Mirror mode peaks: THEMIS observations versus theories

Mirror waves have been observed in many different plasma regions within solar system. In contrast to many other plasma waves, mirror mode waves are rarely observed as a quasiperiodic sinusoidal signature. Mirror structures in the form of magnetic dips are usually observed when the criterion for mirror instability is not satisfied in the plasma surrounding these structures. In contrast, magnetic peaks are observed in mirror unstable plasma. This paper reports on a study of the peaktype mirror structures in the terrestrial magnetosheath conducted using THEMIS magnetic field and plasma measurements. It is shown that the changes observed in the plasma temperature are inconsistent with the previously shared view that magnetic peaks are nonlinear mirror waves immersed in a background plasma. A model in which the nonlinear mirror waves are composed of a series of peaks and troughs can help to explain the apparent contradictions between theory and observations.