Observations of the internal structure of the magnetopause

Abstract

Magnetic field, plasma flux, and ELF wave data have been studied for several encounters of Ogo 5 with the earth's magnetopause. In one case of a crossing in the near-earth region of the geomagnetic tail, the structure agreed closely with a simple Chapman-Ferraro type of model with nearly complete neutralization of the charge separation electric field. The proton turning region was 4 times thicker than the electron turning region. The magnetic pressure was linearly related to the ion flux throughout most of the structure; this relation implies that pressure balance was maintained principally by a variation of proton number density across much of the field strength gradient. The computed gas dynamic constant of 0.77 ± 0.23 is in agreement with the value of 0.83 predicted for such cases of aligned flow. This linear relation between B² and ion flux was not observed at all of the crossings studied. Other departures from the simple structure were observed at other magnetopause crossings: (1) Field strength bumps were sometimes observed just earthward of the ion flux gradients on nontail field lines at which the field underwent a large rotation at the magnetopause. These interior maximums may be a diamagnetic effect due to the loss of trapped particles within one gyrodiameter of the magnetopause. The interior bumps were accompanied by exterior dips or local minimums in field strength. (2) One crossing revealed a well-defined double structure with a large change in field direction closer to the earth than the ion flux and field strength gradients. (3) The thickness of the magnetopause often depended on whether the change in field strength, the change in field direction, or the change in ion flux was being considered. (4) Bursts of ELF waves were occasionally observed at the magnetopause. Although some of these emissions resembled the ‘lion roars’ often observed in the magnetosheath, other magnetopause emissions had spectra unlike the spectra of bursts observed in either the magnetosphere or the magnetosheath.