Relativistic jets from black hole binaries

Relativistic jets are now believed to be a fairly ubiquitous property of accreting compact objects, and are intimately coupled with the accretion history. Associated with rapid changes in the accretion states of the binary systems, ejections of relativistic plasma can be observed at radio frequencies on timescale of weeks before becoming undetectable. However, recent observations point to long term effects of these ejecta on the interstellar medium with the formation of large scale relativistic jets around binary systems.     

Diagnostics of hot thermal plasma in flare-associated X-ray ejections

This is the first analysis of the soft X-ray spectra of flare-associated X-ray ejections. For this aim, the Yohkoh Bragg Crystal Spectrometer data recorded before two behind-the-limb flares were selected. These flares came into view from behind the solar limb but their occurrence was preceded by an X-ray ejection which expanded faster. In consequence, for several minutes the X-ray ejection was seen alone, without the presence of the brighter flare. Parameters of the plasma obtained from a fitting of synthetic spectra to observed spectra are presented and compared to images from the Yohkoh Soft X-ray Telescope. The presence of a minor fraction of the very hot (∼30 MK) thermal plasma in X-ray ejections is confirmed. High values of the non-thermal line broadening are caused by a large extension of X-ray ejections.     

Reconnection in multiple current sheet configurations in the solar corona

We investigate magnetic reconnection in a multiple current sheet configuration by means of three-dimensional resistive MHD simulations. This configuration might be of interest in the solar corona context, e.g. for coronal helmet streamers. We present results of our simulations of the linear and nonlinear development of the tearing mode instability. In particular, we highlight the changes in magnetic topology and the resulting plasma dynamics. Our results indicate that reconnection in complex coronal neighboring magnetic flux systems efficiently converts magnetic field energy into thermal energy and leads to small-scale tongue outflows rather than large-scale coronal mass ejections.     

Determination of temperature maps of EUV coronal hole jets

Coronal hole jets are fast ejections of plasma occurring within coronal holes, observed at Extreme-UltraViolet (EUV) and X-ray wavelengths. Recent observations of jets by the STEREO and Hinode missions show that they are transient phenomena which occur at much higher rates than large-scale impulsive phenomena like flares and Coronal Mass Ejections (CMEs). In this paper we describe some typical characteristics of coronal jets observed by the SECCHI instruments of STEREO spacecraft. We show an example of 3D reconstruction of the helical structure for a south pole jet, and present how the angular distribution of the jet position angles changes from the Extreme-UltraViolet-Imager (EUVI) field of view to the CORonagraph1 (COR1) (height ∼2.0 R_⊙ heliocentric distance) field of view. Then we discuss a preliminary temperature determination for the jet plasma by using the filter ratio method at 171 and 195 Å and applying a technique for subtracting the EUV background radiation. The results show that jets are characterized by electron temperatures ranging between 0.8 and 1.3 MK. We present the thermal structure of the jet as temperature maps and we describe its thermal evolution.     

Numerical Simulation for the 16 August 1999 EUV Brightenings

The 16 August 1999 EUV brightenings are numerically simulated by a third-order upwind compact scheme, basing on the TRACE observation. The present simulation can give a possible explanation to its formation and evolution. The numerical results show that the initial reconnection jets at around X-point are responsible for the occurrence of EUV brightening. The strong and superposed ejections caused by the first and second coalescence of magnetic islands are possibly related to the lifted material which initially appeared as absorption features and Later EUV-emitting structures respectively. The bi-directional reconnection jets may correspond to the lifted material that either continued to move upward along the apparently open field lines or fell down to the surface.      

