Subsolar Magnetopause Research Articles

Joint Observation of a Series of Magnetic Holes by Tianwen-1 and MAVEN on Mars

Magnetic holes (MHs) are transient magnetic structures responsible for energy conversion in space plasma. Using single-spacecraft measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN), the existence of MHs on Mars has been confirmed. However, due to the limitations of single-spacecraft observations, significant uncertainty also arises on the identification of the spatial scale and 3D geometry of MHs. In this study, we report a series of MHs successively detected by Tianwen-1 near the high-latitude magnetopause and by the MAVEN spacecraft near the subsolar magnetopause. The large separation between Tianwen-1 and MAVEN (∼4 R M) suggests these MHs are macroscale structures extending along the axial direction. Additionally, we observe whistler waves generated by electron perpendicular anisotropy in one of the macroscale MHs. This study is the first joint observation of Martian MHs, shedding light on the research of transient magnetic structures on Mars.

IMF Bx, By, and Bz dependence of energetic particle precipitation in the magnetosphere

The orientation of Interplanetary Magnetic Field (IMF B) components plays a major role in magnetospheric Energetic Particles Precipitation (EPP). This is because the IMF B components in the Geocentric Solar Magnetospheric (GSM) coordinate system control magnetic reconnection at the subsolar magnetopause. This study intends to unravel the dependence of the preceding conditions of the orientations of IMF B components that drove geomagnetic storms on the EPPs produced in the Van Allen radiation belt. We classified the geomagnetic storms based on their drivers and IMF Bz orientations prior to the storms’ onset. Energetic particle fluxes, IMF B, and solar wind data were used. The study showed that electrons are more reactive to solar wind speed than protons. Additionally, storms with southward IMF Bz orientation caused more EPP enhancement than with northward orientation. Our results also showed that protons are more influenced by magnetospheric compression than electrons at the northward orientation of IMF Bz. In fact, at this point, the proton’s motion violates adiabatic invariants. As particles interact with electric fields, a significant population of EPPs is produced by pitch angle diffusion. Generally, IMF Bx and IMF By are often anticorrelated. The preceding conditions of orientations of the IMF Bx indicated weaker dependence on auroral indices, and invariably on energetic particles precipitation than the IMF By.

Mercury’s Bow Shock and Magnetopause Variations According to MESSENGER Data

Using data from the MESSENGER spacecraft magnetometer that describes the magnetopause and the bow shock crossing points of the Mercury’s magnetosphere, we have calculated the parameters of the paraboloids of revolution approximating the obtained points. For each spacecraft orbit, the subsolar magnetopause and bow shock standoff distances were obtained, based on the paraboloid parameters for each crossing point. The dependences of the magnetopause and bow shock subsolar standoff distances on the Mercury’s position relative to the Sun have been obtained. These profiles agree with decreases of the solar wind plasma dynamic pressure and the interplanetary magnetic field strength with heliocentric distance. The variations of the interplanetary and magnetosheath magnetic field were investigated. The average subsolar magnetosheath thickness and the value of the magnetic field jump at the bow shock during the transition from the upstream interplanetary magnetic field region to the magnetosheath were obtained.

Plasma Properties in the Earth's Magnetosheath Near the Subsolar Magnetopause: Implications for Geocoronal Density Estimates

AbstractCombined in situ ion measurements and remote sensing of energetic neutral atoms are used to determine the geocoronal Hydrogen density at large (∼10 RE) distances from the Earth. This method for determining the geocoronal density requires global magnetospheric modeling. Observations in the Earth's subsolar magnetosheath from the Magnetospheric Multiscale mission are used to determine the accuracy of using global models to predict the geocoronal density. On average, gas dynamic and magnetohydrodynamic (MHD) models and observations are in reasonable agreement, with differences <25%. In addition, the MHD model subsolar magnetopause is about 0.5 RE sunward of the observed location. However, variations around averages are large (up to a factor of 2), indicating that global models introduce relatively large uncertainties in geocoronal density estimates. Finally, the critical ion flux in the Interstellar Boundary Explorer IBEX‐Hi energy range is often minimally affected by fluctuations of a factor of 2 in the density.

