On the origin of the high-latitude boundary layer

This paper presents a statistical study of the high-latitude boundary layer (HLBL) performed on 53 Interball-1 magnetopause crossings. In the study we verify if antiparallel merging is the main source of HLBL formation when the IMF is nearly horizontal. To provide such a study we designed a new coordinate system which allowed us to analyze HLBL under varied interplanetary conditions. This coordinate system floats over the dayside magnetopause following the changes in the instant location of the reconnection site. Despite very different interplanetary conditions, the observed HLBL plasma regimes manifest systematic behavior in the “reconnection” frame of reference. We explain the observed pattern in terms of sporadic patchy reconnection in the high magnetic shear region of the magnetopause.     

Ion relaxation processes in the heliospheric interface: how perturbed are ion distribution functions?

Charge-exchange processes between interstellar H-/O-atoms and protons of the bulk of the interstellar plasma flow downstream of the outer bowshock in the heliospheric interface induce secondary ions leading to non-relaxated velocity distribution functions. The relaxation of these freshly induced ions towards an equilibrium distribution occurs due to Coulomb interactions and wave–particle interactions with the background turbulence. Since Coulomb interactions are of low relevance, we study here in detail the effect of wave–particle interactions. To find the turbulence levels in the interface we consider the MHD-wave transformation at the outer shock surface between the interface and the local interstellar plasma. The turbulence in the outer interface region is shown to be dominated by incompressible Alfvén waves both for cases of quasiparallel and quasiperpendicular shocks. Also we show that waves propagating towards the shock are more intensive than those propagating away from it. The level of Alfvén turbulence in the interface is estimated using the recent data on local interstellar turbulence deduced from observations of interstellar scintillations of distant radiosources. Two proton relaxation processes are considered: quasilinear resonant interactions with Alfvén waves and non-linear self-induced wave–particle scattering. The corresponding diffusion coefficients are estimated, and typical time periods for protons and oxygen ions relaxation are shown to be of the same order of magnitude as H-/O-atoms passage time over the extent of the interface. This indicates that perturbed ion distribution functions must be expected there.     

Energetic ion observations during comet Giacobini-Zinner encounter

The Energetic Particle Anisotropy Spectrometer (EPAS) on the ICE spacecraft observed large fluxes of energetic ions (E > 65-keV) for a period of one day prior to encounter with comet Giacobini-Zinner to several days afterwards. These observations permit the study of the way in which cometary atoms and molecules are “picked-up” and accelerated by the solar wind flow, such that the flow becomes mass-loaded and slowed in the vicinity of the comet. The ion bulk flow within the mass-loaded region can also be studied together with the nature of the boundary between this region and the outer “pick-up” region. Finally it is also possible to study ion motion close to, and within, the induced magnetotail of the comet.     

太阳风向磁层输入电磁能量研究

利用全球磁流体力学（MHD）模拟结果,通过确立包含磁层顶的太阳风流线内边界来识别三维磁层顶位形,并以极尖区位置作为磁层顶日侧与夜侧的分界线,在此基础上定量研究了不同条件下穿过磁层顶向磁层内输入的电磁能量. 研究发现,磁层顶的能量传输与太阳风条件密切相关,磁重联是控制电磁能量传输的重要机制. 结果表明,当IMF（行星际磁场）南向时,极尖区后方的磁尾附近存在电磁能输入最大值,当IMF北向时,电磁能输入最大值发生在极尖区附近;南向IMF条件下,在IMF强度增大或太阳风密度增大时,磁层顶电磁能传输的电磁能量比北向IMF条件时增加更显著. 太阳风通过调节磁层顶面积间接影响到磁层顶能量传输大小. 研究还发现,北向IMF与南向IMF条件下穿过磁层顶的电磁能输入的比值范围约为10%～30%,此比值一定程度上反映了北、南方向IMF与地磁场磁重联效率的比值.      

高纬磁层顶尾部边界层的离子流剪切不稳定性

研究了高纬磁层顶层部边界层在非均匀磁场条件下的离子流剪切不稳定性，由于考虑了尾向电流，我们发现离子流剪切不稳定性的激发与扰动波长有关，并得到了激发该不稳定性的准临界波长的表达式，对于磁层顶存在的波长为地球半径量级的表面波扰动，离子流剪切不稳定性倾向于发生在磁层顶边界层的内边界，沿磁层顶外边界传播的稳定的表面波和其内边界的离子流剪切不稳定性同时存在，这将有助于解释磁鞘太阳与动量持续不断地向地磁层的传输。     

