Motion and structure of the magnetopause

Data obtained during a 2-hour sequence of multiplecrossings of the magnetopause in the equatorial plane near 0900 LT with the Ogo 5 UCLA triaxial fluxgate magnetometer and electron spectrometer show that the magnetopause motion was composed of two different oscillations: large-amplitude oscillations with periods from 3.5 to 6 min, and smaller amplitude oscillations with periods as short as 10 sec. The amplitude of the short-period oscillation increased abruptly when the magnetosheath field turned 90 ø southward, producing an extremely variable boundary. The particle boundary showed the same oscillations as the magnetic field boundary, but the two were not coincident and their relative position was quite variable. The direction of the normal to the magnetopause during successive crossings shows that these oscillations do not represent pulsation of the whole boundary but are ripples moving tailward with a velocity of the same order as the plasma flow velocity. The observed structure of the boundary was not consistent with a rotational discontinuity. Since the component of the magnetic field normal to the boundary was often nonzero, however, the structure was not consistent with a steady state tangential discontinuity either. The observation of the motion and structure of the magnetopause has been the topic of many papers in recent years [Holzer et al., 1966; Heppner et al., 1967; Sonnerup and Cahill,