An approach of filtering simulated magnetospheric X-ray images based on self-supervised network and random forest

Imaging is an important method for observing the Earth’s space environment. Future missions, such as the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), aim to explore the interaction between the solar wind and the Earth’s magnetosphere via soft x-ray imaging. With the advent of these missions, a large number of magnetospheric images may be acquired. However, as the viewing geometry and solar wind conditions change, satellites sometimes fail to capture the magnetopause inside the field of view. We propose an approach that blends machine learning and deep learning to filter the simulated x-ray images for the SMILE mission, aiming to achieve automatic classification of the detected images. First, we performed magnetohydrodynamic simulations to derive the predicted SMILE x-ray images. Then, we used a self-supervised contrast feature extraction network to study the features of the images. Using this network, the random forest classifier can distinguish whether the subsolar point at the magnetopause has been detected. Finally, we designed the magnetopause filter to obtain the subsolar magnetopause images with observation positions outside the magnetosphere. As a result, the prediction accuracy of the classifier is up to 93%. And the F1 score is up to 95.5%. The stratified predictions allow an automatic screening of whether satellite magnetospheric images cover the subsolar magnetopause. These images, which have observation positions outside the magnetosphere, can be used to invert the three-dimensional magnetopause.

Dynamical Response of Solar Wind Charge Exchange Soft X-Ray Emission in Earth’s Magnetosphere to the Solar Wind Proton Flux

This work studies the dynamic response of solar wind charge exchange (SWCX) soft X-ray emission in the Earth’s magnetosphere to the solar wind proton flux. Unlike previous studies that attempted to use complex magnetohydrodynamic models to match the details of observed SWCX of a necessarily limited number of cases, this work focuses on determining the changes over individual observations in a much larger sample. To provide the cleanest test, we selected XMM-Newton observations when the solar wind proton flux changed suddenly by a factor greater than 1.5 and calculated the correlation coefficient between the SWCX emission in the 0.5–0.7 keV band and the proton flux. We find that the dynamical response is weak when the solar wind proton flux is low (<10,000 n*km/cc/s) because its variation is smaller than the uncertainty due to other emission components, but this response increases with the proton flux and its change value. The response is improved when the valence state of solar wind ions is high, as a higher abundance of ions generating SWCX can produce a greater correlation even though the proton flux is relatively low. It is conducive to the study of interplanetary coronal mass ejections (ICMEs) because ions in ICMEs are usually highly ionized. For XMM-Newton, the 0.5–0.7 keV band shows the strongest correlation, as the instrumental response decreases at lower energies and the SWCX emission decreases at higher energies. Moreover, the closer the satellite line of sight is to the subsolar magnetopause with the strongest SWCX emissivity, the better the correlation.

A Linear Velocity Gradient in the Subsolar Magnetosheath

AbstractTracking the location and motion of the bow shock and magnetopause can reveal many aspects concerning the nature of the solar wind‐magnetosphere interaction. In the absence of missions that image the magnetopause, we propose to detect and track the magnetopause and bow shock motion using multipoint in situ observations of the magnetosheath. Making theoretical assumptions corroborated by numerical simulations, Sibeck et al. (2022, https://doi.org/10.1029/2022JA030704) proposed a method to predict the subsolar magnetopause and the bow shock from the linear variation in magnetosheath plasma velocity expected from global magnetohydrodynamic simulations along the Sun‐Earth line. This paper presents a case study that employs the velocity gradient method using multi‐point THEMIS plasma observations from 21 to 22 June 2007. Using the five THEMIS spacecraft we demonstrate that the radial gradient in plasma velocity remains nearly linear across the subsolar magnetosheath, except when the magnetosheath lies behind the quasi‐parallel bow shock and conditions are very turbulent. The bow shock and magnetopause move outward in response to a gradual decrease in solar wind dynamic pressure. We show that results from the velocity gradient method agree well with predictions of global magnetohydrodynamic and empirical models for this time interval. The velocity gradient method can therefore be used to remotely determine the location of the magnetopause and bow shock, solely based on two‐spacecraft measurements at the subsolar magnetosheath when not downstream of the quasi‐parallel shock.

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.

