The Physics of the Earth’s Collisionless Shock Wave

A fast mode collisionless bow shock is a permanent feature of the solar wind interaction with the earth. The shock is approxim_ately stationary in earth coordinates, its structure, however, changes in space due to different B; values, and in time, due to different solar wind conditions (Mach number M and 8, T,/ TI...). The earth's bow shock has revealed itself as an impressive tool for studying collisionless shock waves. After the large theoretical efforts in studying collisionless shock waves, in the past, it is possible, today, to verify experimentally that different dissipation mechanisms are at work in different plasma regimes. The extensive examination of bow shock morphology allow us, today, to correlate distinctions in bow shock structure, revealed by a variety of diagnostics, with M, B and Bn in the solar wind. For quasi parallel geometries and apparently for any M and 8, the shock layer broadens and breacks up, showing limited level of plasma wave noise and marked precursor effects. For quasi perpendicular shock waves, on the contrary, electromagnetic turbulence increases with P, almost independent of M ; while electrostatic noise level increase with M, almost independent of P. Comparison with the different theories valid in the different plasma regimes is the present task.