Electric field topology near the dayside magnetopause

A direct test for the occurrence of the reconnection electric field ER tangential to the dayside magnetopause is possible by the use of in situ spacecraft observations of magnetic field and plasma parameters. By means of a simple local analysis it is shown that particle energization is given by ϵ = BMS|BMS - BM|/μ0NMS, where the subscripts denote magnetic field strength and plasma density measured in the magnetosheath and the magnetosphere sides. One particular pass of Heos 2 on June 18, 1973, is discussed, although other published passes lead to similar conclusions, without exception. The expected energization is about 1 keV/particle, but none is observed, nor is there any evidence of an enhanced outflow as predicted by reconnection theories. An alternate appraisal, from a global viewpoint, shows that the effect of a reconnection electric field should be one of major proportions and could not be missed. The apparent conclusion is that no reconnection electric field tangential to the dayside magnetopause exists; thus the magnetopause must be very nearly an equipotential surface. The significance of this conclusion, in particular the role of the boundary layer inside the magnetopause, is discussed. This boundary layer must be polarized; it is thus in some respects a dipole sheet, which can transfer the external magnetosheath electric field into the magnetosphere. This may provide an explanation for the dependence of geomagnetic activity on the direction of the interplanetary electric field as an alternative to Dungey's reconnection model. The openness of the polar cap magnetic field lines is not necessarily in question, provided that an electric field component parallel to magnetic field lines is possible.