Generation mechanism of the whistler-mode waves in the plasma sheet prior to magnetic reconnection

The whistler-mode waves and electron temperature anisotropy play a key role prior to and during magnetic reconnection. On August 21, 2002, the Cluster spacecrafts encountered a quasi-collisionless magnetic reconnection event when they crossed the plasma sheet. Prior to the southward turning of magnetospheric magnetic field and high speed ion flow, the whistler-mode waves and positive electron temperature anisotropy are simultaneously observed. Theoretic analysis shows that the electrons with positive temperature anisotropy can excite the whistler-mode waves via cyclotron resonances. Using the data of particles and magnetic field, we estimated the whistler-mode wave growth rate and the ratio of whistler-mode growth rate to wave frequency. They are 0.0016f_ce (Electron cyclotron frequency) and 0.0086f_ce, respectively. Therefore the whistler-mode waves can grow quickly in the current sheet. The combined observations of energetic electron beams and waves show that after the southward turning of magnetic field, energetic electrons in the reconnection process are accelerated by the whistler-mode waves.     

Magnetospheric Boundary Layer Structure and Dynamics as Seen From Cluster and Double Star Measurements

In this review, we discuss the structure and dynamics of the magnetospheric Low-Latitude Boundary Layer (LLBL) based on recent results from multi-satellite missions Cluster and Double Star. This boundary layer, adjacent to the magnetopause on the magnetospheric side, usually consists of a mixture of plasma of magnetospheric and magnetosheath origins, and plays an important role in the transfer of mass and energy from the solar wind into the magnetosphere and subsequent magnetospheric dynamics. During southward Interplanetary Magnetic Field (IMF) conditions, this boundary layer is generally considered to be formed as a result of the reconnection process between the IMF and magnetospheric magnetic field lines at the dayside magnetopause, and the structure and plasma properties inside the LLBL can be understood in terms of the time history since the reconnection process. During northward IMF conditions, the LLBL is usually thicker, and has more complex structure and topology. Recent observations confirm that the LLBL observed at the dayside can be formed by single lobe reconnection, dual lobe reconnection, or by sequential dual lobe reconnection, as well as partially by localized cross-field diffusion. The LLBL magnetic topology and plasma signatures inside the different sub-layers formed by these processes are discussed in this review. The role of the Kelvin-Helmholtz instability in the formation of the LLBL at the flank magnetopause is also discussed. Overall, we conclude that the LLBL observed at the flanks can be formed by the combination of processes, (dual) lobe reconnection and plasma mixing due to non-linear Kelvin-Helmholtz waves.      

The role of magnetic field shear in solar flares

We present observational results and their physical implications garnered from the deliberations of the FBS Magnetic Shear Study Group on magnetic field shear in relation to flares. The observed character of magnetic shear and its involvement in the buildup and release of flare energy are reviewed and illustrated with emphasis on recent results from the Marshall Space Flight Center vector magnetograph. It is pointed out that the magnetic field in active regions can become sheared by several processes, including shear flow in the photosphere, flux emergence, magnetic reconnection, and flux submergence. Modeling studies of the buildup of stored magnetic energy by shearing are reported which show ample energy storage for flares. Observational evidence is presented that flares are triggered when the field shear reaches a critical degree, in qualitative agreement with some theoretical analyses of sheared force-free fields. Finally, a scenario is outlined for the class of flares resulting from large-scale magnetic shear; the overall instability driving the energy release results from positive feedback between reconnection and eruption of the sheared field.     

Study of three dimensional structure of the fast convection flow in the plasma sheet by MHD simulations on the basis of spontaneous fast reconnection model

Three dimensional structure of the fast convection flow in the plasma sheet is examined using magnetohydrodynamic (MHD) simulations on the basis of spontaneous fast reconnection model. The fast flow observed in the near-Earth magnetotail is one of the key phenomena in order to understand the causal relationship between magnetic substorm and magnetic reconnection. In this paper, we focus on this earthward fast flow in the near-Earth magnetotail. Our previous studies have shown that the fast reconnection produces the Alfvénic fast reconnection outflow and drastic magnetic field dipolarization in the finite extent. In this paper, the results of our simulations are compared with those of the in-situ observations in the geomagnetotail. They have consistent temporal profiles of the plasma quantities. It is suggested that the fast convection flows are caused by spontaneous fast reconnection.     