Modeling of the Subsolar Magnetopause Motion Under Interplanetary Magnetic Field Southward Turning

AbstractIn this study, a new analytical model to describe the time‐dependent subsolar magnetopause motion under interplanetary magnetic field (IMF) southward turning has been developed. This model, based on the scenario of magnetopause erosion due to magnetic reconnection, can be approximated by both linear and non‐linear functions. The linear function is simplified under the assumption of constant magnetopause erosion under southward IMF, and the non‐linear function is derived by assuming that the magnetopause erosion decays exponentially. In the limit of a short time, the non‐linear function is essentially the same as the linear function. By comparing with global magnetohydrodynamic simulations, the linear function performs well within the first 10 minutes, and the error then increases with time. The non‐linear function describes the magnetopause motion more accurately with respect to, and consistent with simulations for a time interval of ∼40 min. This model has also been successfully applied to data‐driven simulations of the 17 March 2015 geomagnetic storm event, suggesting the possible applicability of this model in reality.

Magnetosphere-Ionosphere Coupling: Implications of Non-Equilibrium Conditions

The response times of the coupled magnetosphere-ionosphere-thermosphere system are, on average, greater than the autocorrelation timescales of solar wind forcing. This means that the system is rarely, if ever, in equilibrium. Departures from equilibrium are a key component of the Expanding-Contracting Polar Cap (ECPC) model of convection excitation in both the magnetosphere and ionosphere, driven by the Dungey reconnection cycle of opening and re-closing magnetospheric field lines. Averaging over sufficiently long timescales reduces data to the equivalent of steady-state conditions, which hides the physical mechanisms involved and allows us to map electric fields from interplanetary space to the ionosphere–but this is not valid, either physically or generally, because of magnetic induction effects. Only for transient phenomena on sufficiently short timescales do the mechanisms associated with non-equilibrium fully manifest themselves. Nevertheless, because of both ever-changing solar wind conditions and Earth’s dipole tilt, eccentricity and rotation, the magnetosphere is always tending towards a perpetually-evolving equilibrium configuration and there are important implications of transient events for understanding the general behavior of the coupled magnetosphere-ionosphere-thermosphere system and its response to solar wind forcing. We here discuss one example: as a consequence of the importance of departures from equilibrium inherent in the ECPC model, the solar wind dynamic pressure PSW influences the magnetosphere-ionosphere convection response to the generation of open field lines by reconnection in the dayside subsolar magnetopause. We here demonstrate this effect in a statistical survey of observations and show that it is as predicted by the ECPC model and that, through it, PSW has an influence on flux transport in the magnetosphere-ionosphere system.

Overshoot Structure Near the Earth’s Subsolar Magnetopause Generated by Magnetopause Motions

For magnetopause crossing events, the observed magnetospheric magnetic fields in the vicinity of the subsolar magnetopause frequently present an overshoot structure; that is, in small vicinity of the magnetopause, the closer to the magnetopause, the stronger the magnetospheric magnetic field is. In this investigation, an automatic identification algorithm is developed to rapidly and effectively search the magnetopause crossing events using THEMIS data from 2007 to 2021. Nearly 59% of magnetopause crossing events identified near the subsolar region appear an overshoot structure. The statistical result shows that, for overshoot cases, the normalized change rate of magnetospheric magnetic field near the magnetopause is linearly related to the normalized magnetopause velocity, which means that the overshoot structure may be caused by the redistribution of the magnetospheric magnetic field due to the rapid magnetopause motion.

Solar Minimum Exospheric Neutral Density Near the Subsolar Magnetopause Estimated From the XMM Soft X‐Ray Observations on 12 November 2008

