Shape Of Magnetopause Research Articles

A new three‐dimensional magnetopause model with a support vector regression machine and a large database of multiple spacecraft observations

We present results from a new three‐dimensional empirical magnetopause model based on 15,089 magnetopause crossings from 23 spacecraft. To construct the model, we introduce a Support Vector Regression Machine (SVRM) technique with a systematic approach that balances model smoothness with fitting accuracy to produce a model that reveals the manner in which the size and shape of the magnetopause depend upon various control parameters without any assumptions concerning the analytical shape of the magnetopause. The new model fits the data used in the modeling very accurately, and can guarantee a similar accuracy when predicting unseen observations within the applicable range of control parameters. We introduce a new error analysis technique based upon the SVRM that enables us to obtain model errors appropriate to different locations and control parameters. We find significant east‐west elongations in the magnetopause shape for many combinations of control parameters. Variations in the Earth's dipole tilt can cause significant magnetopause north/south asymmetries and deviation of the magnetopause nose from the Sun‐Earth line nonlinearly by as much as 5 Re. Subsolar magnetopause erosion effect under southward IMF is seen which is strongly affected by solar wind dynamic pressure. Further, we find significant shrinking of high‐latitude magnetopause with decreased magnetopause flaring angle during northward IMF.

Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

AbstractPrevious works have emphasized the significant influence of the solar wind Alfvén Mach number (MA) on magnetospheric dynamics. Here we report statistical, observational results that pertain to changes in the magnetosheath flow distribution and magnetopause shape as a function of solar wind MA and interplanetary magnetic field (IMF) clock angle orientation. We use all Cluster 1 data in the magnetosheath during the period 2001–2010, using an appropriate spatial superposition procedure, to produce magnetosheath flow distributions as a function of location in the magnetosheath relative to the IMF and other parameters. The results demonstrate that enhanced flows in the magnetosheath are expected at locations quasi‐perpendicular to the IMF direction in the plane perpendicular to the Sun‐Earth line; in other words, for the special case of a northward IMF, enhanced flows are observed on the dawn and dusk flanks of the magnetosphere, while much lower flows are observed above the poles. The largest flows are adjacent to the magnetopause. Using appropriate magnetopause crossing lists (for both high and low MA), we also investigate the changes in magnetopause shape as a function of solar wind MA and IMF orientation. Comparing observed magnetopause crossings with predicted positions from an axisymmetric semi‐empirical model, we statistically show that the magnetopause is generally circular during high MA, while is it elongated (albeit with moderate statistical significance) along the direction of the IMF during low MA. These findings are consistent with enhanced magnetic forces that prevail in the magnetosheath during low MA. The component of the magnetic forces parallel to the magnetopause produces the enhanced flows along and adjacent to the magnetopause, while the component normal to the magnetopause exerts an asymmetric pressure on the magnetopause that deforms it into an elongated shape.

Solar energetic particles in the Earth magnetosphere: kinematic modeling of the "non-shock" penetration

Penetration of solar energetic particles into the Earth's magnetosphere is quantitatively studied with a simple kinematic model. The goal is to assess, for the first time, how does effectiveness of the penetration depend on such geometry factors as: distance of the magneto-pause (MP) from the Earth; shape of MP; angle at which solar energetic particle crosses MP; location of the crossing point; type of the particle motion in the magnetosphere. To get off excessive details, the model deliberately operates with just equatorial section of the static dipolar magnetic field confined with asymmetric boundary – MP. Several rather obvious facts are illustrated: finite orbits of longitudinal drift reside only inside the circle of the Störmer-unit-length radius; deepest penetration of a particle occurs if the particle crosses MP at the point closest to the Earth and with velocity-vector oriented along the particle's longitudinal drift inside MP (westward for protons); etc. The model's software allows the inquirer to vary geometry of MP, the type, energy and direction of flight of the energetic particle(s), the location(s), aperture and orientation(s) of a virtual sensor, then to run the model and obtain the reference particle distributions either global (for entire magnetosphere) or for specified locations, all along the time, energy and flux-orientation axes. Static and animated plots can be easily produced. The model provides a toolkit allowing one to evaluate and illustrate the process of particle penetration into the magnetosphere under various conditions in space. It may be used for the configuring of the satellite particle sensors; its results may be compared with the observations for to assess how strongly the real magnetosphere differs from its simplified form; it may be used in education.

