STATISTICAL-ANALYSIS OF SHORT LARGE-AMPLITUDE MAGNETIC-FIELD STRUCTURES IN THE VICINITY OF THE QUASI-PARALLEL BOW SHOCK

Short large-amplitude magnetic field structures (SLAMS) appear in the vicinity of the quasi-parallel region of the Earth's bow shock. Using the magnetic field and plasma data of the AMPTE/IRM and UKS satellites, 18 of such structures in the upstream region are statistically investigated. These events occurred on one day. They have typical magnetic amplitudes of 2.6 times the ambient field and are associated with density enhancements of 1.3 times the unperturbed value. Their velocities are roughly 4 times the Alfven speed in the solar wind frame. A relationship between the amplitude of the magnetic field and the density enhancement as well as the propagation velocity of the SLAMS has been derived from the data. This relationship is well reproduced by describing SLAMS in terms of simple nonlinear magnetohydrodynamic waves. Within the SLAMS the magnetic field rotates from a quasi-parallel into a quasi-perpendicular geometry.