Birkeland currents deduced from the APL magnetic field experiments on Freja and UARS. Because of the relatively low inclinations of their orbits (63° for Freja and 57° for UARS) these satellites skim the current systems over a wide range of local time and provide a kind of “global image” of the Birkeland currents. Energetic particle data from the DMSP satellites are used in an attempt to complete the image of the auroral region. The principal findings of this study include the following. The dayside Birkeland current system is displaced 6° to 9° equatorward of the statistical pattern determined by Iijima and Potemra [1978] for undisturbed conditions (|AL|< 100 nT) and 3° equatorward of their disturbed statistical pattern (|AL|≥ 100 nT). The densities of these daytime Birkeland currents are estimated to be 1 to 2 µA/m², comparable to, but not larger than the statistical values determined by Iijima and Potemra for disturbed conditions. The nightside Birkeland currents are located close to the Iijima and Potemra disturbed statistical pattern and they are less intense than the dayside currents.