Magnetosheath-cusp interface

Abstract

Abstract. We advance the achievements of Interball-1 and other contemporary missions in exploration of the magnetosheath-cusp interface. Extensive discussion of published results is accompanied by presentation of new data from a case study and a comparison of those data within the broader context of three-year magnetopause (MP) crossings by Interball-1. Multi-spacecraft boundary layer studies reveal that in ∼80% of the cases the interaction of the magnetosheath (MSH) flow with the high latitude MP produces a layer containing strong nonlinear turbulence, called the turbulent boundary layer (TBL). The TBL contains wave trains with flows at approximately the Alfvén speed along field lines and "diamagnetic bubbles" with small magnetic fields inside. A comparison of the multi-point measurements obtained on 29 May 1996 with a global MHD model indicates that three types of populating processes should be operative:-large-scale (∼few R E ) anti-parallel merging at sites remote from the cusp; -medium-scale (few thousand km) local TBL-merging of fields that are anti-parallel on average;Correspondence to: S. Savin (ssavin@iki.rssi.ru) -small-scale (few hundred km) bursty reconnection of fluctuating magnetic fields, representing a continuous mechanism for MSH plasma inflow into the magnetosphere, which could dominate in quasi-steady cases.The lowest frequency (∼1-2 mHz) TBL fluctuations are traced throughout the magnetosheath from the post-bow shock region up to the inner magnetopause border. The resonance of these fluctuations with dayside flux tubes might provide an effective correlative link for the entire dayside region of the solar wind interaction with the magnetopause and cusp ionosphere. The TBL disturbances are characterized by kinked, double-sloped wave power spectra and, most probably, three-wave cascading. Both elliptical polarization and nearly Alfvénic phase velocities with characteristic dispersion indicate the kinetic Alfvénic nature of the TBL waves. The three-wave phase coupling could effectively support the self-organization of the TBL plasma by means of coherent resonant-like structures. The estimated characteristic scale of the "resonator" is of the order of the TBL dimension over the cusps. Inverse cascades of kinetic Alfvén waves are proposed for forming the larger scale "organizing" structures, which in turn synchronize all nonlinear cascades within the TBL in a self-consistent manner. This infers a qualitative differ-184 S. Savin et al.: Magnetosheath-cusp interface ence from the traditional approach, wherein the MSH/cusp interaction is regarded as a linear superposition of magnetospheric responses on the solar wind or MSH disturbances.

Cluster observations of high-frequency waves in the exterior cusp

“…It has recently been shown as more or less stagnant under northward IMF (Lavraud et al, 2002) while rather largely convective under southward IMF (Cargill et al, 2004). It often shows, however, disordered plasma flows and fields (Haerendel, 1978;Savin et al, 2004) and transient flows associated with recently reconnected flux tubes .…”

“…One region that stands out is the exterior cusp. This region has been the subject of various terminology (Haerendel, 1978;Savin et al, 2004;Lavraud et al, 2002). Here we define it as the low magnetic field region located at large radial distances.…”

Experimental study of nonlinear interaction of plasma flow with charged thin current sheets: 2. Hall dynamics, mass and momentum transfer

“…However, we should mention that bursty reconnection at the cusp MP (see e.g. Savin et al, 2004b) could also provide the leakage of the heated MSH-like plasma from the cusp. But in this case the question arises, why the outcoming population displays only low-energy cutoff without any detectable acceleration and heating?…”

“…as particular scenarios for the current sheet evolution (see e.g. Syrovatskii, 1971;Sibeck et al, 1999;Zelenyi et al, 2002;Silin and Buechner, 2003;Savin et al, 2004bSavin et al, , 2005a. For a simple model Mozer et al (2003) demonstrate that the Hall dynamics per se can explain local plasma structuring without local reconnection features (e.g.…”

Cluster observations of a structured magnetospheric cusp

“…The high altitude region of the cusp is usually termed outer cusp (e.g. Savin et al, 2004). Between the cusp and magnetosheath there is an intermediate region called the exterior cusp, which is bounded on the Earthward side by the cusp-like indentation of the magnetopause and externally by the free-flow stream line of the magnetosheath flow (Spreiter et al, 1967;Sckopke et al, 1976).…”