AbstractThe Earth's magnetosheath and cusps emit soft X‐rays due to the charge exchange between highly charged solar wind ions and exospheric hydrogen atoms. The Lunar Environment Heliospheric X‐ray Imager and Solar wind Magnetosphere Ionosphere Link Explorer missions are scheduled to image the Earth's dayside magnetosphere system in soft X‐rays to investigate global‐scale magnetopause reconnection modes under varying solar wind conditions. The exospheric neutral hydrogen density distribution, especially the value of this density at the subsolar magnetopause is of particular interest for understanding X‐ray emissions near this boundary. This paper estimates the exospheric density during solar minimum using the X‐ray Multimirror Mission (XMM) astrophysics observatory. We selected an event on 12 November 2008 from the XMM data archive, which detects soft X‐rays of magnetosheath origin while solar wind and interplanetary magnetic field conditions are relatively constant. During the event the location of the magnetopause was measured in situ by the THEMIS mission, thus the location of the solar wind ions responsible for the magnetosheath emission is well constrained by observation. We estimated the exospheric density using the Open Geospace Global Circulation Model (OpenGGCM) and a spherically symmetric exosphere model. The ratio of the magnetosheath plasma flux between the OpenGGCM model and the THEMIS, was nearly 1, which means the magnetohydrodynamic model reasonably reproduces the magnetosheath plasma conditions. The OpenGGCM magnetosheath parameters were used to deconvolve soft X‐rays of exospheric origin from the XMM signal. The lower‐limit of the exospheric density of this solar minimum event is 36.8 ± 11.7 cm−3 at 10 RE subsolar location.

Simulating the Solar Wind‐Magnetosphere Interaction During the Matuyama‐Brunhes Paleomagnetic Reversal

AbstractDuring a paleomagnetic reversal, Earth's geomagnetic field is about 10% of today's magnitude and its topology is far more complex than dipolar. Revealing the solar wind energy transmission during the reversal is of importance for understanding the historic space environment. Using a global magnetohydrodynamic model implemented with a data‐reconstructed time‐dependent paleomagnetic field, this study simulates the solar wind‐magnetosphere interactions during the last Matuyama‐Brunhes reversal that occurred about 780,000 years ago. As the magnetosphere shrinks in the midst of the reversal, the stand‐off distance of the subsolar magnetopause approaches as close as 3 Re. Moreover, multiple magnetic reconnection sites emerge due to the irregular geomagnetic configuration, opening up many channels on the magnetopause surface for the solar wind to directly access. The power transmission efficiency through the magnetopause increases to 18%.

Neutral Densities in the Outer Exosphere Near the Subsolar Magnetopause

AbstractIn a seminal analysis, Fuselier et al. (2020) combined subsolar in situ magnetosheath plasma observations with remote energetic neutral atom (ENA) observations to derive an exospheric neutral density of 11 cm−3 in the vicinity of the subsolar magnetopause. This commentary extends the analysis of Fuselier et al. (2020) to (a) employ predicted and observed magnetosheath plasma parameters at times other than those immediately following an MMS magnetopause crossing, (b) take into account the component of the plasma bulk flow transverse to the ENA spacecraft's line‐of‐sight through the flank magnetosheath, and (c) consider the cooling that results from the antisunward expansion of the magnetosheath plasma. With these additional considerations, subsolar exospheric neutral densities rise by a factor of 4.1–4.5 to 45–50 cm−3, consistent with results from previous studies.

Interplanetary Shock‐Induced Magnetopause Motion: Comparison Between Theory and Global Magnetohydrodynamic Simulations

AbstractThe magnetopause marks the outer edge of the Earth's magnetosphere and a distinct boundary between solar wind and magnetospheric plasma populations. In this study, we use global magnetohydrodynamic simulations to examine the response of the terrestrial magnetopause to fast‐forward interplanetary shocks of various strengths and compare to theoretical predictions. The theory and simulations indicate the magnetopause response can be characterized by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compressive phase comprising the majority of the distance traveled, and large‐scale damped oscillations with amplitudes of the order of an Earth radius. The two approaches agree in predicting subsolar magnetopause oscillations with frequencies 2–13 mHz but the simulations notably predict larger amplitudes and weaker damping rates. This phenomenon is of high relevance to space weather forecasting and provides a possible explanation for magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991.

Magnetopause Reconnection and Indents Induced by Foreshock Turbulence.

Based on global hybrid simulation results, we predict that foreshock turbulence can reach the magnetopause and lead to reconnection as well as Earth‐sized indents. Both the interplanetary magnetic field (IMF) and solar wind are constant in our simulation, and hence, all dynamics are generated by foreshock instabilities. The IMF in the simulation is mostly Sun‐Earth aligned with a weak northward and zero dawn‐dusk component, such that subsolar magnetopause reconnection is not expected without foreshock turbulence modifying the magnetosheath fields. We show a reconnection example to illustrate that the turbulence can create large magnetic shear angles across the magnetopause to induce local bursty reconnection. Magnetopause reconnection and indents developed from the impact of foreshock turbulence can potentially contribute to dayside loss of planetary plasmas.

