Statistical Properties of Sub-Ion Magnetic Holes in the Dipolarized Magnetotail: Formation, Structure, and Dynamics

Abstract

Magnetic energy release during magnetic reconnection in the magnetotail leads to fast plasma flows transporting thermal energy toward the inner magnetosphere or deep tail. The interaction of such flows with the ambient plasmas is controlled by forces at the flow's leading edge, manifested as a sharp enhancement of the south-north component of magnetic field there, which has been called the dipolarization front. In this study, we examine the kinetic plasma structure of equatorial magnetic field perturbations observed behind dipolarization fronts. Using statistical observations of dipolarization fronts in the near-Earth magnetotail by Time History of Events and Macroscale Interactions during Substorms mission, we find sub-ion scale (scale is below ion gyroradius) magnetic field depressions (magnetic holes), mostly around the equatorial plane, drifting dawnward. They are populated by hot, transversely anisotropic electrons, likely heated around the front. Combining spacecraft observations, analytical estimates, and particle-in-cell simulations, we suggest that these holes result from the ballooning/interchange instability at the dipolarization front. They may represent the nonlinear stage of magnetic field perturbations associated with front instability, which trap hot electrons behind the front. We also discuss the possible role of these holes in scattering and heating electrons and ions in the dipolarized magnetotail.