Observations of the Earth's polar cleft at large radial distances with the Hawkeye 1 Magnetometer

Abstract

Based on 364 spacecraft passes through the dayside region, the position of the polar cleft at large radial distances was determined with the magnetometer flown on Hawkeye 1. This data set represents one of the largest to investigate the high‐latitude region at large radial distances, making it ideal for the study of the cusp and cleft region. Identification of the cleft depended on noting strong negative deviations of the magnetic field strength in the region from that of the dipole field. In solar magnetic coordinates, cleft observations were found between 40° and 70° latitude and ±75° longitude, while in geocentric magnetospheric coordinates, these observations were found between 20° and 75° latitude and ±75° longitude. The extreme longitudinal extent of 150° is larger than those reported in some previous studies. Large magnetic depressions associated with the cleft extend out to 12 RE. Beyond this point, low model dipole field strengths make the determination of the cleft based on magnetic depressions unreliable. The cleft occurrences fall within an oval in magnetic latitude and longitude, but this oval is of a statistical nature and cannot be interpreted as the shape of the region at a given moment. As reported in other studies, the cleft was observed to shift to lower latitudes as compared to its quiet time geometry during periods when Kp was large and when the interplanetary magnetic field (IMF) pointed in a southerly direction. A southerly shift was also observed when the solar wind bulk flow speed, Vsw, was large (>450 km/s), and the region might have enlarged when solar wind pressure, Psw, was large. The variation of the cleft latitude with Vsw and Psw has not been thoroughly examined in previous studies.