The Control of the Magnetopause by the Interplanetary Magnetic Field

Abstract

The solar wind dynamic pressure determines the “zeroth-order” location of the earth’s magnetopause. However, the normal stresses of the solar wind dynamic pressure are also accompanied by tangential stresses which erode the magnetopause from its equilibrium position and transport magnetic flux into the magnetotail. It is clear that the tangential stress on the magnetopause is at least in part controlled by the southward component of the interplanetary magnetic field. When the interplanetary magnetic field turns from northward to southward, the magnetopause moves in toward the earth, the polar cusp moves equator-ward, and the polar cap increases in size, as does the diameter of the magnetotail. Since particle observations show that the polar cap magnetic field is directly connected to the interplanetary magnetic field, this observation of magnetopause erosion is an unambiguous demonstration that the process of reconnection is occurring. However, it does not elucidate the physical mechanisms by which such reconnection occurs. The study of the physical processes at the magnetopause and their control by the IMF is actively being investigated on the ISEE mission. Initial results indicate that when the magnetosheath magnetic field is southward the connection takes place in a series of flux transfer events capable of transporting 10¹⁶ Mx or more per hour.