QUASI-PARALLEL SHOCKS - A PATCHWORK OF 3-DIMENSIONAL STRUCTURES

Collisionless shocks at quasi-parallel geometries, i.e., for which the average magnetic field direction upstream of the shock is close to the shock normal, reveal temporally varying quantities, a variety of boundary crossing and kinetic signatures, and magnetic structures, often convecting, of finite extent. These results can be put together by a framework in which the shock can be viewed as an extended region containing three-dimensional Short Large Amplitude Magnetic Structures (SLAMS) which represent individual semi-cycles of the ambient upstream low frequency waves associated with diffuse ions in the ULF foreshock. As SLAMS convect with the flow they grow to large amplitudes and entrain inter-SLAMS regions to form an inhomogeneous downstream state. Their finite transverse extent is probably related to, and interacts with, ion beams, to produce a patchy transition zone which accounts for the variety of spacecraft signatures observed.