Shape Of Magnetopause Research Articles

Dawn‐Dusk Asymmetry of the Magnetopause Distance Under the Parker Spiral Configuration of the IMF

AbstractThe dawn‐dusk asymmetry of the magnetopause radial distance under the Parker (and ortho‐Parker) configuration of the interplanetary magnetic field (IMF) is investigated. An extensive data set of about 50,000 magnetopause crossings identified in the data from the THEMIS A‐E, Magion 4, Geotail, and Interball‐1 satellites is used for this purpose. It is shown that the magnetopause radial distances are larger at the side where the IMF is quasi‐parallel to the bow shock normal than at the side where IMF is quasi‐perpendicular to the bow shock normal. The effect is more significant at the flanks than close to the subsolar point. This introduces a significant asymmetry of the magnetopause shape, with the difference in the magnetopause radial distances being as large as about one Earth radius beyond the terminator. The experimental results are confirmed by a global magnetohydrodynamic (MHD) model, demonstrating that the asymmetry can be explained by MHD effects, without considering foreshock transients. This MHD model is further used to investigate the evolution of individual pressure components across the magnetopause and contrast them in quasi‐parallel and quasi‐perpendicular cases.

Magnetopause location modeling using machine learning: inaccuracy due to solar wind parameter propagation

An intrinsic limitation of empirical models of the magnetopause location is a predefined magnetopause shape and assumed functional dependences on relevant parameters. We overcome this limitation using a machine learning approach (artificial neural networks), allowing us to incorporate general, purely data-driven dependences. For the training and testing of the developed neural network model, a data set of about 15,000 magnetopause crossings identified in the THEMIS A-E, Magion 4, Geotail, and Interball-1 satellite data in the subsolar region is used. A cylindrical symmetry around the direction of the impinging solar wind is assumed, and solar wind dynamic pressure, interplanetary magnetic field magnitude, cone angle, clock angle, tilt angle, and corrected Dst index are considered as parameters. The effect of these parameters on the magnetopause location is revealed. The performance of the developed model is compared with other empirical magnetopause models. Finally, we demonstrate and discuss the inaccuracy of magnetopause models due to the inaccurate information about the impinging solar wind parameters based on measurements near the L1 point. This inaccuracy imposes a theoretical limit on the precision of magnetopause predictions, a limit that our model closely approaches.

Localized Magnetopause Erosion at Geosynchronous Orbit by Reconnection

AbstractThis study presents observations of magnetopause reconnection and erosion at geosynchronous orbit, utilizing in situ satellite measurements and remote sensing ground‐based instruments. During the main phase of a geomagnetic storm, Geostationary Operational Environmental Satellites (GOES) 15 was on the dawnside of the dayside magnetopause (10.6 MLT) and observed significant magnetopause erosion, while GOES 13, observing duskside (14.6 MLT), remained within the magnetosphere. Combined observations from the THEMIS satellites and Super Dual Auroral Radar Network radars verified that magnetopause erosion was primarily caused by reconnection. While various factors may contribute to asymmetric erosion, the observations suggest that the weak reconnection rate on the duskside can play a role in the formation of asymmetric magnetopause shape. This discrepancy in reconnection rate is associated with the presence of cold dense plasma on the duskside of the magnetosphere, which limits the reconnection rate by mass loading, resulting in more efficient magnetopause erosion on the dawnside.

Interplanetary Magnetic Field By Controls the Magnetopause Location

AbstractWe present a statistical study of interplanetary magnetic field (IMF) effects on the magnetopause location based on nearly 15,000 magnetopause crossings identified in the THEMIS A‐E, Magion 4, Geotail, and Interball‐1 satellite data. In order to exclude the dominant controlling role of the solar wind dynamic pressure, differences between observed and empirical model magnetopause distances are analyzed. We find that the magnetopause distance is significantly controlled by the IMF clock angle, not considered in the used empirical models. Furthermore, during very low Alfvén Mach numbers (MA < 4), IMF By component can induce a considerable dawn‐dusk asymmetry of the magnetopause shape. The magnetopause distance is found to be larger in the direction of the IMF vector, that is, not only the magnitude but also the orientation of the IMF By component is eventually important. The obtained results are qualitatively consistent with a magnetohydrodynamic model run at the Community Coordinated Modeling Center.

