Magnetopause Standoff Distance Research Articles

Time Sequence of Magnetospheric Responses to a Southward IMF Turning

AbstractWe 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.

Three‐Dimensional Global Hybrid Simulations of Mercury's Disappearing Dayside Magnetosphere

AbstractAn important discovery of MESSENGER is the occurrence of dayside disappearing magnetosphere (DDM) events that occur when the solar wind dynamic pressure is extremely high and the interplanetary magnetic field (IMF) is both intense and southward. In this study, we investigate the DDM events at Mercury under extreme solar wind conditions using a three‐dimensional (3‐D) global hybrid simulation model. Our results show that when the solar wind dynamic pressure is 107 nPa and the magnitude of the purely southward IMF is 50 nT, most of the dayside magnetosphere disappears within 10 s after the interaction between the solar wind and the planetary magnetic field starts. During the DDM event, the ion flux is significantly enhanced at most of the planetary dayside surface and reaches its maximum value of about 1010 cm−2 s−1 at the low‐latitude surface, which is much larger than that under normal solar wind conditions. During the DDM events, the dayside bow shock mostly disappears for about 9 s and then reappears. Moreover, the time evolution of magnetopause standoff distance under different solar wind conditions is also studied. When the solar wind dynamic pressure exceeds 25 nPa and the IMF is purely southward, a part of the dayside magnetosphere disappears. Under the same IMF, the higher the solar wind dynamic pressure, the faster the magnetopause standoff distance reaches the planetary surface. When the solar wind conditions are normal (with a dynamic pressure of 8 nPa) or the IMF is purely northward, the dayside magnetosphere does not disappear. The results provide a clear physical image of DDM events from a 3‐D perspective.

High‐Energy Electron Flux Enhancement Pattern in the Outer Radiation Belt in Response to the Interplanetary Coronal Mass Ejections

AbstractThe high‐energy electron flux enhancement pattern in the outer radiation belt is observed under the influence of the Interplanetary Coronal Mass Ejections (ICMEs). Ten events were selected during the Van Allen Probes era, with the high‐energy electron flux enhancement starting close to L* = 4. A schematic diagram of the main physical processes responsible for this kind of high‐energy electron flux enhancement is presented, considering the energy deposited in the inner magnetosphere under the influence of ICMEs. Superposed Epoch Analysis is applied to the interplanetary medium parameters, the magnetopause standoff distance, the storm‐time geomagnetic activity indices, the Ultra‐Low Frequency (ULF) waves, and the whistler‐mode chorus waves. A compressed magnetopause, Bz component preferentially southward and storm indices considerably high are observed at the beginning of the electron flux enhancements. The modeled parameters of the chorus waves at the dayside/nightside sectors show the high acceleration efficiency at the beginning of the electron flux enhancement pattern II. These results suggest that the local acceleration driven by chorus waves is essential to these electron flux enhancements at low L*. In contrast, although the ULF waves are detected at the beginning of the studied electron flux enhancements, their contribution is insignificant.

Quasi-elastodynamic Processes Involved in the Interaction between Solar Wind and Magnetosphere

The interaction between the solar wind and the magnetosphere is one of the most important research subjects in the fields of astrophysics and space physics. For more than half a century, based on the pressure balance assumption between the solar wind and the magnetosphere and considering other important factors, such as the interplanetary magnetic field and magnetic reconnection process, the dynamic processes at the magnetopause have been extensively analyzed. However, the responses of magnetopause to the solar wind dynamic pressure variations are still complicated to understand. Here, we show that the interaction between the solar wind and the magnetosphere can be regarded as a quasi-elastodynamic process. The driving frequency of the solar wind is determined as a crucial reason for the phase difference between solar wind dynamic pressure variations and magnetopause standoff distance. The low-pass filter effect and oscillation properties of the magnetopause can also be well explained by the forced damped vibrations. Moreover, the quasi-elastodynamic processes predict deformations at the magnetopause, which resemble the magnetopause surface wave. Finally, a three-dimensional time-dependent magnetopause model is constructed and verified by observation. Based on 12,242 magnetopause crossing events, it is found that the new model reveals ∼9.7% better prediction accuracy than the widely used time-independent model. These results can also shed light on our understanding of the solar-wind–magnetopause interaction for other planets.

