Abstract
Magnetopause position is controlled mainly by the solar wind dynamic pressure and north-south interplanetary magnetic field component and these quantities are included in different empirical magnetopause models. We have collected about 50,000 of dayside magnetopause crossings observed by THEMIS in course of 2007-2019 and compared the observed magnetopause position with model prediction.
The difference between observed and predicted magnetopause radial distance, Robs - Rmod is used for quantifying the model-observation agreement. Its median values are well predicted for cases up to Robs approximate to 12 RE for all models but higher positive deviations are found for larger magnetopause distances, mainly under a nearly radial field and low dynamic pressure.
The analysis reveals their connection with transient magnetopause displacements caused by strong sunward flows in the magnetosheath. We discuss the possible origin of the observed magnetosheath flow switching in terms of the interaction of magnetosheath jets with the magnetopause.
Comparison of the observed magnetopause crossings with prediction of the magnetopause models reveals that under nearly radial interplanetary magnetic field can be the magnetopause observed several Earth radii farther from the Earth than the models predict. Comprehensive examination of several cases characterized by suitable spacecraft locations leads to the conclusion that the source of such magnetopause displacements is connected with the reformation of nearly parallel bow shock resulting in a strong antisunward jet in the magnetosheath.
The jet creates a dip in the magnetopause surface that reverses its direction. The sunward flow in the magnetosheath pulls the magnetopause also sunward.
Since these effects are transient in their nature, they cannot be captured by statistical magnetopause models. The magnetopause is often observed several RE upstream its nominal position under nearly radial IMFExtreme magnetopause displacements are accompanied with strong antisunward magnetosheath jetsReversal of the jet direction is associated with the magnetopause outward displacement