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

Abstract

Abstract. 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.

Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

“…[2] Waves on flanks of the magnetosphere have been reported by many observers [e.g., Ogilvie and Fitzenreiter, 1989;Kivelson and Chen, 1995;Takahashi et al, 1991;Kokubun et al, 1994;Seon et al, 1995;Fairfield et al, 2000Fairfield et al, , 2003Farrugia et al, 2000;Owen et al, 2004;Hasegawa et al, 2004]. These waves have frequently been attributed to the Kelvin-Helmholtz (KH) instability caused by shear between the high-velocity magnetosheath plasma and the nearly stagnant magnetosphere plasma.…”

Magnetosheath waves under very low solar wind dynamic pressure: Wind/Geotail observations

“…In particular, compressional power in the main body of the magnetosheath is completely absent, the very reverse of what is typically seen at the dayside (Anderson et al, 1991(Anderson et al, , 1993 and on the flanks (Lucek et al, 1999, Farrugia et al, 2000b. Normal magnetosheath wave activity is present in the last 9-10 h of the pass.…”

Kelvin-Helmholtz Multi-Spacecraft Studies at the Earth’s Magnetopause Boundaries

“…However, the most favorable region in space for its multipoint in-situ observation and analysis is by far the outer boundary of the Earth's magnetosphere -the magnetopause -and its adjacent boundary layers [e.g. 2,3,4].…”

“…In addition, KH surface waves in the magnetospheric context can become non-linear while propagating down the tail. Non-linear effects have been invoked to account for (i) wavelengths of a few R E typically ob-served on the magnetopause, which, as argued in [19], are much longer than those predicted by linear theory; (ii) the tailward steepening of the KH leading fronts observed with Cluster [12,4], consistent with the growing phase of KH waves [20] (note that the inverse dependence found between the boundary layer thickness and the tailward steepening of the leading edge [4] suggest that this effect is affected by changing conditions of the medium in which the waves propagate); (iii) the presence of vortices, a phenomenon supported by the interpretation of data in single or multi-spacecraft analyses [18,21,22,23]; (iv) an inverse dependence between the IMF clock angle and the wavelength at the flank [4] or the geomagnetic pulsation period [3], which confirms the significance of source regions and non-linear development for interpreting observations of remotely generated KH waves.…”