Generation mechanisms of the ELF-ULF waves related to the flux transfer events

Satellite observations near the magnetopause and within the magnetosheath revealed the existence of some structures characterised by specific magnetic field and plasma signatures. They have been called “Flux Transfer Events”. One of the models of FTEs is a reconnected fluxtube, extending from the inner magnetosphere into the magnetosheath. ELF-ULF waves are often observed in together with other FTE's signatures. Wideband emissions are associated with the boundaries of FTEs with characteristic maxima at lower hybrid and ion-cyclotron frequencies. They provide a tool for better timing of the events. These emissions might also be related to the reconnection process. Observations of FTE's by Prognoz-8 satellite are presented in our paper. Wave signatures of the FTEs are described. Various mechanisms of generation of the emissions by instabilities depending on local plasma conditions are discussed along with non-local aspects of such waves. Numerical solutions of the dispersion equation for the typical conditions in FTEs are presented. Possible relation of these waves to the reconnection process are discussed.     

Statistical study of electrostatic solitary waves associated with reconnection: Geotail observations

The role of waves in the dynamics of the magnetotail has long been a topic of interest in magnetospheric physics. The characteristics of Electrostatic Solitary Waves (ESWs) associated with reconnection have been studied statistically in the magnetotail by surveying the large amounts data obtained from Waveform Capture (WFC) which is an important component of Plasma Wave Instrument (PWI) on the Geotail spacecraft. About 150 reconnection events with WFC data available are selected, and approximately 10 thousands of ESW waveforms are picked up by hands for statistical study. The ESWs are observed near diffusion region and near the plasma sheet boundary layer (PSBL). Two kinds of waveforms of ESWs are observed: bi-polar and tri-polar pulses. It is found that the pulse width of the ESWs is in the order of 1–5 ms and the peak-to-peak amplitude is in the order of 0.1–5 mV/m. The amplitudes of ESWs are larger in the near-earth tail region than that in deep tail region. ESWs have been observed with or without guide magnetic field 〈By〉. The characteristics of ESWs in different reconnection region and under different strength of guild magnetic field, their possible generation mechanism will be discussed.     

具有不同等离子体β值的Hall MHD重联中的低频波研究

数值研究具有不同等离子体β值(等离子体热压与磁压之比)的Hall MHD重联过程.对于β值在0.5≤β≤6.5的算例,稳态重联率(6)A/(6)t|st为0.15≥(6)A/(6)t|st≥0.095.本文研究β值在上述范围内的Hall MHD重联中的波动.通过快速傅里叶变换(FFT),将平面外磁场By分量和xz平面内的速度分量vx,vz位于给定点的时间序列转换为功率谱.结果表明,By,vx,vz功率谱的频率范围为Ωci＜ω＜8Ωci,其中Ωci为离子回旋频率.随着β值增大,功率谱峰值能量降低,并且朝着低频端移动.对于电场E采用最小方差分析法(MVA)可以确定波的传播方向k,波矢k与局地磁场B之间的夹角α随β值增大而增大.对于β值最大的算例3(β=6.5),α＜28°.上述结果表明,k准平行于B.为了研究波的偏振特征,在垂直于k的平面内画出E的矢端图.三个算例的矢端图均表明,波动具有右旋偏振特征,在算例1(β=0.5)中矢端图为右旋椭圆偏振,而在算例2(β=2.5)和算例3(β=6.5)中矢端图变成复杂的右旋偏振图形,这些均为哨声波的典型特征.本文的研究为快速磁重联与哨声波之间提供了一种可能的联系.      

