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

Abstract

AbstractIn 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 Bz 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.