Extended magnetic reconnection at the Earth's magnetopause from detection of bi-directional jets

Magnetic reconnection is a process that converts magnetic energy into bi-directional plasma jets; it is believed to be the dominant process by which solar-wind energy enters the Earth's magnetosphere. This energy is subsequently dissipated by magnetic storms and aurorae. Previous single-spacecraft observations revealed only single jets at the magnetopause--while the existence of a counter-streaming jet was implicitly assumed, no experimental confirmation was available. Here we report in situ two-spacecraft observations of bi-directional jets at the magnetopause, finding evidence for a stable and extended reconnection line; the latter implies substantial entry of the solar wind into the magnetosphere. We conclude that reconnection is determined by large-scale interactions between the solar wind and the magnetosphere, rather than by local conditions at the magnetopause.     

Continuous magnetic reconnection at Earth's magnetopause

The most important process that allows solar-wind plasma to cross the magnetopause and enter Earth's magnetosphere is the merging between solar-wind and terrestrial magnetic fields of opposite sense-magnetic reconnection. It is at present not known whether reconnection can happen in a continuous fashion or whether it is always intermittent. Solar flares and magnetospheric substorms--two phenomena believed to be initiated by reconnection--are highly burst-like occurrences, raising the possibility that the reconnection process is intrinsically intermittent, storing and releasing magnetic energy in an explosive and uncontrolled manner. Here we show that reconnection at Earth's high-latitude magnetopause is driven directly by the solar wind, and can be continuous and even quasi-steady over an extended period of time. The dayside proton auroral spot in the ionosphere--the remote signature of high-latitude magnetopause reconnection--is present continuously for many hours. We infer that reconnection is not intrinsically intermittent; its steadiness depends on the way that the process is driven.     

Electron dynamics in collisionless magnetic reconnection

Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy. It is closely associated with many explosive phenomena in space plasma, usually collisionless in character. For this reason, researchers have become more interested in collisionless magnetic reconnection. In this paper, the various roles of electron dynamics in collisionless magnetic reconnection are reviewed. First, at the ion inertial length scale, ions and electrons are decoupled. The resulting Hall effect determines the reconnection electric field. Moreover, electron motions determine the current system inside the reconnection plane and the electron density cavity along the separatrices. The current system in this plane produces an out-of-plane magnetic field. Second, at the electron inertial length scale, the anisotropy of electron pressure determines the magnitude of the reconnection electric field in this region. The production of energetic electrons, which is an important characteristic during magnetic reconnection, is accelerated by the reconnection electric field. In addition, the different topologies, temporal evolution and spatial distribution of the magnetic field affect the accelerating process of electrons and determine the final energy of the accelerated electrons. Third, we discuss results from simulations and spacecraft observations on the secondary magnetic islands produced due to secondary instabilities around the X point, and the associated energetic electrons. Furthermore, progress in laboratory plasma studies is also discussed in regard to electron dynamics during magnetic reconnection. Finally, some unresolved problems are presented.     

A magnetic reconnection X-line extending more than 390 Earth radii in the solar wind

Magnetic reconnection in a current sheet converts magnetic energy into particle energy, a process that is important in many laboratory, space and astrophysical contexts. It is not known at present whether reconnection is fundamentally a process that can occur over an extended region in space or whether it is patchy and unpredictable in nature. Frequent reports of small-scale flux ropes and flow channels associated with reconnection in the Earth's magnetosphere raise the possibility that reconnection is intrinsically patchy, with each reconnection X-line (the line along which oppositely directed magnetic field lines reconnect) extending at most a few Earth radii (R(E)), even though the associated current sheets span many tens or hundreds of R(E). Here we report three-spacecraft observations of accelerated flow associated with reconnection in a current sheet embedded in the solar wind flow, where the reconnection X-line extended at least 390R(E) (or 2.5 x 10(6) km). Observations of this and 27 similar events imply that reconnection is fundamentally a large-scale process. Patchy reconnection observed in the Earth's magnetosphere is therefore likely to be a geophysical effect associated with fluctuating boundary conditions, rather than a fundamental property of reconnection. Our observations also reveal, surprisingly, that reconnection can operate in a quasi-steady-state manner even when undriven by the external flow.     

