2004年4月1日磁通量传输事件特性的研究——通量管内电流密度、粒子运动与管轴方向的对比分析

本文利用Cluster四颗卫星上磁通门磁力计(FGM)的同时观测,采用旋度器方法(Curlometer),计算和分析了2004年4月1日12:24到12:54UT期间Cluster卫星观测的多个磁通量传输事件(FTEs)的特性。结果表明:磁通量管内电流密度较大,可达到约10-7A/m2。应用最小方向微分法(MDD),发现这些FTE事件具有准二维结构,即为圆柱形结构,其通量管轴线方向与管内电流方向及粒子运动方向基本平行。     

The stability of the pristine magnetopause

A MHD theory of combined Kelvin-Helmholtz (KH) and Rayleigh-Taylor (RT) instabilities for a transition layer with two different scale lengths (Δ and δ for the variation of velocity/magnetic fields and density, respectively) is presented. The study is motivated by reports of magnetopauses with no low latitude boundary layer, in which a sharp density drop over a distance δ?Δ is observed (“pristine” magnetopauses (J. Geophys. Res. 101 (1996) 49). The theory ignores compressibility effects and applies to subsonic regions of the dayside magnetopause. The RT effect is included to account for temporary periods of acceleration of the magnetopause, caused by sudden changes of the solar wind dynamic pressure. For small wavelengths λ, such that δ?λ?Δ, a WKB solution shows that the velocity gradient operates, together with magnetic tensions, to attenuate or even stabilize the Rayleigh-Taylor instability within a certain wavelength range. An exact dispersion relation for flute modes, valid for all λ, in the form of a fourth order polynomial for the complex frequency ω, is obtained from a model with a constant velocity gradient, dV/dy within Δ, and with δ→0. Flute modes are possible because of the existence of bands of very small magnetic shear on the dayside magnetopause (J. Geophys. Res. 103 (1998) 6703). The exact solution allows for a study of the change of the action of the velocity gradient with λ from the long-λ range where dV/dy is KH destabilizing to the short-λ range where dV/dy produces a stabilizing effect. Both, the WKB approximation and the well known tangential discontinuity model (Δ→0) are recovered as limiting cases of the exact solution. Properties of the KH and RT instabilities, for different density ratios on either side of the magnetopause, are described. For flute modes, at very small λ the RT instability grows faster and becomes the dominant effect. However, it is shown that the growth rate remains bounded at a finite value as λ→0, when a theory with a finite δ model is considered. To study configurations with finite, arbitrary, δ/Δ ratios, the MHD perturbation equations are solved numerically, using hyperbolic tangent functions for both the density and velocity transitions across the magnetopause. To examine the influence of different δ/Δ ratios on the growth rates of KH and RT, calculations are performed for different δ/Δ, with and without acceleration, and for two different density ratios. It is found that the general features exhibited by the constant dV/dy model, are confirmed by these numerical solutions. The stability of pristine magnetopauses, and the possibility of observing some theoretical predictions during magnetopause crossings in ongoing missions, are discussed.     

磁层顶高纬边界层的瞬时重联过程

用二维可压缩的ＭＨＤ模型模拟研究了北半球背阳面磁层顶区的瞬时重联过程．结果表明，当行星际磁场（ＩＭＦ）具有南向Ｂｚ分量和背太阳向Ｂｘ分量时，ＩＭＦ与地磁场联接，磁层顶向外扩张，在等离子体幔区可形成流体旋涡，磁力线被扭曲，但不易形成磁涡旋；当ＩＭＦ具有北向Ｂｚ分量时，不论Ｂｘ分量是背向太阳或指向太阳，都可发生瞬时重联，而且当ＩＭＦＢｘ分量与地磁场Ｂｘ分量反向时，在等离子体幔区更易形成磁涡旋．这两种情况，在磁层顶边界层区都能产生多层的电流片．     

The shearing instability of ion flow at the high-latitude tail of magnetopause boundary layer

Shearing instability of ion flow in an inhomogeneous plasma background in the magnetopause boundary layer at the high-latitude magnetotail is studied in this paper. By considering tail-aligned currents, we find that the instability excitation strongly depends on the disturbed wavelength. A quasi-critical wave number for instability is obtained. For relatively long perturbations, the instability tends to be excited at the inner edge of the boundary layer. The stable surface waves at the magnetopause and the K-H instability at the inner edge of the boundary layer can exist at the same time. This may contribute to the continuous transfer of momentum toward the magnetosphere.     

Multiple Flux Rope Events at the High-Latitude Magnetopause: Cluster/Rapid Observation on 26 January, 2001

Cluster measurements of the cusp and high latitude magnetopause boundary on 26 January, 2001 confirm that the cusp is a dynamic region full of energetic charged particles and turbulence. An energetic ion layer at high-latitudes beyond and adjacent to the duskside magnetopause exists when the Interplanetary Magnetic Field (IMF) has a southward orientation. Multiple energetic ion flux bursts were observed in the energetic ion layer. Each energetic ion flux burst was closely related to a magnetic flux rope. The axes of the flux ropes lie in the direction pointing duskward/tailward and somewhat upward. An intense axis-aligned current flows inside the ropes, with the current density reaching ∼10⁻⁸ A/m². The main components of the energetic ions are protons, helium and CNO ions, which originate from the magnetosphere, flowing out into the magnetosheath along the axis of the flux ropes. The velocity of the magnetosheath thermal plasma relative to the deHoffman-Teller (DHT) frame is found to be basically along the axis of the flux ropes also, but towards the magnetosphere. These flux ropes seem to be produced somewhere away via magnetic reconnection and move at similar DHT velocities passing over the spacecraft. These observations further confirm that the high-latitude magnetopause boundary region plays an important role in the solar wind-magnetopause coupling.     