An Interpretable Machine Learning Procedure Which Unravels Hidden Interplanetary Drivers of the Low Latitude Dayside Magnetopause

AbstractIn this study, we propose an interpretable machine learning procedure to unravel the importance of multiple interplanetary parameters to the Earth's magnetopause standoff distance (MSD). We construct the interpretable procedure based on SHapley Additive exPlanations. A magnetopause crossings database from the Time History of Events and Macroscale Interactions during Substorms satellites and the multiple interplanetary parameters from OMNI during the period of 2007–2016 are utilized. The solar wind dynamic pressure and the interplanetary magnetic field (IMF) BZ are widely suggested as the important two interplanetary parameters that drive the MSD. However, the examination of the interpretable procedure suggests that the magnitude of the IMF is the second significant parameter after the dynamic pressure. Although the magnetic pressure, which is the function of the IMF magnitude was considered in previous studies, the importance of the IMF magnitude was underestimated. The interpretable procedure also reveals that the IMF magnitude and the BZ have different effects on the MSD. Their joint effect is the formation of the MSD sag near BZ = 5 nT. This is for the first time the interpretable concept is being applied to construct a machine‐learning magnetopause model.

The exoplanetary magnetosphere extension in Sun-like stars based on the solar wind–solar UV relation

ABSTRACT The Earth’s magnetosphere extension is controlled by the solar activity level via solar wind properties. Understanding such a relation in the Solar system is important for predicting the condition of exoplanetary magnetospheres near Sun-like stars. We use measurements of a chromospheric proxy, the Ca ii K index, and solar wind OMNI parameters to connect the solar activity variations, on decennial time-scales, to the solar wind properties. The data span the period 1965–2021, which almost entirely covers the last five solar cycles. Using both cross-correlation and mutual information analysis, we find a 3.2-yr lag of the solar wind speed with respect to the Ca ii K index. Analogously, a 3.6-yr lag is found once we consider the dynamic pressure. A correlation between the solar wind dynamic pressure and the solar ultraviolet emission is found and used to derive the Earth’s magnetopause standoff distance. Moreover, the advantage of using a chromospheric proxy, such as the Ca ii K index, creates the possibility to extend the relation found for the Sun to Sun-like stars, by linking stellar variability to stellar wind properties. The model is applied to a sample of Sun-like stars as a case study, where we assume the presence of an Earth-like exoplanet at 1 au. Finally, we compare our results with previous estimates of the magnetosphere extension for the same set of Sun-like stars.

Finding Magnetopause Standoff Distance Using a Soft X‐Ray Imager: 1. Magnetospheric Masking

AbstractThe 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.

Successive Interacting Coronal Mass Ejections: How to Create a Perfect Storm

Coronal mass ejections (CMEs) are the largest type of eruptions on the Sun and the main driver of severe space weather at the Earth. In this study, we implement a force-free spheromak CME description within 3D magnetohydrodynamic simulations to parametrically evaluate successive interacting CMEs within a representative heliosphere. We explore CME–CME interactions for a range of orientations, launch time variations, and CME handedness and quantify their geo-effectiveness via the primary solar wind variables and empirical measures of the disturbance storm time index and subsolar magnetopause standoff distance. We show how the interaction of two moderate CMEs between the Sun and the Earth can translate into extreme conditions at the Earth and how CME–CME interactions at different radial distances can maximize different solar wind variables that induce different geophysical impacts. In particular, we demonstrate how the orientation and handedness of a given CME can have a significant impact on the conservation and loss of magnetic flux, and consequently B z , due to magnetic reconnection with the interplanetary magnetic field. This study thus implicates the identification of CME chirality in the solar corona as an early diagnostic for forecasting geomagnetic storms involving multiple CMEs.

Variations in Observations of Geosynchronous Magnetopause and Last Closed Drift Shell Crossings With Magnetic Local Time

AbstractWe analyze a set of events in which both electron flux dropouts caused by magnetopause shadowing and geosynchronous magnetopause crossings (GMCs) are observed. These observations are compared to event‐specific last closed drift shell (LCDS) models derived from the TS05 and TS07 external field models and magnetopause standoff distance. The LCDS models show good association with losses due to magnetopause shadowing but fail to reproduce observations of GMCs on the timescale of minutes. We show that different satellites in geostationary orbit observe different trends in electron flux during storm events on timescales of less than a day due to their separation in longitude. These differences demonstrate that both satellite L* and magnetic local time must be taken into account when modeling rapid variations in the outer radiation belt, and at least three satellites in geostationary orbit, ideally more, may be required for accurate forecasting and reconstruction of these events on timescales shorter than days.

