Short Quasi-Periodic MHD Waves in Coronal Structures

The possibility of remote diagnostics of coronal structures with impulsively-generated short-period fast magnetoacoustic wave trains is demonstrated. An initially broad-band, aperiodic fast magnetoacoustic perturbation guided by a 1D plasma inhomogeneity develops into a quasi-periodic wave train with a well-pronounced frequency and amplitude modulation. The quasi-periodicity results from the geometrical dispersion of the modes, determined by the transverse profile of the loop, and hence contains information about the profile. Wavelet images of the wave train demonstrate that their typical spectral signature is of a “crazy tadpole’’ shape: a narrow spectrum tail precedes a broad-band head. The instantaneous period of the oscillations in the wave train decreases gradually with time, with a mean value of several seconds for typical coronal values. The period and the spectral amplitude evolution are determined by the steepness of the transverse density profile and the density contrast ratio in the loop, which offers a tool for estimation of the sub-resolution structuring of the corona.     

Synthesis of CME-Associated Moreton and EIT Wave Features from MHD Simulations

Soft X-ray (SXR) waves, EIT waves, and Hα Moreton waves are all associated with coronal mass ejections (CMEs). The knowledge of the characteristics about these waves is crucial for the understanding of CMEs, and hence for the space weather researches. MHD numerical simulation is performed, with the consideration of the quiet Sun atmosphere, to investigate the CME/flare processes. On the basis of the numerical results, SXR, EUV, and Hα images of the eruption are synthesized, where SXR waves, EIT waves, and Hα Moreton waves are identified. It confirms that the EIT waves, which border the expanding dimmming region, are produced by the successive opening (or stretching) of the closed magnetic field lines. Hα Moreton waves are found to propagate outward synchronously with the SXR waves, lagging behind the latter spatially by ～27 Mm in the simulated scenario. However, the EIT wave velocity is only a third of the Moreton wave velocity. The synthesized results also suggest that Hα± 0.45Å would be the best off-band for the detection of Hα Moreton waves.     

3D nonlinear wave heating of coronal loops

The heating of solar coronal loops by the resonant absorption or phase-mixing of incident wave energy is investigated in the framework of 3D nonlinear magnetohydrodynamics (MHD) by means of numerical simulations.     

Stability and waves of transonic laboratory and space plasmas

The properties of magnetohydrodynamic waves and instabilities of laboratory and space plasmas are determined by the overall magnetic confinement geometry and by the detailed distributions of the density, pressure, magnetic field, and background velocity of the plasma. Consequently, measurement of the spectrum of MHD waves (MHD spectroscopy) gives direct information on the internal state of the plasma, provided a theoretical model is available to solve the forward as well as the inverse spectral problems. This terminology entails a program, viz. to improve the accuracy of our knowledge of plasmas, both in the laboratory and in space. Here, helioseismology (which could be considered as one of the forms of MHD spectroscopy) may serve as a luminous example. The required study of magnetohydrodynamic waves and instabilities of both laboratory and space plasmas has been conducted for many years starting from the assumption of static equilibrium. Recently, there is a outburst of interest for plasma states where this assumption is violated. In fusion research, this interest is due to the importance of neutral beam heating and pumped divertor action for the extraction of heat and exhaust needed in future tokamak reactors. Both result in rotation of the plasma with speeds that do not permit the assumption of static equilibrium anymore. In astrophysics, observations in the full range of electromagnetic radiation has revealed the primary importance of plasma flows in such diverse situations as coronal flux tubes, stellar winds, rotating accretion disks, and jets emitted from radio galaxies. These flows have speeds which substantially influence the background stationary equilibrium state, if such a state exists at all. Consequently, it is important to study both the stationary states of magnetized plasmas with flow and the waves and instabilities they exhibit. We will present new results along these lines, extending from the discovery of gaps in the continuous spectrum and low-frequency Alfvén waves driven by rotation to the nonlinear flow patterns that occur when the background speed traverses the full range from sub-slow to super-fast. This revised version was published online in August 2006 with corrections to the Cover Date.     

MHD Spectroscopy of Transonic Flows

In previous publications (Keppens et al.: 2002, Astrophys. J. 569, L121; Goedbloed et al.: 2004a, Phys. Plasmas 11, 28), we have demonstrated that stationary rotation of magnetized plasma about a compact central object permits an enormous number of different MHD instabilities, with the well-known magneto-rotational instability (Velikhov, E. P.: 1959, Soviet Phys.–JETP Lett. 36, 995; Chandrasekhar, S.: 1960, Proc. Natl. Acad. Sci. U.S.A. 46, 253; Balbus, S. A. and Hawley, J. F.: 1991, Astrophys. J. 376, 214) as just one of them. We here concentrate on the new instabilities found that are driven by transonic transitions of the poloidal flow. A particularly promising class of instabilities, from the point of view of MHD turbulence in accretion disks, is the class of trans-slow Alfv’en continuum modes, that occur when the poloidal flow exceeds a critical value of the slow magnetosonic speed. When this happens, virtually every magnetic/flow surface of the disk becomes unstable with respect to highly localized modes of the continuous spectrum. The mode structures rotate, in turn, about the rotating disk. These structures lock and become explosively unstable when the mass of the central object is increased beyond a certain critical value. Their growth rates then become huge, of the order of the Alfv’en transit time. These instabilities appear to have all requisite properties to facilitate accretion flows across magnetic surfaces and jet formation.     

