Magnetosheath for almost‐aligned solar wind magnetic field and flow vectors: Wind observations across the dawnside magnetosheath at X = −12 Re

Abstract

[1] While there are many approximations describing the flow of the solar wind past the magnetosphere in the magnetosheath, the case of perfectly aligned (parallel or antiparallel) interplanetary magnetic field (IMF) and solar wind flow vectors can be treated exactly in a magnetohydrodynamic (MHD) approach. In this work we examine a case of nearly-opposed (to within 15°) interplanetary field and flow vectors, which occurred on October 24-25, 2001 during passage of the last interplanetary coronal mass ejection in an ejecta merger. Interplanetary data are from the ACE spacecraft. Simultaneously Wind was crossing the near-Earth (X ∼ −13 Re) geomagnetic tail and subsequently made an approximately 5-hour-long magnetosheath crossing close to the ecliptic plane (Z = −0.7 Re). Geomagnetic activity was returning steadily to quiet, "ground" conditions. We first compare the predictions of the Spreiter and Rizzi theory with the Wind magnetosheath observations and find fair agreement, in particular as regards the proportionality of the magnetic field strength and the product of the plasma density and bulk speed. We then carry out a small-perturbation analysis of the Spreiter and Rizzi solution to account for the small IMF components perpendicular to the flow vector. The resulting expression is compared to the time series of the observations and satisfactory agreement is obtained. We also present and discuss observations in the dawnside boundary layer of pulsed, high-speed (v ∼ 600 km/s) flows exceeding the solar wind flow speeds. We examine various generating mechanisms and suggest that the most likely cause is a wave of frequency 3.2 mHz excited at the inner edge of the boundary layer by the Kelvin-Helmholtz instability.

Properties of Kelvin‐Helmholtz waves at the magnetopause under northward interplanetary magnetic field: Statistical study

“…But KHI is still likely to be generated under IMFs greatly deviating from the due north orientation. Farrugia et al [] reported Wind observation of the equatorial boundary layer at dawn possibly presented with the KHI at the inner edge of LLBL under almost aligned solar wind magnetic field and flow vectors. Most recently, Tang et al [] find the KHI can still be excited at both flanks of magnetopause under the radial IMF with a global MHD simulation.…”

Magnetopause inflation under radial IMF: Comparison of models

“…[] and Farrugia et al . [], the Petrinec‐Russell'96 model was used by Dušík et al . [], and the approximation of Fairfield and models by Sibeck et al'91 and Shue et al'97 were used by Farrugia et al .…”

“…The foreshock pressure variations cause transient magnetopause motion [Sibeck et al, 1989]. Various aspects of the transient magnetopause motion at the dayside and flanks under radial IMF were comprehensively studied in many papers [e.g., Labelle et al, 1990;Sibeck and Newell, 1995;Laakso et al, 1998;Sibeck et al, 2000;Merka et al, 2003;Shue et al, 2009;Farrugia et al, 2010;Suvorova et al, 2010;Korotova et al, 2011;Turner et al, 2011]. Generally, the amplitude of transient motion ranges from 0.5 to 2 R E that is within 20% of nominal subsolar magnetopause or 10% of the flank magnetopause.…”

Draping of strongly flow-aligned interplanetary magnetic field about the magnetopause

“…Shue et al [2009] investigated a case with multiple THEMIS spacecraft wherein the magnetopause was found to rebound, with fast sunward flows confined to a relatively small region of the magnetopause. Farrugia et al [2010] examined an interval when Wind was in the magnetosheath and ACE in the solar wind. The interplanetary flow was within 15° of the flow vector, and fair agreement was found with an analytic formulation of magnetic field components derived from perturbations to the Spreiter and Rizzi model (Spreiter and Rizzi [1974]).…”