Solar Wind Control of the Size and Shape of the Magnetosphere

The position of the earth's magnetopause is controlled by the kinetic pressure of the solar wind and further influenced by the north-south component of the interplanetary magnetic field. On average the distance to the subsolar magnetopause is -10RE, but this distance varies with the solar wind pressure during the 11 year sunspot cycle. Maximum solar wind pressure occurs during the declining phase of the cycle and compresses the average subsolar magnetopause inside 10RE while minimum pressure occurs during solar maximum, allowing expansion of the magnetopause outward toward 11RE. The northsouth component of the IMF moves the subsolar magnetopause inward or outward approximately. 25RE for each 1nT unit of negative or positive field strength, with the magnetotail expanding or contracting to contain the flux transported into the tail. Other observations suggest that the distant tail (1) may be flattened into an elliptical cross section with major axis aligned with the YZ component of the IMF and (2) is probably twisted by the torque exerted by interplanetary field lines connected to the tail lobes. Recent evidence suggests that reorientations of the upstream foreshock may produce density enhancements of a few minutes duration that impinge on the magnetopause and compress the magnetosphere.