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(Bz≥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.     

电离层电导对地球磁层顶和舷激波尺度的影响

本文在如下假定下分析电离层电导对地球磁层顶和舷激波尺度的影响：（1）对电离层采用球壳近似,Pedersen电导ΣP均匀,Hall电导为零;（2）地磁偶极矩处于正南方向,行星际磁场（IMF）只有南向分量（BzBz和ΣP,通过三维全球MHD模拟获得系统的准定态.结果表明,在大约1～5 S范围内,ΣP值显著影响磁层顶和舷激波的尺度,而在该范围之外则几乎没有影响.随着ΣP的增加,磁层顶和舷激波整体向外扩张,前者的扩张程度低于后者,以至磁鞘区的范围扩大.磁层顶的侧翼点的位置随ΣP的变化与Bz的幅度有关：在弱南向IMF情况下磁层顶的侧翼点随ΣP的增加向内移动,而在强南向IMF情况下则向外移动.上述结果表明,在构建磁层顶和舷激波的经验模型时,有必要计入电离层电导的影响.     

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.     

地球弓激波的旋转非对称性

通过对太阳风-磁层-电离层系统的全球MHD模拟,研究地球弓激波相对日地连线的旋转非对称性．模拟限于太阳风速度沿日地连线、地球磁偶极矩和行星际磁场（IMF）与日地连线垂直的简单情况．模拟结果表明,即便对于IMF强度为零的情况,弓激波相对日地连线也不具备旋转对称性质：终端面（晨昏子午面）及其向阳侧的弓激波截线的东西宽度大于南北宽度（约9%~11％）,终端面尾侧的弓激波截线东西宽度小于南北宽度（约8％）．在存在IMF的情况下,弓激波的位形同时受到磁层顶的形状和快磁声波速度各向异性的影响．磁层顶向外扩张并沿IMF方向拉伸,且其扩张和拉伸程度随IMF由北转南而增强．在磁鞘中,垂直于磁场方向的快磁声波速度高于平行方向．因此,磁层顶拉伸方向与快磁声波速度最大方向垂直,它们对弓激波位置的效应恰好相反;弓激波的最终形状取决于何种效应占据主导地位．对于终端面尾侧,快磁声波速度的各向异性起主导作用,弓激波截线沿IMF垂直方向的宽度大于平行方向．对于终端面及其向阳侧,弓激波截线的形状与IMF取向有关：在准北向或晨昏向IMF情况下,弓激波截线沿IMF垂直方向的宽度仍大于平行方向;在准南向IMF情况下,弓激波截线沿IMF垂直方向的宽度小于平行方向的．鉴于弓激波形状同IMF取向之间的密切关系,我们提议以IMF为基准方向,提取弓激波截线的平行半宽度R_b∥和垂直半宽度R_b⊥作为尺度参数．这些尺度参数和通常引入的弓激波截线的东西半宽度y_b和南北半宽度z_b相比,更为合理地表征了弓激波的几何性质．模拟结果表明,在终端面上,y_b/z_b和R_b∥/R_b⊥在IMF各向同性取向下的统计平均值均低于1,与观测得到的结论一致.     

太阳风在地球激波前兆区减速的统计研究

本文首次利用完全相同两颗卫星（CLUSTER C1和C3）的数据对地球激波前兆区太阳风的减速和偏转特性进行了统计研究.结果表明,在激波前兆坐标系中,太阳风减小的速度随观测点到激波的距离D_BS增大而减小,随行星际磁场与激波法向夹角θ_BN增大也减小,在ULF波动区深度D_WS小于6R_e(R_e为地球半径)的范围内最为显著;伴随着太阳风减速的另外一个现象——太阳风的偏转,也存在相似的规律.其最大减速和最大偏转角度分别为10 km/s和3°.太阳风减速和偏转,以及随之变化的太阳风动压,可能会引起地球磁层顶位置和形状发生改变,同时也为激波前兆区弥散（diffuse）离子的起源及加热提供了一种可能的机制.     

