The low-energy H+ data with 6-s resolution from the retarding ion mass spectrometer instrument on DE 1 have been used to reveal an ion boundary structure at midaltitudes that is associated with the nightside convection reversal. The convection reversal provides a free energy source for the perpendicular ion heating that generates ion conics. A detailed analysis of a Y shape distribution observed in the spin-spectrogram (Plates 1--4) shows that the altitude of the ion heating decreases monotonically as latitude decreases along the satellite orbit. This suggests that the convection boundary makes an angle of approximately 3¿ to 10¿ with the magnetic field line. Near the convection reversal, the nonlinear transverse heating leads to a local charge separation, which forms a double layer along the boundary. The observations of dc electric field are fitted by a model with a Gaussian charge distribution. ¿ American Geophysical Union 1994