Anti-sunward high-speed jets in the subsolar magnetosheath

F Plaschke,V Angelopoulos,H Hietala

doi:10.5194/angeo-31-1877-2013

Abstract

Abstract. Using 2008–2011 data from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in Earth's subsolar magnetosheath, we study high-speed jets identified as intervals when the anti-sunward component of the dynamic pressure in the subsolar magnetosheath exceeds half of its upstream solar wind value. Based on our comprehensive data set of 2859 high-speed jets, we obtain the following statistical results on jet properties and favorable conditions: high-speed jets occur predominantly downstream of the quasi-parallel bow shock, i.e., when interplanetary magnetic field cone angles are low. Apart from that, jet occurrence is only very weakly dependent (if at all) on other upstream conditions or solar wind variability. Typical durations and recurrence times of high-speed jets are on the order of tens of seconds and a few minutes, respectively. Relative to the ambient magnetosheath, high-speed jets exhibit higher speed, density and magnetic field intensity, but lower and more isotropic temperatures. They are almost always super-Alfvénic, often even super-magnetosonic, and typically feature 6.5 times as much dynamic pressure and twice as much total pressure in anti-sunward direction as the surrounding plasma does. Consequently, they are likely to have significant effects on the magnetosphere and ionosphere if they impinge on the magnetopause.

Highlights

Within the dayside, subsolar magnetosheath, the plasma dynamic pressure is in general much lower than in the pristine solar wind
We calculated the distances of the THEMIS spacecraft to a model magnetopause and a model bow shock along lines radially connecting the center of the Earth with the spacecraft
Our results show that neither the solar wind Mach numbers nor the density, velocity, and magnetic field strength, nor the variability in these solar wind quantities seem to be strongly controlling the occurrence of high-speed jets (HSJs) in the subsolar magnetosheath

Summary

Introduction

Subsolar magnetosheath, the plasma dynamic pressure is in general much lower than in the pristine solar wind. Transient enhancements of the dynamic pressure that stand out from the usual magnetosheath turbulence are observed. If these enhancements are directed toward the magnetopause, their impact on the magnetopause–magnetosphere–ionosphere system can be remarkably strong (see, e.g., Shue et al, 2009; Amata et al, 2011). In the absence of such upstream causes, other generation mechanisms have to be responsible These mechanisms (i) can result in global or localized dynamic pressure enhancements; (ii) can be related to certain steady upstream solar wind conditions or variations thereof; (iii) can act in the foreshock, at the bow shock, or within the magnetosheath itself

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