Design and construction of the magnetopause shape control coils of the Space Plasma Environment Research Facility to regulate the field configuration of the simulated Earth's magnetopause magnetic reconnection

Two groups of magnetopause shape control coils are used on the SPERF (Space Plasma Environment Research Facility) facility to regulate the magnetic field configuration of the simulated Earth's magnetopause magnetic reconnection. Here, design and construction of the magnetopause shape control coils is presented to resolve the issues regarding the strong pulsed current, high voltage, and vacuum sealing, etc. At first, the physical requirement imposed on the coils is explained, followed by clarification of the key technological issues facing the coils' design. Afterwards, the fundamental design of the coils is shown, with emphasis on the segmented structure and reliable vacuum sealing. Then, the supporting and motion mechanism of the coils are designed, and the layout of the leading wires in the supporting tubes is optimized to alleviate the impact of the magnetic forces between them. Further, the connection structure is designed to connect the leading wires of the coils with the pulsed power supply. At last, the critical measurements done for the magnetopause shape control coils are given to demonstrate the design. Presently, installation of the coils on the SPERF has completed, and they will be used in the physical experiments in the future.

An 18.3 MJ charging and discharging pulsed power supply system for the Space Plasma Environment Research Facility (SPERF): The subsystem for the magnetopause shape control coils.

The Space Plasma Environment Research Facility currently under construction at the Harbin Institute of Technology in China uses four magnetosheath coils (flux cores) and a dipole coil to generate the magnetic field required for the study on the magnetopause magnetic reconnection. Two groups of magnetopause shape control coils (labeled CK coils) are used to slightly adjust the magnetic field distribution on the magnetopause. A capacitive pulsed power supply (PPS) system with a modular design is developed to excite CK coils. The PPS system consists of six sets of PPS with the same principle and structure, which are used to excite six sub-coils of the CK coils, respectively. Each set of PPS consists of ten modules and one local controller, and each module consists of a charger and discharge unit. Each set of PPS can provide a pulsed current of no less than 400 kA for the corresponding sub-coil at 0.11ms when the charging voltage is 20 kV, and the duration of the pulsed current from the peak to 10% of the peak is no more than 0.6ms. The detailed design of the PPS is discussed in this paper, and the discharge test of the PPS is carried out to verify the design of the PPS. Because there are acquisition and control devices in the discharge unit, the electromagnetic interference immunity design is also discussed to ensure the normal operation of the PPS.

Evolution of a Foreshock Bubble in the Midtail Foreshock and Impact on the Magnetopause: 3‐D Global Hybrid Simulation

AbstractForeshock transients have been observed in the midtail foreshock. How a foreshock transient evolves in nightside foreshock and how it may impact the midtail bow shock, magnetosheath, and magnetopause has not been simulated. In this study, we present three‐dimensional global hybrid simulation results of a foreshock bubble (FB) driven by an interplanetary magnetic field (IMF) rotational discontinuity. As the FB propagates in the nightside foreshock, its spatial size grows to >10 RE, and its density and dynamic pressure perturbations are enhanced by >50% of the solar wind values. As the FB interacts with the nightside bow shock, the bow shock is temporarily eroded, and the corresponding dynamic pressure perturbations in the magnetosheath penetrate deep to the magnetopause, causing localized distortion of the magnetopause shape. As the FB moves tailward, it continuously generates new perturbations in the magnetosheath and magnetopause following FB's propagation.