Spatial distribution of the magnetosheath ion flux

The magnetosheath plays a crucial role in solar wind-magnetosphere interaction because it is the magnetosheath magnetic field and plasma that interact with the magnetopause and magnetosphere, not the unshocked solar wind. We are presenting ion flux measurement statistics at both the dawn and dusk flanks of the magnetosheath and their comparison with a gasdynamic magnetosheath model. The study is based on three years of INTERBALL-1 measurements supported by simultaneous WIND solar wind and magnetic field observations. Statistical processing has shown (1) the limitations of the gasdynamic model, (2) the conditions favorable for the creation of a plasma depletion layer adjacent to the flank magnetopause, (3) strong dawn-dusk asymmetry of the ion fluxes, and (4) an evidence for the presence of a slow mode front adjacent to the magnetopause.     

Sharp boundaries of solar wind plasma structures and their relationship to solar wind turbulence

Sharp (<10 min) and large (>20%) solar wind ion flux changes are common phenomena in turbulent solar wind plasma. These changes are the boundaries of small- and middle-scale solar wind plasma structures which can have a significant influence on Earth’s magnetosphere. These solar wind ion flux changes are typically accompanied by only a small change in the bulk solar wind velocity, hence, the flux changes are driven mainly by plasma density variations. We show that these events occur more frequently in high-density solar wind. A characteristic of solar wind turbulence, intermittency, is determined for time periods with and without these flux changes. The probability distribution functions (PDF) of solar wind ion flux variations for different time scales are calculated for each of these periods and compared. For large time scales, the PDFs are Gaussian for both data sets. For small time scales, the PDFs from both data set are more flat than Gaussian, but the degree of flatness is much larger for the data near the sharp flux change boundaries.     

The plasma sheet and boundary layers under northward IMF: A multi-point and multi-instrument perspective

During conditions of northward interplanetary magnetic field (IMF), the near-tail plasma sheet is known to become denser and cooler, and is described as the cold-dense plasma sheet (CDPS). While its source is likely the solar wind, the prominent penetration mechanisms are less clear. The two main candidates are solar wind direct capture via double high-latitude reconnection on the dayside and Kelvin–Helmholtz/diffusive processes at the flank magnetopause. This paper presents a case study on the formation of the CDPS utilizing a wide variety of space- and ground-based observations, but primarily from the Double Star and Polar spacecraft on December 5th, 2004. The pertinent observations can be summarized as follows: TC-1 observes quasi-periodic (∼2 min period) cold-dense boundary layer (compared to a hot-tenuous plasma sheet) signatures interspersed with magnetosheath plasma at the dusk flank magnetopause near the dawn-dusk terminator. Analysis of this region suggests the boundary to be Kelvin–Helmholtz unstable and that plasma transport is ongoing across the boundary. At the same time, IMAGE spacecraft and ground based SuperDARN measurements provide evidence of high-latitude reconnection in both hemispheres. The Polar spacecraft, located in the southern hemisphere afternoon sector, sunward of TC-1, observes a persistent boundary layer with no obvious signature of boundary waves. The plasma is of a similar appearance to that observed by TC-1 inside the boundary layer further down the dusk flank, and by TC-2 in the near-Earth magnetotail. We present comparisons of electron phase space distributions between the spacecraft. Although the dayside boundary layer at Polar is most likely formed via double high-altitude reconnection, and is somewhat comparable to the flank boundary layer at Double Star, some differences argue in favour of additional transport that augment solar wind plasma entry into the tail regions.     

The magnetopause erosion and the magnetosheath magnetic field penetration into the dayside magnetosphere

The earthward displacement of the magnetopause observed during a southward IMF (or the magnetopause erosion) and its dependence on the solar wind plasma and magnetic field parameters is studied by investigating data of about 30 magnetopause crossings by the ISEE 1 and 2 spacecraft. It is shown that the magnetopause erosion may be explained by a depression of the magnetic field intensity in the dayside magnetosphere caused by the penetration of the magnetosheath magnetic field (component perpendicular to the reconnection line) into the magnetosphere. The penetration coefficient (the ratio of the intensity of the penetrated field to the intensity of the magnetosheath magnetic field) is estimated and found to equal approximately 1.     

磁层顶附近流场剪切度的确定

磁层顶附近的流场剪切度与磁层顶附近能量转换的程度有关.很多磁层顶数值模拟用到流场剪切度这个输入参数,但一直是假设的.本文利用Cluster多卫星同时观测数据及独特的时空分辨功能,采用线性插值和重心坐标的方法确定了磁层顶附近流场的剪切度.通过对晨侧和昏侧磁层顶及附近磁层磁鞘流场剪切度的真实空间分布的研究结果表明,在平静的太阳风条件和地磁条件下磁层顶附近流场剪切度有时也很大,可达每百公里330 km/s的相对速度差.但在很多情况下流场是弱剪切的,在上千公里的距离上只有每秒几十公里的相对速度差.本文确定流场剪切度的方法可以推广用来确定任一位置的流场剪切度.      

