Department of Mechanical Engineering, Catholic University of America, Washington, D.C.

This paper reports the observations of a compound structure in space plasma composed of a slow shock layer and an adjoining rotational discontinuity layer on the postshock side. The compound structure looks like a new kind of MHD discontinuity; it may be called a double discontinuity. Two dual-spacecraft observations of double discontinuities using high-resolution magnetic field data were made in interplanetary space in 1995. The first one observed from Wind and IMP 8 is composed of a reversed slow shock and a rotational discontinuity. The second one observed from Wind and Geotail is composed of a forward slow shock and a rotational discontinuity. A double discontinuity is a stable, large-scale solar wind structure. The thickness of the slow shock layer is of a few ion inertial lengths. The flow in the preshock region has a low b_i value and a large shock angle.

J. Geophys. Res., 103, No. A4, 6513-6520, Apr. 1998