Coordinated Wind, Interball/tail, and ground observations of Kelvin-Helmholtz waves at the near-tail, equatorial magnetopause at dusk: January 11, 1997


We analyze ground magnetograms and magnetic field, ion, and electron data from Interball/tail (IT) for the period 0030–0530 UT on January 11, 1997, focusing on waves at the near-tail (∼−13 RE), duskside, equatorial flank, a locale whose physical and wave properties have not been as well studied as those on the dayside. Two major interplanetary features, monitored by Wind, are relevant to this work: The very high and variable dynamic pressure and the strongly northward and generally increasing magnetic field. In this paper, we report, first, on magnetosonic waves in the magnetosheath of frequency ∼0.15 Hz, probably generated by the mirror instability, which are Doppler shifted with respect to similar waves on the dayside. Second, we discuss Kelvin-Helmholtz (KH) waves on the magnetopause, of wavelength ∼13–14 RE and frequency ∼3.6 mHz, i.e., in the Pc 5 range. At IT, these waves appear as an envelope modulation of the magnetosonics and are recorded on ground stations at dusk. We argue that the large magnetic shear across the magnetopause and a magnetosheath flow aligned almost normal to the field stabilized the magnetopause locally. Thus these waves were generated on the dayside and propagated to the flank. Third, we examine a low-latitude boundary layer (LLBL), whose tailward stretched field and average antisunward flow were perturbed quasi-periodically. This, together with the particle behavior, suggests a complex billowy structure where hot plasma sheet and cold magnetosheath populations wind around each other while drifting antisunward. A numerical calculation using IT parameters suggests that the inner edge of the LLBL was at this time KH unstable. Fourth, over the 5-hour period the power of the KH oscillations drifts to lower frequencies which we attribute to the progressive decrease in clock angle. Fifth, transients induced by dynamic pressure pulses include a 7.5-min single, free oscillation upon arrival of a fourfold pressure release. Sixth, the long-term effect on the magnetosphere of the increasing northward pointing magnetic field and the stepwise decreasing dynamic pressure is to make the shape of the cavity progressively less blunt. A conclusion of this work is that the equatorial magnetopause can be very oscillatory with various, distinct periodicities even when the interplanetary magnetic field is strongly north. The solar wind dynamic pressure, while responsible for some, cannot explain all of this wave activity.

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JGR: Space Physics



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