Equatorial trench at the magnetopause under saturation

Magnetic data from GOES geosynchronous satellites were applied for statistical study of the low‐latitude dayside magnetopause under a strong interplanetary magnetic field of southward orientation when the reconnection at the magnetopause was saturated. From minimum variance analysis, we determined the magnetopause orientation and compared it with predictions of a reference model. The magnetopause shape was found to be substantially distorted by a duskward shifting such that the nose region appeared in the postnoon sector. At equatorial latitudes, the shape of magnetopause was characterized by a prominent bluntness and by a trench formed in the postnoon sector. The origin of distortions was regarded in the context of the storm‐time magnetospheric currents and the large‐scale quasi‐state reconnection at the dayside magnetopause.

Conditions at the expanded Jovian magnetopause and implications for the solar wind interaction

Using idealized models of the magnetosheath and magnetosphere magnetic fields, plasma densities, and plasma flow, we test for the steady state viability of processes mediating the interaction between the solar wind and the Jovian magnetosphere. The magnetopause is modeled as an asymmetric paraboloid with variable asymmetry. The subsolar standoff of the magnetopause has been shown to exhibit a bimodal probability distribution (Joy et al., 2002). Only the expanded magnetopause is considered, with a standoff of ∼90 RJ. We test where on the magnetopause surface large‐scale reconnection may be affected by either a shear flow or diamagnetic drift due to a pressure gradient across the magnetopause boundary. We also test for the onset of the Kelvin‐Helmholtz instability. We find that reconnection is inhibited on the dawn flank due to the large shear flows in this region, regardless of magnetopause shape or interplanetary magnetic field orientation. The presence of a high energy plasma population in the magnetosphere may inhibit reconnection over much of the magnetopause area, except when the fields are antiparallel. Additionally, most of the dawn flank of the magnetopause is Kelvin‐Helmholtz unstable, regardless of magnetopause asymmetry; and the dusk flank tailward of the planet is Kelvin‐Helmholtz unstable when the magnetopause is highly oblate.

Predictive model of magnetosheath plasma flow and its validation against Cluster and THEMIS data

Abstract. An analytical model of magnetosheath plasma flow is described and compared with a large dataset of magnetosheath ion flow velocity measurements from Cluster and THEMIS spacecraft. The model is based on previous works by Kobel and Flückiger (1994) and Génot et al. (2011) and has been modified to overcome the restrictions of these models on the shape of model magnetopause and bow shock. Our model is compatible with any parabolic bow shock model and arbitrary magnetopause model. The model is relatively simple to implement and computationally inexpensive, and its only inputs are upstream solar wind parameters. Comparison with observed data yields a good correspondence: median error in the direction of flow velocity is comparable with the instrumental error, and flow magnitude is predicted with a reasonable accuracy (relative error in flow speed was less than 25% for 86.5% of observations).

An MHD simulation study on the location and shape of magnetopause in equatorial plane

The determination of the varying position and shape of magnetopause is one of the important Gordian knots in geophysics and space physics. According to the solar wind-magnetosphere-ionosphere coupling global magnetohydrodynamic (MHD) simulation, and with the maximum electric current criterion, we study the position and shape of the magnetopause under several solar wind dynamic pressure (Dp) and interplanetary magnetic field conditions. The simulation results show that the subsolar position (r0) of the magnetopause is controlled mainly by Dp with the significant decrease of r0 as Dp increases. At a certain Dp, when southword Bz (Bz0) decreases to zero, then shifts to northward (Bz0) and increases, the subsolar position r0 keeps increasing. For all cases studie here, the flare angle () of the magnetopause experiences small changes. This provide an evidence for the structural self-similarity of magnetopause in equatorial plane. Compared with the empirical low-latitude magnetopause model of Shue98, MHD simulation can reproduce the dependence of the subsolar point r0 on Dp, while the saturation effect of r0 varying with Bz in empirical model is represented only with slow solar wind. As to the flare angle , although the difference between MHD simulation and empirical model is less than 2.5, the variation of with Bz in MHD simulations is nonlinear and different from the linear trend in empirical model.

