Supersonic mixing layers: Stability of magnetospheric flanks models

Abstract

Abstract. Compressibility has a strong influence on the stability of velocity shear layers when the difference of velocity ∆V across the flow becomes supersonic. The flanks of the Earth's magnetopause are normally supersonic M s > 1, and super-Alfvénic M A > 1, depending on the distance from the dayside terminator (M s and M A are the sonic and Alfvén Mach numbers of the magnetosheath plasma, respectively). The stability of MHD supersonic flows depends, also on several other features, such as the finite thickness ∆ of the boundary layer, the relative orientation of velocity and magnetic fields, the density jump across the boundary and the magnetic shear angle. We analyze the MHD stability of some representative flank sites modeled after data from spacecraft crossings of the magnetopause under different interplanetary conditions, complementing these cases with extrapolations of likely conditions upstream, and downstream of the crossing site. Under northward interplanetary magnetic field conditions, there are solar wind regimes such that the near, but already supersonic, flank of the magnetopause may be locally stable. Stability is possible, e.g., when M s becomes larger than ~1.2-1.4 while M A remains smaller than 1.2, and there is magnetic shear between the geomagnetic and the interplanetary magnetic field. Solar winds favouring local stability of the boundary layer are cold, not-too-dense plasmas, with strong magnetic fields, so that M A is smaller, while M s is larger, than normal values of the magnetosheath flow. A gap between dayside and tail amplifying regions of Kelvin-Helmholtz disturbances over the magnetopause may exist when the above conditions are realized.

The Magnetosphere Mixing Layer: Observations, MHD Stability, and Large Eddy Simulations

“…The research is a follow-up of [1,26,27,29] that produced original results studying the consequences of increasing the flow speed of the basic configuration .We present new computational plots, and also an analysis of data correlations. The following is a summary of the facts and results obtained, in which we examined the influence of increasing M and M A in all the points from (a) to (h).…”

“…How does the broadening of the flank BL produced by the KH instability depend on (i) the local orientation of B and V; (ii) the physical conditions at the boundary; and (iii) its distance from the subsolar point? Some of these questions have been addressed by members of our team, and by coworkers in recent studies [1,23,24,26,27,28,29,30].…”

“…The strength and orientation of the magnetosheath's magnetic field, both magnetic and velocity shears across the MP, the flow speed (supersonic, superAlfvénic? ), the density gradients across the MP, the scale lengths of the magnetic and velocity transitions (which may be different (e.g., [25,29]), and the influence of supersonic flows where compressibility, a stabilizing agent, comes into play. When is a flank region KH-stable under supersonic flows?…”

“…The phenomenology of this event and the associated theoretical and computational models are given in detail in [1,26,27,29]. Only a minimal description is given here to allow the reader to proceed, skipping a search of references if he/she so prefers.…”

“…Due to the minimal magnetic shear, the MHD theory of the Kelvin-Helmholtz instability predicts significant growth rates for flute modes that have a most favorable angle between the k -vector and the direction of the bulk flow at φ=32° [26,29]. Modes with deviations of k by 10 -15 degrees from this orientation do not grow.…”