Structure and kinetic properties of slow-mode shocks associated with magnetic reconnection in the near-Earth magnetotail

We study the structure and kinetic properties of slow-mode shocks near the plasma sheet boundary layer (PSBL) associated with magnetic reconnection by Cluster observation. The presence of slow-mode shocks is confirmed by traditional Rankine–Hugoniot (RH) analysis and Monte-Carlo shock fitting method. The Walén analysis, applied to the tailward flow associated with slow-mode shocks, also supports that plasma was accelerated across a Petschek-type slow-mode shock connected to the diffusion region. Back-streaming ions were observed on the shock layer, and cold ions were accelerated and heated by slow-mode shocks. In addition, whistler and electrostatic solitary waves were observed around the slow-mode shocks. These waves might be excited by the observed field-aligned electron beams near the shocks.     

The plasma sheet and boundary layers under northward IMF: A multi-point and multi-instrument perspective

During conditions of northward interplanetary magnetic field (IMF), the near-tail plasma sheet is known to become denser and cooler, and is described as the cold-dense plasma sheet (CDPS). While its source is likely the solar wind, the prominent penetration mechanisms are less clear. The two main candidates are solar wind direct capture via double high-latitude reconnection on the dayside and Kelvin–Helmholtz/diffusive processes at the flank magnetopause. This paper presents a case study on the formation of the CDPS utilizing a wide variety of space- and ground-based observations, but primarily from the Double Star and Polar spacecraft on December 5th, 2004. The pertinent observations can be summarized as follows: TC-1 observes quasi-periodic (∼2 min period) cold-dense boundary layer (compared to a hot-tenuous plasma sheet) signatures interspersed with magnetosheath plasma at the dusk flank magnetopause near the dawn-dusk terminator. Analysis of this region suggests the boundary to be Kelvin–Helmholtz unstable and that plasma transport is ongoing across the boundary. At the same time, IMAGE spacecraft and ground based SuperDARN measurements provide evidence of high-latitude reconnection in both hemispheres. The Polar spacecraft, located in the southern hemisphere afternoon sector, sunward of TC-1, observes a persistent boundary layer with no obvious signature of boundary waves. The plasma is of a similar appearance to that observed by TC-1 inside the boundary layer further down the dusk flank, and by TC-2 in the near-Earth magnetotail. We present comparisons of electron phase space distributions between the spacecraft. Although the dayside boundary layer at Polar is most likely formed via double high-altitude reconnection, and is somewhat comparable to the flank boundary layer at Double Star, some differences argue in favour of additional transport that augment solar wind plasma entry into the tail regions.     

Theoretical model of steady-state magnetic reconnection in collisionless incompressible plasma based on the Grad–Shafranov equation solution

The problem of steady-state magnetic reconnection in an infinite current layer in collisionless, incompressible, nonresistive plasma, except of the electron diffusion region, is examined analytically using the electron Hall magnetohydrodynamics approach. It is found that this approach allows reducing the problem to the magnetic field potential finding, while last one has to satisfy the Grad–Shafranov equation. The obtained solution demonstrates all essential Hall reconnection features, namely proton acceleration up to Alfvén velocities, the forming of Hall current systems and the magnetic field structure expected. It turns out that the necessary condition of steady-state reconnection to exist is an electric field potential jump across the electron diffusion region and the separatrices. Besides, the powerful mechanism of electron acceleration in X-line direction is required. It must accelerate electrons up to the electron Alfvén velocity inside the diffusion region and on the separatrixes. This is a necessary condition for steady-state reconnection as well.     

Interconnection of high-latitude and low-latitude boundary layers when IMF BY is dominant

The antiparallel merging model places the location of the reconnection region for a dominant interplanetary magnetic field (IMF) B_Y at high latitudes at the dayside magnetopause and predicts that the low-latitude boundary layer (LLBL) is located on open field lines of the magnetospheric flanks. Interball-1 data obtained in the wide local time range near the low-latitude magnetopause makes it possible to analyze the LLBL plasma population and to find a link between possible reconnection at high latitudes and LLBL occurrence. We found that no boundary layer was observed in the regions which have no topological connection with the merging site. All cases of LLBL observations are located downstream from a specific boundary. This boundary coincides with the first magnetospheric field line touching the reconnection region and can be located in a wide local time region depending on the instant IMF direction. Even the LLBL on closed field lines shows the tendency to be concentrated in the vicinity of this boundary. Thus we show that all types of observed LLBLs are linked to reconnection sites predicted by the antiparallel merging model.     