等离子体彗尾动力学的数值研究

本文基于可压缩磁流体动力学模型,数值研究了尾瓣巾具有超Alfven速流动的等离子体彗尾的动力学特征。结果表明,等离子体片和尾瓣之间的剪切等离子体流动将会激发流动撕裂模不稳定性,引起彗尾等离子体片中发生磁场重联,形成磁岛和高密度的等离子体团。进而模拟了太阳风引起的局部驱动力对等离子体彗尾中磁场重联的影响,其特征时间远大于流动撕裂模。我们认为一些观测到的等离子体彗尾中的四块和彗尾截断事件可能主要与彗尾中剪切等离子体流动所引起的流动撕裂模不稳定性有关。      

Energy requirement of magnetic reconnection during magnetospheric substorms

Methods are discussed to estimate energy transfer from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the magnetotail. The major avenues of energy dissipation are examined and the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, plasmoids, and energetic particle production during substorms is assessed. Energy sources are evaluated to drive substorm evolution in a particularly well-observed case: stored tail-lobe energy is sufficient to drive observed substorm dissipation processes (by large margins). On the other hand, energy in the closed field lines of the plasma sheet is insufficient to supply the substorm energy. Hence, magnetic reconnection is required on energy grounds during well-observed substorm cases.     

双带耀斑磁重联过程的数值研究

本文用多步隐格式求解包含电阻的磁流体力学方程组, 对双带耀斑的主相作数值模拟, 清晰地展示了中性片区由撕裂模线性重联向准稳态重联的过渡以及后随耀斑环的产生和等离子体团的喷发过程.对于在能量方程中计及和忽略焦耳加热两种情况, 分别作了计算.结果表明, 计及焦耳加热时, 电流片中等离子体的温度显著增加(是初始温度的2—3倍), 但等离子体的运动速度却变化不大.两种情况的计算结果均表明:等离子体的运动速度低于声速, 因此不会形成快激波.计及焦耳加热的计算结果显示了两个新的特征:其一是中性片高密度等离子体的受热膨胀, 增大了电流片的有效厚度, 它使重联速率降低, 并逐渐趋于饱和, 其二是同时形成上升和沉降等离子体团, 后者与耀斑环碰撞, 并合并于后随耀斑环内.      

磁云边界层中的重联慢激波观测分析

在Petschek模型中，排空区边界处的一对慢激波是能量耗散的重要机制.已有大量行星际空间的Petschek型磁场重联排空区观测事件被报道，但是只有少量的排空区边界处观测到了慢激波.针对一例位于磁云边界层中的Petschek型磁场重联排空区观测事件，在排空区靠近磁云一侧边界处证认了一例慢激波.激波跃变层两侧的磁场和等离子体参数满足Rankine-Hugoniot关系，且激波上下游的中间马赫数均小于1，上游的慢马赫数为2.94（>1），下游的慢马赫数为0.65（<1），符合慢激波的观测特征.磁云内部的等离子体β值很低，局地阿尔芬速度高，同时磁云边界层中可能发生丰富的磁场重联活动，这可能是磁云前边界处慢激波形成的原因.      

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.     

An investigation of small GOES flares with intense hard X-ray bursts

Most solar flare observations show that intense hard X-ray bursts come from large flares that have a large GOES classification (large peak 1 – 8 Å flux). This correlation, known as the “Big Flare Syndrome”, suggests that more intense flares tend to have harder spectra. We have observed 7 flares that are exceptions to this. These flares have small GOES classifications ranging from B1.4 to C5.5 and peak hard X-ray count rates similar to those often observed from M class flares. This paper examines the cause of this anomoly using the Yohkoh Soft X-Ray Telescope, Hard X-Ray Telescope, and Bragg Crystal Spectrometer. Two hypotheses are proposed for the exceptions: (1) flares with multiple magnetic loops and common footpoints, producing multiple hard X-ray emission regions and low density thermal plasma distributed over a large volume, and (2) high densities in the magnetic loops restricting the propagation of the non-thermal electrons in the loop after magnetic reconnection has occurred and suppressing chromospheric evaporation. Two of the flares support the first hypothesis. The other flares either have data missing or are too small to be properly analysed by the Yohkoh instruments.     