远磁尾磁层顶位形的偏转

利用WIND和ARTEMIS卫星观测数据，分析远磁尾磁层顶对行星际和太阳风变化的响应，尤其是偏离日地连线的太阳风速度改变对远磁尾磁层顶的影响.研究发现在2011年9月13日的事件中，P2卫星观测到高速且高密度的磁鞘流.利用最小变量法进行分析发现，磁层顶沿着偏离日地连线的太阳风速度方向发生偏转.根据相似三角形定理，推断出本次事件中磁层顶在y方向和z方向上的偏转幅度分别达到10R_e和6R_e.P1和P2卫星的相对位置也证实了这一观点.因此，偏离日地连线的太阳风速度对远磁尾磁层顶的位形影响很大.研究结果可为建立包含太阳风速度v_y和v_z效应的磁层顶模型提供观测证据.      

Magnetopause poleward of the cusp: Comparison of plasma and magnetic signature of the boundary for southward and northward directed interplanetary magnetic field

A coherent data set of high-latitude dayside magnetopause encounters by old (Heos 2, Hawkeye, Prognoz 7, 8) and new (Polar, Interball Tail, Cluster) spacecraft is needed to build a realistic model of the magnetopause (MP) including an indentation in the cusp. In building such a coherent data set a caution is necessary as the dayside magnetopause at high-latitudes may be less clearly defined than in the case of observations at low latitudes. It is due to expected presence of bundles of newly-reconnected magnetic field lines forming an extended boundary layer on the magnetosheath (MS) side of the magnetopause in the cusp region. Moreover, numerical magnetohydrodynamic (MHD) models of the solar wind-magnetosphere interaction predict that under northward interplanetary magnetic field (IMF) an additional thin current sheet should form inside the magnetopause at high latitudes on the dayside (e.g., Wu, 1983; Palmroth et al., 2001). Such a thin currect sheet is absent in empirical magnetosphere models. This internal current sheet, if a real one, may be mistaken for the magnetopause if magnetic field data are only taken into account and/or plasma data are unavailable. The Interball-Tail orbit allows for a full transition of magnetopause boundary layers at high-latitudes. We compare plasma and magnetic field signatures of the magnetopause poleward of the cusp for southward and northward IMF. The distance between the magnetic signature of the magnetopause (the current layer) and a cold and laminarly antisunward flowing MS plasma (so called free-flow MS) was found to be 0.5 to 1 R_E, at least. These observations were made under nominal solar wind of v350 km/s and p_dyn=1 to 4 nPa. We also observed several transient magnetic field reversals in the cusp related to pulses of solar wind dynamic pressure and/or the IMF discontinuity arrival. These transient reversals occurred at the same distance to the model MP as well defined full MP crossing, so most probably they represent just short encounters with the magnetopause current layer. Our analysis suggests that an indentation of the magnetopause with a subtle structure dependent on the local magnetic shear would explain and allow to predict the magnetic configuration in the high-altitude cusp.     

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.      

极端太阳风条件下的磁层顶位形

基于极端太阳风条件下的三维MHD数值模拟数据, 构建了一种极端太阳风条件下的三维非对称磁层顶位形模型. 所提出的模型考虑了行星际南向磁场(IMF) B_z日下点距离侵蚀的饱和效应, 太阳风动压B_d对磁层顶张角影响的饱和效应, 赤道面、昼夜子午面磁层顶的不对称性以及极尖区的内凹结构和内凹中心的移动, 并利用Levenberg-Marquart多参量非线性拟合方法拟合了模型参数. 数值模拟研究表明, 在极端太阳风条件下, 随B_d增大, 磁层顶日下点距离减小, 磁层顶磁尾张角几乎不变; 随南向(IMF)B_z增大, 磁层顶日下点距离略有减小, 磁层顶磁尾张角减小, 极尖区内凹中心向低纬移动. 通过对2010年8月1日太阳风暴事件验证发现, 本文所建立的模型能够描述极端太阳风条件下的三维磁层顶位形.      

