Global Hybrid Simulation of Magnetic Reconnection in the Magnetosheath

A three-dimensional (3-D) global hybrid simulation is carried out for the generation and structure of magnetic reconnection in the magnetosheath due to interaction of an interplanetary Tangential Discontinuity (TD) with the bow shock and magnetosphere. Runs are performed for solar wind TDs possessing different initial half-widths. As the TD propagates through the bow shock toward the magnetopause, it is greatly narrowed by a two-step compression processes, a "shock compression' followed by a subsequent ``convective compression'. In cases with a relatively thin solar wind TD, 3-D patchy reconnection is initiated in the transmitted TD, forming magnetosheath flux ropes. Multiple components of ion particles are present in the velocity distribution in the magnetosheath merging, accompanied by ion heating. For cases with a relatively wide initial TD, a dominant single X-line appears in the subsolar magnetosheath after the transmitted TD is narrowed. A shock analysis is performed for the detailed structure of magnetic reconnection in the magnetosheath. Rotational Discontinuity (RD)/Time-Dependent Intermediate Shock (TDIS) are found to dominate the reconnection layer, which and some weak slow shocks are responsible for the ion heating and acceleration.      

Rapid and Asymmetric Response of the Earth's Bow Shock:Multipoint Observations

We analyze observations of three bow shock crossings which occurred during 2007, using upstream data from STEREO A/B, ACE and WIND, combined with multi-point THEMIS and Cluster data, and TC-1 data located near noon. During the crossing of 7 May 2007, we find that following a rapid reduction in solar wind ram pressure and subsequent pressure pulse seen by ACE and WIND upstream, the bow shock responds asymmetrically from dawn to dusk. Cluster data on the dawn-side suggest the bow shock is significantly flared and responds rapidly to the pulse arrival, while TC-1 at noon, and THEMIS on the dusk-side, are well matched to the model bow shock, but show a delayed response. The crossings observed on 21 May and 2 June show contrasting response matching the model boundary for northward Interplanetary Magnetic Field (IMF). The IMF and solar wind plasma data suggest that the bow shock crossing at dawn-dusk side and subsolar point were mainly caused by large and smaller scale features of the solar wind ram pressure rise rather than the influence of IMF.      

Survey on the Magnetic Structure of the Neutral Sheets in Earth's Magnetotail

Based on the multipoint magnetic observations of Cluster from 2001 to 2004, the magnetic field structure in magnetotail Neutral Sheet （NS） is statistically surveyed. The results are as follows. In NS, a cubic function is selected to reveal the relation between y （GSM） and positional parameter z. The relation between y and magnetic field values indicates that the magnetic field is weak at midnight region and strengthens gradually at the duskside and dawnside. The relation between y and curvature radius is expressed by a quadratic function. And R_c of flattened CS is less than that of the normal CS. B_y determines the orientation of MFLs' configuration. The polar angle of the curvature vector is affected by the NS configuration. In addition, the correlation between the polar angle of the curvature vector and z is higher. The polar angle of the normal of the osculating plane is uncertain in the center area. The relation between the azimuthal angles of the curvature vector （the normal of the osculating plane） and y is negatively correlated. An empirical model applied to yz plane of the three-dimensional structure of the magnetic field lines in the NS are developed, and it is represented as a function of the positional parameter y. Finally, the current density is also statistically surveyed.      

Influence of Fast Global Variations of Solar Magnetic Fields on Space Weather in Cycle 23

It is established that the large-scale and global magnetic fields in the Sun's atmosphere do not change smoothly, and long-lasting periods of gradual variations are superseded by fast structural changes of the global magnetic field. Periods of fast global changes on the Sun are accompanied by anomalous manifestations in the interplanetary space and in the geomagnetic field. There is a regular recurrence of these periods in each cycle of solar activity, and the periods are characterized by enhanced flaring activity that reflects fast changes in magnetic structures. Is demonstrated, that the fast changes have essential influencing on a condition of space weather, as most strong geophysical disturbances are connected to sporadic phenomena on the Sun. An explanation has been offered for the origin of anomalous geomagnetic disturbances that are unidentifiable in traditionally used solar activity indices. Is shown, main physical mechanism that leads to fast variations of the magnetic fields in the Sun's atmosphere is the reconnection process.     