MHD study of the planetary magnetospheric response during extreme solar wind conditions: Earth and exoplanet magnetospheres applications

Context.The stellar wind and the interplanetary magnetic field modify the topology of planetary magnetospheres. Consequently, the hazardous effect of the direct exposition to the stellar wind, for example, regarding the integrity of satellites orbiting the Earth or the habitability of exoplanets, depends upon the space weather conditions.Aims.The aim of the study is to analyze the response of an Earth-like magnetosphere for various space weather conditions and interplanetary coronal mass ejections. The magnetopause standoff distance, the open-close field line boundary, and plasma flows toward the planet surface are calculated.Methods.We used the magnetohydrodynamics code PLUTO in spherical coordinates to perform a parametric study of the dynamic pressure and temperature of the stellar wind as well as of the interplanetary magnetic field intensity and orientation. The range of the parameters we analyzed extends from regular to extreme space weather conditions, which is consistent with coronal mass ejections at the Earth orbit for the present and early periods of the solar main sequence. In addition, implications of sub-Afvénic solar wind configurations for the Earth and exoplanet magnetospheres were analyzed.Results.The direct precipitation of the solar wind at the Earth dayside in equatorial latitudes is extremely unlikely even during super coronal mass ejections. On the other hand, for early evolution phases during the solar main sequence, when the solar rotation rate was at least five times faster (<440 Myr), the Earth surface was directly exposed to the solar wind during coronal mass ejections. Today, satellites at high, geosynchronous, and medium orbits are directly exposed to the solar wind during coronal mass ejections because part of the orbit at the Earth dayside is beyond the nose of the bow shock.

Global Magnetohydrodynamic Simulations: Performance Quantification of Magnetopause Distances and Convection Potential Predictions

The performance of three global magnetohydrodynamic (MHD) models in estimating the Earth's magnetopause location and ionospheric cross polar cap potential (CPCP) have been presented. Using the Community Coordinated Modeling Center's Run-on-Request system and extensive database on results of various magnetospheric scenarios simulated for a variety of solar weather patterns, the aforementioned model predictions have been compared with magnetopause standoff distance estimations obtained from six empirical models, and with cross polar cap potential estimations obtained from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) Model and the Super Dual Auroral Radar Network (SuperDARN) observations. We have considered a range of events spanning different space weather activity to analyze the performance of these models. Using a fit performance metric analysis for each event, the models' reproducibility of magnetopause standoff distances and CPCP against empirically-predicted observations were quantified, and salient features that govern the performance characteristics of the modeled magnetospheric and ionospheric quantities were identified. Results indicate mixed outcomes for different models during different events, with almost all models underperforming during the extreme-most events. The quantification also indicates a tendency to underpredict magnetopause distances in the absence of an inner magnetospheric model, and an inclination toward over predicting CPCP values under general conditions.

Modelling the imposed magnetospheres of Mars-like exoplanets: star–planet interactions and atmospheric losses

ABSTRACT Based on 3D compressible magnetohydrodynamic simulations, we explore the interactions between the magnetized wind from a solar-like star and a Mars-like planet – with a gravitionally stratified atmosphere – that is either non-magnetized or hosts a weak intrinsic dipolar field. The primary mechanism for the induction of a magnetosphere around a non-magnetized conducting planet is the pile-up of stellar magnetic fields in the day-side region. The magnetopause stand-off distance decreases as the strength of the planetary dipole field is lowered and saturates to a minimum value for the case of a planet with no magnetic field. Global features such as bow shock, magnetosheath, magnetotail, and strong current sheets are observed in the imposed magnetosphere. We explore variations in atmospheric mass loss rates for different stellar wind strengths to understand the impact of stellar magnetic activity and plasma winds – and their evolution – on (exo)planetary habitability. In order to simulate a case analogous to the present-day Mars, a planet without atmosphere is considered. Our simulations are found to be in good agreement with observational data from Mars Global Surveyor and Mars Atmosphere and Volatile EvolutioN missions and is expected to complement observations from the Emirates (Hope) Mars Mission, China's Tianwen-1 and NASA's Mars 2020 Perseverance mission.