In situ detection of collisionless reconnection in the Earth's magnetotail

Magnetic reconnection is the process by which magnetic field lines of opposite polarity reconfigure to a lower-energy state, with the release of magnetic energy to the surroundings. Reconnection at the Earth's dayside magnetopause and in the magnetotail allows the solar wind into the magnetosphere. It begins in a small 'diffusion region', where a kink in the newly reconnected lines produces jets of plasma away from the region. Although plasma jets from reconnection have previously been reported, the physical processes that underlie jet formation have remained poorly understood because of the scarcity of in situ observations of the minuscule diffusion region. Theoretically, both resistive and collisionless processes can initiate reconnection, but which process dominates in the magnetosphere is still debated. Here we report the serendipitous encounter of the Wind spacecraft with an active reconnection diffusion region, in which are detected key processes predicted by models of collisionless reconnection. The data therefore demonstrate that collisionless reconnection occurs in the magnetotail.     

Rapid magnetic reconnection in the Earth's magnetosphere mediated by whistler waves

Magnetic reconnection has a crucial role in a variety of plasma environments in providing a mechanism for the fast release of stored magnetic energy. During reconnection the plasma forms a 'magnetic nozzle', like the nozzle of a hose, and the rate is controlled by how fast plasma can flow out of the nozzle. But the traditional picture of reconnection has been unable to explain satisfactorily the short timescales associated with the energy release, because the flow is mediated by heavy ions with a slow resultant velocity. Recent theoretical work has suggested that the energy release is instead mediated by electrons in waves called 'whistlers', which move much faster for a given perturbation of the magnetic field because of their smaller mass. Moreover, the whistler velocity and associated plasma velocity both increase as the 'nozzle' becomes narrower. A narrower nozzle therefore no longer reduces the total plasma flow-the outflow is independent of the size of the nozzle. Here we report observations demonstrating that reconnection in the magnetosphere is driven by whistlers, in good agreement with the theoretical predictions.     

电子磁流体模型及Biermann电池效应和位移电流效应

电子磁流体（Electron Magnetohydrodynamics,EMHD）理论在空间、天体等离子体物理以及纯电子等离子体物理领域,特别是磁场的产生、磁力线的重联等重要物理现象的研究中有广泛应用．近年来对一些空间、天体物理过程的实验室模拟以及相关的激光等离子体物理与高能量密度物理的研究中,等离子体物理过程的快尺度时空演化导致位移电流效应、Biermann电池效应等对电子磁流体模型的一些基本假设产生重要的修正．本文在讨论分析电子磁流体理论模型主要近似的基础上,研究了Biermann电池效应与位移电流引起的效应,并进一步讨论了这一些效应对一些重要等离子体物理过程的影响．     

Responses of the magnetotail plasma sheet to two interplanetary shocks: TC-1 observations

Two interplanetary shocks are examined to determine the responses of the magnetic field and plasma in the plasma sheet upon the shock impacts by using TC-1 observational data.The two shocks are observed by WIND on November 7,2004.Prior to and after the shock,the IMF is either weakly southward or northward.The responses of the plasma sheet to the two shocks are intense and much similar.When the shock interacts with the magnetosphere,the magnetic field impulsively increases 1-2 min after the geomagnetic field...     

Magnetic reconnection from a multiscale instability cascade

Magnetic reconnection, the process whereby magnetic field lines break and then reconnect to form a different topology, underlies critical dynamics of magnetically confined plasmas in both nature and the laboratory. Magnetic reconnection involves localized diffusion of the magnetic field across plasma, yet observed reconnection rates are typically much higher than can be accounted for using classical electrical resistivity. It is generally proposed that the field diffusion underlying fast reconnection results instead from some combination of non-magnetohydrodynamic processes that become important on the 'microscopic' scale of the ion Larmor radius or the ion skin depth. A recent laboratory experiment demonstrated a transition from slow to fast magnetic reconnection when a current channel narrowed to a microscopic scale, but did not address how a macroscopic magnetohydrodynamic system accesses the microscale. Recent theoretical models and numerical simulations suggest that a macroscopic, two-dimensional magnetohydrodynamic current sheet might do this through a sequence of repetitive tearing and thinning into two-dimensional magnetized plasma structures having successively finer scales. Here we report observations demonstrating a cascade of instabilities from a distinct, macroscopic-scale magnetohydrodynamic instability to a distinct, microscopic-scale (ion skin depth) instability associated with fast magnetic reconnection. These observations resolve the full three-dimensional dynamics and give insight into the frequently impulsive nature of reconnection in space and laboratory plasmas.     