The Shape of Mercury's Magnetopause: The Picture From MESSENGER Magnetometer Observations and Future Prospects for BepiColombo

AbstractThe MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft orbited Mercury between March 2011 and April 2015, typically passing through the magnetosphere, magnetosheath, and interplanetary magnetic field, on each orbit. Using data from the Magnetometer, we identify magnetopause crossings for the complete orbital mission and model the average boundary shape. We find that Mercury's average magnetopause is well modeled by both an axisymmetric shape and a three‐dimensional shape containing indentations in the cusp regions and a magnetotail that is wider in the north‐south versus east‐west direction. Examination of MESSENGER data and the use of simulated crossings show that coverage is very limited in the cusp region. Parameters describing the indentation are poorly constrained, and model fits may be biased due to the combination of orbital geometry and magnetospheric dynamics. The distribution of MESSENGER data inside the magnetopause, together with minimum variance analysis of a representative subset of magnetopause crossings, hints at the presence of a cusp indentation, though likely shallower than that estimated from the observed crossing positions. The BepiColombo spacecraft will arrive at Mercury in 2025. We simulate expected data coverage and magnetopause crossings using both axisymmetric and three‐dimensional magnetopause models. BepiColombo will improve our understanding of the magnetopause shape by providing data over the southern hemisphere and regions of the northern cusp indentation not covered by MESSENGER. There is also the potential for simultaneous measurements of the interplanetary magnetic field and magnetopause position.

Beyond the Minisolar Maximum of Solar Cycle 24: Declining Solar Magnetic Fields and the Response of the Terrestrial Magnetosphere

AbstractThe present study examines the response of the terrestrial magnetosphere to the long‐term steady declining trends observed in solar magnetic fields and solar wind microturbulence levels since mid‐1990s that has been continuing beyond the minisolar maximum of cycle 24. A detailed analysis of the response of the terrestrial magnetosphere has been carried out by studying the extent and shape of the Earth's magnetopause and bow shock over the past four solar cycles. We estimate subsolar stand‐off distance of the magnetopause and bow shock and the shape of the magnetopause, using numerical as well as empirical models. The computed magnetopause and bow shock stand‐off distances have been found to be increasing steadily since around mid‐1990s, consistent with the steady declining trend seen in solar magnetic fields and solar wind microturbulence levels. Similarly, we find an expansion in the shape of the magnetopause since 1996. The implications of the increasing trend seen in the magnetopause and bow shock stand‐off distances are discussed and a forecast of the shape of the magnetopause in 2020, the minimum of cycle 24, has been made. Importantly, we also find two instances between 1968 and 1991 when the magnetopause stand‐off distance dropped to values close to 6.6 Earth radii, the geostationary orbit, for duration ranging from 9–11 hr and one event in 2005, post 1995 when the decline in photospheric fields began. Though there have been no such events since 2005, it represents a clear and present danger to our satellite systems.

Shape of the equatorial magnetopause affected by the radial interplanetary magnetic field

The ability of a prediction of the magnetopause location under various upstream conditions can be considered as a test of our understanding of the solar wind-magnetosphere interaction. The present magnetopause models are parametrized with the solar wind dynamic pressure and usually with the north-south interplanetary magnetic field (IMF) component. However, several studies pointed out an importance of the radial IMF component, but results of these studies are controversial up to now. The present study compares magnetopause observations by five THEMIS spacecraft during long lasting intervals of the radial IMF with two empirical magnetopause models. A comparison reveals that the magnetopause location is highly variable and that the average difference between the observed and predicted positions is ≈+0.7RE under this condition. The difference does not depend on the local times and other parameters, like the upstream pressure, IMF north-south component, or tilt angle of the Earth dipole. We conclude that our results strongly support the suggestion on a global expansion of the equatorial magnetopause during intervals of the radial IMF.

