K-H不稳定性在多电流片系统磁场重联中的效应

等离子体系统中存在两个或多个电流片时,电流片中发生的不稳定性可能会相互作用．行星际磁场北向时,背阳面碰层顶电流片与磁尾等离子体片之间可能发生相互作用,高纬边界层强烈的流场剪切可能促进磁场重联,产生磁层亚暴．本文运用二维可压缩磁流体模拟研究具有强流场剪切的多个电流片系统中磁场重联的演化．结果表明,Kelvin－Helmholtz不稳定性使多电流片系统的磁场重联过程明显加快;相邻电流片之间的距离越近,两者相互作用越强,重联增长率越大;在三电流片系统中,超Alfven速度强流场导致外侧两个电流片中出现强烈的磁场重联,并引发中心电流片的磁场重联．行星际磁场北向时,也可能发生磁层亚暴．     

A Statistical Study on the Reconnection in Boundary Layers of Small-scale Magnetic Flux Tubes in Solar Winds

With the data from WIND satellite in 1995—2005, the small-scale magnetic ?ux tubes determined with two methods, i.e., the arti?cial recognition and program selection respectively, are compared. It is found that there are magnetic reconnections in the boundary layers of 41% small-scale magnetic ?ux tubes determined with the program selection method, which is similar to the re- sult of small-scale magnetic ?ux tubes determined with the method of arti?cial recognition. The features of magnetic reconnections, such as the magnetic shear angle, magnetic ?eld strength, and duration of the dissipation region of recon- nection at the small-scale ?ux tubes’ boundaries determined from both methods have the same statistical tendency. This shows that there is no essential differ- ence in the properties of reconnections in the boundary layers of the small-scale magnetic ?ux tubes determined with the two methods. Hence the data yielded by both methods can be used as the samples for statistically studying the events of reconnection in the front and back boundary layers of small-scale ?ux ropes. There are totally 71 magnetic reconnection events selected in this paper. Our statistical result shows that in 50 events (70%) the decrease of magnetic ?eld strength in the dissipation region of reconnection is larger than 20%, and in 47 events (66%) the magnetic shear angle is larger than 90 degrees. These indicate that the magnetic reconnections in boundaries of the small-scale magnetic ?ux tubes are more likely to be anti-parallel. The statistics has been performed sep- arately in the reconnections of the front and back boundary layers of small-scale magnetic ?ux tubes. The results show that the features of reconnections in the front and back boundaries are similar to each other, which is different from the various properties in the front and back boundaries of the magnetic clouds, and this means that the expansion in the large-scale magnetic ?ux tubes, such as magnetic clouds, does not happen in the small-scale magnetic ?ux tubes.     

Flushing effect in reconnection: Effects of minority species of oxygen ions

Effects of O⁺ ions on magnetic reconnection in the magnetotail are examined in a Harris equilibrium using a combination of linear Vlasov theory and large-scale driven and non-driven two-dimensional fully kinetic particle simulations. Linear theory of multiple species plasma indicates that the growth rate is rather insensitive to the composition of the background (lobe) or its temperature but more sensitive to the properties of the current carriers. Thus O⁺ can affect significant changes to the linear growth rate of tearing mode only as a current carrier. However, it is demonstrated that in the nonlinear stage reconnection can effectively move trace material from the lobes into the current sheet proper. If the supply of lobe markers (like O⁺) is sufficiently permanent in the lobe, an initially proton-dominated current sheet can be virtually replaced by the marker ions through this “flushing effect”. The dominance of marker ions introduces finite Larmor radius signatures with marker gyroradii scales rather than that of the protons. In this way, the presence of heavier marker species in the lobe can lead to (i) reduced efficiency of energy conversion, (ii) reduction in the number and repetition frequency of secondary islands, (iii) broadening of the quadrupole magnetic structure, (iv) slowing down of the coalescence process and (v) modification of the composition of the ion current carriers. Using Cluster observations, we show the evidence for the “flushing effect” in the data. Detailed comparison with observations is planned for future work.     

非稳态多重X线重联的类型与特征

为了进一步认识间歇性多重Ｘ线重联的特征，本文分析计算结果，发现入流自边界进入后向两侧发散，沿着ｘ轴（ｚ＝０）和出流边界（ｚ＝２），压强Ｐ和磁场Ｂ的分布形态表明这种非稳态重联属快模式扩张型．数值结果还表明，进入扩散区的入流马赫数Ｍ_ｉ与磁雷诺数Ｒｍ之间基本满足关系式Ｍ_ｉ≈１．５Ｒ_ｍ^－１／２─１．７５Ｒ_ｍ^－１／２，而且等离子体的加速与压强Ｐ_ｔ(Ｐ_ｔ     

太阳大气中自发磁重联的数值模拟(Ⅱ)重力、热传导下二维三分量的计算结果

在无力场中引入反常电阻,数值模拟电阻撕裂模不稳定性引起的均匀重力场中的二维三分量磁重联过程．首先研究了Vc（反常电阻模型中的临界漂移速度）和β0（气压磁压比）对磁重联的影响,结果表明：（1）在Vc大的情况下,重联率开始很小,但最终出现爆发现象．另外,温度的增加不如Vc小的情况明显。（2）Vc相同而β0不同的情况具有相似的演化特征,但β0越小,动力学过程越剧烈。（3）Vc大巨β0也大的情况不能产生爆发现象．还研究了各向同性热传导对磁重联过程及扩散区结构的影响．结果表明热传导加快磁重联速率．在无热传导的情况中,开始时温度增加迅速,重联率较低,但最终出现磁重联率迅速上升的现象．对上述结果及其在解释耀斑、日冕物质抛射及冕流等太阳大气现象方面进行了讨论．     

