Soft X-rays are emitted in the magnetosheath and cusps because of solar wind charge exchange. The soft X-ray Imager (SXI) on board Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) will measure these X-rays. We developed a new method for finding the magnetopause standoff distance from simulations that reproduce the expected X-ray images using software developed by the SXI instrument team. We consider three points near the SMILE apogee. We apply this method to a three-hour interval with an interplanetary shock and a southward interplanetary magnetic field turning when the magnetosphere was moderately compressed. The results show that the magnetopause position can be reconstructed with an accuracy better than 0.5 R-E for a five-minute integration time, which matches the SMILE scientific requirements. Moreover, we can even determine the magnetopause position using one-minute integration when the magnetosphere is strongly compressed and the spacecraft's position and SXI's orientation are favorable for magnetopause observations.

The joint mission between the European Space Agency and the Chinese Academy of Sciences, the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), is due to launch in spring 2026. The Soft X-ray Imager (SXI) on board SMILE will measure X-rays emitted from the magnetosheath and cusps. These data will help trace variations in the positions of the magnetopause and cusps in response to changes in the solar wind. We present a fast, computationally inexpensive method for determining the magnetopause standoff distance using a set of simulated X-ray images. We demonstrate that the standoff distance can be obtained with an accuracy better than 0.5 RE using a 1-minute integration time when the magnetosphere is significantly compressed. We also discuss the differences between emissions produced by the magnetosheath and the cusps, as well as the role of spacecraft position in SXI data analysis.

The interaction between the solar wind and Earth's magnetosphere is a critical area of research in space weather and space physics. Accurate determination of the magnetopause position is essential for understanding magnetospheric dynamics. While numerous magnetopause models have been developed over past decades, most are time-independent, limiting their ability to elucidate the dynamic movement of the magnetopause under varying solar wind conditions. This study introduces the first time-dependent three-dimensional magnetopause model based on quasi-elastodynamic theory, named the POS (Position–Oscillation–Surface wave) model. Unlike existing time-independent models, the POS model physically reflects the dynamic responses of magnetopause position and shape to time-varying solar wind conditions. The predictive accuracy of the POS model was evaluated using 38 887 observed magnetopause-crossing events. The model achieved a root-mean-square error of 0.774 Earth radii (RE, representing a 17.9 % improvement over five widely used magnetopause models. Notably, the POS model demonstrated superior accuracy under highly disturbed solar wind conditions (22.1 % better) and in higher-latitude regions (27.0 % better) and flank regions (33.3 % better) of the magnetopause. The POS model's remarkable accuracy, concise formulation, and fast computational speed enhance our ability to predict magnetopause position and shape in real time. This advancement is significant for understanding the physical mechanisms of space weather phenomena and improving the accuracy of space weather forecasts. Furthermore, this model may provide new insights and methodologies for constructing magnetopause models for other planets.

State-of-the-art numerical models have been developed to reproduce magnetospheric dynamics in response to solar wind variations. However, we do not understand how accurate the predictions of the models would be in different solar wind and magnetospheric conditions. In this study, we consider the two relatively simple cases with southward interplanetary magnetic field turnings which have been simulated by several MHD models (SWMF, LFM, PPMLR-MHD, PLUTO). We compare numerical results with observations in terms of global magnetospheric characteristics such as the polar cap open flux and the indices of magnetospheric activity. Our purpose is to understand why some models can make better predictions. To answer this question we also compare the results of the same MHD model with different numerical resolutions and ionospheric conductances and show that both resolution and conductance are important for accurate predictions. By comparing simulations with observations, we can figure out the optimal parameters in the models which should be used in the future.

The soft X-ray imager (SXI) on board the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission will measure X-rays emitted in the Earth’s magnetosheath and cusps. Using these measurements, we will find the magnetopause positions and shape for variable solar wind conditions. However, the recently developed methods of magnetopause finding do not accurately consider the differences in magnetosheath configuration for northward and southward IMF. Analysing MHD results, we show that the plasma depletion layer occurring in the magnetosheath close to the magnetopause for a northward IMF may shift the maximum of X-ray emission farther from the Earth. It requires corrections in calculations of the magnetopause position obtained from the maximum integrated X-ray emissivity.