Cross‐Scale Quantification of Storm‐Time Dayside Magnetospheric Magnetic Flux Content

AbstractA clear understanding of storm‐time magnetospheric dynamics is essential for a reliable storm forecasting capability. The dayside magnetospheric response to an interplanetary coronal mass ejection (ICME; dynamic pressure Pdyn > 20 nPa and storm‐time index SYM‐H < −150 nT) is investigated using in situ OMNI, Geotail, Cluster, MMS, GOES, Van Allen Probes, and THEMIS measurements. The dayside magnetic flux content is directly quantified from in situ magnetic field measurements at different radial distances. The arrival of the ICME, consisting of shock and sheath regions preceding a magnetic cloud, initiated a storm sudden commencement (SSC) phase (SYM‐H ~ +50 nT). At SSC, the magnetopause standoff distance was compressed earthward at ICME shock encounter at an average rate ~−10.8 Earth radii per hour for ~10 min, resulting in a rapid 40% reduction in the magnetospheric volume. The “closed” magnetic flux content remained constant at 170 ± 30 kWb inside the compressed dayside magnetosphere, even in the presence of dayside reconnection, as evident by an outsized flux transfer event containing 160 MWb. During the storm main and recovery phases, the magnetosphere expanded. The dayside magnetic flux did not remain constant within the expanding magnetosphere (110 ± 30 kWb), resulting in a 35% reduction in pre‐storm flux content during the magnetic cloud encounter. At that stage, the magnetospheric magnetic flux was eroded resulting in a weakened dayside magnetospheric field strength at radial distances R ≥ 5 RE. It is concluded that the inadequate replenishment of the eroded dayside magnetospheric flux during the magnetosphere expansion phase is due to a time lag in storm‐time Dungey cycle.

Is the Relation Between the Solar Wind Dynamic Pressure and the Magnetopause Standoff Distance so Straightforward?

AbstractWe present results of global magnetohydrodynamic simulations which reconsider the relationship between the solar wind dynamic pressure (Pd) and magnetopause standoff distance (RSUB). We simulate the magnetospheric response to increases in the dynamic pressure by varying separately the solar wind density or velocity for northward and southward interplanetary magnetic field (IMF). We obtain different values of the power law indices N in the relation depending on which parameter, density, or velocity, has been varied and for which IMF orientation. The changes in the standoff distance are smaller (higher N) for a density increase for southward IMF and greater (smaller N) for a velocity increase. An enhancement of the solar wind velocity for a southward IMF increases the magnetopause reconnection rate and Region 1 current that move the magnetopause closer to the Earth than it appears in the case of density increase for the same dynamic pressure.

Convection in the Magnetosphere of Saturn During the Cassini Mission Derived From MIMI INCA and CHEMS Measurements

AbstractAn extensive analysis of Cassini Ion and Neutral Camera (INCA) and Charge Energy Mass Spectrometer (CHEMS) measurements of ~6–231 keV ion anisotropies acquired during selected spin and stare periods for nearly all mission orbits has been completed. Based on this analysis, we find that the computed azimuthal speed of Saturn's magnetodisk plasma increases within the inner and middle magnetosphere. Beyond the orbit of Titan, in the magnetotail, the calculated rotation speed remains roughly constant with increasing distance from Saturn. The component of convection parallel to Saturn's spin axis is smaller and on average nearly zero. The radial speed of plasma shows distinct local time dependence and increases outward with increasing distance down the magnetotail. The magnetodisk flow remains primarily azimuthal to large distances, indicating the plasma is still under the influence of Saturn as it transits across the nightside. Tailward flows have been observed in the region near the dawn and dusk magnetotail flanks. The plasma flow in the predawn quadrant is much more disorganized than that in the premidnight quadrant. The hydrogen and oxygen hot ion temperatures increase with decreasing distance to Saturn. Some plasma evidently escapes into a dusk boundary layer at the dusk magnetotail flank, while the remaining plasma primarily moves across the magnetotail and is entrained into the flow of a boundary layer at the dawn flank of the magnetotail. When scaled to the magnetopause standoff distance and corotation fraction, the convection speed of plasma in the magnetospheres of Jupiter and Saturn is similarly organized.

Magnetohydrodynamic simulations of a Uranus-at-equinox type rotating magnetosphere

