Fluid signatures of rotational discontinuities at the Earth's magnetopause

Abstract

Fluid signatures in the MHD approximation at rotational discontinuities (RD) of finite width called rotational shear layers (RSL) are examined for general flow and magnetic geometries. Analytical and geometrical arguments illustrate that the fluid speed can either go up or down across an RSL for a fixed normal mass flux. The speed profile may or may not be monotonic with the phase of the magnetic rotation depending on the boundary conditions. The flow velocity may or may not be field aligned or “jetting” as a result of traversing the RSL. In general, significant “convection” is expected in the layer. The observable signatures of (MHD) RSL's depend on seven (boundary condition) parameters: (1) the mass density, (2)–(5) the incident normal and transverse components of the magnetic field and fluid velocity, (6) the angle ε between the incident tangential flow velocity and tangential magnetic field, and (7) the size of the magnetic angular rotation implemented by the layer Δϕ. This general survey illustrates the singular character of the Petschek geometry and that the model predictions of jetting and monotonic speed increases through the layer may not be used as general signatures of RSL's. This is especially true along the Levy, Petschek, and Siscoe curved magnetopause boundary, where the fluid flow, Δϕ, and ε are generally all different from the Petschek regime. Accordingly, the lack of observed speed increases across the magnetopause can no longer be used by itself to infer that the magnetopause is locally closed. Of the spectrum of MHD RSL's that are possible, the “accelerating” ones require special boundary conditions most generally occurring near the nose of the magnetopause; the “decelerating” RSL's become increasingly more probable at magnetopause crossings removed from the subsolar point. Documented RSL's have been located within the spectrum of possible signatures; the number of RSL's reported to date implies only a lower bound for their frequency of occurrence, since surveys conducted to date have excluded the decelerating ones.

Temporal-spatial structure of magnetic merging at the magnetopause inferred from 557.7-nm all-sky images

“…Flow enhancements may be associated with each region when it activates, as observed by Maynard et al (2003b) in the SuperDARN and Cluster data, but the net flow is spread out in the large convection cell. Merging is an intermittent process consistent with expectations for regions where the tangential component of magnetosheath plasma velocity exceeds the Alfvén speed (Rodger et al, 2000;Scudder, 1984). These conclusions apply to high-latitude merging in the local hemisphere.…”

“…Just as ions may be decelerated rather than accelerated by merging in the upstream exhaust (Scudder, 1984;Aggson et al, 1984), we may expect electrons that propagate away from the separator to the near (far) ionosphere to be at higher (lower) energies. During this whole sequence, no 557.7-nm emissions were seen in the prenoon segment of the cusp, a region which we have suggested is tied to a Southern Hemisphere source.…”

Observations of simultaneous effects of merging in both hemispheres

“…It extends back toward the flank on the dusk (dawn) side of the cusp for IMF By positive (negative). Topologically, the sash is similar in location to the separator line determined by Luhmann et al [1984], using the antiparallel merging model [Crooker, 1979] From theoretical considerations Scudder [1984] showed that deceleration at a magnetopause crossing occurs for many conditions when the separator is off the equator. ,,,, .,,.…”

Polar, Cluster and SuperDARN evidence for high-latitude merging during southward IMF: temporal/spatial evolution

“…Moreover, they emanated from a merging site poleward of the satellite. Scudder (1984) calculated that, as the merging site moved off the equator, the exhaust velocity from the X-line should equal the sum from the merging acceleration and the local magnetosheath velocity at the merging site. Hence, the outflow velocity in inertial space may be significantly less than the Alfvén speed.…”