Characteristics of Magnetic Structures Observed Upstream of Supercritical Quasiperpendicular Shocks

Upstream magnetic perturbations play an important role in particle acceleration and energy conversion across collisionless shocks. Supercritical shocks reflect part of the incident particles back upstream to maintain a stable shock layer. At quasiparallel shocks backstreaming ion travel long distances upstream along the magnetic field and create plethora of plasma instabilities and steepened waves. Reflected particles at quasiperpendicular shocks extend up to an ion gyroradius before returning to the shock. 

We investigate formation and properties of quasiperpendicular shocks using high time resolution multipoint in-situ spacecraft observations from the Magnetospheric Multiscale (MMS) mission. We find that short large amplitude magnetic structures form in the foot region of some quasiperpendicular shocks. Upon formation, these structures propagate in oblique angles to the magnetic field and are carried towards the shock. During this transport, the structures are steepened and magnetosonic whistler waves are developed within the structures. We show that these quasi-periodic pulsations modulate the shock layer in similar ways as their counterparts in quasiparallel shocks.