Observations of Persistent Downstream Magnetic Oscillations at the Earth Bow Shock

The redistribution of the directed flow energy in a collisionless shock is the central problem of shock physics. The incident ion energy is transferred to ion and electron heating, acceleration of a small fraction of particles, and enhancement of the magnetic field. The mean magnetic field enhancement is determined by the standard boundary conditions. Recently, shocks were observed in which the amplitude of the persisting downstream magnetic fluctuations exceeded the mean downstream field. The question of the ubiquity of the phenomenon is of utmost importance since it would require re-consideration of the boundary conditions. It may also mean that the effective magnetic field in supernova remnant shocks may be currently grossly underestimated. Using the Magnetospheric Multiscale observations of the Earth bow shock, we analyze the dependence of the relative amplitude of the downstream magnetic fluctuations on the main shock parameters. It is found that large fluctuations are typical for quasi-parallel shocks and higher Mach numbers.