Interaction of the bow shock with a tangential discontinuity and solar wind density decrease: Observations of predicted fast mode waves and magnetosheath merging

Shortly after 0600 UT on 7 April 2000 a tangential discontinuity (TD) in the solar wind passed the Advanced Composition Explorer satellite (ACE). It was characterized by a rotation of the interplanetary magnetic field (IMF) by ∼145° and more than a factor‐of‐2 decrease in the plasma density. About 50 min later, Polar encountered more complex manifestations of the discontinuity near noon in the magnetosheath outside the Northern Hemisphere cusp. On the basis of Polar observations, theoretical modeling, and MHD simulations we interpret the event as demonstrating that (1) a fast mode rarefaction wave was generated during the TD‐bow shock interaction, (2) the fast wave carried a significant fraction of the density change to the magnetopause while the remainder stayed with the transmitted discontinuity, and (3) magnetic merging occurred between IMF field lines within the magnetosheath on opposite sides of the discontinuity's surface as it approached the magnetopause. Before the discontinuity passed the spacecraft, Polar detected ions accelerated antiparallel to B in the fast wave and perpendicular to B in a weak slow mode structure located adjacent to and just downstream of the fast wave. The antiparallel accelerated ions in the fast wave had no measurable ion‐velocity dispersion signature, placing their source a few R_E equatorward of Polar. Simulation results, a Walén test, detections of wave Poynting flux parallel to B, bidirectional electron heat flux, and ion velocity enhancements all indicate that the three ion bursts associated with the passage of the discontinuity were signatures of time‐dependent, magnetic merging events within the magnetosheath.