Formation mechanism of foreshock transients

Terry Liu

Geophysical Institute UAF

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. Using 1-D particle-in-cell simulations, we show that electric fields play a critical role in shaping theshock's magnetic field structure, as well as in coupling the energy of hot ions to that of the shock. From a more realistic 2D simulation, we examine how a discontinuity interacts with foreshock ions leading to the formation of a foreshock transient and a secondary shock. To further examine these, 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. 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. 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.