Magnetospheric Boundary Layer Structure and Dynamics as Seen From Cluster and Double Star Measurements

In this review, we discuss the structure and dynamics of the magnetospheric Low-Latitude Boundary Layer (LLBL) based on recent results from multi-satellite missions Cluster and Double Star. This boundary layer, adjacent to the magnetopause on the magnetospheric side, usually consists of a mixture of plasma of magnetospheric and magnetosheath origins, and plays an important role in the transfer of mass and energy from the solar wind into the magnetosphere and subsequent magnetospheric dynamics. During southward Interplanetary Magnetic Field (IMF) conditions, this boundary layer is generally considered to be formed as a result of the reconnection process between the IMF and magnetospheric magnetic field lines at the dayside magnetopause, and the structure and plasma properties inside the LLBL can be understood in terms of the time history since the reconnection process. During northward IMF conditions, the LLBL is usually thicker, and has more complex structure and topology. Recent observations confirm that the LLBL observed at the dayside can be formed by single lobe reconnection, dual lobe reconnection, or by sequential dual lobe reconnection, as well as partially by localized cross-field diffusion. The LLBL magnetic topology and plasma signatures inside the different sub-layers formed by these processes are discussed in this review. The role of the Kelvin-Helmholtz instability in the formation of the LLBL at the flank magnetopause is also discussed. Overall, we conclude that the LLBL observed at the flanks can be formed by the combination of processes, (dual) lobe reconnection and plasma mixing due to non-linear Kelvin-Helmholtz waves.      

A 3rd Order WENO GLM-MHD Scheme for Magnetic Reconnection

A new numerical scheme of 3rd order Weighted Essentially Non-Oscillatory (WENO)type for 2.5D mixed GLM-MHD in Cartesian coordinates is proposed. The MHD equations are modified by combining the arguments as by Dellar and Dedner et al to couple the divergence constraint with the evolution equations using a Generalized Lagrange Multiplier (GLM). Moreover, the magnetohydrodynamic part of the GLM-MHD system is still in conservation form. Meanwhile, this method is very easy to add to an existing code since the underlying MHD solver does not have to be modified. To show the validation and capacity of its application to MHD problem modelling,interaction between a magnetosonic shock and a denser cloud and magnetic reconnection problems are used to verify this new MHD code. The numerical tests for 2D Orszag and Tang's MHD vortex,interaction between a magnetosonic shock and a denser cloud and magnetic reconnection problems show that the third order WENO MHD solvers are robust and yield reliable results by the new mixed GLM or the mixed EGLM correction here even if it can not be shown that how the divergence errors are transported as well as damped as done for one dimensional ideal MHD by Dedner et al.      

Mid-Latitude Pc1, 2 Pulsations Induced by Magnetospheric Compression in the Maximum and Early Recovery Phase of Geomagnetic Storms

We investigate the properties of interplanetary inhomogeneities generating long-lasting mid-latitude Pc1, 2 geomagnetic pulsations. The data from the Wind and IMP 8 spacecrafts, and from the Mondy and Borok midlatitude magnetic observatories are used in this study. The pulsations under investigation develop in the maximum and early recovery phase of magnetic storms. The pulsations have amplitudes from a few tens to several hundred pT andlast more than seven hours. A close association of the increase (decrease) in solar wind dynamic pressure (Psw) with the onset or enhancement (attenuation or decay) of these pulsations has been established. Contrary to high-latitude phenomena, there is a distinctive feature of the interplanetary inhomogeneities that are responsible for generation of long-lasting mid-latitude Pc1, 2. It is essential that the effect of the quasi-stationary negative Bz-component of the interplanetary magnetic field on the magnetosphere extends over 4 hours. Only then are the Psw pulses able to excite the above-mentioned type of mid-latitude geomagnetic pulsations. Model calculations show that in the cases under study the plasmapause can form in the vicinity of the magnetic observatory. This implies that the existence of an intense ring current resulting from the enhanced magnetospheric convection is necessary for the Pc1, 2 excitation. Further, the existence of the plasmapause above the observation point (as a waveguide) is necessary for long-lasting Pc1 waves to arrive at the ground.      

A POSSIBLE MECHANISM FOR CORONAL MASS EJECTIONS

This paper examines a possible mechanism for the initiation of coronal mass ejections by means of the eruption of magnetic flux beneath a closed magnetic structure analogous to a coronal helmet structure. Anumerical computation shows that such an eruption deforms the helmet structure into an expanding bright "loop", followed by a dark cavity due to the expansion of the erupted magnetic field. The morphology and the internal structure of the loop are mainly determined by the background magnetic field whereas its ejective speed is largely controlled by the rate of flux ejection. The present model identifies the loop-like mass ejections with the above plasma structures moving with the frozen-in magnetic field and it may well explain some general properties of such ejections.      