The properties of hard X-ray emitting symbiotic stars

Hard X-ray emitting symbiotic stars are candidates for SN Ia progenitors. The importance of Type Ia SNe as standard candles for cosmology makes the study of their progenitor systems particularly important. Additionally, they provide one of the most promising laboratories for the study of astrophysical jets. Typically, the X-ray emission in these systems is modeled with a collisional plasma model, sometimes with an emission measure distribution taken from a cooling flow model. The lack of any coherent periods in both X-rays and optical wave band strongly suggests that the accreting white dwarfs in the hard X-ray symbiotic stars are non-magnetic. Although relatively few have been discovered to date, but we believe that there are very many of them in our galaxy and could be possible candidates for the Galactic Ridge X-ray Emissions (GRXE).     

Energetic electrons in solar flares as viewed in X-rays

Hard X-ray observations provide the most direct diagnostic we have of the suprathermal electrons and the hottest thermal plasma present in solar flares. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is obtaining the most comprehensive observations of individual solar flares ever available in hard X-rays. For the first time, high-resolution spectra are available for a large number of flares that accurately display the spectral shape and its evolution and, in many cases, allow us to identify the transition from the bremsstrahlung X-rays produced by suprathermal electrons to the bremsstrahlung at lower energies emitted by thermal plasma. Also, for the first time, images can be produced in arbitrary energy bands above 3–4 keV, and spectra of distinct imaged components can be obtained.I review what we have learned from RHESSI observations about flare suprathermal electron distributions and their evolution. Next, I present computations of the energy deposited by these suprathermal electrons in individual flares and compare this with the energy contained in the hot thermal plasma. I point out unsolved problems in deducing both suprathermal electron distributions and the energy content of the thermal plasma, and discuss possible solutions. Finally, I present evidence that electron acceleration is associated with magnetic reconnection in the corona.     

Two energy release processes for CMEs: MHD catastrophe and magnetic reconnection

It remains an open question how magnetic energy is rapidly released in the solar corona so as to create solar explosions such as solar flares and coronal mass ejections (CMEs). Recent studies have confirmed that a system consisting of a flux rope embedded in a background field exhibits a catastrophic behavior, and the energy threshold at the catastrophic point may exceed the associated open field energy. The accumulated free energy in the corona is abruptly released when the catastrophe takes place, and it probably serves as the main means of energy release for CMEs at least in the initial phase. Such a release proceeds via an ideal MHD process in contrast with nonideal ones such as magnetic reconnection. The catastrophe results in a sudden formation of electric current sheets, which naturally provide proper sites for fast magnetic reconnection. The reconnection may be identified with a solar flare associated with the CME on one hand, and produces a further acceleration of the CME on the other. On this basis, several preliminary suggestions are made for future observational investigations, especially with the proposed Kuafa satellites, on the roles of the MHD catastrophe and magnetic reconnection in the magnetic energy release associated with CMEs and flares.     

Interplanetary coronal mass ejections during the descending cycle 23: Sheath and ejecta properties comparison

We have used Omniweb data in order to identify the sheath and the ejecta boundaries of 67 shock-driving interplanetary coronal mass ejections during the time period 2003–2006. We examine and compare their statistical properties (speed, magnetic field strength, proton density and temperature, proton plasma beta), with those of the typical solar wind. We also calculate their passage time and radial width. We study the correlation between the ejecta and sheath characteristics.     

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.     

