Magnetosheath waves under very low solar wind dynamic pressure: Wind/Geotail observations

Abstract

Abstract. The expanded bow shock on and around "the day the solar wind almost disappeared" (11 May 1999) allowed the Geotail spacecraft to make a practically uninterrupted 54-h-long magnetosheath pass near dusk (16:30-21:11 magnetic local time) at a radial distance of 24 to 30 R E (Earth radii). During most of this period, interplanetary parameters varied gradually and in such a way as to give rise to two extreme magnetosheath structures, one dominated by magnetohydrodynamic (MHD) effects and the other by gas dynamic effects. We focus attention on unusual features of electromagnetic ion wave activity in the former magnetosheath state, and compare these features with those in the latter. Magnetic fluctuations in the gas dynamic magnetosheath were dominated by compressional mirror mode waves, and leftand right-hand polarized electromagnetic ion cyclotron (EIC) waves transverse to the background field. In contrast, the MHD magnetosheath, lasting for over one day, was devoid of mirror oscillations and permeated instead by EIC waves of weak intensity. The weak wave intensity is related to the prevailing low solar wind dynamic pressures. Left-hand polarized EIC waves were replaced by bursts of right-hand polarized waves, which remained for many hours the only ion wave activity present. This activity occurred when the magnetosheath proton temperature anisotropy (=T p,⊥ /T p, −1) became negative. This was because the weakened bow shock exposed the magnetosheath directly to the (negative) temperature anisotropy of the solar wind. Unlike the normal case studied in the literature, these right-hand waves were not by-products of left-hand polarized waves but derived their energy source directly from the magnetosheath temperature anisotropy. Brief entries into the low latitude boundary layer (LLBL) and duskside magnetosphere occurred under such inflated conditions that the magnetospheric magnetic pressure was insufficient to maintain pressure balance. In theseCorrespondence to: C. J. Farrugia (charlie.farrugia@unh.edu) crossings, the inner edge of the LLBL was flowing sunward. The study extends our knowledge of magnetosheath ion wave properties to the very low solar wind dynamic pressure regime.

Tenuous solar winds: Insights on solar wind–magnetosphere interactions

“…A reliable normal was found (ratio of intermediateto-minimum eigenvalues ¼ 4.5) with a normal component (B k B n ) % 0 nT. Further, from the direction of the normal, we find that the IP structure associated with the density decrease was in this case a planar IP structure containing the Parker spiral (Nakagawa et al, 1989;Farrugia et al, 2005). On May 11, 1999, the magnetic field is tilted antisunwards (IMF B x o0), and northward (average clock angle % 45 ).…”

“…), according to the measurements of Wind (Farrugia et al, 2005). In this case, if there are RH EICWs they would have been generated directly from the negative A p of the solar wind (see below).…”

A comparison of bow shock models with Cluster observations during low Alfvén Mach number magnetic clouds

“…Outside MCs, a low M A solar wind is generally due to low density and low speed, as it happened during the famous "day the solar wind almost disappeared", 11 May 1999, described, for example, by Farrugia et al (2005). The M A was low, but the IMF strength remained close to its usual value, and the dynamic pressure was very small because of the tenuous and slow solar wind.…”

Some tests of a new magnetosheath model via comparison with satellite measurement

“…It is worth noting that the ''gasdynamic magnetoshearth'' application utilized here is well justified in the literature (e.g. Farrugia et al, 2005), especially for large enough solar wind Alfven Mach number (M A ). This is the case of the example considered below, where M A $6:4.…”