Multi-case Study of Current Sheet Flapping Motions Induced by Non-adiabatic Ions

In this paper, the y-component of magnetic field line curvature in the plasma sheet was analyzed, and two kinds of shear structures of the flapping current sheet were found, i.e. symmetric and antisymmetric. The alternating bending orientations of the guiding field are exactly corresponding to alternating north-south asymmetries of the bouncing ion population in the sheet center. Those alternating asymmetric plasma sources consequently induce the current sheet flapping motion as a driver. In addition, a substantial particle population with downward motion was observed in the center of a bifurcated current sheet. This population is identified as the quasi-adiabatic particles, and provides a net current opposite to the conventional cross-tail current.      

The evolution of mirror type magnetic fluctuations in the magnetosheath based on multipoint observations

Two orbits were selected in January–February 2006 when the separation between the Cluster spacecraft was large and mirror type magnetic field fluctuations were observed by all spacecraft in different regions of the terrestrial magnetosheath. Minimum variance analysis was applied to find the mirror type fluctuations, and the amplitude of the fluctuations was determined individually. Mirror mode structures are moving along the streamlines frozen in the plasma. A model was developed for the calculation of plasma flowtime from the bow shock to the observation point. The growth rate of the field strength perturbations was estimated by comparing the amplitudes of fluctuations observed simultaneously at distant locations (∼10,000 km) based on the assumption that δB ∼ exp(γt). The obtained growth rate values were about an order of magnitude smaller than those provided by linear models and they decreased in the inner regions of the magnetosheath, indicating some saturation in the growth of the waves when proceeding towards the magnetopause. The results of these two case studies suggest that mirror type fluctuations originate from the compression region downstream of the quasi-perpendicular bow shock, and the growth of the fluctuations cannot be described by linear approximations.     

Dayside reconnection during IMF northward: A possible foreshock effect

The northward and southward orientation of the interplanetary magnetic field (IMF) is usually considered as providing the external boundary conditions in the solar wind interaction with the Earth's magnetopause but it is the magnetic field in the magnetosheath that interacts with the Earth's magnetic field. In this paper, we consider the possibility that the wave activity in the foreshock region may affect the magnetic field orientation in the magnetosheath with time scales that might be geomagnetically effective. If magnetosheath magnetic field becomes disturbed on plasma streamlines which are connected to the quasi-parallel bow shock and foreshock, the magnetic field orientation on the inner magnetosheath may differ significantly from the undisturbed IMF. We present a model of dayside reconnection which may occur when the IMF northward and illustrate its effects on the erosion of the magnetopause.     

地球磁鞘中磁场起伏特性的分布

本文利用行星际起伏通过激波后的变化的MHD模型, 具体讨论了地球磁鞘中磁场起伏特性在黄道面内的分布.主要结果是:(1)行星际磁场起伏的强度和各向异性在磁鞘中被显著放大;(2)行星际磁场基本位于黄道面内时, 磁鞘中磁场起伏特性(强度、相对起伏和各向异性等)呈现明显的晨一昏不对称性, 早晨侧(准平行激波)显著地高于黄昏侧(准垂直激波);(3)行星际磁场方向对磁场起伏特性在磁鞘中的分布有强烈的控制作用, 早晨侧响应灵敏, 黄昏侧反响不大.相对地讲, 黄昏侧的磁活动较之早晨侧稳定;(4)行星际磁场转南的增强将导致磁鞘中磁场起伏的最大区域自黄道面低纬向北极高纬移动, 南-北不对称性磁活动随之加强, 最强大致出现在磁场与黄道面相交成大约45°时, 而晨-昏不对称性的强弱程度则发生相反变化;(5)行星际磁场的相对起伏增加, 晨-昏不对称性反随之减弱.磁鞘中磁场起伏分布的特性与卫星观测大体符合, 是磁顶、边界层某些晨-管不对称性出现的可能起因之一.      

Foreshock and magnetosheath waves at Uranus and Neptune studied with wavelet analysis

Foreshock and magnetosheath waves in Uranus and Neptune magnetospheres are studied in this work with wavelet analysis. In order to conduct this study, Voyager-2 magnetometer 3-s averaged data are used. The Morlet wavelet transform is applied to the magnetic field vector data. Waves present in the magnetosheath and foreshock regions are highly non-stationary, showing large amplitude variations. It was found that the dominant periods of these waves are longer than the H+ cyclotron period. Overall, high frequency waves are seen near the bow shock crossing and low frequency oscillations near the magnetopause crossing. It can be concluded that non-stationary foreshock and magnetosheath planetary waves can be well characterized with wavelet analysis.     