XMM-Newton observation of Vela shrapnel D

The ROSAT all-sky survey discovered several ‘shrapnels’, showing boomerang structures outside the Vela SNR. We observed the shrapnel D in the eastern limb of the Vela SNR with the XMM-Newton satellite. There is an X-ray bright ridge structure in our FOV running from north to south. There is also an optical bright ridge structure running parallel to that in X-ray about 3′ in the east. These suggest that the shrapnel D is now interacting with an interstellar cloud. Applying the VNEI model to X-ray spectra of various regions, we find that the plasma state of the eastern region of the X-ray ridge is significantly different from that of the western region. The X-ray spectra in the western region can be well fitted with a single temperature component. Abundances of heavy elements are almost uniform, whereas they are heavily overabundant (except Fe): the relative abundances to the solar values are O ∼5, Ne ∼8, Mg ∼8, Fe ∼1. It must have originated from the ejecta of the SN. We find that the plasma in the eastern part of the ridge consists of two temperature components: one component (hot plasma) representing the ejecta while the other (cold plasma) represents the interstellar cloud or swept-up ISM. These two components must have contacted to each other, indicating to form a contact discontinuity. Around northern part of the contact discontinuity, we found wavelike structures of which the typical scales are comparable with that of the Rayleigh–Taylor instability.     

太阳表面水平运动驱动的磁圈与开放磁场重联的模拟研究

太阳大气的诸多观测事件(如耀斑、喷流等)均被归因于磁重联产生的能量转换. 近年来, 关于太阳风起源, 有研究提出了磁重联使闭合磁圈开放为太阳风供应物质的新模式. 在该模式中, 闭合磁圈被光球超米粒组织对流携带, 向超米粒边界运动, 与位于边界的开放磁场相碰撞进而发生磁重联. 该模式中磁重联的驱动及其效应是本文的研究目标. 磁流体力学(MHD)数值模拟是研究太阳大气磁重联物理过程的重要途径. 本文建立了一个二维MHD数值模型, 结合太阳大气温度和密度的分层分布, 在超米粒组织尺度上模拟了水平流动驱动的闭合磁圈与开放磁场的重联过程. 通过对模拟结果的定量分析, 认为磁重联确实能够将闭合磁圈的物质释放, 进而供应给新的开放磁结构并产生向上流动. 该结果为进一步模拟研究太阳风初始外流奠定了基础.      

Black hole spectral states and physical connections

The dramatic changes seen in the X-ray spectral and timing properties of accreting black hole candidates (BHCs) provide important clues about the accretion and jet formation processes that occur in these systems. Dividing the different source behaviors into spectral states provides a framework for studying BHCs. To date, there have been three main classification schemes with Luminosity-based, Component-based, or Transition-based criteria. The canonical, Luminosity-based criteria and physical models that are based on this concept do not provide clear explanations for several phenomena, including hysteresis of spectral states and the presence of jets. I discuss the re-definitions of states, focusing on an application of the Component-based states to more than 400 RXTE observations of the recurrent BHC 4U 1630–47. We compare the X-ray properties for the recent 2002–2004 outburst to those of an earlier (1998) outburst, during which radio jets were observed. The results suggest a connection between hysteresis of states and major jet ejections, and it is possible that both of these are related to the evolution of the inner radius of the optically thick accretion disk.     

Combining MHD and kinetic modelling of solar flares

Solar flares are explosive events in the solar corona, representing fast conversion of magnetic energy into thermal and kinetic energy, and hence radiation, due to magnetic reconnection. Modelling is essential for understanding and predicting these events. However, self-consistent modelling is extremely difficult due to the vast spatial and temporal scale separation between processes involving thermal plasma (normally considered using magnetohydrodynamic (MHD) approach) and non-thermal plasma (requiring a kinetic approach). In this mini-review we consider different approaches aimed at bridging the gap between fluid and kinetic modelling of solar flares. Two types of approaches are discussed: combined MHD/test-particle (MHDTP) models, which can be used for modelling the flaring corona with relatively small numbers of energetic particles, and hybrid fluid-kinetic methods, which can be used for modelling stronger events with higher numbers of energetic particles. Two specific examples are discussed in more detail: MHDTP models of magnetic reconnection and particle acceleration in kink-unstable twisted coronal loops, and a novel reduced-kinetic model of particle transport in converging magnetic fields.