Abstract
We study the Kelvin‐Helmholtz (KH) instability at the dayside magnetopause, modeling the flow, the magnetic field, and the density profiles in the transition from magnetosheath to magnetosphere with hyperbolic tangent functions. The strength and the direction of the fields on the sunward sides of the magnetopause are obtained from a MHD simulation code of the magnetosheath, which includes the magnetic tension forces on the plasma in the plasma depletion layer. The theory is applied to strongly northward interplanetary magnetic fields. We work at slightly off‐noon local times and compute at three different latitudes. We find that as the latitude increases the instability growth rate becomes negligible due to the increasing local magnetic shear, which reaches ∼21° at the highest latitude examined. The KH growth rates for the most unstable modes are given as functions of λ/Δ, the ratio of the wavelength to the width of the transition. The KH perturbation tends to be localized on the magnetospheric side when the configuration is most unstable, whereas it shifts increasingly toward the magnetosheath side of the velocity gradient region as the latitude increases and the growth rate diminishes substantially. Growth rates for a tangential discontinuity model are within a 10% of those corresponding to continuous profiles when λ > 15Δ. The influence of temporary (sunward or earthward) accelerations of the magnetopause on the KH modes is examined. The effect of a difference between the scale length of the density profile and the width of the current sheath on the KH instability, as in pristine magnetopauses, is also studied.
Highlights
2724 Magnetospheric Physics: Magnetopause, cusp, and boundary layers; 2752 Magnetospheric Physics: MHD waves and instabilities; 2753 Magnetospheric Physics: Numerical modeling; 7871 Space Plasma Physics: Waves and instabilities; KEYWORDS: Dayside magnetopause, magnetopause instabilities, KelvinHelmholtz instability, solar wind - magnetosphere interaction, Rayleigh-Taylor instability
[4] This paper presents a theoretical study of the KH instability at the dayside magnetopause for northward interplanetary magnetic field (IMF) conditions, where we compute the stability of models with continuous profiles for the magnetic field and the velocity field at different latitudes on the magnetopause, to examine the stabilizing effect of the local magnetic shear angle
From a statistical study of lowlatitude, multiple crossings of the magnetopause made by ISEE 1 and 2 over a 10-year period, Song et al [1988] concluded that for northward IMF the solar wind dynamic pressure may be sufficient to explain magnetopause surface oscillations with periods in the 2– 30 min range, and so KH would be at most a secondary effect
Summary
[2] Very early in the development of space physics, the Kelvin-Helmholtz (KH) instability at the magnetopause was suggested as a means for enhancing the transfer of solar wind momentum to the magnetosphere [Dungey, 1954]. [4] This paper presents a theoretical study of the KH instability at the dayside magnetopause for northward IMF conditions, where we compute the stability of models with continuous profiles for the magnetic field and the velocity field (represented by hyperbolic tangent functions) at different latitudes on the magnetopause, to examine the stabilizing effect of the local magnetic shear angle. The velocity and the magnetic fields change direction and intensity with position on the magnetopause, and we take into account the variations at different locales by using values for the field and flow given by results of a magnetosheath MHD simulation model [Erkaev, 1988; Farrugia et al, 1998b]. We normalize our variables using the same units, as follows
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