磁层顶压缩事件的磁场分析

向日面磁层顶在平静太阳风条件下,处于10RE（RE为地球半径）左右．但在异常的太阳风条件下,即南向行星际磁场很强和（或）太阳风的动压很大时,会被压缩,甚至到达同步轨道附近．集中分析2001年4月11日的磁暴事件,研究当磁层顶发生强烈压缩以后。在地球空间和地面上产生的磁场影响．磁层顶位形选取Shue（1998）模型计算．当把计算结果与GOESl0卫星的观测数据对比时发现：磁层顶在强的太阳风条件下的确会被压缩到同步轨道以内．Shue（1998）模型的预测基本正确,通常的漏报可能是由于预报的位置误差所致．实际磁层顶电流片的位置和强度与我们假设的理想磁层顶间断面计算结果基本吻合．在分析大磁暴过程时,磁层顶压缩使磁层顶电流对于中低纬度地磁场扰动有突出的贡献,在2001年4月事件中,这个贡献可以大于50nT,占主相的1／6左右．这一贡献可以使Dst指数产生相应的误差．     

太阳风压力系数的研究

利用全球磁流体力学(MHD)的模拟结果,研究了太阳风压力系数与上游太阳风参数和日下点磁层顶张角的相关性.在识别出日下点附近磁层顶位置后,通过拟合得到日下点附近的磁层顶张角.在考虑上游太阳风中的磁压和热压以及磁层顶外侧的太阳风动压的情况下,计算了太阳风压力系数.通过分析行星际磁场不同方向时太阳风动压在日地连线上与磁压和热压的转化关系,详细研究了太阳风参数和日下点磁层顶张角对太阳风压力系数的影响,得到以下相关结论:(1) 在北向行星际磁场较大(B_z≥5 nT)时,磁层顶外侧磁压占主导,南向行星际磁场时磁层顶外侧热压占主导;(2) 太阳风压力系数随着行星际磁场的增大而增大,随着行星际磁场时钟角的增大而减小;并且在行星际磁场大小和其他太阳风条件相同时,北向行星际磁场时的太阳风压力系数要大于南向行星际磁场时的;北向行星际磁场时,太阳风压力系数随着太阳风动压的增大而减小,南向行星际磁场时,太阳风压力系数随着太阳风动压的增大而增大;以上结论是对观测结果的扩展;(3) 最后,我们还发现太阳风压力系数随着日下点磁层顶张角的增大而增大.     

Rayleigh-Taylor-Kelvin-Helmholtz combined instability at the magnetopause

The magnetopause separates the geomagnetic field from the interplanetary plasma and performs finite motions under the action of the solar wind pressure variable in time. Accelerations originating in this case result in that the necessary condition for the development of the Rayleigh-Taylor instability is formed at quite a definite motion phase. We can anticipate that the instability will develop during compression of the magnetosphere. It should be taken into account that the magnetopause is a potential tangential discontinuity. On the one hand, a plasma flow along the magnetopause results in a decrease in the Rayleigh-Taylor instability threshold. On the other hand, the Kelvin-Helmholtz instability threshold, typical of the tangential discontinuity, also decreases during the magnetosphere compression phase. Thus, if we speak about the magnetopause, it is natural and necessary to jointly consider both types of instability. Main information on the combined Rayleigh-Taylor-Kelvin-Helmholtz instability is presented, the dispersion equation determining the evolution of small perturbations is considered, and the possible geophysical applications to the theory (e.g., penetration of the solar plasma into the magnetosphere, excitation of global Pc5 oscillations) are indicated.     