Effect of the interplanetary electric field on the magnetopause from global MHD simulations

The north-south component <I>B</I><SUB>z</SUB> of the Interplanetary Magnetic Field (IMF) and solar wind dynamic pressure <I>P</I><SUB>d</SUB> are generally treated as the two main factors in the solar wind that determine the geometry of the magnetosphere. By using the 3D global MHD simulations, we investigate the effect of the Interplanetary Electric Field (IEF) on the size and shape of magnetopause quantitatively. Our numerical experiments confirm that the geometry of the magnetopause are mainly determined by <I>P</I><SUB>d</SUB>B</I><SUB>z</SUB>, as expected. However, the dawn-dusk IEFs have great impact on the magnetopause erosion because of the magnetic reconnection, thus affecting the size and shape of the magnetopause. Higher solar wind speed with the same <I>B</I><SUB>z</SUB> will lead to bigger dawn-dusk IEFs, which means the higher reconnection rate, and then results in more magnetic flux removal from the dayside. Consequently, the dayside magnetopause moves inward and flank magnetopause moves outward.

Three dimensional shape of the magnetopause: Global MHD results

[1] The numerical results from a physics‐based global magnetohydrodynamic (MHD) model are used to examine the relationship between the shape and size of the magnetopause and the solar wind conditions. The magnetopause location is identified by tracing three‐dimensional streamlines through the simulation domain and is fitted by simple analytical functions. The resulting model is applicable for approximating magnetopause location for dipole tilt angle ∼0° and interplanetary magnetic field (IMF) BX and BY = 0 nT at both low and high magnetospheric latitudes. In both regions the results are compared with available empirical models. It is shown that IMF BZ mainly affects the flaring angle (the magnetopause shape) and has smaller effects on the magnetopause size. In contrast, the solar wind DP mainly affects the magnetopause standoff distance (magnetopause size) and has little effect on the magnetopause shape. Both conclusions are consistent with empirical models.

Size and Shape of Magnetopause in the Noon-midnight Meridian Plane Based on the MHD Simulation

通过分析太阳风-磁层-电离层系统的三维全球磁流体力学(MHD)模型的计算数据, 给出了正午-午夜子午面磁层顶位形的定量模型. 分析表明, 正午-午夜子午面磁层顶位形可以用文献[3]提出的基于卫星观测数据的、描述赤道面磁层顶位形的函数来描述. 与赤道面磁层顶不同, 正午-午夜子午面磁层顶位形更为复杂. 在忽略极尖区(cusp)的简化条件下, 磁层顶位形仍需利用两条曲线来拟合. 太阳风动压Dp与行星际磁场分量Bz是控制磁层顶位形的主要因素. 行星际磁场为北向时, 磁场增强, 日下点距离r0增大; 行星际磁场为南向时, 磁场增强, 磁层顶日下点距离r0减小. 整体而言, 行星际磁场分量Bz由南转北时, r0增大, 且Bz对r0的影响减弱. 太阳风动压Dp是控制磁层顶日下点的主要因素, Dp增大, r0减小. 磁层顶位形的另一个参数磁层顶磁尾张角α, 随着行星际磁场南向分量增强而增大, 即磁层顶张开程度更加显著, 更多的磁通量由向阳侧传输到夜侧; Dp增大, α略增大, 这意味着Dp对磁通量由日侧向夜侧的传输也有一定的贡献.

A new form of Saturn's magnetopause using a dynamic pressure balance model, based on in situ, multi‐instrument Cassini measurements

The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the boundary. Previous studies have found the kronian magnetosphere to show rigidity (like that of Earth) as well as compressibility (like that of Jupiter) in terms of its dynamics. In this paper we expand on previous work and present a new model of Saturn's magnetopause. Using a Newtonian form of the pressure balance equation, we estimate the solar wind dynamic pressure at each magnetopause crossing by the Cassini spacecraft between Saturn Orbit Insertion in June 2004 and January 2006. We build on previous findings by including an improved estimate for the solar wind thermal pressure and include low‐energy particle pressures from the Cassini plasma spectrometer's electron spectrometer and high‐energy particle pressures from the Cassini magnetospheric imaging instrument. Our improved model has a size‐pressure dependence described by a power law DP−1/5.0 ± 0.8. This exponent is consistent with that derived from numerical magnetohydrodynamic simulations.

