Dawn-Dusk Asymmetry of the Magnetopause Distance Under the Parker Spiral Configuration of the IMF

The dawn-dusk asymmetry of the magnetopause radial distance under the Parker (and ortho-Parker) configuration of the interplanetary magnetic field (IMF) is investigated. An extensive data set of about 50,000 magnetopause crossings identified in the data from the THEMIS A-E, Magion 4, Geotail, and Interball-1 satellites is used for this purpose. It is shown that the magnetopause radial distances are larger at the side where the IMF is quasi-parallel to the bow shock normal than at the side where IMF is quasi-perpendicular to the bow shock normal. The effect is more significant at the flanks than close to the subsolar point. This introduces a significant asymmetry of the magnetopause shape, with the difference in the magnetopause radial distances being as large as about one Earth radius beyond the terminator. The experimental results are confirmed by a global magnetohydrodynamic (MHD) model, demonstrating that the asymmetry can be explained by MHD effects, without considering foreshock transients. This MHD model is further used to investigate the evolution of individual pressure components across the magnetopause and contrast them in quasi-parallel and quasi-perpendicular cases.