Dynamics and local boundary properties of the dawn-side magnetopause under conditions observed by Equator-S

Magnetic field measurements, taken by the magnetometer experiment (MAM) on board the German Equator-S spacecraft, have been used to identify and categorise 131 crossings of the dawn-side magnetopause at low latitude, providing unusual, long duration coverage of the adjacent magnetospheric regions and near magnetosheath. The crossings occurred on 31 orbits, providing unbiased coverage over the full range of local magnetic shear from 06:00 to 10:40 LT. Apogee extent places the spacecraft in conditions associated with intermediate, rather than low, solar wind dynamic pressure, as it processes into the flank region. The apogee of the spacecraft remains close to the magnetopause for mean solar wind pressure. The occurrence of the magnetopause encounters are summarised and are found to compare well with predicted boundary location, where solar wind conditions are known. Most scale with solar wind pressure. Magnetopause shape is also documented and we find that the magnetopause orientation is consistently sunward of a model boundary and is not accounted for by IMF or local magnetic shear conditions. A number of well-established crossings, particularly those at high magnetic shear, or exhibiting unusually high-pressure states, were observed and have been analysed for their boundary characteristics and some details of their boundary and near magnetosheath properties are discussed. Of particular note are the occurrence of mirror-like signatures in the adjacent magnetosheath during a significant fraction of the encounters and a high number of multiple crossings over a long time period. The latter is facilitated by the spacecraft orbit which is designed to remain in the near magnetosheath for average solar wind pressure. For most encounters, a well-ordered, tangential (draped) magnetosheath field is observed and there is little evidence of large deviations in local boundary orientations. Two passes corresponding to close conjunctions of the Geotail spacecraft are analysed to confirm boundary orientation and motion. These further show evidence of an anti-sunward moving depression on the magnetopause (which is much smaller at Equator-S). The Tsyganenko model field is used routinely to assist in categorising the crossings and some comparison of models is carried out. We note that typically the T87 model fits the data better than the T89 model during conditions of low to intermediate K_p index near the magnetopause and also near the dawn-side tail current sheet in the dawnside region.     

Response of currents in Earth’s and Saturn’s dayside magnetopause to a sudden change in the solar wind density

When the effect of a solar wind dynamic pressure pulse on the magnetospheric and ionospheric dynamics is studied, it is usually difficult to detect the effect of a sudden change in the density against the background of the other varying solar wind parameters, which often play a most pronounced role. Cases in which the solar wind plasma density gradient dominated in the dynamics of the different parameters of an interplanetary medium and its magnetic field are considered in this work. Variations in the Earth’s dayside magnetopause current caused by a change in the solar wind ion density are presented for two such cases (February 11 and January 11, 1997) based on the method developed by us previously. Variations in the dayside magnetopause current for collisions of the magnetosphere with corotating interacting flows in January 2004, studied in detail by us previously, are also presented for Saturn. The estimates are comparable with the current values in the transitional three-dimensional current systems of Saturn that were previously calculated by us.     

Solar wind—magnetosphere coupling: A review of recent results

This paper reviews some aspects of solar wind–magnetosphere–ionosphere interaction. It is shown that in addition to the interplanetary electric field, the solar wind dynamic pressure also has a significant role in determining the state, dynamics, and energetics of the system. It is demonstrated how the state of the magnetosphere and the prior driving affect the amount of energy input to the system, which highlights the capability of the magnetosphere to control the energy flow. The active role of the magnetosphere in determining the dynamics is illustrated by statistical results of the flux balance in the magnetotail and the various dynamic cycles the system can enter. The inner magnetosphere processes during storms are shown to be a result of a complex interplay of processes at the magnetopause and in the magnetotail in response to the solar wind driving. The conclusions are drawn from statistical observational results, empirical models, and global MHD simulations.     