Manifestation of magnetic reconnection in coronal streamer current sheets

We investigate the possibility of observing the effects of magnetic reconnection inside a current sheet forming in a coronal streamer in the extended corona. In particular we study the possibility to observe with the UVCS of SOHO the excitation of the tearing instability in the current sheet.     

Magnetohydrodynamic simulation of a streamer beside a realistic coronal hole

Existing models of coronal streamers establish their credibility and act as the initial state for transients. The models have produced satisfactory streamer simulations, but unsatisfactory coronal hole simulations. This is a consequence of the character of the models and the boundary conditions. The models all have higher densities in the magnetically open regions than occur in coronal holes (Noci,et al., 1993).     

Magneto-gravity Waves Trapped in the Lower Solar Corona

The possibility of trapped magneto-gravity waves in the lower solar corona with an open magnetic field is discussed. Intensity variations and/or Doppler shifts of relevant UV, EUV and x-ray spectral lines in the chromosphere, transition region and lower corona may reveal the existence of such low-frequency modes (with periods longer than ∼ 1.5 hour). The spectrum may be either discrete or continuous depending on the reflection property of the narrow transition region. These modes can be utilized to probe the dynamics of the upper chromosphere, transition region and lower corona; they may also play an important role in coronal heating. This revised version was published online in June 2006 with corrections to the Cover Date.     

三维等离子体MHD气动热环境数值模拟

电磁流动控制技术是一个多学科交叉融合的重要研究方向,在高超声速飞行器气动特性优化、气动热环境减缓、边界层转捩和等离子体分布等流动控制方面显示出广阔的应用前景。考虑高超声速飞行器绕流流场中发生的离解、复合、电离和置换等化学反应,气体分子振动能激发以及化学非平衡效应,耦合电磁场作用并基于低磁雷诺数假设,通过数值模拟求解三维非平衡Navier-Stokes流场控制方程和Maxwell电磁场控制方程,建立磁场与三维等离子体流场耦合数值模拟方法及程序,采用典型算例进行考核。在此基础上,开展不同条件下磁场对再入三维等离子体流场以及气动热环境影响分析。研究表明:建立的高超声速飞行器的等离子体流场与磁场耦合计算方法及程序,其数值模拟结果与文献符合,外加磁场使飞行器头部弓形激波外推,磁场强度越强,激波面外推距离越大;不同磁场强度环境下,流场中温度峰值大小略有变化,变化幅度较小;磁场对绝大部分区域的热流有减缓作用,作用的大小与飞行高度、马赫数以及磁场的配置紧密相关;当前的计算条件下,飞行的高度越高,磁场的作用越明显。     

Shock Wave Interaction with the Magnetopause

The magnetopause is in continuous motion and shock waves and impulsive acceleration events can occur. As an example, we show that the interaction of an interplanetary shock with the bow shock can generate a shock wave that after passing through the magnetosheath can interact with the magnetopause. In fluid dynamics, when a shock wave encounters a fluid discontinuity, the interface may become unstable and form bubbles and spikes. We consider this Richtmyer-Meshkov instability in magnetohydrodynamics. At the dayside magnetopause, the instability tends to be stabilized by the magnetic field. However, the shock wave interaction can initiate magnetic field reconnection for the southward IMF, which may be important in strong interplanetary shock events. This revised version was published online in August 2006 with corrections to the Cover Date.     

Alfvén wave heating

This review discusses Alfvén wave heating in non-uniform plasmas as a possible means for explaining the heating of the solar corona. It focusses on recent analytical results that enable us to understand the basic physics of Alfvén wave heating and help us with the interpretation of results of numerical simulations. First we consider the singular wave solutions that are found in linear ideal MHD at the resonant magnetic surface where the frequency of the wave equals the local Alfvén frequency. Next, we use linear resistive MHD for describing the waves in the dissipative region and explain how dissipation modifies the singular solutions found in linear ideal MHD.     

