Magnetospheric Multiscale (MMS) Observations of Foreshock Transients at Their Very Early Stage

Abstract:

Foreshock transients are ion kinetic structures in the ion foreshock. Due to their dynamic pressure perturbations, they can disturb the bow shock and consequently the magnetosphere-ionosphere system. Recently, they are also found to accelerate particles contributing to particle acceleration at the bow shock. However, it is still unclear how exactly they form. Using data from the Magnetospheric Multiscale (MMS) mission, we 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 time variation of the magnetic field caused by the Hall current induced electric field that drove cold plasma moving outward together 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 charge separation between electrons and foreshock ions, nonlinearly further enhanced the Hall current. Based on our observations, we propose a physical formation process of foreshock transients 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.