Observations of Mirror Mode Structures in the Dawn‐Side Magnetosphere

Abstract

AbstractWe present observations of mirror mode structures in the dawn‐side magnetosphere from the Magnetospheric Multiscale Mission. The observations were made during a period of relatively stable northward interplanetary magnetic field conditions. We observed magnetic troughs only with reductions of up to 90% in the magnitude of the local magnetic field. The structures were closely aligned with the local magnetic field and had sizes perpendicular to the magnetic field of the order of 1Re and ion densities up to twice that of the surrounding regions. We used ion, magnetic field, and electric field data to examine the plasma conditions inside and outside of these structures to investigate the stability and evolution of the structures and estimate the magnitude of changes in the ion distributions. Comparing the ion bulk properties to theoretical predictions, we concluded that variations in the parameters with trough depth were due to the spacecraft traversing them at different locations along their length. Our calculations of heating and cooling of the ions at different pitch angles is consistent with theory. We conclude that these troughs were likely long‐lived, bistable structures that originated in the magnetosheath near the nose of the magnetopause and were captured into the magnetosphere by dual‐lobe reconnection.