Magnetospheric Multiscale (MMS) observations of foreshock transients at their very early stage

TL;DR

This study uses MMS observations to investigate the early formation stages of foreshock transients, revealing the critical role of Hall currents and ion dynamics in their development.

Contribution

It provides the first detailed observational evidence of the formation process of foreshock transients, emphasizing the importance of Hall currents and ion demagnetization.

Findings

Hall currents driven by demagnetized ions shape magnetic field profiles

Magnetic and mass fluxes are transported outward, steepening boundaries

Positive feedback from ions and Hall currents enables instability growth

Abstract

Foreshock transients are ion kinetic structures in the ion foreshock. Due to their dynamic pressure perturbations, they can disturb the bow shock and magnetosphere-ionosphere system. They can also accelerate particles contributing to shock acceleration. However, it is still unclear how exactly they form. Recent particle-in-cell simulations point out the important role of electric field and Hall current in the formation process. To further examine this, we use data from the Magnetospheric Multiscale (MMS) mission to apply case studies on two small (1000-2000 km) foreshock transient events that just started to form. In event 1 where MMS were in a tetrahedral formation, we show that the current density configuration, which determined the magnetic field profile, was mainly driven by Hall currents generated by demagnetized foreshock ions. The resulting time variation of the magnetic field induced electric field that drove cold plasma moving outward with magnetic field lines. In event 2 where MMS were in a string-of-pearls formation, we analyze the evolution of field and plasma parameters. We show that the magnetic flux and mass flux were transported outward from the core resulting in the steepening of the boundary. The steepened boundary, which trapped more foreshock ions and caused stronger demagnetization of foreshock ions, nonlinearly further enhanced the Hall current. Based on our observations, we propose a physical formation process that the positive feedback of foreshock ions on the varying magnetic field caused by the foreshock ion Hall current enables an instability and the growth of the structure.