A stationary bow shock model for plasmas: The spherical blunt obstacle problem

We present an analytic model of a stationary bow shock which describes the interaction between a supermagnetosonic ambient wind and an obstacle with spherical-like frontal shape. We develop expressions for the bow shock’s geometry and the physical properties of the plasma sheath as functions of the upstream conditions. The solution is limited to magnetic fields parallel to the upstream velocity. The model allows to use any value of the upstream alfvenic and sonic Mach numbers and the polytropic index (γ$\gamma$), pointing out the influence of γ$\gamma$ for the magnetosheath compression and the bow shock shape. When both Mach numbers are small, the upstream magnetic field intensity affects also the bow shock shape. We compare our results with other models finding important consistencies. We also compare our results with in-situ data, we fund a reasonable qualitative agreement; however, it seems that our model underestimates the magnetosheath size.     

On the modular approach of a 3D model of the system magnetosheath–magnetosphere

This article presents some preliminary features of a new self consistent model of the system magnetosheath–magnetosphere, recently developed in the Institute of Mechanics, Sofia, Bulgaria. The flow in the magnetosheath is governed by 3D ideal gas-dynamic equations. The positions and the shapes of the bow shock and the magnetopause are calculated iteratively as a part of the solution. These surfaces are essentially three-dimensional (generally non-axially-symmetric). The self-consistency between the regions is ensured via the boundary conditions. The magnetopause cusp indentations are formed, influencing essentially the magnetosheath flow. Prediction of the position and the shape of the bow shock for different values of the sonic Mach number are derived. Distribution of some flow parameters in the magnetosheath is presented. 3D numerical finite element model, calculating the field due to the magnetopause currents for an arbitrary magnetopause geometry, is used in the magnetosphere. The fields due to the current systems inside the magnetosphere(cross-tail current, ring current, and Birkeland current) are taken from the Tsyganenko empirical model. The magnetopause surface is calculated from the requirement the outside gas-dynamic pressure to be balanced by the magnetic pressure inside. The magnetosphere model can be viewed as an improved version of the empirical model but with more realistic magnetopause form and shielding field. Not a final but a beta version is used in this approach. The final model version as well the model details will be presented elsewhere.     

太阳风状态参数的变化对地球磁鞘中磁场起伏特性的影响

本文利用MHD激波跳跃条件的精确解,具体讨论了行星际背景太阳风状态参数Alfvén马赫数M₁、等离子体β₁参数和磁场角θ₁的变化对地球磁鞘区中磁场起伏特性及其分布的影响.主要结果是:马赫数M₁的变化主要控制磁场起伏特性:放大倍数、相对起伏和各向异性程度的水准高低.磁场角θ₁的变化控制磁场起伏的空间分布特性.等离子体β₁参数的变化,不引起磁场起伏特性的明显变化(对于实际经常发生的情况M₁ 8而言).M₁、θ₁是强控制参数,而β₁是弱控制参数;磁鞘区磁场起伏对太阳风状态参数的变化响应呈现明显的晨-昏不对称性(行星际磁场位于黄道面时),响应主要发生在晨侧.晨侧的磁场起伏(或湍动)相当活跃,而昏侧相当稳定;磁鞘中不同地点磁场起伏特性对太阳风状态参数M₁、β₁的变化响应有大致相同的形式,而对其磁场角度θ₁的变化却有迥然不同的形式.      

Small-scale deformation of the bow shock

The prediction of the bow shock location is a proof of our understanding of the processes governing the solar wind – magnetosphere interaction. However, the models describing the bow shock location as a function of upstream parameters are based on a statistical processing of bow shock crossings observed by a single spacecraft. Such crossings locate the bow shock in motion, i.e., in a non-equilibrium state and this fact can be a source of significant errors. We have carefully analyzed a long interval of simultaneous observations of the bow shock and magnetopause and another interval of bow shock observations at two well-separated points. Our results suggest that often a small-scale deformation of the bow shock front due to magnetosheath fluctuations is the most appropriate interpretation of observations. Since the low-frequency magnetosheath variations exhibit largest amplitudes, a simultaneous bow shock displacement over a distance of 10–15 R_E can be observed. We suggest that bow shock models can be probably improved if the tilt angle would be implemented as a parameter influencing the bow shock location in high latitudes.     

Energetic ions upstream of the Earth''s bow shock: INTERBALL-1 survey

A statistical survey of energetic ions (> 20 keV) observed by the DOK2 experiment on Interball-1 during foreshock intervals from 1996 to 1998 is presented. Flux levels depend on the connection geometry, with higher values at quasi-parallel shocks, particularly at lower energies. The decrease of flux in diffusive events with distance from the bow shock is consistent with results from earlier surveys performed closer to the bow shock. The energy spectra are softer for quasi-parallel than quasi-perpendicular connections. The reflected ions have musch softer spectra. The cumulative frequencies for flux level occurrence patterns exhibit differences for quasi-parallel and quasi-perpendicular geometries up to approximately 200 keV.