Dipole tilt controls bow shock location and flaring angle

The dipole tilt angle has been found to affect Earth’s bow shock. This work presents a quantitative relationship between the dipole tilt angle and the bow shock location and flaring angle. We collected a large data set of bow shock crossings from four different satellites (IMP 8, Geotail, Magion 4, and Cluster), including some recent crossings obtained during 2012–2013. The results from a statistical analysis demonstrate that: (1) the subsolar standoff distance increases but the flaring angle decreases with increasing dipole tilt angle; (2) when the dipole tilt angle changes sign from negative to positive, the dayside bow shock moves toward Earth and the shift can be as much as 2.29 R _E, during which the flaring angle increases; and (3) the shape of bow shock in the northern and southern hemispheres differs. For the northern hemisphere bow shock, with increasing positive/negative dipole tilt angle, the flaring angle increases/decreases. While for the southern hemisphere, the trend is the opposite; with increasing positive/negative dipole tilt angle, the flaring angle decreases/increases. These results are helpful for future bow shock modeling that needs to include the effects of dipole tilt angle.     

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.     

地球弓激波及其与行星际激波相互作用

给定地球轨道附近的行星际条件和地球弓激波的几何位形,本文分析向阳侧弓激波强度参数沿激波阵面的分布,以及行星际激波与弓激波的相互作用.对于弓激波阵面相对日地连线轴对称的情形,得到如下结论：（1）弓激波强度参数分布相对由行星际磁场和日地连线构成的基准面对称,各激波强度参数的最大值出现在基准面上.磁压比在垂直激波线一侧较大,而气压比在平行激波点一侧较大,导致总压比相对日地连线大致呈轴对称分布.（2）随着行星际磁场与日地连线夹角的增大,弓激波强度参数的最大值有所减小,且位置朝远离日下点方向偏移;但气压比和总压比的分布基本上不受影响.（3）行星际激波透过弓激波之后,切向磁场比更接近于1,但总磁场跳变幅度增大.（4）透过弓激波之后,行星际准垂直激波的总磁场比更接近于1,准平行激波的总磁场比则反之.     

Shape and position of Earth’s bow shock near-lunar orbit based on ARTEMIS data

Earth’s bow shock is the result of interaction between the supersonic solar wind and Earth’s magnetopause. However, data limitations mean the model of the shape and position of the bow shock are based largely on near-Earth satellite data. The model of the bow shock in the distant magnetotail and other factors that affect the bow shock, such as the interplanetary magnetic field (IMF) B_y, remain unclear. Here, based on the bow shock crossings of ARTEMIS from January 2011 to January 2015, new coefficients of the tail-flaring angle α of the Chao model (one of the most accurate models currently available) were obtained by fitting data from the middle-distance magnetotail (near-lunar orbit, geocentric distance -20R_E>X>-50R_E). In addition, the effects of the IMF B_y on the flaring angle α were analyzed. Our results showed that: (1) the new fitting coefficients of the Chao model in the middle-distance magnetotail are more consistent with the observed results; (2) the tail-flaring angle α of the bow shock increases as the absolute value of the IMF B_y increases. Moreover, positive IMF B_y has a greater effect than negative IMF B_y on flaring angle. These results provide a reference for bow shock modeling that includes the IMF B_y.     

Magnetic field behavior in the magnetosheath when the IMF slightly differs from the solar wind velocity vector

The magnetic field behavior in the magnetosheath, when the IMF and the solar wind velocity are almost collinear, has been analyzed based on the perturbation method. Magnetic disturbances are considered against a background of the stationary MHD solar wind flow around the magnetosphere when the magnetic field and the solar wind velocity are strictly collinear. It has been indicated that the angle between the magnetic field and velocity vectors increases considerably in a relatively thin layer near the magnetopause. The angle rise factor profiles have been determined for different distances from the subsolar point. The thickness of the layer, where the angle reaches values of about unity and more, has been estimated. It is important to take this layer into account when the magnetopause stability with respect to Kelvin-Helmholtz waves is analyzed.     

