The influence of the interplanetary magnetic field and thermal pressure on the position and shape of the magnetopause

Abstract

An ellipsoidal model is used to represent the location of the dayside magnetopause and to study the influences of the interplanetary magnetic field and thermal pressure on its location. The position and shape of the dayside magnetopause in an unmagnetized cold solar wind can be approximately represented by an ellipsoid of revolution with a constant eccentricity. The size of the ellipsoid, its semilatus rectum l0, is inversely proportional to the sixth root of the stream pressure of the solar wind. The effects of the IMF and thermal pressure on the location of magnetopause are calculated analytically by using the Chapman‐Ferraro theory. When the IMF B0 is included in the boundary calculation, the changes in the magnetopause size, shape, and orientation are inversely proportional to the square of the Alfven Mach number of the solar wind (MA²) and depend on the direction of B0. For B0 directed along an Archimedian spiral and MA=7.3, the symmetry axis of the ellipsoid will deflect 0.6° from the sun‐earth line to the east of the sun. The changes in the magnetopause size, shape, and orientation caused by including the thermal pressure are inversely proportional to the square of the sonic Mach number of the solar wind (MS²). These effects are sufficient to explain the observed slight departure of the magnetotail from the expected aberration due to the earth's orbital motion. There is no requirement, based on the tail observations to invoke a nonradial solar wind component. Hence our results suggest that little angular momentum is presently being carried away from the Sun by the solar wind.