On the generation of magnetosheath high-speed jets by bow shock ripples.

H Hietala,F Plaschke

doi:10.1002/2013ja019172

Abstract

[1]The terrestrial magnetosheath is embedded with coherent high-speed jets of about 1RE in scale, predominantly during quasi-radial interplanetary magnetic field (IMF). When these high dynamic pressure (Pdyn) jets hit the magnetopause, they cause large indentations and further magnetospheric effects. The source of these jets has remained controversial. One of the proposed mechanisms is based on ripples of the quasi-parallel bow shock. In this paper, we combine for the first time, 4 years of subsolar magnetosheath observations from the Time History of Events and Macroscale Interactions during Substorms mission and corresponding NASA/OMNI solar wind conditions with model calculations of a rippled bow shock. Concentrating on the magnetosheath close to the shock during intervals when the angle between the IMF and the Sun-Earth line was small, we find that (1) 97% of the observed jets can be produced by local ripples of the shock under the observed upstream conditions; (2) the coherent jets form a significant fraction of the high Pdyn tail of the magnetosheath flow distribution; (3) the magnetosheath Pdyn distribution matches the flow from a bow shock with ripples that have a dominant amplitude to wavelength ratio of about 9% (∼0.1RE/1RE) and are present ∼12% of the time at any given location.

Highlights

[2] The terrestrial magnetosheath is a highly turbulent region between the bow shock and the magnetosphere of the Earth
Observations have revealed striking transient enhancements of plasma flow, denoted as HighSpeed Jets (HSJs) in this paper. In various studies they have been characterized by slightly different quantities, such as high dynamic pressure, kinetic energy density, or flux [e.g., Nemecek et al, 1998; Savin et al, 2008; Hietala et al, 2009; Amata et al, 2011; Archer and Horbury, 2013; Plaschke et al, 2013]
Subsolar HSJs occur predominantly during intervals of low cone angle—an interplanetary magnetic field (IMF) geometry that takes place 16% of the time [Suvorova et al, 2010]. They can be found throughout the quasi-parallel magnetosheath when their detection is done by requiring a >100% increase over the ambient magnetosheath dynamic pressure

Summary

Introduction

Observations have revealed striking transient enhancements of plasma flow, denoted as HighSpeed Jets (HSJs) in this paper In various studies they have been characterized by slightly different quantities, such as high dynamic pressure, kinetic energy density, or flux [e.g., Nemecek et al, 1998; Savin et al, 2008; Hietala et al, 2009; Amata et al, 2011; Archer and Horbury, 2013; Plaschke et al, 2013]. When the identification is made by comparison with the solar wind dynamic pressure, the HSJ occurrence probability is largest close to the bow shock and in the sunward half of the sheath [Plaschke et al, 2013] Based on these statistical results and previous case studies [e.g., Nemecek et al, 1998; Shue et al, 2009; Hietala et al, 2009, 2012], HSJs are thought to be connected to quasi-parallel shock geometry and ion foreshock processes. We consider the implications of the results for other plasma environments

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