The plasma and magnetic field characteristics of a double discontinuity in interplanetary space

A double discontinuity is a rarely observed compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer in the downstream region. In this paper, we report the observations of a double discontinuity detected by Wind on May 15, 1997. This double discontinuity is found to be the front boundary of a magnetic cloud boundary layer. We strictly identify the shock layer and the rotational discontinuity layer by using the high-resolution plasma and magnetic field data from Wind. The observed jump conditions of the upstream and downstream region of the slow shock layer are in good agreement with the Rankine-Hugoniot relations. The flow speeds in the shock frame U (n) < V-A cos theta(Bn) on both sides of the slow shock layer. In the upstream region, the slow Mach number M (s1)=U (n1)/V (s1) is 1.95 (above unity), and in the downstream region, the slow Mach number M (s2)=U (n2)/V-s2 is 0.31 (below unity). Here V-A and V-s represent the Alfven speed and the local slow magnetosonic speed, respectively, and theta (Bn) is the angle between the direction of the magnetic field and the shock normal. The magnetic cloud boundary layer observed by Wind was also detected by Geotail 48 min later when the spacecraft was located outside the bow shock of the magnetosphere. However, Geotail observations showed that its front boundary was no longer a double discontinuity and the rotational discontinuity layer disappeared, indicating that this double discontinuity was unstable when propagating from Wind to Geotail.