Observation of anomalous slow‐mode shock and reconnection layer in the dayside magnetopause

Abstract

Plasma and magnetic field data from the ISEE 2 spacecraft recorded during an outbound crossing of the dayside, northern hemisphere magnetopause on October 29, 1979, provide evidence for a slow shock (SS) in the observed reconnection layer. This layer is found to be bounded on the magnetosheath side by the SS; near the magnetospheric side of the layer, a second current sheet is found that may have been a rotational discontinuity (RD). The direction of the accelerated plasma flow, the earthward sense of the normal magnetic field across the SS and RD, and the relative orientation of the SS and the RD all indicate that the reconnection site was located south of the spacecraft. Quantitative tests show that, allowing for experimental uncertainties, data taken upstream and downstream of the SS are consistent with coplanarity and other Rankine‐Hugoniot (RH) conditions. Examination of the flow parameters indicates two anomalous properties of the SS: the upstream flow, viewed in the deHoffmann‐Teller frame, is superalfvénic and the downstream plasma is firehose unstable. In comparison to the long‐wavelength slow‐mode phase speed, however, the flow in the upstream region is super slow, while in the downstream region it is subslow, as required for a slow‐mode shock. Further properties of the shock include a large decrease in total enthalpy across it, indicating the escape of a sizable heat flux from the shock structure, and the occurrence of a polarization reversal of the tangential magnetic field within the shock layer, a feature that is predicted by linear double‐polytropic Hall‐MHD and results from a large increase in pressure anisotropy from the upstream region, where p∥ ≅ p⊥, to the downstream region, where p∥ > p⊥. Quantitative tests of the RD‐like discontinuity show that it satisfies the necessary RH conditions within experimental uncertainties and indicate that the flow across it is earthward, that is, it is the same as for the SS. Thus the RD is propagating behind the SS in the direction away from the Earth. This reversal in the order of the discontinuities is attributed to the reversal of the slow and intermediate phase speeds produced by the pressure anisotropy in the region between the RD and the SS. The linear dispersion relation and polarization properties of dispersive MHD waves in an anisotropic plasma are examined in the appendix.