The magnetospheric cusps are populated by the magnetic field lines that connect upward to the magnetosheath and extend downward to the ionosphere, therefore the magnetosheath plasma has direct access to the polar ionosphere in these regions. The location of the cusp responds dynamically to solar wind conditions and geomagnetic field, influencing magnetosphere-ionosphere coupling. The equatorward boundary of the cusp is adjacent to the low-latitude boundary layer (LLBL)/cleft, where the dayside open-closed boundary (OCB) is typically located. The polar cap boundary (PCB) delineates the extent of open magnetic flux, and its midday position is associated with the cusp and OCB. Particles precipitating in the cusp contribute to midday auroral emissions and field-aligned currents. The latitude of midday auroral equatorward boundary varies with the cusp's equatorward boundary, OCB, and the thickness of the LLBL. Field-aligned currents connect magnetospheric currents with ionospheric currents, with the Region 1 currents observed on both open and closed field lines. Consequently, the Region 1 current's high-latitude boundary near local noon relate to the cusp’s equatorward boundary dynamics. Despite the known associations between these cusp-related boundaries, their dynamic responses to variations in solar wind parameters and dipole tilt have not been fully characterized. This study investigates the latitude variations of midday auroral equatorward boundary, OCB footprint in the ionosphere, PCB, Region 1 current poleward boundary, utilizing DMSP auroral observations and CCMC MHD simulation results. The analysis reveals that:All boundaries shift equatorward with increasing southward IMF Bz, consistent with enhanced dayside reconnection. The boundaries exhibit systematic responses to IMF By and solar wind velocity, reflecting asymmetric convection and magnetospheric compression. All boundaries in the Northern Hemisphere shift with dipole tilt. The latitude of the midday auroral lowest-latitude boundary shows seasonal variations and solar cycle dependence. These findings provide insights into the dependence of cusp location on solar wind conditions and dipole tilt, as well as the dynamic relationships between cusp-related boundaries, emphasizing the cusp’s role in solar wind-magnetosphere-ionosphere coupling.

The Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) is a joint European and Chinese spacecraft scheduled to launch in 2025 into a highly elliptical polar orbit. It will carry four instruments: the Soft X-ray Imager (SXI), the UltraViolet Imager (UVI), the Light Ion Analyzer (LIA), and the MAGnetometer (MAG). SMILE will image the dayside magnetosheath and cusps in soft X-ray, as well as the northern auroral oval in ultraviolet, for ∼41 continuous hours per orbit while simultaneously measuring plasma and magnetic field along its path. SMILE aims to advance our understanding of global solar wind – magnetosphere – ionosphere interactions. The Modeling Working Group (MWG), established in 2018, has fostered various modeling studies to ensure the successful scientific outcome of the SMILE mission. This paper overviews several MWG activities related to the SMILE SXI and UVI instruments. Firstly, we introduce the simulation of soft X-ray images of the Earth’s dayside magnetosphere, the SMILE orbit, and the SXI target visibilities. Secondly, we discuss multiple techniques developed for soft X-ray image analysis and the SXI’s capability to capture multi-scale interactions between the solar wind and Earth’s magnetosphere. Thirdly, we focus on the role of exospheric hydrogen density in determining near-Earth soft X-ray emissions, introducing several studies that estimate the exospheric density near the subsolar magnetopause location and its variability during geomagnetic storms. Finally, we present the modeling efforts for simulating the UVI instrument performance and the kinetic transport of suprathermal electrons and their impact on UV emissions.

We present a case study of large amplitude, compressional Pc5 waves observed by GOES 13 and 15 at geosynchronous orbit near noon, driven by two consecutive solar wind dynamic pressure spikes on September 26th, 2011.We present clear evidence that suggests that the first pressure spike gave rise to enhanced pressure anisotropy to drive the drift-mirror mode, whilst the second pressure, associated with a large and rapid southward turning of the Interplanetary Magnetic Field (IMF), spike allowed this anisotropy to increase in spatial extent. Magnetopause oscillations at a similar frequency were observed by the THEMIS spacecraft in the post-noon magnetosphere, however without clear signatures of compressional waves.Additionally, we analysed the ground response near the GOES foot-points, located close to the Churchill line of magnetometer stations. We observed an increase in wave power following each spike, although with different polarisations.Finally, following the second pressure spike’s arrival at the magnetosphere, we found evidence of Field line Resonances (FLR) in the Northward component of the magnetic field, which suggests coupling with the compressional waves observed at GOES.

Plasma convection on a global scale is a fundamental feature of planetary magnetosphere. The Dungey cycle explains that steady-state convection within the closed part of the magnetosphere relies on magnetic reconnection in the nightside magnetospheric tail. Nevertheless, time-dependent models of the Dungey cycle suggest an alternative scenario where magnetospheric convection can be solely driven by dayside magnetic reconnection. In this study, we provide direct evidence supporting the scenario of dayside-driven magnetosphere convection. The driving process is closely connected to the evolution of Region 1 and Region 2 field-aligned currents. Our global simulations demonstrate that intensified magnetospheric convection and field-aligned currents progress from the dayside to the nightside within 10-20 minutes, following a southward turning of the interplanetary magnetic field. Observational data within this short timescale also reveal enhancements in both magnetosphere convection and the ionosphere's two-cell convection. These findings provide insights into the mechanisms driving planetary magnetosphere convection, with implications for the upcoming Solar-Wind-Magnetosphere-Ionosphere Link Explorer (SMILE) mission.

The Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) Soft X-ray Imager (SXI) will shine a spotlight on magnetopause dynamics during magnetic reconnection. We simulate an event with a southward interplanetary magnetic field turning and produce SXI count maps with a 5-minute integration time. By making assumptions about the magnetopause shape, we find the magnetopause standoff distance from the count maps and compare it with the one obtained directly from the magnetohydrodynamic (MHD) simulation. The root mean square deviations between the reconstructed and MHD standoff distances do not exceed 0.2 RE (Earth radius) and the maximal difference equals 0.24 RE during the 25-minute interval around the southward turning.

Magnetospheric convection is a fundamental process in the coupling of the solar wind, magnetosphere, and ionosphere. Recent studies have shown that dayside magnetopause reconnection drives magnetospheric convection, progressing from the dayside to the nightside within approximately 10-20 min in response to southward turning of the interplanetary magnetic field. In this study, we use global magnetohydrodynamic (MHD) simulations to investigate the influence of ionospheric conductance on dayside-driven convection. We conduct three simulation runs: two with normal ionospheric conductance and one with nearly infinite conductance. The temporal and spatial pattern of magnetospheric convection largely remain consistent across all three simulation runs. Comparing the results, we observe a reduction of 20% in magnetospheric convection and a 30% increase of ionospheric Region 1 field-aligned current (FAC) and Pedersen current in the run with nearly infinite conductance, compared to the normal conductance model. The results indicate that ionospheric conductance does not affect the response time of enhanced magnetospheric convection to the solar wind. We suggest that the 10-20 min timescale for establishing magnetospheric convection corresponds to the anti-sunward drag of reconnected magnetic field lines from the sub-solar point to the flank magnetopause. In cases of larger ionospheric conductance, the ionosphere footprints of dragged field lines become more stationary, potentially resulting in larger Region 1 FAC and ionosphere Pedersen current. A larger Pedersen current is associated with stronger sunward J x B force in the ionosphere, which corresponds to a stronger anti-sunward force in the magnetosphere, thereby reducing sunward convection of closed field lines. An approximately infinite ionospheric conductance reduces the magnetospheric convection on the equatorial plane by similar to 20% The ionospheric FAC and Pedersen current increase similar to 30% as the magnetospheric convection is reduced The response time of the enhanced magnetospheric convection to solar wind is not impacted by the ionosphere conductance

The Soft X-ray Imager (SXI) is part of the scientific payload of the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. SMILE is a joint science mission between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS) and is due for launch in 2025. SXI is a compact X-ray telescope with a wide field-of-view (FOV) capable of encompassing large portions of Earth's magnetosphere from the vantage point of the SMILE orbit. SXI is sensitive to the soft X-rays produced by the Solar Wind Charge eXchange (SWCX) process produced when heavy ions of solar wind origin interact with neutral particles in Earth's exosphere. SWCX provides a mechanism for boundary detection within the magnetosphere, such as the position of Earth's magnetopause, because the solar wind heavy ions have a very low density in regions of closed magnetic field lines. The sensitivity of the SXI is such that it can potentially track movements of the magnetopause on timescales of a few minutes and the orbit of SMILE will enable such movements to be tracked for segments lasting many hours. SXI is led by the University of Leicester in the United Kingdom (UK) with collaborating organisations on hardware, software and science support within the UK, Europe, China and the United States.

The interplanetary magnetic field (IMF) north-south component, B-z, plays a crucial role in the interaction between the solar wind and the Earth's magnetosphere. We analyze 98 intervals in which B-z changed from >3 nT to <-3 nT in 5 min and for which these rapid southward turnings (STs) were surrounded by consistently northward or southward IMF. We separate out events in proximity of interplanetary coronal mass ejections and corotating interaction regions. We find that IMF magnitude, solar wind dynamic pressure and proton density (but also flow speed in ICME-related events) near the turnings are enhanced above their medians. We analyze the maximum responses of the SML, SMU, SYM-H, and PCN magnetospheric indices and their timescales, along with the occurrence of geomagnetic phenomena. We find that most STs were followed by either substorms (60.20%) or enhanced convection (37.76%). While SML has similar median minima (similar to-460 nT) and timescales (similar to 56 min) for substorm and convection events, SMU has noticeable differences. STs were followed by geomagnetic storms (SYM-H <= -50 nT) in 46.94% of events within 12 hr, with more storms following ICME-related turnings. PCN has peaks (median 3.8 mV/m) around 30 min after the turning, and larger ones (median 4.9 mV/m) later. Stronger solar wind driving and magnetospheric responses are observed for ICME-related events. The correlation between the geomagnetic and solar wind parameters around STs reveals a more direct link between solar wind driving and geomagnetic response for STs than at other times.

