Pulsations in magnetic field and ion flux observed at L = 4.5 on August 5, 1972

Abstract

After the unusually strong compression of the earth's magnetosphere associated with the August 1972 geomagnetic storm, large amplitude ULF pulsations were observed for several hours in space by Explorer 45 magnetic field and particle instruments near L = 4.5 and at magnetic observatories on the ground over a large range of latitudes. Spectral analysis of Explorer 45 magnetometer data suggests that a compressional mode oscillation coupled to a transverse mode oscillation. Enhancements in amplitude of a 300‐s period wave near 0040 UT August 5 coincide with an intensification of 100‐ to 1000‐Hz magnetic and electric field oscillations and with the appearance of enhancements of fluxes of energetic ions. During this period the ion pitch angle distribution in each available energy channel (24–300 keV) followed a periodic sequence, apparently synchronized with the magnetic pulsations, from normal trapping (highest fluxes near 90° and lowest near 0° and 180°) to a nearly isotropic particle distribution. During the transitions the particle flux near 90° pitch angle was alternately larger earthward of the satellite (before isotropy) and larger radially outward from the satellite (after isotropy). Intense field‐aligned fluxes of lower energy ions (E ≤ 5 keV) were observed periodically throughout the interval. Possible configurations of the magnetosphere consistent with the wave and particle observations are discussed, the most likely candidate being the presence of a wavelike boundary near the satellite. It is possible that the satellite sensed the low‐latitude boundary layer at L = 4.5 during this period of extreme magnetospheric compression.