[1] We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at $1500 magnetic local time (MLT) by the Geotail spacecraft and at $1900 MLT by the Cluster spacecraft, which detected rolled-up MHDscale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 Â 10 4 km) is considerably longer than predicted by the linear theory from the thickness ($1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length $100 km and of magnetic shear $60°existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvénic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.

[1] The 'whistler critical Mach number', M crit w , is one of the dimensionless parameters that characterizes collisionless shocks. Originally, it was introduced to indicate the critical point above which whistler waves do not propagate upstream. Indeed our analysis of Geotail data at the Earth's bow shock shows intense whistler waves in the sub-critical regime, M A < M crit w , but not in the super-critical regime. In this paper, we further report that M crit w seems to regulate the electron acceleration efficiency at the shocks. At the shock transition layer it is found that the spectral index G of electron energy spectra defined by f(E) / E ÀG is distributed between 3.5 and 5.0 in the sub-critical regime, while the hardest energy spectra with G = 3-3.5 are detected in the super-critical regime. We discuss a possible relationship between M crit w and the electron acceleration.

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