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The soft X‐ray emissions from the Earth's magnetosheath and cusp regions are simulated under different solar wind conditions, based on the PPMLR‐MHD code. The X‐ray images observed by a hypothetical telescope are presented, and the basic responses of the magnetopause and cusp regions are discernable in these images. From certain viewing geometries, the magnetopause position in the equatorial plane, as well as the latitudinal scales and azimuthal extent of cusp can be directly extracted from the X‐ray images. With these reconstructed positions, the issues we are able to analyze include but are not limited to the compression of magnetopause and widening of the cusp after an enhancement of solar wind flux, as well as the erosion of the magnetopause and equatorward motion of cusp after the southward turning of the interplanetary magnetic field. Hence, the X‐ray imaging is an appropriate technique to study the large‐scale motion of magnetopause and cusps in response to solar wind variations. Key Points: X‐ray images are estimated based on MHD simulations under different solar wind conditionsResponses of the magnetopause to solar wind changes are analyzed from X‐ray imagesResponses of the cusp to solar wind variations are presented via X‐ray images [ABSTRACT FROM AUTHOR]

We compare the relative importance of two competing mechanisms (Alfvén wing vs. magnetosheath force balance) in determining the saturation of the reverse convection potential under northward interplanetary magnetic field. We run the Lyon‐Fedder‐Mobarry magnetohydrodynamic model under strong northward interplanetary magnetic field (20 nT) twice: once with a solar wind number density of 20 cm−3 and once with a density of 2 cm−3. We find that the reverse convection potential is reduced for the low‐density run, as predicted by both models. We also find that despite the formation of Alfvén wings, the sunward flows in the magnetosphere are governed by the local reconnection physics. Further, the length of the x‐line poleward of the cusp is reduced by a factor of 2 for the low‐density run. This suggests that the reverse convection potential is governed by the local reconnection physics at the x‐line and the geoeffective length. Plain Language Summary: One major challenge in understanding solar wind‐magnetosphere‐ionosphere coupling is understanding why the electric potential across the polar ionosphere saturates in response to increasing solar wind driving. At present, there are two competing hypotheses for why this happens, and phenomena associated with both are found in simulations of the magnetosphere. In the present study, we simulate a special magnetospheric case where we can separate out the contributions of each mechanism. We find that only one of the hypotheses is necessary to explain the saturation of the polar ionospheric potential. This is a new finding with respect to the phenomena of polar cap potential saturation, and may help explain why it occurs. Key Points: Reverse convection potential reduced at lower solar wind densityAlfven wings also form at low density, but they mostly are connected to the interplanetary magnetic fieldReverse convection potential reduction likely due to reduced geoeffective length [ABSTRACT FROM AUTHOR]