Magnetopause Indentation Induced by the Magnetosheath Fast Flow

Recent studies indicated that the magnetopause indentation plays an important role in magnetosphere-ionosphere coupling. Confirmation of magnetopause indentation requires joint observations with multiple satellites. So far, there have been few magnetopause indentation events reported. In this paper, a case of magnetopause indentation induced by fast magnetosheath flow is reported with multiple spacecraft analysis based on the observations of five THEMIS probes. During the interval from 10:00 UT to 10:45 UT on 21 July 2007, when the five THEMIS probes are located near the subsolar magnetopause, a fast anti-sunward flow (with a velocity of 400 km·s^-1) was observed in the magnetosheath just before THEMIS crossed the magnetopause to the magnetosphere. A magnetopause local indentation event was identified by comparing the nominal magnetopause and the tangential magnetopause plane calculated using the MVA method. In order to explore the origin of this magnetosheath fast flow, solar wind data observed by WIND satellite at L1 point were analyzed. It is found that the solar wind is very stable during this period. The Interplanetary Magnetic Field (IMF) is mainly radial and the component of the north-south direction is very small. It is speculated that the generation of this magnetosheath fast anti-sunward flow may be related to the radial IMF.

Simultaneous Polar and Cluster Observations in the Northern and Southern Middle‐Altitude Polar Cusps Around Equinox

AbstractWe present an event of simultaneous observations of the northern and southern middle‐altitude polar cusps by the Polar spacecraft and Cluster fleet that occurred on 23 September 2004. We examine the possible asymmetries in the fields and plasma parameters, although the proximity of the equinox should limit these asymmetries. Ion sensors reveal two dispersions in both cusps, and data analysis leads to the conclusion that those dispersions are due to pulsed reconnection at a single X‐line, which runs along the subsolar magnetopause. While the electromagnetic and particle energy fluxes injected in both cusp are globally very similar, we report significant differences in ion dispersions, width of the low‐latitude boundary layer, and peak convection velocities. We ascribe these differences to the dipole tilt that introduces an asymmetry in the magnetosheath flow at the exterior cusps.

Neutral Atom Imaging of the Solar Wind-Magnetosphere-Exosphere Interaction Near the Subsolar Magnetopause.

Energetic neutral atoms (ENAs) created by charge‐exchange of ions with the Earth's hydrogen exosphere near the subsolar magnetopause yield information on the distribution of plasma in the outer magnetosphere and magnetosheath. ENA observations from the Interstellar Boundary Explorer (IBEX) are used to image magnetosheath plasma and, for the first time, low‐energy magnetospheric plasma near the magnetopause. These images show that magnetosheath plasma is distributed fairly evenly near the subsolar magnetopause; however, low‐energy magnetospheric plasma is not distributed evenly in the outer magnetosphere. Simultaneous images and in situ observations from the Magnetospheric Multiscale (MMS) spacecraft from November 2015 (during the solar cycle declining phase) are used to derive the exospheric density. The ~11–17 cm−3 density at 10 RE is similar to that obtained previously for solar minimum. Thus, these combined results indicate that the exospheric density 10 RE from the Earth may have a weak dependence on solar cycle.

Hybrid-Vlasov modeling of three-dimensional dayside magnetopause reconnection

Dayside magnetic reconnection at the magnetopause, which is a major driver of space weather, is studied for the first time in a three-dimensional (3D) realistic setup using a hybrid-Vlasov kinetic model. A noon–midnight meridional plane simulation is extended in the dawn–dusk direction to cover 7 Earth radii. The southward interplanetary magnetic field causes magnetic reconnection to occur at the subsolar magnetopause. Perturbations arising from kinetic instabilities in the magnetosheath appear to modulate the reconnection. Its characteristics are consistent with multiple, bursty, and patchy magnetopause reconnection. It is shown that the kinetic behavior of the plasma, as simulated by the model, has consequences on the applicability of methods such as the four-field junction to identify and analyze magnetic reconnection in 3D kinetic simulations.