A cluster multi-spacecraft study of Earth's bow shock

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University of New Brunswick
The location, shape and motion of Earth’s bow shock are investigated using observations based on measurements made by the Cluster spacecraft quartet. Several bow shock crossings have been identified and carefully characterized according to relevant plasma parameters; a collection of 133 shocks has been selected and analysed using a timing method. The shock crossings cover orbits in which the spacecraft separation is of the order of ∼ 600 km or less. When present, the magnetic field fluctuations are suppressed using the conventional low-pass filtering technique prior to implementing timing method. The results of this investigation are compared with both Gas Dynamics and Magnetohydrodynamics (MHD) bow shock models.We have found, on a statistical basis, that the shock standoff position derived from the timing method agrees well with the Gas Dynamics predictions for high Mach-number cases only. We have also found that for half the crossings, the timing and the conic-based shock normals agree within an 11 degree-angle. Our results strongly indicate that the motion of the shock is predominantly along the Sun-Earth direction; a departure from this direction is not related to the shock-crossing location. Shock velocities below ∼ 80 km/s satisfactorily follow a nearly Gaussian distribution with zero mean and a standard deviation of ∼ 42 km/s. We show that high speed motions are correlated with sharp increases in the solar wind upstream ram pressure, and are consistent with gas dynamics model predictions.