关于动力Alfven波的色散与朗道阻尼

根据完全动力论理论,证明了可由简化漂移动力论方程求得动力Ａｌｆｖｅｎ波色散方程,即横向用磁流体力学方程,纵向用Ｖｌａｓｏｖ方程,在初级近似下由此推导出适合于日冕和太阳风的色散关系和Ｌａｎｄａｕ阻尼,得到在性质上与ＭＨＤ　Ａｌｆｖｅｎ波完全不同的动力Ａｌｆｖｅｎ波,太阳风中Ａｌｆｖｅｎ湍流很容易由动力Ａｌｆｖｅｎ波演化而来。提出由动力Ａｌｆｒｅｎ波构筑太阳风高速流模型将更符合观测结果。     

Energy as a tracer of magnetospheric processes: GUMICS-4 global MHD results and observations compared

This paper reviews quantitative analysis results of the energy transfer and dissipation processes in the GUMICS-4 global MHD simulation. Reconnection power dissipating magnetic energy, dynamo power transferring energy from plasma to the field, and energy flux transport across the magnetopause surface are all examined separately and shown to yield consistent results. This is used to argue that magnetic reconnection is the process controlling the energy transfer, even though it is not localized near the reconnection line. The most important factors controlling the reconnection efficiency are the interplanetary magnetic field (IMF) orientation and the solar wind speed, while the IMF magnitude and solar wind density play a lesser role. During northward IMF, the reconnection efficiency is larger for high speed and low IMF than for low speed and high IMF magnitude, even though the solar wind electric field in both cases is the same. Moreover, increasing pressure by increasing density has a different effect from equal increase of pressure by increasing the solar wind speed. Comparison with statistical observational results shows that the simulation results are in qualitative agreement with the observations, which significantly increases our confidence in interpreting the simulation results.     

Dependence of magnetic field parameters at the subsolar point of the magnetosphere on the interplanetary magnetic field according to the data of the THEMIS experiment

We analyze the dependence of the magnitude of the magnetic field, its three components, and the clock angle in the magnetosheath just in front of the magnetopause on the same values in the solar wind before a shock wave using the data of the THEMIS experiment. We take into account the time delay of the solar wind arrival at the subsolar point of the magnetopause. We obtain dependencies of the components of the magnetic field and the clock angle at the magnetopause on the corresponding quantities in the solar wind for different averaging intervals. We point to the events for which the direction of the magnetic field at the magnetopause is highly different from the direction of the magnetic field in the solar wind up to the sign change.     

Impact of solar wind tangential discontinuities on the Earth’s magnetosphere

The collision of a solar wind tangential discontinuity with the bow shock and magnetopause is considered in the scope of an MHD approximation. Using MHD methods of trial calculations and generalized shock polars, it has been indicated that a fast shock refracted into the magnetosheath originates when density increases across a tangential discontinuity and a fast rarefaction wave is generated when density decreases at this discontinuity. It has been indicated that a shock front shift under the action of collisions with a tangential discontinuity is experimentally observed and a fast bow shock can be transformed into a slow shock. Using a specific event as an example, it has been demonstrated that solar wind tangential discontinuity affects the geomagnetic field behavior.     

同步轨道磁场和地面磁场对太阳风动压变化事件的响应

本文对磁宁静时的123个动压变化事件(不包含激波事件)进行了统计研究.研究表明,在白天侧(9～15MLT)同步轨道磁场z分量对太阳风动压增大、减小事件具有较强的正响应,而在夜侧(21～3MLT)响应明显减弱,响应幅度具有明显的磁地方时分布.对动压增大事件的平均响应幅度在午前最大,而对动压减小事件的平均响应幅度在午后达到...     

Analytical description of electric currents in the magnetosheath region

Volume currents in the magnetosheath region are calculated within the framework of a new analytical model. Magnetic field structure in the region is found, satisfying boundary conditions on the bow shock and the magnetopause, and then volume currents are calculated using the Maxwell equation. Surface bow shock and magnetopause currents are calculated, too. Free parameters of the model are interplanetary magnetic field, Mach number of the solar wind flow, distances to the bow shock and to the magnetopause, and field compression at the magnetopause.     

