High energy jets in the Earth’s magnetosheath: Implications for plasma dynamics and anomalous transport

Abstract

593The region downstream of a supercritical collisionless shock, the magnetosheath (MSH), is known to be in a highly disturbed turbulent state [1][2][3]. The undisturbed solar wind (SW) streams with supermagnetosonic velocity V > c ms at a magnetosonic Mach number up to M ms ~ 15. At the Earth's bow shock (BS), the SW decelerates to Mach numbers M ms < 1, thermalizes, and, when entering the MSH, is compressed by roughly a factor of 4. The flow downstream of the BS is highly disturbed and turbulent. However, the MSH is not spacious enough for the turbulence to reach a quasi-sta- ¶ The text was submitted by the authors in English.tionarity. It remains not fully developed, intermittent, and structured in time and space. In this framework, high-energy density jets have been observed in the past in the magnetosheath [1,5]. As a development of such earlier studies, we have found more than 140 events of an anomalously high kinetic energy density in the MSH during 20 orbits of Interball-1 , Cluster , Polar , and Geotail . Here, we concentrate on two MSH crossings-by Interball-1 and Cluster [11], respectively-characterized by the bursts of an extraordinarily high ion flux and kinetic energy density. High energy density jets in the magnetosheath near the Earth magnetopause were observed by Interball-1 [1]. In this paper, we continue the investigation of this important physical phenomenon. New data provided by Cluster show that the magnetosheath kinetic energy density during more than one hour exhibits an average level and a series of peaks far exceeding the kinetic energy density in the undisturbed solar wind. This is a surprising finding because the kinetic energy of the upstream solar wind in equilibrium should be significantly diminished downstream in the magnetosheath due to plasma braking and thermalization at the bow shock. We suggest resolving the energy conservation problem by the fact that the nonequilibrium jets appear to be locally superimposed on the background equilibrium magnetosheath, and, thus, the energy balance should be settled globally on the spatial scales of the entire dayside magnetosheath. We show that both the Cluster and Interball jets are accompanied by plasma superdiffusion and suggest that they are important for the energy dissipation and plasma transport. The character of the jet-related turbulence strongly differs from that of known standard cascade models. We infer that these jets may represent the phenomenon of the general physical occurrence observed in other natural systems, such as heliosphere, astrophysical, and fusion plasmas [2][3][4][5][6][7][8][9][10]. High Energy Jets in the Earth

Plasma flow patterns in and around magnetosheath jets

“…Plasma jets defined as transient increases in dynamic pressure are a very common phenomenon in the subsolar magnetosheath (Němeček et al, 1998;Savin et al, 2008;Karlsson et al, 2012;Plaschke et al, 2016). In particular, the magnetosheath region downstream of the quasi-parallel shock is permeated by magnetosheath jets.…”

Plasma penetration of the dayside magnetopause

“…High energy density jets that are thought to be accelerated through a Fermi-type mechanism were observed in the magnetosheath by Savin et al 29 . Such jets have been shown to be deflected toward the magnetopause, where they may constitute a source of penetrating plasma 30 . Recently, magnetopause deformation by supermagnetosonic plasma streams has been reported 31 .…”

Anti-sunward high-speed jets in the subsolar magnetosheath

“…Savin et al (2012) presented a study of the relationship between magnetosheath plasma jets and hot flow anomalies upstream of the bow shock (e.g., Schwartz et al, 2000). The authors argue that hot flow anomalies (obstacles to the solar wind flow) trigger high-speed jets (HSJs) locally to restore solar wind flux balance (see also, Savin et al, 2004Savin et al, , 2008. Because interactions of current sheets in the solar wind with the bow shock are required for hot flow anomaly occurrence (e.g., Schwartz et al, 2000;Omidi and Sibeck, 2007), the work of Savin et al (2012) again relates the appearance of magnetosheath plasma jets to discontinuities in the IMF.…”