Geomagnetopause Position and Shape Dependence on Solar Wind Plasma and IMF Parameters: Analytic Model Comparison with Observations and 3-D MHD Runs

AbstractIt is generally accepted to use the solar wind ram pressure ρV2 and the IMF Bz component for empirical description of the geo-magnetopause position and shape. A specific feature of the present paper is not to use the solar wind ρV2 but the thermal Pth and magnetic field Pmag pressures adjacent to the magnetopause for proper modelling. These pressures are deduced from the results of 3-D MHD runs and analytic solutions for post bow shock MHD flow in Lagrangian variables. The magnetopause shape variation due to Bz component changes the so called ‘doubling factor’ fd, which can be analytically deduced from a Tsyganenko magnetospheric field ellipsoidal model. Including all the above effects in our analytical model leads to a good description of ‘rapid’ magnetopause approach to the Earth for southward IMF and to its ‘stagnant’ behaviour with the increase of northward IMF component.

Nowcasting and forecasting of the magnetopause and bow shock—A status update

AbstractThere has long been interest in knowing the shape and location of the Earth's magnetopause and of the standing fast‐mode bow shock upstream of the Earth's magnetosphere. This quest for knowledge spans both the research and operations arenas. Pertinent to the latter, nowcasting and near‐term forecasting are important for determining the extent to which the magnetosphere is compressed or expanded due to the influence of the solar wind bulk plasma and fields and the coupling to other magnetosphere‐ionosphere processes with possible effects on assets. This article provides an update to a previous article on the same topic published 15 years earlier, with focus on studies that have been conducted, the current status of nowcasting and forecasting of geophysical boundaries, and future endeavors.

Global expansion of the dayside magnetopause for long‐duration radial IMF events: Statistical study on GOES observations

AbstractFor decades, unusual locations of the magnetopause under radial interplanetary magnetic field (IMF) have been reported in many studies using in situ satellite observations. These studies have shown that the magnetopause is expanded either over all magnetic local times (MLTs) on the dayside (global expansion) or just near the noon (bullet‐like expansion) during radial IMF conditions. With limited observations near the magnetopause, the type of the magnetopause expansion is still undetermined. In this study, 19 years of dayside geosynchronous magnetic field data obtained from Geostationary Operational Environmental Satellites (GOES) are statistically processed to infer the shape of the dayside magnetopause under radial IMF conditions. The MLT distributions of geosynchronous magnetic fields for radial IMF are compared with those for northward IMF with the correction of solar wind dynamic pressure distributions between two different IMF conditions. After removal of the solar wind dynamic pressure effect, we have found that the geosynchronous magnetic field for radial IMF is smaller than that for northward IMF over almost all MLTs on the dayside regardless of magnetic latitudes and seasons. The difference between geosynchronous magnetic fields for the northward and radial IMFs supports the suggestion that the dayside magnetopause is globally expanded during the radial IMF conditions.

Do we know the actual magnetopause position for typical solar wind conditions?

AbstractWe compare predicted magnetopause positions at the subsolar point and four reference points in the terminator plane obtained from several empirical and numerical MHD models. Empirical models using various sets of magnetopause crossings and making different assumptions about the magnetopause shape predict significantly different magnetopause positions (with a scatter >1 RE) even at the subsolar point. Axisymmetric magnetopause models cannot reproduce the cusp indentations or the changes related to the dipole tilt effect, and most of them predict the magnetopause closer to the Earth than nonaxisymmetric models for typical solar wind conditions and zero tilt angle. Predictions of two global nonaxisymmetric models do not match each other, and the models need additional verification. MHD models often predict the magnetopause closer to the Earth than the nonaxisymmetric empirical models, but the predictions of MHD simulations may need corrections for the ring current effect and decreases of the solar wind pressure that occur in the foreshock. Comparing MHD models in which the ring current magnetic field is taken into account with the empirical Lin et al. model, we find that the differences in the reference point positions predicted by these models are relatively small for Bz=0. Therefore, we assume that these predictions indicate the actual magnetopause position, but future investigations are still needed.