太阳风动量涨落激发磁层亚暴的机制

本文将太阳风涨落传输能量产生磁层亚暴的机制推广到无碰撞等离子体过程。太阳风的涨落在磁层顶激发压缩阿尔文波,并在磁尾的无碰撞等离子体中传播。尾瓣中满足条件β?1,而等离子体片中β≥1,其中β为等离子体压力与磁压之比。这样,快磁声波在尾瓣中几乎不衰减,而在等离子体片中很快衰减,将波动能量耗散在等离子体片中使等离子体加热或者粒子加速。这种机制还表明,磁尾等离子体片中的高能粒子可以由太阳风涨落动能耗散而被加速,不一定是直接源于太阳。     

Magnetopause stand-off distance in dependence on the magnetosheath and solar wind parameters

A model of the magnetosheath structure proposed in a recent paper from the authors is extended to estimate the magnetopause stand-off distance from solar wind data. For this purpose, the relationship of the magnetopause location to the magnetosheath and solar wind parameters is studied. It is shown that magnetopause erosion may be explained in terms of the magnetosheath magnetic field penetration into the magnetosphere. The coefficient of penetration (the ratio of the magnetospheric magnetic field depression to the intensity of the magnetosheath magnetic field B_mz = -B_m sin²/2, is estimated and found approximately to equal 1. It is shown that having combined a magnetosheath model presented in an earlier paper and the magnetosheath field penetration model presented in this paper, it is possible to predict the magnetopause stand-off distance from solar wind parameters.     

Analytical model of the near-Earth magnetopause according to the data of the Prognoz and Interball satellite data

Based on the magnetopause observations near the Earth by the Prognoz/Interball satellites in 1972–2000, the empirical model of this boundary has been proposed, and the magnetopause behavior at different parameters of the oncoming solar wind has been studied. For the first time, it has been detected that the Earth’s magnetopause is compressed by ∼5% in the direction perpendicular to the plane including the vectors of the solar wind velocity and IMF. At the same time, any dependence of the subsolar magnetopause position on the IMF B _z component has not been revealed in the Progrnoz/Interball data. The proposed magnetopause model can be used to model the position and shape of the near-Earth bow shock.     

Low-latitude variations in the geomagnetic field caused by solar wind disturbances

In view of the actual question regarding the effect of a solar-wind pressure jump on disturbances in the Earth’s magnetosphere, events with high velocity and density gradients are of special interest. In this work, we consider the response of the current at the dayside magnetopause to these events and the corresponding strengthening of the geomagnetic field in the low-latitude magnetosphere. A transient process is studied that accompanies reconfiguration of the magnetosphere under the effect of disturbances of solar wind parameters. An analytical equation is received for estimation of an increase in the northern component of low-latitude magnetic field of the magnetosphere in a transient current system (transient ring current) versus initial values of the solar-wind velocity and density and their disturbances.     

The Influence of Parameters of the Interplanetary Medium and Magnetosheath Boundaries on the Correlation Coefficient between the Ion Flux Measured in the Solar Wind and the Magnetosheath

In this paper, the correlation coefficient between the ion fluxes in the solar wind and the magnetosheath is analyzed with the use of data of two satellites of the THEMIS mission and the THEMIS/Spektr-R satellites obtained in 2008 and 2011?2014, respectively. We have distinguished the conditions in which a high level of correlation between the measurements in the solar wind and the magnetosheath is observed, i.e., the correlation coefficient exceeds 0.7. As key factors, we consider both direct parameters of the solar wind, such as the density, the magnetic field magnitude, the magnetosonic Mach number, and the ratio β of the thermal pressure to the magnetic, and a more general factor—the type of large-scale structure of the solar wind. In addition, the effect of the satellite location in the magnetosheath relative to its boundaries—the bow shock and the magnetopause—on the correlation level is considered. It has been shown that, in roughly one third of cases, the plasma structures of the solar wind undergo a strong modification at the bow shock and in the magnetosheath, which results in a low correlation level corresponding to a correlation coefficient of less than 0.5; a high correlation level is observed in half of cases, i.e., the plasma structures are weakly disturbed. It has been determined that (1) the low correlation level in the magnetosheath behind quasi-perpendicular bow shock is more often observed near the magnetopause than in region just behind the bow shock, (2) the probability of observations of a high correlation level is independent of the profile shape of the quasi-perpendicular bow shock, and (3) the high correlation is more probable for the events corresponding to the solar wind of the Corotating Interaction Region (CIR) type than for those with the other solar wind types observed in the considered period.     

