Nature of Magnetic Holes above Ion Scales: A Mixture of Stable Slow Magnetosonic and Unstable Mirror Modes in a Double-polytropic Scenario?

Abstract

Abstract Magnetic holes are common features with a prominent dip of magnetic field strength in space plasma turbulence. As to their nature, there exists a dispute of explanations among discontinuities, magnetic reconnection, solitons, kinetic-scale electron vortexes, slow waves, and mirror-mode instability. As magnetic holes are often accompanied by thermal anisotropy, at magnetohydrodynamic scales double-polytropic equations can serve as an appropriate description. The reason for the long-lasting dispute lies in the fact that both mirror-mode structures and oblique slow-mode waves are characterized with anticorrelation between plasma density (or temperature) and magnetic field strength, which, as often used in preceding works, is also the characteristic feature of magnetic holes. Therefore, to finally and unambiguously diagnose the nature of magnetic holes above ion scales, we propose to resort to other features, among which v ∥ and its phase relation with and n behave differently between mirror-mode structures and slow-mode waves. Herewith we establish a model with superposition of both slow and mirror modes to reproduce the observed types of behaviors . This model inspires new understanding of the nature of magnetic holes: the magnetic hole in reality is not solely contributed by only one mode, but a mixture of the two modes with an adjustable amplitude ratio.