行星际磁场B_y对弓激波影响的MHD结果

利用全球磁流体模拟的结果,本文研究了行星际磁场B_y对弓激波位型的影响.结果显示:随着行星际磁场B_y绝对值的增大,弓激波的日下点距离、旋转对称张角、南北非对称性以及旋转非对称性均随之增加.其中,B_y对弓激波日下点距离的影响可达5 RE左右.东向B_y和西向B_y对弓激波位型影响具有对称性,东向B_y和西向B_y大小相同时弓激波日下点距离、旋转对称张角以及旋转非对称性参数均相同,而南北非对称性参数大小相同正负相反.行星际磁场B_y占主导时弓激波尾部横截面在南北方向上拉伸,并且拉伸程度随着B_y绝对值的增大而增大,弓激波尾部横截面的拉伸现象与磁声波马赫数密切相关.     

A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar

Observations of a flux transfer event (FTE) have been made simultaneously by the Equator-S spacecraft near the dayside magnetopause whilst corresponding transient plasma flows were seen in the near-conjugate polar ionosphere by the CUTLASS Finland HF radar. Prior to the occurrence of the FTE, the magnetometer on the WIND spacecraft ≈226 R_E upstream of the Earth in the solar wind detected a southward turning of the interplanetary magnetic field (IMF) which is estimated to have reached the subsolar magnetopause ≈77 min later. Shortly afterwards the Equator-S magnetometer observed a typical bipolar FTE signature in the magnetic field component normal to the magnetopause, just inside the magnetosphere. Almost simultaneously the CUTLASS Finland radar observed a strong transient flow in the F region plasma between 78° and 83° magnetic latitude, near the ionospheric region predicted to map along geomagnetic field lines to the spacecraft. The flow signature (and the data set as a whole) is found to be fully consistent with the view that the FTE was formed by a burst of magnetopause reconnection.     

Specific features of magnetic barrier formation near the magnetopause

The numerical three-dimensional MHD model is used to study the formation of the magnetic barrier in the inner part of the magnetosheath near the magnetopause. The set of the quasistationary solutions for several characteristic directions of the interplanetary magnetic field (IMF) has been obtained: for northward and southward IMF, for the direction along the Parker helix (at an angle of 45° with respect to the Sun-Earth line), and for the predominantly radial direction (at an angle of 20° with respect to the Sun-Earth line). The mechanism used to take into account the effect of magnetic reconnection at the magnetopause on a flow in the magnetosheath is introduced in the case of southward IMF. The results of the calculations indicate that the magnetic field absolute value in the magnetic barrier reaches its maximal value when IMF is northward. The introduction of magnetic reconnection at southward IMF can result in an insignificant decrease in the field value. However, the model predicts that a decrease in the magnetic field is much more substantial when the IMF direction is close to radial.     

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.     

午后极光强度与行星际磁场的相关

利用1997年和199年南极中山站多通道扫描光度计的地面观测数据和WIND卫星在弓激波上游对行星际磁场(IMF)的观测数据,对午后高纬极光强度与IMF各分量、以及时钟角之间的相关进行了定量研究. 统计表明,630nm的强度Ir随IMF Bx的增大而减小,其线性相关系数为-0.3;而557.7nm的变化趋势与此相反,其相关系数要低得多. 630nm的强度随IMF By的变化曲线为一"V"形结构,其谷底在By=-3nT附近;557.7nm的强度也有相似的变化趋势,其谷底的位置在By=-2nT附近. 极光强度随IMF Bz的变化曲线为一倒着的"Z"字形结构. 630nm的强度随IMF的模B的增大而增强,其线性相关系数达到0.9,而557.7nm与B之间的相关性要差得多. 极光强度随IMF时钟角的变化曲线为一倒"V"结构,其反转点在θ=130°附近.     

午后极光强度与行星际磁场的相关

利用1997年和199年南极中山站多通道扫描光度计的地面观测数据和WIND卫星在弓激波上游对行星际磁场(IMF)的观测数据,对午后高纬极光强度与IMF各分量、以及时钟角之间的相关进行了定量研究. 统计表明,630nm的强度Ir随IMF Bx的增大而减小,其线性相关系数为-0.3;而557.7nm的变化趋势与此相反,其相关系数要低得多. 630nm的强度随IMF By的变化曲线为一"V"形结构,其谷底在By=-3nT附近;557.7nm的强度也有相似的变化趋势,其谷底的位置在By=-2nT附近. 极光强度随IMF Bz的变化曲线为一倒着的"Z"字形结构. 630nm的强度随IMF的模B的增大而增强,其线性相关系数达到0.9,而557.7nm与B之间的相关性要差得多. 极光强度随IMF时钟角的变化曲线为一倒"V"结构,其反转点在θ=130°附近.     

