2-DIMENSIONAL SIMULATIONS OF MAGNETIC PULSATIONS UPSTREAM OF THE EARTHS BOW SHOCK

The development of turbulence upstream of the quasi-parallel portion o f the Earth's bow shock is investigated using two-dimensional hybrid s imulations involving the injection of a very hot ion beam against a co ld incident ion flow. Initially, waves are produced in the right-hand resonant ion beam mode and with wave vectors mainly parallel to the am bient magnetic field B-0 as expected in linear theory. The waves attem pt to propagate upstream but are convected back toward the shock by th e incident flow and strongly grow in amplitude as they encounter large r beam densities. Wave growth is associated with reduction of both inc ident and beam ion bulk velocities. Differential slowing of the incide nt flow between the different wave fronts and, when the angle theta(Bn ) between B-0 and the shock normal n is non zero, along the fronts the mselves, leads to wave front refraction parallel to the shock and frag mentation into smaller structures. Simultaneously, strong scattering b y the waves generates beam ion clumps containing a significant fractio n of ions with velocities antiparallel to the beam bull velocity, whic h enables the destabilization of the left-hand resonant ion beam mode. The beam ions ultimately form a diffuse distribution whose density ra tio relative to the solar wind is a few percents in average, but incre ases up to about 20% in the immediate vicinity of the shock. These pro cesses result in the formation of magnetic pulsations whose extension parallel and perpendicular to n is typically 15 and a few tens of ion inertial lengths, respectively. These structures can reach total ampli tudes delta B of 3 to 4 B-0, and exhibit most of the time a left-hande d polarization in the plasma rest frame, in agreement with spacecraft observations upstream of the Earth's bow shock. The simulations show t hat pulsations induce slowing and moderate heating of the incident flo w and play an active role in the quasi-parallel shock transition.