Context. As proven by measurements at Uranus and Neptune, the magnetic dipole axis and planetary spin axis can be off by a large angle exceeding 45°. The magnetosphere of such an (exo-)planet is highly variable over a one-day period and it does potentially exhibit a complex magnetic tail structure. The dynamics and shape of rotating magnetospheres do obviously depend on the planet’s characteristics but also, and very substantially, on the orientation of the planetary spin axis with respect to the impinging, generally highly supersonic, stellar wind. Aims. On its orbit around the Sun, the orientation of Uranus’ spin axis with respect to the solar wind changes from quasi-perpendicular (solstice) to quasi-parallel (equinox). In this paper, we simulate the magnetosphere of a fictitious Uranus-like planet plunged in a supersonic plasma (the stellar wind) at equinox. A simulation with zero wind velocity is also presented in order to help disentangle the effects of the rotation from the effects of the supersonic wind in the structuring of the planetary magnetic tail. Methods. The ideal magnetohydrodynamic (MHD) equations in conservative form are integrated on a structured spherical grid using the Message-Passing Interface-Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC). In order to limit diffusivity at grid level, we used background and residual decomposition of the magnetic field. The magnetic field is thus made of the sum of a prescribed time-dependent background field B0(t) and a residual field B1(t) computed by the code. In our simulations, B0(t) is essentially made of a rigidly rotating potential dipole field. Results. The first simulation shows that, while plunged in a non-magnetised plasma, a magnetic dipole rotating about an axis oriented at 90° with respect to itself does naturally accelerate the plasma away from the dipole around the rotation axis. The acceleration occurs over a spatial scale of the order of the Alfvénic co-rotation scale r*. During the acceleration, the dipole lines become stretched and twisted. The observed asymptotic fluid velocities are of the order of the phase speed of the fast MHD mode. In two simulations where the surrounding non-magnetised plasma was chosen to move at supersonic speed perpendicularly to the rotation axis (a situation that is reminiscent of Uranus in the solar wind at equinox), the lines of each hemisphere are symmetrically twisted and stretched as before. However, they are also bent by the supersonic flow, thus forming a magnetic tail of interlaced field lines of opposite polarity. Similarly to the case with no wind, the interlaced field lines and the attached plasma are accelerated by the rotation and also by the transfer of kinetic energy flux from the surrounding supersonic flow. The tailwards fluid velocity increases asymptotically towards the externally imposed flow velocity, or wind. In one more simulation, a transverse magnetic field, to both the spin axis and flow direction, was added to the impinging flow so that magnetic reconnection could occur between the dipole anchored field lines and the impinging field lines. No major difference with respect to the no-magnetised flow case is observed, except that the tailwards acceleration occurs in two steps and is slightly more efficient. In order to emphasise the effect of rotation, we only address the case of a fast-rotating planet where the co-rotation scale r* is of the order of the planetary counter-flow magnetopause stand-off distance rm. For Uranus, r*≫ rm and the effects of rotation are only visible at large tailwards distances r ≫ rm.

Solar Wind Interaction With Jupiter's Magnetosphere: A Statistical Study of Galileo In Situ Data and Modeled Upstream Solar Wind Conditions

AbstractJupiter's magnetosphere is often said to be rotationally driven, with strong centrifugal stresses due to large spatial scales and a rapid planetary rotation period. While the solar wind is therefore expected to have a relatively small influence on Jupiter's magnetosphere and aurora, there is considerable observational evidence that the solar wind does affect the magnetopause standoff distance, auroral radio emissions, and the ultraviolet auroral position and brightness. We report on the results of a comprehensive, quantitative study of the influence of the solar wind dynamic pressure on magnetospheric data sets measured by the Galileo mission (1996–2003). Using model predictions of the solar wind conditions near Jupiter calculated by propagating measurements made near the Earth, we have established how predicted changes in the solar wind affect the magnitude and direction of the magnetic field in the magnetosphere, the hectometric auroral radio emissions, and energetic particles in Jupiter's magnetosphere. We find that increases in the solar wind dynamic pressure are statistically associated with magnetospheric compression events but that tail reconnection and plasmoid release is most likely driven internally by the Vasyliunas cycle. We find no link between solar wind conditions and the occurrence of quasiperiodic modulations in the magnetosphere. Overall, we conclude that activity in Jupiter's magnetosphere is both internally and solar wind driven. Finally, we examine the effects of solar wind‐induced magnetospheric compressions on Jupiter's auroral brightness and mapping, finding that a solar wind compression could shift the auroral mapping of a given point in the magnetosphere by ~3° on average.

