Magnetosheath Microstructure: Mirror Mode Waves and Jets during Southward IP Magnetic Field

Abstract

In the Earth's magnetosheath plasma waves, nonlinear structures associated with characteristic ion populations can occur. Understanding the interaction of the solar wind with the magnetosphere requires a deeper knowledge of the underlying magnetosheath kinetic microstructure. We study a 45 min interval when the MMS spacecraft observed a southward magnetic field (Bz < 0 nT) in the dayside magnetosheath. Using magnetic field and plasma data, we analyze three transient dynamic pressure enhancements identified as magnetosheath jets. The characteristics of these jets are different, suggesting different origins. While two of them, called J1 and J2, exhibit large increment in velocity and almost no density increment, the third jet (J3) shows large density enhancements with almost no velocity increment. The duration of J3 is ∼7 times longer than those of J1 and J2. J3 occurs at the region where the negative B z becomes positive. Ion distributions inside the jets are different. J1 and J2 show a secondary field-aligned beam, which is not present in J3, suggesting that magnetic reconnection at the magnetopause is responsible for their formation. Distributions inside J3 are more isotropic. B field and plasma signatures inside J3 correspond to the crossing of a sector boundary, similar to the heliospheric plasma sheet, suggesting that J3 forms by the evolution and interaction of a solar wind structure with the bow shock and magnetosheath. Fluctuations inside J3 have larger transverse components, although they propagate at large angles to B. In contrast, waves in regions between the jets are compressive and are identified as mirror mode waves.

Causes of Jets in the Quasi‐Perpendicular Magnetosheath

“…However, a bipolar IMF signature is missing in our case. Finally, this event is similar to reconnection jets due to magnetopause reconnection (Blanco‐Cano et al., 2020), although these authors showed that such events exhibit ion distributions with two distinct populations, which is not the case here (Figure 4c iii). Hence, we will refer to this event simply as a “structure”.…”

“…Blanco‐Cano et al. (2020) found jets formed by magnetic reconnection at the magnetopause. Some jets were observed to impact and sometimes penetrate the magnetopause (Dmitriev & Suvorova, 2012; H. Hietala et al., 2018; Plaschke & Glassmeier, 2011; Plaschke et al., 2016; Savin et al., 2012; Shue et al., 2009; Wang et al., 2018) and even perturb the ionosphere (Archer & Horbury, 2013; N. Hietala et al., 2012; Wang et al., 2018).…”

Parametric Study of Magnetosheath Jets in 2D Local Hybrid Simulations

“…Raptis et al (2020b) have shown that a minority of the magnetosheath jets can also be found downstream of the quasi-perpendicular bow-shock and that they are on average related to lower solar wind velocities than the jets observed downstream of the quasi-parallel bow-shock. It has been shown by Blanco-Cano et al (2020) and that the jets downstream of the quasi-perpendicular bowshock may have very different origin. As such, they are not produced in our simulations.…”

“…Raptis et al (2020b) have shown that magnetosheath jets can sometimes also be found downstream of quasi-perpendicular bow-shock. These may have very different origin than jets downstream of quasi-parallel bow shock, as has been discussed by Blanco-Cano et al (2020) and (these authors talk about the jets in the quasi-parallel and quasiperpendicular magnetosheath). Raptis et al (2020b) showed that on average, the jets in the quasi-parallel magnetosheath occur for higher solar wind speeds than jets in the quasiperpendicular magnetosheath.…”

On the Generation of Pi2 Pulsations due to Plasma Flow Patterns Around Magnetosheath Jets

“…Furthermore, it is permeated both by waves that have been transmitted through the bow shock, as well as by fluctuations that have been generated locally (Blanco-Cano et al, 2006;Lucek et al, 2005).Upstream of the quasi-parallel bow shock, solar wind ions are reflected forming a region called the ion foreshock which is filled by backstreaming ion populations. These ions can generate various wave modes and-more often than not-ULF waves (Blanco-Cano et al, 2020) which is suggested to be transmitted, through the magnetosheath, into the magnetosphere (Clausen et al, 2009). On the other hand, at the quasi-perpendicular bow shock, the initially reflected ions are mostly heated in the direction perpendicular to the magnetic field exhibiting enhanced temperature anisotropy.…”