太阳风中磁场重联离子扩散区的大尺度特征

利用ACE和WIND卫星2007年1月6日的联合探测, 在1AU附近发现了一个等离子体密度极低的Petschek-like重联喷流区. 该喷流区内部出现了非常明显的Hall双极磁场、等离子体密度下降区以及与Hall电流相符的低能段电子投掷角分布. 这些特征与重联离子扩散区的Hall效应非常吻合, 说明很可能在太阳风中观测到了一个离子扩散区. 分析表明, 与之相关的磁场重联为准稳态快速完全反向重联, 其扩散区以一对慢模波为边界, 空间尺度达到80个离子惯性长度, 表现出了大尺度重联的特征.      

Plasma flow in the near and distant geomagnetic tail

Measurements of the bulk flow of plasma in the outer magnetosphere were first made a little over a decade ago with Los Alamos instruments on the Vela satellites. During the intervening years, as flow measurements have been made with improved instruments and by other satellites they have come to play a crucial role in the development of our understanding of the structure and dynamics of the magnetosphere. For example, they were the means of discovery of the magnetosphere's boundary layer and of plasma vortices within the plasma sheet. They were the essential ingredient in the identification of signatures of magnetic reconnection at the magnetopause. And they were indispensible in clarifying the complex phenomena in the magnetotail accompanying substorms and in showing that these phenomena are consistent with a substorm model involving magnetic reconnection at a near-earth neutral line. Most recently, magnetotail plasma flow measurements by the ISEE-3 satellite at distances as great as 230 R_E have been instrumental in fixing the average location of the “distant” neutral line at ～ 60 to 120 R_E and in identifying plasmoids (i.e., severed sections of the plasma sheet), released during substorms and escaping down-tail. This paper reviews the features of magnetotail plasma flow, describes the most recent observations, and discusses their implications for magnetospheric physics.     

Magnetic reconnection in solar flares and corresponding radio bursts

The 2D MHD model of the flare magnetic reconnection shows that a reconnection activity, changes of the magnetic field topology and generation of waves are connected. It is found that after the phase of a quasi-stationary reconnection in the extended current sheet above the flare arcade the tearing mode instability produces the plasmoids which then can interact and generate MHD waves. Results of particle-in-cell simulations of the tearing processes, which accelerate electrons, are mentioned. Then all these processes are discussed from the point of view of possible radio emissions. While shocks can contribute to the type II radio burst, the superthermal electrons trapped in plasmoids can generate so called drifting pulsating structures. Furthermore, regions with the MHD turbulence may manifest themselves as the lace or dm-spike bursts.     

ISEE-1 and -2 observations of magnetotail flux ropes: FTEs in the plasma sheet?

ISEE-1 and 2 observations from about 20 Re down the near-Earth magnetotail indicate the presence of magnetic flux ropes in the neutral sheet. Magnetic and electric field and fast plasma data show that these structures convect across the spacecraft at speeds of 200–600-km/s, and have scale sizes of roughly 3–5-Re. The rope axis orientation is approximately cross-tail. Their magnetic structure is similar to Venus ionospheric flux ropes, and to flux transfer events at the dayside magnetopause. These structures may arise from patchy reconnection or tearing mode reconnection within the plasma sheet.     