Coupling processes at the magnetopause

This review covers several aspects of magnetopause research during the two-year period from mid-1991 to mid-1993. It focusses upon three topics which received renewed attention: the structure of the steady-state magnetopause, the origin of the transient events which are superimposed upon it, and the cause of transient signatures observed by high-latitude dayside ground magnetometers. Case and statistical studies defined the relatively unknown characteristics of the magnetosheath plasma layers lying outside the magnetopause, while theoretical studies provided alternative explanations for the presence of magnetosheath plasma within the LLBL. Evidence was presented for a steady transition from magnetosheath to magnetospheric plasma parameters. Detailed studies described the plasma, energetic particle, and magnetic field characteristics of transient events in the outer dayside magnetosphere, and multipoint studies provided important new information concerning the ionospheric response to sudden changes in solar wind parameters. This review emphasizes the competing explanations which have been advanced to explain these phenomena.     

亚暴事件中磁尾多重等离子体团的数值研究（Ⅰ）

GEOTAIL卫星于1994年1月15日亚暴期间，在深磁尾（x＝96R_E）观测到多重等离子体团及与之相对应的高能离子爆，作者以宁静磁尾平衡位形为初态，考虑介质的可压缩性，数值研究亚暴期间磁尾动力学过程．计算结果展现了等离子体团间歇性形成及其运动发展过程．体现了强亚暴事件中储存于碰尾的能量，通过多重等离子体团的排放而逐渐释放的进程．数值结果还表明：持续施加于边界上的晨昏电场及由此引发的驱动重联是导致等离子体团准周期形成的主要因素．此外，作者还考察尾瓣内任一点磁场强度及其分量随时间的演化，它与行进压缩区（TCRs）的观测特征基本相符．     

多层电流片中双撕裂模的耦合

应用边界层理论研究了多层电流片中m≥2的双撕裂模的耦合发展行为．在一阶近似下,解析求得了环位形中撕裂模的外区匹配参数Δ,所得结论对撕裂模的线性和非线性发展均有意义．结果表明,两有理面模之间的耦合总是使双撕裂模更不稳定．耦合的强度不仅与两有理面之间的距离和平衡状态有关,而且与模在两有理面发展的相对强弱有关．两有理面相距愈远,耦合相对愈弱;发展强的有理面处的模对另一有理面处的模影响更大．本文用所得结论对双撕裂模的初始非统性发展行为进行了分析．多层电流片中撕裂模不稳定性引起的自发重朕,可能对太阳耀斑、日冕加热、太阳风和磁层耦合等有重要影响．     

引力场中磁场重联的数值研究──（Ⅰ）磁岛的形成和合并

数值研究了引力场中电阻撕裂模不稳定性所引起的磁场重联，结果表明，在电流片长度Ｌ＝１Ｈ、半宽度δ＝Ｈ／２４的情况下，电流片两端附近将出现两个Ｘ线的磁场重联，并形成磁岛和高温高密度的等离子体团．磁岛宽度随着时间而增长，在ｔ≈５５τＡ时达到饱和，最大饱和岛宽约为４δ．同时从ｔ≈３７τＡ开始，磁岛中心位置逐渐下降；在ｔ＝５７τＡ时发生结合不稳定性，磁岛与底部附近的闭合磁场区合并，导致磁场湮灭和磁能的快速释放．另一方面，由于引力场和等离子体非均匀性的影响，顶部附近随时间而增长的等离子体外流速度达１．１４ＶＡ∞；磁岛中等离子体向下运动的最大速度达１．４１ＶＡ∞，且大于局地声速；在磁岛前方可形成快激波．这些结果可用于解释双带耀斑中后随耀斑环的形成、磁场湮灭和磁能释放、以及Ｄｏｐｐｌｅｒ速度图上观测到的红移现象．     

不同行星际磁场条件下极向运动极光结构的多手段联合观测

本文利用北极黄河站三波段全天空成像仪（ASI）的高分辨率的地面极光观测数据,联合欧洲非相干散射雷达（EISCAT）Svalbard雷达（ESR）,超级双子极光雷达网（SuperDARN）雷达等观测,研究了发生在2003年12月22日0900-1010 UT时间段内极区电离层极光和等离子体的变化特征。研究发现在不同行星际磁场（IMF）条件下,黄河站极光ASI均观测到了一系列极向运动极光结构（PMAFs）,其特征为极光在赤道向一侧点亮后向高纬扩展。这些PMAFs都伴随有明显的粒子沉降特征,且在IMF北向时该粒子沉降能达到更低的电离层E层区域,而此时这些PMAFs相应位置的高纬电离层出现了一个典型反向对流涡,这是高纬（尾瓣）重联的典型特征之一。这些结果表明北向IMF条件下的这些PMAFs是由高纬重联所产生。在IMF南向时,黄河站观测到的PMAFs可跨越更广的地磁纬度,表明其演化时间亦较长,其相应区域的电离层特征也表明该PMAFs由日侧磁层顶低纬磁重联所产生。