New type of laser-plasma experiments to simulate an extreme and global impact of giant coronal mass ejections onto earth' magnetosphere*

The goals, technical approach and first results of a new-type Laser-Plasma experiment AMEX for the simulation of over-compression effect of the Earth' magnetosphere by Giant Coronal Mass Ejections are described. Parameters of large-scale KI-1 facility of ILP with kJ-laser provide such values of dimensionless criteria which are in the range of very rare and enormous Solar flares that could cause a shift of magnetopause from the usual distance of 10RE down to ∼ (2-3)RE. These extreme ‘Artificial Magnetosphere’ states with highly compressed dipole field could result in world-wide damage phenomena in various networks and its physics could be explored in laboratory only by using of compact dipoles with a large moment. In a series of experiments with a variable magnetic moment and plasma blobs of effective energy up to 500 J an important role of plasma instabilities at magnetopause was revealed. The data on the magnetopause shape and stand-off size show a good correspondence both to general magnetospheric models and to expected scaling of ‘Artificial Magnetosphere’.

Interball contribution to the high-altitude cusp observations

The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent observations and theoretical considerations revealed that the formation of the cusp cannot be treated separately from the processes along the whole dayside magnetopause and that the plasma in regions like cleft or low-latitude boundary layer is of the same origin. Our review of statistical results as well as numerous case studies identified the anti-parallel merging at the magnetopause as the principal source of the magnetosheath plasma in all altitudes. Since effective merging requires a low plasma speed at the reconnection spot, we have found that the magnetopause shape and especially its indentation at the outer cusp is a very important part of the whole process. The plasma is slowed down in this indentation and arising multiscale turbulent processes enhance the reconnection rate.

Strong bulk plasma acceleration in Earth's magnetosheath: A magnetic slingshot effect?

In the near‐Earth environment, strong bulk plasma accelerations are frequently taken to be the diagnostic of the occurrence of magnetic reconnection. In this letter, we report new and unambiguous spacecraft observations and corresponding magnetohydrodynamic (MHD) simulation of strong bulk plasma acceleration in the terrestrial magnetosheath during low Alfvén Mach number solar wind conditions, which is demonstrably not associated with magnetic reconnection. We illustrate this effect with Cluster spacecraft data that show plasma accelerations up to speeds of 1040 km/s, while the ambient solar wind speed is only 650 km/s (i.e., in excess by 60%). Based on a comparison with global MHD simulations of the magnetosphere, we show that the acceleration results from enhanced magnetic forces exerted on the plasma by “stiff” magnetic flux tubes in a low‐β magnetosheath that result from the low Alfvén Mach number solar wind. The MHD simulations demonstrate that the acceleration is asymmetric, as well as the magnetopause shape, and is the result of both magnetic pressure gradient and tension forces, showing that this effect is not a simple analogy to a “slingshot effect” for which magnetic tension would dominate. Like magnetic reconnection, this mechanism is capable of producing strong plasma acceleration in the near‐Earth's environment. The low Alfvén Mach number solar wind condition leading to this mechanism is often characteristic of coronal mass ejections (CMEs).

Modeling the size and shape of Saturn's magnetopause with variable dynamic pressure