Simulation Studies of High-Latitude Magnetospheric Boundary Dynamics

Magnetic reconnection at the high-latitude magnetopause is studied using 2–1/2-dimensional (2−1/2-D) Hall-MHD simulation. Concentric flow vortices and magnetic islands appear when both Hall effect and sheared flow are considered. Plasma mixing across the magnetopause occurs in the presence of the flow vortices. Reconnected structure generated in the vicinity of the subsolar point changes its geometry with increasing flow shear while moving to high latitudes. In the presence of flow shear, with the Hall-MHD reconnection a higher reconnection rate than with the traditional MHD is obtained. The out-of-plane components of flow and magnetic field produced by the Hall current are redistributed under the action of the flow shear, which makes the plasma transport across the boundaries more complicated. The simulation results provide some help in understanding the dynamic processes at the high-latitude magnetopause.     

Magion-4 High-Altitude Cusp Study

The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath ones, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent middle- to high-altitude observations (i.e., the Interball, Hawkeye, Polar, Image, and Cluster spacecraft) reported the cusps to encompass a broad area near local noon. The present paper focuses on a statistical study of the high-altitude cusp and surrounding magnetosheath regions as well as on some peculiarities of the cusp-magnetosheath transition. For a comparison of high- and low-altitude cusp determination, we present a mapping of two-year Magion-4 (a part of the Interball project) observations of cusp-like plasma along model magnetic field lines (according to the Tsyganenko 96 model) down to the Earth’s surface. The footprint positions show a substantial latitudinal dependence on the dipole tilt angle. The dependence can be fitted by a line with a slope of 0.14° MLAT per 1° of tilt. In contrary to previously reported IMF or solar wind influences on the cusp shape or location, some differences exist: (1) a possible IMF B_X dependence of the cusp location, (2) a split cusp for B_Y≠ 0, and (3) a smaller cusp during periods of higher solar wind dynamic pressure. The conclusions following from the statistical analysis are confirmed by case studies which reveal the physical mechanisms leading to the observed phenomena. Results have shown that (1) reconnection near the cusp does not necessarily lead to observable precipitation, (2) the cusp precipitation in one hemisphere can be supplied from the conjugate hemisphere, and (3) the cusp geometry at a certain time depends on the IMF history.     

涡旋诱发重联模型（Ⅱ）——通量传输事件理论和模拟

本文应用涡旋诱发重联理论研究了地球磁层顶区发生的瞬时局部重联现象.对向阳面磁顶区通量传输事件(FTEs)的形成、结构和运动进行了理论和模拟研究,并与卫星观测结果作了比较.结果表明,涡旋诱发重联可能是产生FTEs的重要机制.利用这一理论模型能解释FTEs的一些主要观测现象.此外,对背阳面磁顶区的局部重联从理论上作了分析,指出在背阳面磁顶区可能存在类似于向阳面磁顶区的通量传输事件.     

The ionospheric footprint of antiparallel merging regions on the dayside magnetopause

The antiparallel merging hypothesis states that reconnection takes place on the dayside magnetopause where the solar and geomagnetic fields are oppositely directed. With this criterion, we have mapped the predicted merging regions to the ionosphere using the Tsyganenko 96 magnetic field model, distinguishing between regions of sub-Alfvénic and super-Alfvénic magnetosheath flow, and identifying the day-night terminator. We present the resulting shape, width and latitude of the ionospheric dayside merging regions in both hemispheres, showing their dependence on the Earths dipole tilt. The resulting seasonal variation of the longitudinal width is consistent with the conjugate electric fields in the northern and southern cusps, as measured by the SuperDARN HF radars, for example. We also find a seasonal shift in latitude similar to that observed in satellite cusp data.     

Simultaneous optical and radar signatures of poleward-moving auroral forms

Dayside poleward moving auroral forms (PMAFs) were detected between 06:30 and 07:00 UT on December 16, 1998, by the meridian scanning photometer and the all-sky camera at Ny Ålesund, Svalbard. Simultaneous SuperDARN HF radar measurements permitted the study of the associated ionospheric velocity pattern. A good general agreement is observed between the location and movement of velocity enhancements (flow channels) and the PMAFs. Clear signatures of equatorward flow were detected in the vicinity of PMAFs. This flow is believed to be the signature of a return flow outside the reconnected flux tube, as predicted by the Southwood model. The simulated signatures of this model reproduce globally the measured signatures, and differences with the experimental data can be explained by the simplifications of the model. Proposed schemes of the flow modification due to the presence of several flow channels and the modification of cusp and region 1 field-aligned currents at the time of sporadic reconnection events are shown to fit well with the observations.