Abstract
Abstract. A double discontinuity is a compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer on the postshock side. We use high-resolution data from Geotail and Wind spacecraft to examine the interior structure within the finite thickness of the discontinuity at the plasma sheet-lobe boundary and found that recognizable MHD structures at the boundary can be stand-alone slow shocks or double discontinuities. The plasma density increases significantly and the magnetic field intensity decreases significantly across the interior of the slow shock layer. Through the rotational layer, the magnetic field rotates about the normal direction of the shock surface, as the plasma density and the magnetic field intensity remain nearly unchanged. The rotational angle can vary over a wide range. We notice that the observations of double discontinuities are no less frequent than the observations of stand-alone slow shocks. Identification of slow shocks and double discontinuities infers that plasma and magnetic field lines continuously move across the boundary surface from the lobe into the plasma sheet, and there is a conversion of magnetic field energy into plasma thermal energy through the slow shock layer. The double discontinuities also allows for a rapid rotation of the postshock magnetic field lines immediately behind the shock layer to accommodate the environment of the MHD flow in the plasma sheet region.Key words. Magnetospheric physics (plasma sheet) Space plasma physics (discontinuities; shock waves)
Highlights
Based on classical theory, there are four kinds of magnetohydrodynamics (MHD) discontinuities: MHD fast or slow shock, rotational discontinuity, tangential discontinuity, and contact discontinuity
This research is concerned with the observations of a compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer on the postshock side
We found that some of them are accompanied by an adjoining rotational discontinuity layer on the postshock side and others are stand-alone slow shocks not accompanied by rotational layers; the discontinuity at the plasma sheet-lobe boundary can be a slow shock or a double discontinuity
Summary
There are four kinds of magnetohydrodynamics (MHD) discontinuities: MHD fast or slow shock, rotational discontinuity, tangential discontinuity, and contact discontinuity. There is a mass flow across the MHD shock or rotational discontinuity, but there is no mass flow across the tangential discontinuity or contact discontinuity These discontinuities are well understood in theory and their existence in space plasma has been verified using observational data from spacecraft. The Geotail spacecraft crossed the double discontinuity 51 min later than the crossing time of the Wind spacecraft, yet the magnetic field structures observed from the two spacecraft at two locations have nearly identical signatures During this time interval, the discontinuity surface has traveled over a distance of > 106 km, and the two crossing points on the surface of the observed double discontinuity are at a distance of > 0.4 × 106 km apart. This paper is the third journal article dealing with the observations of double discontinuities
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