Evidence for ion energy dispersion in the polar cusp related to a northward-directed IMF

Data from the particle experiment aboard the AUREOL-3 polar satellite show that about 30% of the summer cusp crossings are characterised by a clear latitudinal energy dispersion of the solar wind ions. This energy-latitude correlation is observed at very high latitudes, 80° – 85°, near the polar boundary of the cusp, as an increase of the ion average energy with latitude. These structures have a typical latitude extent of 1° – 2° at ionospheric heights and correspond to a northward-directed IMF. These observations are consistent with a sunward convection of the foot of the magnetic flux tubes recently merged with a northward directed interplanetary magnetic field.     

行星际起伏向磁层顶的输运

时间尺度为分钟数量级的太阳风速度和行星际磁场大幅度扰动实际上始终存在于行星际空间的。这些扰动一直传输到紧贴磁层边界面外侧的区域。它们在磁鞘等离子体和磁层顶的相互作用过程中可能起很重要的作用。行星际起伏中的磁场分量在通过地球弓激波时首先经历一次跳跃,然后一部分扰动被带到磁层边界面处。在边界面附近磁场扰动幅度被大大地放大了。弓激波上游的太阳风条件控制了放大因子。本文所作的数值模拟研究结果表明,如果上游有大幅度的扰动,在边界面附近就有大幅度的Alfven起伏的磁场分量。当上游磁场接近垂直于日地联线时,放大因子变得相当大,而且放大因子随上游的等离子体β值和/或Alfven马赫数的增加而增加。上游各向异性对放大因子的影响不大。在磁层边界附近存在大幅度起伏表明这里不存在稳定的片流。      

磁鞘快速流引起的磁层顶凹陷事件

最近研究表明，磁层顶凹陷对磁层-电离层耦合具有重要作用.但是，磁层顶凹陷现象的确认需要多颗卫星的联合观测，目前为止报道的磁层顶凹陷事例非常少.本文利用THEMIS5颗卫星的联合观测结果，分析了一例由磁鞘快速流引起的磁层顶凹陷事件.2007年7月21日10:00 UT—10:45 UT，位于日下点磁层顶附近的THEMIS卫星在磁鞘观测到很强的地向流（约400km·-1），随后THEMIS5颗卫星相继穿越磁层顶进入磁层.通过最小方差MVA方法确认局部磁层顶法向，与经典磁层顶模型比较发现，磁鞘快速流压缩磁层顶形成局部凹陷.为了探究此磁鞘快速流的起源，对位于L1点的WIND卫星观测到的太阳风数据进行分析发现:在这个时间段内太阳风条件非常稳定，行星际磁场主要为径向，磁场南北向分量非常小.由此推测此磁鞘快速流的产生很可能与径向行星际磁场有关.      

A Theoretic Interpretation of Movement of the Cusp Equatorward Boundary

A preliminary model is proposed to describe quantitatively the position and movement of cusp equatorward boundary. This integrated model, consisting of an empirical model of the magnetopause and a compressed dipolar model of Open/Closed field line, connects quantitatively the solar wind conditions, subsolar magnetopause and cusp equatorward boundary. It is shown that the increasing solar wind dynamic pressure and the increasing southward Interplanetary Magnetic Field (IMF) component drive the magnetopause to move inward and the cusp equatorward. This model is adopted to interpret quantitatively the cusp movement of August 14, 2001 observed by Cluster. The results show that the subsolar magnetopause moved earthward from 10.7 He to 9.0 Re during the period of 002300-002800 UT, and correspondingly the cusp equatorward boundary shifted equatorward. The observations of Cluster C1 and C4 show the cusp equatorward boundary that Cluster Cl and C4 were crossing during same interval moved equatorward by 4.6°. The cusp equatorward boundary velocity computed in the theoretical model (10.7km/s) is in good agreement with the observed value (9.4km/s) calculated from the data of CIS of Cluster C4 and C1.     

A solar wind driven model of geosynchronous plasma moments

We describe a tabular specification model of the density and temperature of ions and electrons at geosynchronous orbit as a function of magnetic local time and solar wind parameters. This model can be used to provide boundary conditions for numerical ring current models. Unlike previous specification models of geosynchronous plasma moments, this model is parameterized by upstream solar wind conditions. We find that solar wind parameters are a better predictor of geosynchronous ion density than magnetospheric indices, and as upstream parameters they are often more appropriate as model inputs since they causally precede the model outputs. Of the upstream parameters that were tested, the best predictors of geosynchronous conditions were the solar wind flow pressure and the magnitude and Z-component of the interplanetary magnetic field.     

Effect of the Interplanetary Electric Field on the Magnetopause From Global MHD Simulations

The north-south component B_z of the Interplanetary Magnetic Field (IMF) and solar wind dynamic pressure P_d 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 P_dB_z, 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 B_z 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.