The location and shape of a planetary magnetopause is principally determined by the dynamic pressure, Dp, of the solar wind, the orientation of the planet's magnetic dipole with respect to the solar wind flow, and by the distribution of stresses inside the magnetosphere. The magnetospheres of Saturn and Jupiter have strong internal plasma sources compared to the solar wind source and also rotate rapidly, causing an equatorial inflation of the magnetosphere and consequently the magnetopause. Empirical studies using Voyager and Pioneer data concluded that the kronian magnetopause was Earth‐like in terms of its dynamics (Slavin et al., 1985) as revealed by how the position of the magnetopause varies with dynamic pressure. In this paper we present a new pressure‐dependent model of Saturn's magnetopause, using the functional form proposed by Shue et al. (1997). To establish the pressure‐dependence, we also use a new technique for fitting a pressure‐dependent model in the absence of simultaneous upstream pressure measurements. Using a Newtonian form of the pressure balance across the magnetopause boundary and using model rather than minimum variance normals, we estimate the solar wind dynamic pressure at each crossing. By iteratively fitting our model to magnetopause crossings observed by the Cassini and Voyager spacecraft, in parallel with the pressure balance, we obtain a model which is self‐consistent with the dynamic pressure estimates obtained. We find a model whose size varies as ∼Dp−1/4.3 and whose flaring decreases with increasing dynamic pressure. This is interpreted in terms of a different distribution of fields and particles stresses which has more in common with the jovian magnetosphere compared with the terrestrial situation. We compare our model with the existing models of the magnetopause and highlight the very different geometries. We find our results are consistent with recent MHD modeling of Saturn's magnetosphere (Hansen et al., 2005).

A flexible approach to modeling the storm time Region 2 and magnetopause currents

We present flexible approaches to modeling the Region 2 (R2) Birkeland current system with the associated partial ring current (PRC) and the Chapman‐Ferraro currents for arbitrarily shaped magnetopauses. The R2 field‐aligned current sheet and the PRC are constructed with an array of overlapping conical diffuse current segments. The dawnside R2 current is connected to the duskside R2 current azimuthally in the equatorial plane, forming a spread‐out PRC. The Chapman‐Ferraro shielding field due to an arbitrary magnetopause shape (on the basis of empirical approximations) is derived as a linear combination of spherical harmonics and image dipoles by a least squares minimization. We generalize the Rice Field Model, a modular open field model, to include the effects of these current systems, which are important in the storm time magnetosphere. We demonstrate the feasibility and flexibility of these techniques by modeling a simple R2 Birkeland current and PRC system.

Experimental study of nonlinear interaction of plasma flow with charged thin current sheets: 2. Hall dynamics, mass and momentum transfer

Abstract. Proceeding with the analysis of Amata et al. (2005), we suggest that the general feature for the local transport at a thin magnetopause (MP) consists of the penetration of ions from the magnetosheath with gyroradius larger than the MP width, and that, in crossing it, the transverse potential difference at the thin current sheet (TCS) is acquired by these ions, providing a field-particle energy exchange without parallel electric fields. It is suggested that a part of the surface charge is self-consistently produced by deflection of ions in the course of inertial drift in the non-uniform electric field at MP. Consideration of the partial moments of ions with different energies demonstrates that the protons having gyroradii of roughly the same size or larger than the MP width carry fluxes normal to MP that are about 20% of the total flow in the plasma jet under MP. This is close to the excess of the ion transverse velocity over the cross-field drift speed in the plasma flow just inside MP (Amata et al., 2005), which conforms to the contribution of the finite-gyroradius inflow across MP. A linkage through the TCS between different plasmas results from the momentum conservation of the higher-energy ions. If the finite-gyroradius penetration occurs along the MP over ~1.5 RE from the observation site, then it can completely account for the formation of the jet under the MP. To provide the downstream acceleration of the flow near the MP via the cross-field drift, the weak magnetic field is suggested to rotate from its nearly parallel direction to the unperturbed flow toward being almost perpendicular to the accelerated flow near the MP. We discuss a deceleration of the higher-energy ions in the MP normal direction due to the interaction with finite-scale electric field bursts in the magnetosheath flow frame, equivalent to collisions, providing a charge separation. These effective collisions, with a nonlinear frequency proxy of the order of the proton cyclotron one, in extended turbulent zones are a promising alternative in place of the usual parallel electric fields invoked in the macro-reconnection scenarios. Further cascading towards electron scales is supposed to be due to unstable parallel electron currents, which neutralize the potential differences, either resulted from the ion- burst interactions or from the inertial drift. The complicated MP shape suggests its systematic velocity departure from the local normal towards the average one, inferring domination for the MP movement of the non-local processes over the small-scale local ones. The measured Poynting vector indicates energy transmission from the MP into the upstream region with the waves triggering impulsive downstream flows, providing an input into the local flow balance and the outward movement of the MP. Equating the transverse electric field inside the MP TCS by the Hall term in the Ohm's law implies a separation of the different plasmas primarily by the Hall current, driven by the respective part of the TCS surface charge. The Hall dynamics of TCS can operate either without or as a part of a macro-reconnection with the magnetic field annihilation.