We use MHD simulations to study the time sequence of magnetospheric responses to a synthetic event with a southward interplanetary magnetic field (IMF) turning. The onset of dayside magnetopause reconnection launches a weak rarefaction wave and sunward flow in the equatorial magnetosphere simultaneously with a tailward flow through the polar cap. This convection results in the accumulation of magnetic flux in the tail lobes and thinning of the tail current layer which provides favorable conditions for the onset of nightside reconnection. The onset of nightside reconnection about 40 min later closes the Dungey convection cycle, resulting in a second increase in the sunward flow in the equatorial plane. Variations of the magnetopause standoff distance as well as the size of the polar cap (PC) may indicate the onsets of the dayside and nightside reconnections. We compare the results of two MHD models and discuss their differences. The auroras and other space weather phenomena are produced by the interaction of the solar wind with the Earth's magnetic environment. The fundamental plasma process of magnetic reconnection modulates this interaction, which then drives a circulation of magnetic field and plasma within the magnetosphere known as the Dungey cycle. Reconnection occurs when the magnetic field in the solar wind points southwards but turns off when it is northwards. The sequence of events within the magnetosphere in response to a southwards turning of the magnetic field is currently poorly understood. In this study, we model the interaction with two magnetohydrodynamic simulation codes. These show a rarefaction wave from the front of the magnetosphere toward the magnetotail in response to the turn-on of reconnection. Thereafter magnetic reconnection occurs in the magnetotail to complete the cycle. Although the simulations differ in the details, the two agree on this sequence of events, providing new insights into the dynamics of the magnetosphere. We investigate magnetospheric convection after southward turning and highlight the differences between the two MHD models Two-step response in convection after dayside and nightside reconnection Variations in the magnetopause standoff distance may indicate variations in the magnetic flux

We study the magnetospheric response to solar wind discontinuities with a southward interplanetary magnetic field (IMF) turning. We find two events characterized by a strong positive IMF Bz before the discontinuity and a strong negative Bz after the discontinuity. The magnetosphere stays in quiet conditions until the southward turning in both cases, then the dayside reconnection starts and the electromagnetic energy is accumulated in the magnetotail. We simulate these cases using several MHD models and compare numerical predictions of the global parameters such as the magnetopause standoff distance, open flux in the polar cap, auroral indices, and cross polar cap potential. We also make several runs of one model with different spatial resolutions and ionospheric conductivities. Summarizing this study, we discuss the differences between the MHD models and speculate about the reasons why one model is able to better predict observations than the other. We also discuss the reasons for different magnetospheric responses observed in two cases.

Abstract The Dungey cycle is a fundamental concept in understanding magnetosphere convection in the presence of the interplanetary magnetic field (IMF). It explains that the convection within the closed part of the magnetosphere is primarily driven by magnetic reconnection in the nightside magnetospheric tail. Here we provide evidence that magnetospheric convection can also be driven solely by dayside magnetic reconnection, specifically through the ionosphere's two-cell convection. The enhanced convection progresses from the dayside to the nightside and responds rapidly, within 10-20 minutes, to a southward turning of the IMF. Uncovering this physical process provides new insights into the driving mechanisms behind planetary magnetosphere convection.

The Earth's magnetosheath and cusps are the sources of soft X‐rays. In the accompanying paper (Part 1) and this paper, we discuss the methods of finding the magnetopause position by analyzing the X‐ray images. We use the software developed for the Soft X‐ray Imager (SXI) on board the forthcoming Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. We show how to find the maximum SXI count rate in noisy count maps. We verify the assumption that the maximum of the X‐ray emissivity integrated along the Line‐of‐Sight ( Ix ) is tangent to the magnetopause. We consider two cases using two MHD models and apply different methods of magnetospheric masking. Overall, the magnetopause is located close to the maximum Ix gradient or between the maximum Ix gradient and the maximum Ix depending on the method used. But since the angular distance between the maximum Ix gradient and the maximum Ix is relatively small (about 3°), the maximum Ix might be used as an indicator of the outer boundary of a wide magnetopause layer usually obtained in MHD simulations.

The magnetopause standoff distance characterizes global magnetospheric compression and deformation in response to changes in the solar wind dynamic pressure and interplanetary magnetic field orientation. We cannot derive this parameter from in situ spacecraft measurements. However, time series of the magnetopause standoff distance can be obtained in the near future using observations by soft X-ray imagers. In two companion papers, we describe methods of finding the standoff distance from X-ray images. In Part 1, we present the results of MHD simulations which we use for the calculation of the X-ray emissivity in the magnetosheath and cusps. Some MHD models predict relatively high density in the magnetosphere, larger than observed in the data. Correcting this, we develop magnetospheric masking methods to separate the magnetosphere from the magnetosheath and cusps. We simulate the X-ray emissivity in the magnetosheath for different solar wind conditions and dipole tilts.