Global aspects of solar wind–ionosphere interactions

Recent observations have quantified the auroral wind O⁺ outflow in response to magnetospheric inputs to the ionosphere, notably Poynting energy flux and precipitating electron density. For moderate to high activity periods, ionospheric O⁺ is observed to become a significant or dominant component of plasma pressure in the inner plasma sheet and ring current regions. Using a global circulation model of magnetospheric fields and its imposed ionospheric boundary conditions, we evaluate the global ionospheric plasma response to local magnetospheric conditions imposed by the simulation and evaluate magnetospheric circulation of solar wind H⁺, polar wind H⁺, and auroral wind O⁺. We launch and track the motions of millions of test particles in the global fields, launched at randomly distributed positions and times. Each particle is launched with a flux weighting and perpendicular and parallel energies randomly selected from defined thermal ranges appropriate to the launch point. One sequence is driven by a two-hour period of southward interplanetary magnetic field for average solar wind intensity. A second is driven by a 2-h period of enhanced solar wind dynamic pressure for average interplanetary field. We find that the simulated ionospheric O⁺ becomes a significant plasma pressure component in the inner plasma sheet and outer ring current region, particularly when the solar wind is intense or its magnetic field is southward directed. We infer that the reported empirical scalings of auroral wind O⁺ outflows are consistent with a substantial pressure contribution to the inner plasma sheet and plasma source surrounding the ring current. This result violates the common assumption that the ionospheric load is entirely confined to the F layer, and shows that the ionosphere is often an important dynamic element throughout the magnetosphere during moderate to large solar wind disturbances.     

高密度极尖区的形成原因

极尖区是太阳风进入磁层的一个重要窗口,极尖区密度是反映这一物理过程的重要参量,通常情况下极尖区密度约为1~10 cm^-3,但有时卫星会观测到密度大于40 cm^-3的极尖区,本文称之为高密度极尖区.我们分析了Cluster卫星2001—2009年的观测数据,在470个极尖区穿越中找到28个高密度极尖区穿越事件并进行了统计研究,分析了高密度极尖区事件的形成原因,进而讨论了太阳风高效地进入极尖区的外部条件.结果表明：距正午的距离（｜MLT-12｜）较小,太阳风的密度高,低纬有磁层顶磁重联发生以及正偶极倾角都是观测到高密度极尖区事件的有利条件,并且当同时满足上述4个条件时,高密度极尖区事件发生率为100%;而低纬磁层顶磁重联以及大的正偶极倾角被认为是太阳风高效地进入极尖区的重要条件.这些研究结果有助于我们更进一步地理解太阳风进入极尖区的物理机制.     

On the determination of the Hermaean magnetic moment: A critical review

The determination of Mercury's magnetic moment from the spatially and temporally limited observations obtained by the Mariner 10 mission is dependent upon the assumed nature of both the intrinsic planetary magnetic field and that of the magnetospheric current systems. In this paper the methods that have been used for this purpose are reviewed. The results that have been obtained are then considered in comparison with the constraints placed on the planetary field by the solar wind dynamic pressures and stand-off distances inferred from the Mariner 10 magnetic field data by Slavin and Holzer (1979) which are consistent with an effective dipole moment of 6 ± 2 × 10²² G cm³. It is found that the models which fit the observations with substantial quadrupole and octupole moments are not consistent with the magnetospheric boundary conditions, presumably owing to incorrect assumptions regarding the magnetopause position, incorrect assumptions regarding solar wind dynamic pressure, and/or averaging over temporal variations in the Mariner 10 data.