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 f rom Interball/tail (IT) for the period 0030-0530 UT on January 11, 1997, fo cusing on waves at the near-tail (similar to-13 R-E), duskside, equatorial flank, a locale whose physical and wave properties have not been as well st udied as those on the dayside. Two major interplanetary features, monitored by Wind, are relevant to this work: The very high and variable dynamic pre ssure and the strongly northward and generally increasing magnetic field. I n this paper, we report, first, on magnetosonic waves in the magnetosheath of frequency similar to 0.15 Hz, probably generated by the mirror instabili ty, which are Doppler shifted with respect to similar waves on the dayside. Second, we discuss Kelvin-Helmholtz (KH) waves on the magnetopause, of wav elength similar to 13-14 R-E and frequency similar to 3.6 mHz, i.e., in the Pc 5 range. At IT, these waves appear as an envelope modulation of the mag netosonics and are recorded on ground stations at dusk. We argue that the l arge magnetic shear across the magnetopause and a magnetosheath flow aligne d almost normal to the field stabilized the magnetopause locally. Thus thes e waves were generated on the dayside and propagated to the flank. Third; w e examine a low-latitude boundary layer (LLBL), whose tailward stretched fi eld and average antisunward flow were perturbed quasi-periodically. This, t ogether with the particle behavior, suggests a complex billowy structure wh ere hot plasma sheet and cold magnetosheath populations wind around each ot her while drifting antisunward. A numerical calculation using IT parameters suggests that the inner edge of the LLBL was at this time KH unstable. Fou rth, over the 5-hour period the power of the KH oscillations drifts to lowe r frequencies which we attribute to the progressive decrease in clock angle . Fifth, transients induced by dynamic pressure pulses include a 7.5-min si ngle, free oscillation upon arrival of a fourfold pressure release. Sixth, the long-term effect on the magnetosphere of the increasing northward point ing 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 stron gly north. The solar wind dynamic pressure, while responsible for some, can not explain all of this wave activity.