Scaling relations in the paleomagnetosphere derived from MHD simulations

[1] On geological timescales the Earth's dipole moment can vary between 0.1 and 2 times the present value. The weakest internal magnetic fields occur during geomagnetic polarity transitions when the Earth's internal dipole field reverses. Theoretically, the size of the paleomagnetosphere is expected to change in the function of the dipole moment according to a power law scaling relation. We carried out a series of numerical magnetohydrodynamic (MHD) simulations of axial dipolar paleomagnetospheres, gradually decreasing the relative dipole moment from 1 to 0.1, in order to test the validity of the theoretical scaling relations for different values of the north-south interplanetary magnetic field component (IMF B z ). We study the dipole moment dependence of the standoff distance, the flank distances, and the polar cap size, and derive power law scaling relations with significantly differing scaling exponents as compared to the theoretically expected ones. The extent of deviation from the theoretical scaling exponents is controlled by the magnitude of the southward B z . We quantify the B z -dependence of the size of the magnetosphere and validate our results with the Roelof-Sibeck bivariate function of magnetopause shape obtained from in-situ satellite measurements for the present-day magnetosphere. We conclude that the Roelof-Sibeck function cannot be applied for southward B z values stronger than 5 nT. A new B z -dependent dipole scaling relation is suggested for the magnetospheric scale size. Inserting our corrected scaling relation in the Hill model of magnetosphere-ionosphere coupling, a better fit can be obtained between simulated and theoretically calculated paleomagnetospheric transpolar potentials.

The shape and location of the high-latitude magnetopause

Recent empirical magnetopause models are bi-variate with respect to both solar wind dynamic pressure and B Z component of the interplanetary magnetic field (IMF). Models use various functional forms to represent the shape and location of the magnetopause. Moreover, the models have different ranges of validity because, among other things, the data sets used for their development were usually different. A majority of these models were created for magnetopause crossings regardless of latitude. For the shape of the near-Earth high-latitude magnetopause, a new empirical model was published by Boardsen et al. [Boardsen, S.A., Eastman, T.E., Sotirclis, T, Green, J. L. An empirical model of the high-latitude magnetopause. J. Geophys. Res. 105, 23,193, 2000]. This model is parameterized by solar wind dynamic pressure, IMF B Z and dipole tilt angle. However, this model covers only a high-latitude part of the magnetopause in a limited range of the X GSE coordinate (∼±8 R E). We have used a new set of high-latitude magnetopause crossings and compared their coordinates with predictions of several magnetopause models with motivation to analyze differences between measurements and models. Our study reveals that the effect of the near-cusp magnetopause indentation can be reflected by an addition of a simple geometrical formula to the generally accepted model. This approach provides an universal model which describes the whole magnetopause surface with sufficient accuracy regardless of latitude.

Earth's bow shock and magnetopause in the case of a field‐aligned upstream flow: Observation and model comparison

On 5 May 1996 the Interball‐1 and IMP 8 spacecraft crossed the bow shock boundary. The upstream conditions were special in two factors: (1) the interplanetary magnetic field was anti‐parallel to the solar wind flow within 15° and (2) the conditions were stable for a prolonged period (∼9 hours). At the nose of the magnetosphere, the Interball‐1 data revealed that the magnetopause was farther outward by ∼2 RE than model predictions and the subsolar magnetosheath was unusually thin, at most 10% of the magnetopause standoff distance. Both results stand in contrast to predictions of existing magnetopause/bow shock models. Assuming a hyperboloidal (paraboloidal) shock wave, the calculated shock's standoff distance was 13.7 (13.6) RE, and the focus was located on the x axis at 4.5 (4.2) RE. On the basis of the IMP 8 observation, the bow shock flares significantly less than MHD simulations predict for a field‐aligned bow shock at the magnetospheric flanks. This study discusses differences between the observations and existing MHD bow shock simulations for field‐aligned upstream flow. Furthermore, it is suggested that the flow‐aligned IMF orientation causes a significant change of the magnetopause shape into a bullet‐like obstacle.