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This research examines the ability of current physics‐based models to predict the magnetopause location. We use the Run‐On‐Request capabilities at the Community Coordinated Modeling Center at NASA GSFC with 4 magnetohydrodynamic (MHD) models. The magnetopause position prediction and response time to the solar wind changes is then compared to extreme solar wind conditions where magnetopause crossing of geosynchronous orbit have been observed by the Geostationary Operational Environmental Satellite (GOES) satellites. Rigorous analysis/comparison of observations and models is critical in determining magnetosphere dynamics for model validation. This paper is a preliminary effort defining the metrics necessary to understand the current magnetosphere model capabilities and challenges. Results show that there are discrepancies between the MHD models' standoff positions of the dayside magnetopause for the same solar wind conditions on events that included (a) an increase in solar wind dynamic pressure and a step function in the Interplanetary Magnetic Field Bz component; (b) nominal solar wind conditions (values of approximated 400 km/s for solar wind speed and 5 nT for the magnetic field magnitude) with a northward IMF; and (c) compression caused by several coronal mass ejections impacting the near Earth environment. Overall, the models predicted different magnetopause subsolar locations sometimes in the order of 3 RE. Contingency tables were calculated to show model performance in comparison with the data observed with the GOES 13/15 geosynchronous orbit for extreme events and skill scores were calculated. Plain Language Summary: Earth's magnetopause is the boundary that separates the solar wind from Earth's magnetosphere. Its location and dependence on solar wind parameters has been studied via simulations and observations. In this study, we analyzed events where the solar wind conditions were very dynamic, including extreme conditions during which geosynchronous orbit crossings were detected by the Geostationary Operational Environmental Satellite 13 and 15 satellites. Our results show that the models give very different magnetopause positions sometimes showing geosynchronous orbit crossings where they were not observed. Contingency tables and skill scores were calculated to analyze the discrepancies between the models for the extreme events. Key Points: A preliminary effort into defining metrics necessary to understand the current magnetosphere models' capabilities and challengesResults show big discrepancies between the magnetohydrodynamic (MHD) models' standoff positions of the dayside magnetopause for the same solar wind conditionsSkill scores were calculated comparing the predictions by the MHD models and the observations from Geostationary Operational Environmental Satellite satellites [ABSTRACT FROM AUTHOR]

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]

Abstract: During southward interplanetary magnetic field (IMF) conditions, the subsolar magnetopause moves inward even under steady solar wind pressure. This is generally referred to as “erosion.” Empirical and computational studies have been conducted to quantify the effect of magnetopause erosion; however, they generally use steady state IMF conditions. Solar wind conditions are not usually constant for very long, especially during such conditions as are present in high‐speed streams, which typically have large fluctuations in the IMF. These IMF fluctuations can occur on a range of times scales and amplitudes, resulting in a series of northward and southward IMF intervals imposed on the magnetosphere. We have simulated the effect of such IMF variations on the magnetopause position using a global magnetohydrodynamic model. We find that there are both time lag and hysteresis‐like effects on the motion of the magnetopause due to the variation in the magnitude of the southward IMF and that the instantaneous position of the magnetopause can be significantly different from what one would expect from steady state conditions. [ABSTRACT FROM AUTHOR]

In this study we examine the recovery of relativistic radiation belt electrons on 15-16 November 2014, after a previous reduction in the electron flux resulting from the passage of a corotating interaction region (CIR). Following the CIR, there was a period of high-speed streams characterized by large, nonlinear fluctuations in the interplanetary magnetic field (IMF) components. However, the outer radiation belt electron flux remained at a low level for several days before it increased in two major steps. The first increase is associated with the IMF background field turning from slightly northward on average to slightly southward on average. The second major increase is associated with an increase in the solar wind velocity during a period of southward average IMF background field. We present evidence that when the IMF B z is negative on average, the whistler mode chorus wave power is enhanced in the outer radiation belt, and the amplification of magnetic integrated power spectral density in the ULF frequency range, in the nightside magnetosphere, is more efficient as compared to cases in which the mean IMF B z is positive. Preliminary analysis of the time evolution of phase space density radial profiles did not provide conclusive evidence on which electron acceleration mechanism is the dominant. We argue that the acceleration of radiation belt electrons requires (i) a seed population of keV electrons injected into the inner magnetosphere by substorms and both (ii) enhanced whistler mode chorus waves activity as well as (iii) large-amplitude MHD waves. [ABSTRACT FROM AUTHOR]