Energetic electrons associated with magnetic reconnection in the sheath of interplanetary coronal mass ejection

Magnetic reconnection is an important universal plasma dissipation process that converts magnetic energy into plasma thermal and kinetic energy,and simultaneously changes the magnetic field topology.In this paper,we report the first observation of energetic electrons associated with asymmetric reconnection in the sheath of an interplanetary coronal mass ejection.The magnetic field shear angle was about 151°,implying guide-field reconnection.The width of the exhaust was about 8×104 km.The reconnection rate was estimated as 0.044-0.08,which is consistent with fast reconnection theory and previous observations.We observed flux enhancements of energetic electrons with energy up to 400 keV in this reconnection exhaust.The region where ener- getic electron fluxes were enhanced is located at one pair of separatrices in the higher density hemisphere.We discuss these observation results,and compare with previous observations and recent kinetic simulations.     

Enhanced anti-sunward flow near local noon during a period of horizontal IMF and high solar wind velocity <Emphasis Type="Italic">V</Emphasis><Subscript><Emphasis Type="Italic">Y</Emphasis></Subscript>

It is believed that a southward interplanetary magnetic field (IMF) is mainly responsible for the energy input from solar wind into the magnetosphere. This paper presents an unusual case of strong anti-sunward plasma flow (up to 2 km/s) in the polar cap ionosphere and large cross-polar cap potential (CPCP) during a period of horizontal IMF (|B _Z | < 2 nT) observed by both ACE (at the L1 point) and Geotail (on the dusk flank of the magnetosheath). The CPCP is even higher than that under preceding B _Z ≈ −23 nT. Furthermore, GOES8 observed that the magnetosheath field turns northward as the anti-sunward plasma flow and CPCP start to increase, which implies that the magnetosheath field interacting with the Earth’s magnetopause has significantly rotated and differs from the IMF observed by ACE and Geotail. In accordance with previous theoretical work, we suggest that the magnetic field line draping produces a southward magnetosheath field and enhances anti-sunward plasma flow and the CPCP.     

A current filamentation mechanism for breaking magnetic field lines during reconnection

During magnetic reconnection, the field lines must break and reconnect to release the energy that drives solar and stellar flares and other explosive events in space and in the laboratory. Exactly how this happens has been unclear, because dissipation is needed to break magnetic field lines and classical collisions are typically weak. Ion-electron drag arising from turbulence, dubbed 'anomalous resistivity', and thermal momentum transport are two mechanisms that have been widely invoked. Measurements of enhanced turbulence near reconnection sites in space and in the laboratory support the anomalous resistivity idea but there has been no demonstration from measurements that this turbulence produces the necessary enhanced drag. Here we report computer simulations that show that neither of the two previously favoured mechanisms controls how magnetic field lines reconnect in the plasmas of greatest interest, those in which the magnetic field dominates the energy budget. Rather, we find that when the current layers that form during magnetic reconnection become too intense, they disintegrate and spread into a complex web of filaments that causes the rate of reconnection to increase abruptly. This filamentary web can be explored in the laboratory or in space with satellites that can measure the resulting electromagnetic turbulence.     

Dependence of magnetic field just inside the magnetopause on subsolar standoff distance: Global MHD results

The subsolar magnetopause is the boundary between the solar wind and the Earth’s magnetosphere,where reduced solar wind dynamic pressure is equal to the magnetic pressure of the Earth’s outer magnetosphere.We use a global magnetohydrodynamic (MHD)model to estimate the ratio f of the compressed magnetic field just inside the subsolar magnetopause to the purely dipolar magnetic field.We also compare our numerical results to a similar work by Shue,which used Time History of Events and Macroscale Interactions during Substorms(THEMIS)data.Our results show that the ratio f is linearly proportional to the subsolar magnetopause standoff distance(r0)for both the northward and southward interplanetary magnetic field,properties consistent with Shue but with a smaller proportionality constant.However,previous theoretical studies show that f is nearly independent of the subsolar standoff distance.The global model results also show that f is smaller for the southward Interplanetary Magnetic Field(IMF)under the same r0,and that the proportionality constant for the southward IMF is larger than that for the northward IMF.Both conclusions agree with statistical results from observations by Shue.     

