Explorer 12 observations of the magnetopause current layer

The variation in magnetic field magnitude at magnetopause is examined with the objective of separating boundaries of the rotational-discontinuity type from those of the tangential-discontinuity type. The field magnitude should remain constant throughout the current layer in the former case but may vary considerably in the latter. Also, the field component normal to the layer may be nonzero in the former case but not in the latter. It is estimated that approximately one-third of the 91 cases examined may have been of the rotational-discontinuity type. The field component along a theoretical vector normal to the current layer is compared for the two groups. The result indicates that the normal field component, on the average, may be less than 5 γ for the rotational-discontinuity group also. For the tangential discontinuities, a normal vector, calculated from the data under the assumption of a vanishing normal-field component, is compared with a corresponding vector obtained from the Mead-Beard theoretical magnetosphere model. Substantial individual, as well as systematic, deviations are found. The latter are related to the angle between the solar-wind and the earth's dipole axis. Finally, the sense of rotation of the tangential magnetic-field component at magnetopause is examined. For rotational discontinuities, the sense of rotation is found to be opposite above and below the solar-magnetospheric equatorial plane. A tentative explanation of this observation, in terms of the electrical structure of the current layer, is proposed.