Lessons from laboratory experiments on reconnection

Magnetic reconnection has been studied in a laboratory experiment designed to model the basic two-dimensional neutral sheet configuration. However, the focus has been put on the inner region of the neutral sheet where the ions are effectively unmagnetized and MHD concepts are violated. In this parameter regime driven reconnection is governed by the fast dynamics of electrons. In true neutral sheets (B_z ⋍ 0) the current is carried by electrons. Thin current sheets (Δz ≳ c/ω_pe) rapidly form multiple X and 0 points due to the onset of the collisionless electron tearing mode. Magnetic energy is transported along the separator at the speed of whistler waves rather than Alfvén waves. Due to space charge separation the reconnection electric field E_y is, in general, not constant along the separator but localized near boundaries, nonuniformities in density and magnetic fields which limit the current I_y. This leads to localized particle acceleration, formation of anisotropic velocity distributions and instabilities. Reconnection and energization can be spatially separated which shows the importance of investigating both the global current system as well as critical local plasma properties. Experiments of current sheet disruptions are performed which demonstrate the processes of magnetic energy storage, transport, conversion and dissipation. Double layers and shock waves can be produced by current disruptions. The laboratory experiments show new dynamic features of reconnection processes not considered in MHD models yet relevant to narrow current sheets or the center of thick sheets.     

Interpretation of observations on July 24, 1996 detected by Interball-1 as pulsed reconnection at low latitude magnetopause

A method for investigating reconnection events is presented. The approach is based on advantages of non-linear spectral analysis named by the Method of Global Minimum and magnetic field measurements. The technique allows to reveal a presence and plasma properties of different particle species, waves, and time intervals of development of non-linear processes in the reconnection layers. We put into practice the approach to study the event on July 24, 1996 detected by Interball-1 and interpreted as gross deformation of the dayside magnetopause of about 5 R_E caused by a process at the bow shock by (Sibeck et al., 1998). Our method of spectral analysis of the data reveals that plasma detected during the event consists of ions both solar wind and ionosphere origin. The spectrum of the magnetic field data is determined by periods caused by gyro-motions of different species of the solar wind plasma (Fe⁺⁶, O⁺⁶, He⁺⁺) and the ionosphere ions (N⁺⁺, He⁺), and power non-stationary (transient) oscillations at period T120 sec. (frequency8 mHz. We reveal the exchange of plasma from the solar wind to the magnetosphere during the event too. Based on results of our analysis we suggest that the simplest explanation of the discussed event is that these are signatures of transient reconnection of interplanetary and terrestrial magnetic fields.     

Acceleration of charged particles by fluctuating and steady electric fields in a X-type magnetic field

Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. Collisionless magnetic reconnection could provide both the energy release mechanism and the particle accelerator in space plasmas. Here we studied particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbance. Our results are compared to particle acceleration from constant electric field superposed on an X-type magnetic field. The constant electric field configuration represents the effects of steady state magnetic reconnection. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For frequencies (ω in dimensioless units) in the range 0.5 ? ω ? 1.0 a substantial fraction (20%–30%) of the proton distribution is accelerated to gamma-ray producing energies. For frequencies in the range 1 ? ω ? 100.0 the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas.     

Multi-point observations of earthward fast flow in the plasma sheet by virtual satellites located in the MHD simulation domain

Time profiles of some physical values in earthward fast flows in the plasma sheet are observed at three dimensionally different positions by employing virtual satellites located in the three-dimensional magnetohydrodynamic simulation domain, and these simulations are done on the basis of the spontaneous fast reconnection model. In the spontaneous fast reconnection evolution, the width of the flow channel is narrow in the dawn-dusk direction, and it does not spread until the plasma collides with the magnetic loop. The enhancements in B_z and V_x are larger at the center of the fast flow channel than those at its dawn and dusk edges, reflecting the differences in the reconnection rate in the diffusion region. The enhancement in V_x is shorter near the plasma sheet boundary layer than that near the neutral sheet, reflecting the changes in the thickness of the flow channel.     

On the origin of the high-latitude boundary layer

This paper presents a statistical study of the high-latitude boundary layer (HLBL) performed on 53 Interball-1 magnetopause crossings. In the study we verify if antiparallel merging is the main source of HLBL formation when the IMF is nearly horizontal. To provide such a study we designed a new coordinate system which allowed us to analyze HLBL under varied interplanetary conditions. This coordinate system floats over the dayside magnetopause following the changes in the instant location of the reconnection site. Despite very different interplanetary conditions, the observed HLBL plasma regimes manifest systematic behavior in the “reconnection” frame of reference. We explain the observed pattern in terms of sporadic patchy reconnection in the high magnetic shear region of the magnetopause.