Is the Near-Earth Current Sheet Prior to Reconnection Unstable to Tearing Mode？

The tearing mode instability plays a key role in the triggering process of reconnection. The triggering collisionless tearing mode instability has been theoretically and numerically analyzed by many researchers. However, due to the difficulty in obtaining the observational wave number, it is still unknown whether the tearing mode instability can be excited in an actual plasma sheet prior to reconnection onset. Using the data from four Cluster satellites prior to a magnetospheric reconnection event on 13 September 2002, we utilized the wave telescope technique to obtain the wave number which corresponds to the peak of power spectral density. The wavelength is about 18RE and is consistent with previous theoretic and numerical results. After substituting the wave vector and other necessary parameters of the observed current sheet into the triggering condition of tearing mode instability, we find that the near-Earth current sheet prior to reconnection is unstable to tearing mode.     

太阳爆发的抵近探测

本工作将介绍一项具有重大科学和实际意义的深空探测任务,这项任务的顺利实施将允许我们在一个前所未有的近距离上以遥感和实地探测手段相结合的方式观测和研究一颗恒星的磁活动以及磁重联区域.首先,我们将首次直接进入太阳风暴的核心能量释放区——磁重联电流片内部,对其中的磁场耗散、能量转换、带电粒子加速等重要过程的细节进行精细实地测量和研究.其次,我们将对太阳风暴,即日冕物质抛射(Coronal Mass Ejection, CME)的物质成分和内部结构进行直接探测,帮助我们深入研究和了解CME的爆发机制和其中的物质来源;实地探测快CME前面的快模激波,被磁重联和CME激波加速的带电粒子及其所产生的电磁辐射.第三,我们将在离开太阳5-10个太阳半径的距离上直接测量日冕磁场-太阳活动的能量来源.第四,利用成像和光谱观测手段,我们能够近距离地观测和研究太阳高层大气中的动力学过程.目前在地球附近对日冕常规观测的分辨率在1.5′′,甚至更差,而通过抵近观测可以将同样设备的分辨能力提高5-30倍,将为我们提供在地球附近无法获得的太阳超清晰图像以及相应的物理信息,让我们在一个前所未有的平台上来研究、认识和了解距离我们最近、对我们最重要的恒星,从而解决太阳爆发和日冕加热等长期困扰太阳物理研究领域的难题.这也将使我们获得唯一的、能够对发生在恒星大气中的磁重联过程进行直接或者是抵近探测的机会!     

Collisionless magnetic reconnection in the magnetosphere

Magnetic reconnection underlies the physical mechanism of explosive phenomena in the solar atmosphere and planetary magnetospheres, where plasma is usually collisionless. In the standard model of collisionless magnetic reconnection, the diffusion region consists of two substructures: an electron diffusion region is embedded in an ion diffusion region, in which their scales are based on the electron and ion inertial lengths. In the ion diffusion region, ions are unfrozen in the magnetic fields while electrons are magnetized. The resulted Hall effect from the different motions between ions and electrons leads to the production of the in-plane currents, and then generates the quadrupolar structure of out-of-plane magnetic field. In the electron diffusion region, even electrons become unfrozen in the magnetic fields, and the reconnection electric field is contributed by the off-diagonal electron pressure terms in the generalized Ohm's law. The reconnection rate is insensitive to the specific mechanism to break the frozen-in condition, and is on the order of 0.1. In recent years, the launching of Cluster, THEMIS, MMS, and other spacecraft has provided us opportunities to study collisionless magnetic reconnection in the Earth's magnetosphere, and to verify and extend more insights on the standard model of collisionless magnetic reconnection. In this paper, we will review what we have learned beyond the standard model with the help of observations from these spacecraft as well as kinetic simulations.     

ULF Waves Associated with Solar Wind Deceleration in the Earth＇s Foreshock

Characteristics of ULF waves associated with the solar wind deceleration in the Earth＇s foreshock on 6-7 April 2003 is studied using the wave telescope technique. In the satellite frame, the ULF waves are the left-handed polarized and quasi anti-parallel propagating mode, with a power peak at about 18.63mHz. The wave vector in the GSE coordinates is estimated to be k = （-4.29, 2.28, 1.21）×10^-4 km^-1, In the solar wind frame, the frequency of waves becomes - 9.39 mHz after the Doppler shift correction. The propagation direction of the waves is thus reversed and correspondingly the polarization of the waves becomes right-handed. The above-mentioned characteristics of the ULF waves in the solar wind frame indicate that the ULF waves associated with the solar wind deceleration are the Alfven-whistler waves, which have been frequently reported in both the observations and computer simulations.