Accelerated motions of the magnetopause as a trigger of the Kelvin‐Helmholtz instability

Abstract

We consider the effect of accelerated magnetopause motions arising from the arrival of a modified-pressure region of the solar wind with a sharp forefront upon the growth rate of the velocity shear layer instability. The range of values of relative pressure variation on the front was taken to lie within 1.25-2, and the front thickness ranged from 200 km to 1RE (6400 km). At different phases of motion of the boundary, the instability growth rate can change in either direction as a consequence of the Rayleigh-Taylor effect, in accordance with a change of sign of acceleration g of the boundary. The typical time during which of the boundary remains inside the new equilibrium position (_• I min) is sufficient for the instability to reach the nonlinear regime. It is possible that plasma flutes on the boundary penetrate the magnetosphere. Generation of disturbances can be modulated by boundary oscillations inside the new equilibrium position. We discuss the interplanetary magnetic field influence upon the growth rate of the MHD instability under consideration. An analysis is made of a number of daytime geophysical conditions which cannot be understood when interpreted in terms of the instability on a stationary boundary. the prime denotes differentiation with respect to the normal to the boundary. If the first factor is absent and the second is present, then the arising instability is said to be the Rayleigh-Taylor (R-T) or flute instability. A relevant classical example is the instability of a heavy fluid above a heated, hghter-weight fluid in the gravity field. In the general case of the presence of both factors the influence of the second mechanism upon the value of the K-H growth rate will be henceforth referred to as the R-T effect. In this case, by the positive and negative R-T effects, we mean an enhancement and an attenuation of the K-H growth rate, respectively. In the vicinity of the subsolar magnetopause the density Copyright 1993 by the American Geophysical Union. Paper number 93JA00417. 0148--0227/93/93 JA-00417505.00 changes abruptly, i.e., plI/pI > 4 (where indices I and II refer to regions inside and outside the boundary), and the flow velocity is minimal [Paschmann et al., 1978]. Since the radius of curvature of the geomagnetic field here is also minimal, R = l0 s km [Guterich and Shcherbakov, 1971], the R-T effect here must manifest itself most conspicuously. Its negative character is attributable to the convexity of the boundary and is associated with two factors' the trapped particles travelling in the magnetosphere, and the flow round

Charts of joint Kelvin‐Helmholtz and Rayleigh‐Taylor instabilites at the dayside magnetopause for strongly northward interplanetary magnetic field

“…While the occasions when these conditions are met are perhaps infrequent, KH growth rates are modified by any accelerated motion of the magnetopause. Specifically, they are increased during sunward accelerations, and reduced during earthward accelerations, as first pointed out by Mishin [1993]. (We note that while these are considerations within the fluid picture, they are also valid in a particle description, as the compatibility of the two pictures has been established by many past studies.)…”

“…In the particle and field environment of the dayside magnetopause, this brings on the possibility of a second instability: Rayleigh-Taylor. Since all accelerated motions of the magnetopause affect KH growth rates [Mishin, 1993], this effect should clearly be considered.…”

“…Another possible transfer mechanism is RayleighTaylor (RT) instability [Mishin, 1993;Gratton et al, 1996]. At the magnetopause the dense magnetosheath plasma interfaces with the muah more tenuous magne- [1996] showed that under well-specified conditions it can go RT-unstable.…”

Spatio-temporal structure of Alfvén waves excited by a sudden impulse localized on an L-shell

“…by Kelvin-Helmholtz or Kruskal-Schwarzschild instability (e.g. Mishin, 1993;Fujita et al, 1996;Plaschke and Glassmeier, 2011;Mazur and Chuiko, 2011).…”

Richtmyer‐Meshkov Instability and the Dynamics of the Magnetosphere

“…Thus, one can expect the RM instability to occur on the magnetosphere interfaces. Indeed, the possibility of RT instability at the magnetopause has recently been considered [Mishin, 1993;Gratton et al, 1996]. RM instability may be related to pressure pulse events [Sibeck, 1990] and the impulsive penetration model [Lemaire et al, 1979;Huba, 1996] a rotational discontinuity with fast and slow shocks is also unstable and will be presented elsewhere.…”