Scale, geometry and stability of bi-stable mirror mode structures in the magnetosheath

Abstract:

Mirror mode waves in the magnetosheath are typically observed in the form of non-periodic low frequency plasma oscillations, characterized by magnetic field magnitude alternating between two preferred values (high and low field level). This property of fully saturated mirror modes is often referred to as bi-stability in the literature. We investigated mirror structures of this form observed by Cluster spacecraft focusing on the width and and magnitude of the "low field" and "high field" areas. In a statistical study it was observed that the plasma in the high field areas is usually stable or marginally stable with respect to mirror instability, but plasma corresponding to the low field level is unstable. The relative width of the low and high field areas is thus proportional to the degree of instability of the plasma. This relationship appears to be a generalization of the previously described association between mirror mode shape and distance to mirror instability threshold, where mirror "dips" (localized drops in field magnitude) are favored in stable plasma, and mirror "peaks" (localized field increases) occur in unstable plasmas. The spatial scale and geometry of the structures has also been investigated and compared to mirror instability theory. It is shown that the areas of low magnetic field level and unstable plasma are usually comparable or smaller than the mirror mode wavelength, indicating fully saturated state where the mirror instability is no longer efficient.