During a geomagnetic substorm on April 12, 1979, the ISEE 1 and 2 spacecraft entered the plasma sheet, crossing the boundary layer several times. The spacecraft were located approximately 21 Earth radii from the Earth on the nightside. Using the time delay between the two satellites, one can determine the speed of the plasma sheet surface only by making assumptions about its orientations. By contrast, in this work, the plasma sheet boundary is investigated with the method of remote sensing using energetic proton data from the medium energy particle experiment (MEPE) to measure the speed and the orientation simultaneously. It is shown that, in this case, the plasma sheet boundary is oriented more than 45¿ to the GSM equator and has a speed normal to its surface of the order of tens of kilometers per second in the dawn direction. The ion flux gradient scale length at the boundary is estimated to be some hundred kilometers. The orientations of the subsequent exit and reentry are such as to indicate curvature of the plasma sheet surface on the order of several thousand kilometers, the whole surface structure moving in the dawn direction at speeds of about 70 km s-1. In addition, close analysis of the data suggest that the plasma sheet boundary may be locally aligned nearly perpendicular to the magnetic equator. These dawnward moving surface structures are possibly related to eastward moving auroral features observed on the ground. ¿ American Geophysical Union 1988