Motion and structure of the magnetopause

Abstract

Data obtained during a 2-hour sequence of multiple'crossings 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. We will conclude that our observations do not 1673

Waves in the Vicinity of the Magnetopause

“…Tail-:nagnetosheath crossings have significantly lower angles (median 81.6 0) than magnetosheath-tail crossings (median 87.4 0). This result is similar to that seen by Aubry et al (1971) and Ledley (1971) near the subsolar magnetopause and can be interpreted, Gs these authors did, in terms of surface waves propagating in a tailward direction (see also Kaufmann and Konradi, 1969) . This interpretation is illustrated schematically by the sketch in Figure 7, where normal vectors have been drawn at the median angles for coossings in the two directions.…”

“…Although careful searches have revealed a few examples of such normal components (Sonnerup, 1971;Sonnerup and Ledley, 1974), c is clear (see also Aubry 1971;Ledley, 1971) that any identifiable normal component is small or larking most of the time.…”

“…Fairfield, 1971) and there is a growing body of evidence in support of the Kelvin-Helmholtz instability (e.g. Dungey and Southwood, 1970;Ledley, 1971;Aubry et al 1971;Wolfe and Kaufmann, 1975 (Aubry et al 1971;Ogilvie et al 1971;Neugebauer et al, 1974;Sonnerup and Ledley, 1974) and the presence of waves near the proton gyrofrequency has been only briefly noted.…”

“…The use of Method 2 can be justified a priori when a tangential discontinuity is found, whereas the component along 6 "n " determined from a rotational discontinuity should display a characteristic variation across the boundary. Whether the average component across a discontinuity of 30 seconds width is meaningful or whether one should talk about variations on a time scale of seconds (Aubry et al , 1971) is not completely clear. It should be recalled that a time scale of seconds is approaching the proton gyrofrequency which is the limit of the hydromagnetic discontinuity theory.…”

Structure of the magnetopause: Observations and implications for reconnection

“…A wave propagating sunward would reverse the vector tilts for 4 the same sense of crossing. Aubry et al (1971) II the 14.1.11 i\'4. \'t • lt t k'it y hiltw4.,'n the boundary and the , spacecraft Is known, the thickness is easily determined as the product of velocity and the observed crossing time.…”