Generation of Inverted Rope-like Structures in the Transition Region of Earth’s Bow Shock

Abstract:

Observations of Earth’s bow shock have shown that a disordered or turbulent transition region can generate thin current sheets at ion and electron kinetic scales. Indeed, a recent survey of current sheets in the shock has shown that active reconnection occurs across the full range of shock parameters associated Earth’s bow shock. Furthermore, hybrid and particle-in-cell simulations have also shown that reconnection in the transition region can generate magnetic islands (in 2D), flux ropes (in 3D) and similar rope-like "twisted field" current structures. Using observations of Earth’s bow shock by NASA’s Magnetospheric Multiscale mission (MMS), we have identified several of these rope-like current structures embedded in the shock transition region. In each case the associated shock crossings also have observations of active magnetic reconnection. In contrast to typical models of flux ropes, several identified events exhibit an "inverted" rope-like structure for which the axial, core field is anti-parallel to the background, resulting in a full 360° rotation of the magnetic field. By examining the generation of reconnecting structures in hybrid particle-in-cell simulations, we show that these inverted rope-like structures can be generated by instabilities of whistler waves in the shock foot. The structures then contract as they are convected downstream towards the magnetosheath. Using multi-spacecraft analyses to measure size and contraction rates of the inverted rope-like structures observed by MMS, we associate our case studies with different stages of the life of these structures as seen in hybrid simulations. We also explore these current structures as sites for heating and particle acceleration.