极区电离层对IMF Bz 4次快速转向的响应——EISCAT/ESR雷达观测

利用EISCAT VHF和EISCAT Svalbard（ESR）雷达观测数据,对2003年2月12日IMF Bz分量4次快速方向转换期间,极区电离层,尤其是极尖/极隙区的响应特征进行了分析研究.随着IMF Bz方向的多次快速变化,地面雷达观测到极尖/极隙区所在位置随着开放-闭合磁力线边界在纬度方向上来回移动.在此期间,极区电离层等离子体水平对流多次反向,表现出与IMF Bz分量强的负相关性.进一步分析表明：极区磁层-电离层系统在日侧对IMF极性变化的平均响应时间约为3 min.     

Spatial-Energy Characteristics of Cosmic Rays and Parameters of Magnetospheric Current Systems in March and June 2015

The data from terrestrial observations of cosmic rays at the global network of stations by the method of spectrographic global survey were used to analyze two Forbush decreases during the geomagnetic storms in March and June 2015. The spectra of cosmic ray variations, pitch angle anisotropy of cosmic rays at different phases of Forbush decrease development, and the changes in the planetary system of geomagnetic cutoff rigidities are presented. It is shown that, during the approximation of the spectra of variations by the power function of particle rigidity in the interval of 10–50 GV, the spectrum index is softer at the maximum modulation phase than during the phases of cosmic ray intensity decline and recovery. In the axisymmetric model of the bounded magnetosphere of the Earth, which takes into account the currents at the magnetopause and the ring current, the distance to the subsolar point and the radius of the ring current, as well as the contribution of the ring current to the changes in geomagnetic cutoff rigidity and to the Dst index during the studied events, are determined.     

Spatial distribution of upstream magnetospheric ≥50 keV ions

We present for the first time a statistical study of 50 keV ion events of a magnetospheric origin upstream from Earths bow shock. The statistical analysis of the 50–220 keV ion events observed by the IMP-8 spacecraft shows: (1) a dawn-dusk asymmetry in ion distributions, with most events and lower intensities upstream from the quasi-parallel pre-dawn side (4 LT-6 LT) of the bow shock, (2) highest ion fluxes upstream from the nose/dusk side of the bow shock under an almost radial interplanetary magnetic field (IMF) configuration, and (3) a positive correlation of the ion intensities with the solar wind speed and the index of geomagnetic index Kp, with an average solar wind speed as high as 620 km s^–1 and values of the index Kp 2. The statistical results are consistent with (1) preferential leakage of 50 keV magnetospheric ions from the dusk magnetopause, (2) nearly scatter free motion of 50 keV ions within the magnetosheath, and (3) final escape of magnetospheric ions from the quasi-parallel dawn side of the bow shock. An additional statistical analysis of higher energy (290–500 keV) upstream ion events also shows a dawn-dusk asymmetry in the occurrence frequency of these events, with the occurrence frequency ranging between 16%-34% in the upstream region.     

Interplanetary fast forward shocks and their geomagnetic effects: CAWSES events

The seven CAWSES interplanetary fast forward shocks and their geomagnetic effects during 2004–2005 have been analyzed. It is found that the arrival time of the shocks at Earth can be estimated within an accuracy of ～5 min. Furthermore, AL decreases are found to occur within 10 min of shock impingement on the magnetopause. It was also determined that there is a direct correlation between the interplanetary magnetic field southward directed (IMF B_s) prior to shock arrival and substorms triggered by the shocks. If the IMF is northward prior to shock arrival, the geomagnetic activity is present but is low. One interpretation of this result is that the preconditioning energy stored in the magnetotail leaks away rapidly. A correlation between substorm peak AL and shock strength (Mach number) has also been noted, which could imply that shock strength is important for the amount of energy released into the magnetosphere/ionosphere.