The evolution of Earth’s magnetosphere during the solar main sequence

ABSTRACT As a star spins-down during the main sequence, its wind properties are affected. In this work, we investigate how Earth’s magnetosphere has responded to the change in the solar wind. Earth’s magnetosphere is simulated using 3D magnetohydrodynamic models that incorporate the evolving local properties of the solar wind. The solar wind, on the other hand, is modelled in 1.5D for a range of rotation rates Ω from 50 to 0.8 times the present-day solar rotation (Ω⊙). Our solar wind model uses empirical values for magnetic field strengths, base temperature, and density, which are derived from observations of solar-like stars. We find that for rotation rates ≃10 Ω⊙, Earth’s magnetosphere was substantially smaller than it is today, exhibiting a strong bow shock. As the Sun spins-down, the magnetopause standoff distance varies with Ω−0.27 for higher rotation rates (early ages, ≥1.4 Ω⊙) and with Ω−2.04 for lower rotation rates (older ages, <1.4 Ω⊙). This break is a result of the empirical properties adopted for the solar wind evolution. We also see a linear relationship between the magnetopause distance and the thickness of the shock on the subsolar line for the majority of the evolution (≤10 Ω⊙). It is possible that a young fast rotating Sun would have had rotation rates as high as 30–50 Ω⊙. In these speculative scenarios, at 30 Ω⊙, a weak shock would have been formed, but for 50 Ω⊙, we find that no bow shock could be present around Earth’s magnetosphere. This implies that with the Sun continuing to spin-down, a strong shock would have developed around our planet and remained for most of the duration of the solar main sequence.

Long‐Term Variations in Solar Wind Parameters, Magnetopause Location, and Geomagnetic Activity Over the Last Five Solar Cycles

AbstractWe use both solar wind observations and empirical magnetopause models to reconstruct time series of the magnetopause standoff distance for nearly five solar cycles. Since the average annual interplanetary magnetic field (IMF) Bz is about zero, and the annual IMF cone angle varies between 54.0° and 61.2°, the magnetopause standoff distance on this timescale depends mostly on the solar wind dynamic pressure. The annual IMF magnitude well correlates with the sunspot number (SSN) with a zero time lag, while the annual solar wind dynamic pressure (Pdyn) correlates reasonably well with the SSN but with 3‐year time lag. At the same time, we find an anticorrelation between Pdyn and SSN in cycles 20–21 and a correlation in cycles 22–24 with 2‐year time lag. Both the annual solar wind density and velocity well correlate with the dynamic pressure, but the correlation coefficient is higher for density than for velocity. The 11‐year solar cycles in the dynamic pressure variations are superimposed by an increasing trend before 1991 and a decreasing trend between 1991 and 2009. The average annual solar wind dynamic pressure decreases by a factor of 3 from 1991 to 2009. Correspondingly, the predicted standoff distance in Lin et al.'s (2010, https://doi.org/10.1029/2009JA014235) magnetopause model increases from 9.7 RE in 1991 to 11.6 RE in 2009. The annual SSN, IMF magnitude, and magnetospheric geomagnetic activity indices display the same trends as the dynamic pressure. We calculate extreme solar wind parameters and magnetopause standoff distance in each year using daily values and find that both extremely small and large standoff distances during a solar cycle preferably occur at solar maximum rather than at solar minimum.

MESSENGER Observations and Global Simulations of Highly Compressed Magnetosphere Events at Mercury

AbstractWe identify and examine all MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER) crossings of Mercury's dayside magnetopause with magnetospheric field intensities ≥300 nT. The eight such events, which occurred under highly compressed magnetosphere conditions, are analyzed in the identical manner utilized by Slavin et al. (2014, https://doi.org/10.1002/2014JA020319). The results suggest that the eight highly compressed magnetosphere events represent the highest solar wind dynamic pressures for which the MESSENGER's orbit still passed below the magnetopause and provided measurements of the dayside magnetosphere. Using the magnetohydrodynamic model by Jia et al. (2015, https://doi.org/10.1002/2015JA021143) that electromagnetically couples Mercury's interior with its magnetosphere, a series of global simulations are conducted to quantitatively characterize the response of Mercury's magnetosphere to solar wind forcing. Combining the MESSENGER observations with the simulations, we have obtained a consistent picture of how Mercury's dayside magnetospheric configuration is controlled, separately and in combination, by induction‐driven shielding and reconnection‐driven erosion. For solar wind pressures of ∼40–90 nPa, compared with the average ∼10–15 nPa at Mercury's orbit, the shielding effects of induction in Mercury's core in standing‐off the solar wind typically exceed the erosion of the dayside magnetosphere due to reconnection for these events, most of which occurred under low magnetic shear conditions. For high magnetic shear across the magnetopause our simulation predicts that reconnection would dominate. Mercury's effective magnetic moment as inferred from magnetopause standoff distance ranges from 170 to 250 for these events. These findings are of crucial importance for understanding the space weathering at Mercury and its contribution to the generation of Mercury's exosphere.