Balloon observations of prebay X ray events and satellite observations of 40-keV electron precipitation preceding magnetic substorms are compared. The electron precipitation has a small latitudinal but large longitudinal extent and extremely sharp latitudinal gradients. Maximum intensities are near the detection threshold for X ray observations by balloons. This explains the relatively low intensity of the prebay X ray events. Smooth variations of the X ray intensity may be a geometrical effect due to changing relative positions of the observational site and the precipitation zone. It is suggested that the prebay electron precipitation is caused by strong pitch angle scattering of trapped electrons near the earthward moving inner edge of the plasma sheet.