Absolute and Convective Instabilities of Circularly Polarized Alfvén Waves

We discuss the recent progress in studying the absolute and convective instabilities of circularly polarized Alfvén waves (pump waves) propagating along an ambient magnetic field in the approximation of ideal magnetohydrodynamics (MHD). We present analytical results obtained for pump waves with small dimensionless amplitude a, and compare them with numerical results valid for arbitrary a. The type of instability, absolute or convective, depends on the velocity U of the reference frame where the pump wave is observed with respect to the rest plasma. One of the main results of our analysis is that the instability is absolute when U _l < U < U _r and convective otherwise. We study the dependences of U _l and U _r on a and the ratio of the sound speed to the Alfvén speed b. We also present the results of calculation of the increment of the absolute instability on U for different values of a and b. When the instability is convective (U < U _l or U > U _r) we consider the signalling problem, and show that spatially amplifying waves exist only when the signalling frequency is in two symmetric frequency bands. Then, we write down the analytical expressions determining the boundaries of these frequency bands and discuss how they agree with numerically calculated values. We also present the dependences of the maximum spatial amplification rate on U calculated both analytically and numerically. The implication of the obtained results on the interpretation of observational data from space missions is discussed. In particular, it is shown that circularly polarized Alfvén waves propagating in the solar wind are convectively unstable in a reference frame of any realistic spacecraft.     

A Self-Consistent Determination of the Temperature Profile and The Magnetic Field Geometry in Winds of Late-Type Stars

Cool giant and supergiant stars generally present low velocity winds with high mass-loss rates. Several models have been proposed to explain the acceleration process of these winds. Although dust is known to be present in these objects, the radiation pressure on these particles is uneffective in reproducing the observed physical parameters of the wind. The most promising acceleration mechanism cited in the literature is the transference of momentum and energy from Alfvén waves to the gas. Usually, these models consider the wind to be isothermal. We present a stellar wind model in which the Alfvén waves are used as the main acceleration mechanism, and determine the temperature profile by solving the energy equation taking into account both the radiative losses and the wave heating. We also determine, self-consistently, the magnetic field geometry as the result of the competition between the magnetic field and the thermal pressure gradient. As the main result, we show that the magnetic geometry presents a super-radial index in the region where the gas pressure is increasing. However, this super-radial index is greater than that observed for the solar corona.     

Dissipative Solitary Kinetic Alfvén Waves and Electron Acceleration in the Solar Corona

Nonthermal electrons play a major role during solar flares since not only they contain a large amount of the released energy but also they provide important information of the flaring physics through their nonthermal radiation in radio and hard X-ray bands. In a recent work Wu (Phys. Plasmas 10 (2003) 1364) proposed that dissipative solitary kinetic Alfvén wave (DSKAW) with a local shock-like structure could provide an efficient acceleration mechanism for energetic electrons in a low-β plasma. In the present paper dynamical characteristics of the DSKAW acceleration mechanism in solar coronal plasmas are studied and its application to the acceleration of flaring electrons is discussed.     

考虑烧蚀情况下的表面热流辨识

针对烧蚀传热问题,在热解面模型的基础上通过伴随方程推导建立了基于测点温度辨识表面热流的方法,并进行了算例考核。结果表明:热流辨识结果与真值符合较好,辨识结果与真值之间的偏差随测量误差的增加而增加;烧蚀后退量测量结果的误差对辨识结果有较为显著的影响。然后,将该辨识方法用于钝头型碳酚醛材料Narmco4028试件在陶瓷加热风洞中的烧蚀试验结果分析,结果表明辨识出的表面热流与加热功率基本符合,辨识方法是有效的,在工程实际中有较好的应用前景。     

Working Group 2 Report: Energy Input, Heating, and Solar Wind Acceleration in Coronal Holes

Theories and observations of energy input, heating and acceleration mechanisms in the low corona were presented and discussed. The main topics of discussion were large-scale solar wind simulations, theoretical heating mechanisms, observational constraints, confronting theory with observations and observational issues. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Solar Wind - Inner Heliosphere

The solar wind in the inner heliosphere, inside ~ 5 AU, has been almost fully characterized by the addition of the high heliographic latitude Ulysses mission to the many low latitude inner heliosphere missions that preceded it. The two major omissions are the high latitude solar wind at solar maximum, which will be measured during the second Ulysses polar passages, and the solar wind near the Sun, which could be analyzed by a Solar Probe mission. Here, existing knowledge of the global solar wind in the inner heliosphere is summarized in the context of the new results from Ulysses.     

表面热流可辨识性初步分析

表面热流的可辨识性分析可用于飞行器防热层内温度测量精度和测温点位置的确定,在工程上有较强的实用意义。从无量纲分析和仿真辨识出发,根据防热层材料热物性系数、测点位置、表面热流的频域特性等参数对表面热流辨识结果的影响规律,总结出了表面热流辨识问题的相似参数：基于表面热流频率参数的傅立叶数。此后,以这一傅立叶数为判据,针对不同测量误差值的情况初步建立起了表面热流可辨识性的准则和分析方法。