Abstract Solar wind densities and dynamic pressures can fall abruptly upon the arrival of a magnetic cloud. Results from the SWMF BATS‐R‐US global magnetohydrodynamic (MHD) simulation indicate that the arrival of a factor of 10 decrease in the solar wind density associated with a tangential discontinuity moving antisunward with the solar wind immediately causes the subsolar bow shock to move outward at a velocity half that of the solar wind. Shortly thereafter, the subsolar magnetopause moves outward and the magnetic field strengths within the dayside magnetosphere fall. The sudden drops in solar wind densities and dynamic pressure reverse flows within the pre‐existing magnetosheath, resulting in cold dense inner magnetosheath plasma moving sunward and away from the Sun‐Earth line. Meanwhile, the newly arriving magnetosheath plasma comes to a halt, resulting in a stagnant hot tenuous plasma in the outer subsolar magnetosheath. Transient sunward plasma velocities peak at velocities half those in the solar wind in the outer magnetosphere at and/or just inside the subsolar magnetopause. The magnetopause overshoots its final position and begins to move antisunward, indicating the start of an oscillation, while the speed of the outward bow shock motion diminishes. Plasma flows antisunward and away from the Sun‐Earth line throughout the magnetosheath.
Sibeck et al. (Sun,) studied this question.