Asymmetries observed in Saturn's magnetopause geometry.

For over 10 years, the Cassini spacecraft has patrolled Saturn's magnetosphere and observed its magnetopause boundary over a wide range of prevailing solar wind and interior plasma conditions. We now have data that enable us to resolve a significant dawn‐dusk asymmetry and find that the magnetosphere extends farther from the planet on the dawnside of the planet by 7 ± 1%. In addition, an opposing dawn‐dusk asymmetry in the suprathermal plasma pressure adjacent to the magnetopause has been observed. This probably acts to reduce the size asymmetry and may explain the discrepancy between the degree of asymmetry found here and a similar asymmetry found by Kivelson and Jia (2014) using MHD simulations. Finally, these observations sample a wide range of season, allowing the “intrinsic” polar flattening (14 ± 1%) caused by the magnetodisc to be separated from the seasonally induced north‐south asymmetry in the magnetopause shape found theoretically (5 ± 1% when the planet's magnetic dipole is tilted away from the Sun by 10–17°).

Modular model for Mercury's magnetospheric magnetic field confined within the average observed magnetopause.

Accurate knowledge of Mercury's magnetospheric magnetic field is required to understand the sources of the planet's internal field. We present the first model of Mercury's magnetospheric magnetic field confined within a magnetopause shape derived from Magnetometer observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. The field of internal origin is approximated by a dipole of magnitude 190 nT RM 3, where RM is Mercury's radius, offset northward by 479 km along the spin axis. External field sources include currents flowing on the magnetopause boundary and in the cross‐tail current sheet. The cross‐tail current is described by a disk‐shaped current near the planet and a sheet current at larger (≳ 5 RM) antisunward distances. The tail currents are constrained by minimizing the root‐mean‐square (RMS) residual between the model and the magnetic field observed within the magnetosphere. The magnetopause current contributions are derived by shielding the field of each module external to the magnetopause by minimizing the RMS normal component of the magnetic field at the magnetopause. The new model yields improvements over the previously developed paraboloid model in regions that are close to the magnetopause and the nightside magnetic equatorial plane. Magnetic field residuals remain that are distributed systematically over large areas and vary monotonically with magnetic activity. Further advances in empirical descriptions of Mercury's magnetospheric external field will need to account for the dependence of the tail and magnetopause currents on magnetic activity and additional sources within the magnetosphere associated with Birkeland currents and plasma distributions near the dayside magnetopause.

The dipole tilt angle dependence of the bow shock for southward IMF: MHD results

The location and shape of the Earth׳s bow shock depend on the properties of the upstream solar wind, as well as the size and shape of the downstream magnetopause. Many studies have suggested that the influence of the dipole tilt angle on the magnetopause is significant, especially at the high-latitude region, however, to date there is no bow shock model which depends on the dipole tilt angle. Using a physics-based global magnetohydrodynamic (MHD) model, the Space Weather Modeling Framework (SWMF), we investigate the effect of the dipole tilt angle on the location and shape of the bow shock, and our results show that (1) the subsolar standoff distance and the north-south asymmetry of bow shock increase with the increasing dipole tilt angle; (2) with the dipole tilt angle positively increasing, the flaring angle of the bow shock increases in the northern hemisphere but keeps almost unchanged in the southern hemisphere, and the rotational asymmetry slightly decreases in the northern hemisphere and rapidly decreases in the southern hemisphere; and (3) the influence of dipole tilt angle on the shape of the bow shock is north-south symmetric.

