  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.
