A model for the transient magnetospheric response to sudden solar wind dynamic pressure variations

Abstract

Brief, impulsive, large‐amplitude (δp/p ∼ 1) solar wind dynamic pressure pulses, recurring on time scales of 5 to 15 min, are common just upstream of the Earth's bow shock. When each pulse strikes the magnetopause, it launches a fast‐mode compressional wave in the magnetosphere that can propagate antisunward faster than the magnetosheath flow. Consequently, the magnetopause bulges outward ahead of each contraction associated with a pressure pulse. These ridges generally propagate antisunward, although sunward motion is common on the early post‐noon magnetopause. The greatest amplitude (∼1 to 2 RE) magnetopause motion occurs on the prenoon magnetopause, at high‐latitudes, and during periods of southward interplanetary magnetic field. The signatures of the pressure‐pulse‐driven magnetopause motion include a bipolar magnetic field signature normal to the nominal magnetopause, a rotation of the magnetic field away from both magnetosheath and magnetospheric orientations, a mixture of magnetosheath and magnetospheric plasmas, and high‐speed magnetosheath plasma flows. The magnetopause boundary motion, in turn, drives transient compressions and shears in the dayside magnetospheric magnetic field. These compressions and shears map to the dayside auroral ionosphere, where the ground signatures produced by a single, brief, solar wind dynamic pressure pulse are an antisunward moving (sunward at early post‐noon local times) double‐convection vortex, associated with north–south magnetic field perturbations, increased ELF/VLF wave activity, precipitating particles, and cosmic noise absorption. The ionospheric and magnetospheric signatures driven by solar wind pressure pulses greatly resemble those previously associated with flux transfer events.