OBSERVED CURRENTS ON THE EARTHS HIGH-LATITUDE MAGNETOPAUSE

This paper reports a survey of electrical currents on the Earth's magnetopause, principally at high latitudes, as inferred from magnetic vector data acquired by the University of Iowa/Langley Research Center/NASA satellite Hawkeye 1. The very eccentric orbit of this satellite had an inclination to the equator of approximately 90-degrees, an initial apogee at a radial distance of about 21 R(E) (Earth radii) over the north pole, a relatively low altitude perigee, an essentially constant semi-major axis of 11.0 R(E), and a corresponding period of 51.3 hours. A total of 536 candidate crossings of the magnetopause were examined. Many of these were in bundles of multiple crossings and were considered essentially redundant. Others were poorly defined. A reduced data set of 139 selected cases was analyzed in detail though solar wind dynamic pressure data were available for only 117 of these cases. Some 17 crossings of the cusp/cleft region were included. Inferred values of the lineal current densities J on the magnetopause are in the range 5.5 to 157.5 mA m-1 over a wide range of solar wind dynamic pressure P from 1.17 to 16.1 nPa. On the high-latitude magnetopause near the noon-midnight meridian plane, a suggested dependence of absolute value of J on P is absolute value of J2 = 95 P2 + 280 P among widely scattered data. The sense of the current system at high latitudes is counterclockwise as viewed from the Sun, in qualitative agreement with the Chapman-Ferraro theory. The apparent normal thickness of the magnetopause current sheet ranges from 30 to 850 km with mean and median values of 185 and 158 km, respectively. It is argued provisionally that the radial rate of motion of the magnetopause is of the order of 2 km s-1 and hence that its true thickness is of similar magnitude, being of the order of one gyroradius for a 500-eV proton moving perpendicular to the mean magnetic field in the current sheet or several gyroradii if moving more nearly parallel to the field. The relationship of our results to models of the geomagnetic field and to other related work is discussed briefly.