Depression of the Magnetic Field in an Ion-scale Flux Rope

Abstract:

We report an ion-scale magnetic flux rope (˜9.8 ion inertial lengths) at the trailing edge of Kelvin-Helmholtz (KH) waves observed by the Magnetospheric Multiscale (MMS) mission on September 27 2016, which is likely generated by multiple X-line reconnections. The currents of this flux rope are highly filamentary: inside the flux rope, the current flows are mainly parallel to the magnetic field, while at the edges, the current filaments are predominantly along the anti-parallel direction. The current filaments at the edges induce an opposing field that causes the observed |B| depression in the central flux rope, meaning the magnetic field inside this flux rope is generally depressed comparing with the ambient magnetic field. Thus, this flux rope is referred as a crater type, and corresponds to a non-force-free structure. Furthermore, intense lower hybrid drift waves (LHDW) are found at the magnetospheric edge of the flux rope, associated with a wave potential as large as 17% of the electron temperature. Though LHDWs may be stabilized by the mechanism of electron resonance broadening, these waves could still effectively scatter electrons by the wave electric field, corresponding to a local density dip. This indicates LHDWs could play important roles in plasma transports of crater flux rope evolutions.