Properties of low‐latitude mantle plasma in the Earth's magnetotail: ARTEMIS observations and global MHD predictions

AbstractThe Earth's plasma mantle is one of the major suppliers of particles for the plasma sheet. To understand its plasma characteristics, spatial distributions, and dependencies on interplanetary magnetic field (IMF) direction, we statistically analyzed the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) observations in the low‐latitude magnetotail (~10 RE above and below the current sheet) and investigated the predictions from global Block Adaptive Tree Solar wind‐Roe‐Upwind Scheme MHD simulations. The mantle plasma flows tailward along magnetic field lines (~50–200 km/s) and at the same time drifts toward midnight and toward the current sheet. The mantle plasma has similar temperature (~0.05–0.2 keV) to the magnetosheath plasma but has lower density (~0.1–1 cm−3). The mantle appearance is dawn‐dusk asymmetric depending mainly on the IMF By direction. The occurrence rates, density, and V|| all decrease with decreasing |Y|. This density cross‐tail profile suggests that the low‐latitude mantle plasma mainly comes from the magnetosheath entering through the tail magnetopause along the open field lines. Density is highly and positively correlated with V||. These observations are qualitatively consistent with the MHD results. The simulations indicate that as IMF By becomes dominant, the source locations at the magnetopause for the mantle move to lower latitudes and become dawn‐dusk asymmetric, and the tail cross section also becomes distorted with the magnetopause shape elongating and the current sheet tilting significantly. Degrees of these changes also vary with the downtail distances and IMF Bz direction. The source location change leads to the dawn‐dusk asymmetric mantle appearance. The tail cross‐section change alters the distance from the sources to the current sheet and thus the resulting mantle density distributions just outside the plasma sheet.

Data‐based modeling of the geomagnetosphere with an IMF‐dependent magnetopause

The paper presents first results of the data‐based modeling of the geomagnetospheric magnetic field, using the data of Polar, Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms satellites, taken during the period 1995–2012 and covering 123 storm events with SYM‐H ≥ −200 nT. The most important innovations in the model are (1) taking into account the interplanetary magnetic field (IMF)‐dependent shape of the model magnetopause, (2) a physically more consistent global deformation of the equatorial current sheet due to the geodipole tilt, (3) symmetric and partial components of the ring current are calculated based on a realistic background magnetic field, instead of a purely dipolar field, used in earlier models, and (4) the validity region on the nightside is extended to ∼40–50 RE. The model field is confined within a magnetopause, based on Lin et al. (2010) empirical model, driven by the dipole tilt angle, solar wind pressure, and IMF Bz. A noteworthy finding is a significant dependence of the magnetotail flux connection across the equatorial plane on the model magnetopause flaring rate, controlled by the southward component of the IMF.

Mercury's magnetopause and bow shock from MESSENGER Magnetometer observations

AbstractWe have established the average shape and location of Mercury's magnetopause and bow shock from orbital observations by the MESSENGER Magnetometer. We fit empirical models to midpoints of boundary crossings and probability density maps of the magnetopause and bow shock positions. The magnetopause was fit by a surface for which the position R from the planetary dipole varies as [1 + cos(θ)]−α, where θ is the angle between R and the dipole‐Sun line, the subsolar standoff distance Rss is 1.45 RM (where RM is Mercury's radius), and the flaring parameter α = 0.5. The average magnetopause shape and location were determined under a mean solar wind ram pressure PRam of 14.3 nPa. The best fit bow shock shape established under an average Alfvén Mach number (MA) of 6.6 is described by a hyperboloid having Rss = 1.96 RM and an eccentricity of 1.02. These boundaries move as PRam and MA vary, but their shapes remain unchanged. The magnetopause Rss varies from 1.55 to 1.35 RM for PRam in the range of 8.8–21.6 nPa. The bow shock Rss varies from 2.29 to 1.89 RM for MA in the range of 4.12–11.8. The boundaries are well approximated by figures of revolution. Additional quantifiable effects of the interplanetary magnetic field are masked by the large dynamic variability of these boundaries. The magnetotail surface is nearly cylindrical, with a radius of ~2.7 RM at a distance of 3 RM downstream of Mercury. By comparison, Earth's magnetotail flaring continues until a downstream distance of ~10 Rss.