宇宙等离子体中调制相互作用

该文对等离子体天体物理中的强湍动进行了评述并强调地等离子体的基本相互作用。分析了控制湍动现象的Ｚａｋｈａｒｏｖ方程讨论了天体物理中一些有些兴趣的课题，诸如加速、爆发、有动力所引发的磁重联，等离激元所诱发的自生场以及自引力系统的非线性结构。     

Electron acceleration from contracting magnetic islands during reconnection

A long-standing problem in the study of space and astrophysical plasmas is to explain the production of energetic electrons as magnetic fields 'reconnect' and release energy. In the Earth's magnetosphere, electron energies reach hundreds of thousands of electron volts (refs 1-3), whereas the typical electron energies associated with large-scale reconnection-driven flows are just a few electron volts. Recent observations further suggest that these energetic particles are produced in the region where the magnetic field reconnects. In solar flares, upwards of 50 per cent of the energy released can appear as energetic electrons. Here we show that electrons gain kinetic energy by reflecting from the ends of the contracting 'magnetic islands' that form as reconnection proceeds. The mechanism is analogous to the increase of energy of a ball reflecting between two converging walls--the ball gains energy with each bounce. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. The back pressure of the energetic electrons throttles reconnection so that the electron energy gain is a large fraction of the released magnetic energy. The resultant energy spectra of electrons take the form of power laws with spectral indices that match the magnetospheric observations.     

Jovian-like aurorae on Saturn

Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. Earth's main auroral oval is formed through interactions with the solar wind, whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is approximately 25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter's main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter's volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.     

Numerical study of magnetic reconnection process near interplanetary current sheet

The third order accurate upwind compact difference scheme has been applied to the numerical study of the magnetic reconnection process possibly occurring near the interplanetary current sheet, under the framework of the two-dimensional compressible magnetohydrodynamics (MHD). Our results here show that the driven reconnection near the current sheet can occur within 10—30 min for the interplanetary high magnetic Reynolds number, R_M =2 000—10 000, the stable magnetic reconnection structure can be formed in hour-order of magnitude, and there are some ba- sic properties such as the multiple X-line reconnections, vortical velocity structures, filament current systems, splitting and collapse of the high-density plasma bulk. These results are helpful in understanding and identifying the magnetic reconnection phenomena near the interplanetary current sheets.     

空间灾害性扰动事件数值预报初探——行星际激波动力学问题6步求解法

空间灾害性天气的预报是日地物理学界及高科技领域的热门话题.未来预测太阳剧烈扰动所造成的行星际风暴到达地球空间的状态势必借助于数值方法.浅析了空间灾害性扰动事件数值预报存在的问题及未来设想, 针对这一目的对一维球对称问题提出了处理行星际激波的 6步求解方法,指出未来空间灾害性扰动事件预报模式应是一个基于三维的以真实太阳风为背景自洽建立起来的、以太阳等离子体输出及磁场全球结构为初边值、太阳、行星际、地磁因果耦合模式.     

Fast acceleration of “killer” electrons and energetic ions by interplanetary shock stimulated ULF waves in the inner magnetosphere

Energetic electrons and ions in the Van Allen radiation belt are the number one space weather threat. Understanding how these energetic particles are accelerated within the Van Allen radiation belt is one of the major challenges in space physics. This paper reviews the recent progress on the fast acceleration of "killer" electrons and energetic ions by ultralow frequency (ULF) waves stimulated by the interplanetary shock in the inner magnetosphere. Very low frequency (VLF) wave-particle interaction is considered to be one of the primary electron acceleration mechanisms because electron cyclotron resonances can easily occur in the VLF frequency range. Recently, using four Cluster spacecraft observations, we have found that, after interplanetary shocks impact the Earth’s magnetosphere, energetic electrons in the radiation belt are accelerated almost immediately and continue to accelerate for a few hours. The time scale (a few days) for traditional acceleration mechanisms, based on VLF wave-particle interactions to accelerate electrons to relativistic energies, is too long to explain our observations. Furthermore, we have found that interplanetary shocks or solar wind pressure pulses, with even small dynamic pressure changes, can play a non-negligible role in radiation belt dynamics. Interplanetary shocks interaction with the Earth’s magnetosphere manifests many fundamental space physics phenomena including energetic particle acceleration. The mechanism of fast acceleration of energetic electrons in the radiation belt responding to interplanetary shock impacts consists of three contributing parts: (1) the initial adiabatic acceleration due to strong shock-related magnetic field compression; (2) followed by the drift-resonant acceleration with poloidal ULF waves excited at different L-shells; and (3) particle acceleration due to the quickly damping electric fields associated with ULF waves. Particles end up with a net acceleration because they gain more energy in the first half of this cycle than they lose in the second. The results reported in this paper cast a new light on understanding the acceleration of energetic particles in the Earth’s Van Allen radiation belt. The results of this study can likewise be applied to interplanetary shock interaction with other planets such as Mercury, Jupiter, Saturn, Uranus and Neptune, and other astrophysical objects with magnetic fields.