Modeling coronal streamers and their eruption

Numerical solutions of the time-dependent MHD equations are used to generate ambient coronal streamer structures in a corona characteristic of that near solar minimum. The streamers are then disrupted by slow photospheric shear motion at the base of magnetic field lines within the closed field region, which is currently believed to be responsible for producing at least some CMEs. In contrast to several other simulations of this phenomena, the polytropic index is maintained at a value of 5/3 through the addition of coronal heating. Observations are used as a guide in determining the thermodynamic structure and plasma beta in the ambient corona. For a shear speed of 2.5 km/sec, the streamer configuration evolves slowly for about 65 hours before erupting outward with the formation of a CME. The bright CME leading edge travels outward at a speed of about 240 km/sec, and the sheared field lines follow at a somewhat slower speed. A closed magnetic field region is ejected as the magnetic field lines that were opened by the CME reconnect and reform the streamer.     

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfvén speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfvén wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfvén-speed profile. In a ??=0 plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfvén wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfvén wave accumulates along the separatrices (in 2D) or along the spine or fan-plane (in 3D). Hence, Alfvén wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, mode-coupling, quasi-periodic pulsations and phase-mixing.     

三维磁流体强化超燃冲压发动机数值模拟

建立了三维磁流体强化超燃冲压发动机内部黏性流场的求解模型.针对马赫数为6设计了联合应用磁控进气道和磁流体能量旁路的磁流体强化超燃冲压发动机模型.针对该模型进行了数值模拟研究,分析其中的三维流场结构、电参数分布规律以及能量转换特性.结果表明:当飞行马赫数为8时,磁控进气道的应用能够使头部压缩激波回到唇口,使分离区消失,内进气道中的流动恢复到设计状态.磁流体能量旁路可有效降低燃烧室入口处的马赫数,从而改善发动机性能.其中发生器中的流动参数和电参数的分布比较理想,效果显著;而加速器要取得显著的加速效果则需要大量的能量输入.在加速器中,电极附近焦耳耗散严重,导致局部高温以及流动的复杂性,性能不够理想.      

Coronal streamers' theories

Some theoretical aspects of solar coronal streamers are discussed with emphasis on the current sheet and reconnection processes going on along the axis of the streamer. The dynamics of the streamer is a combination of MHD and transport, with acceleration of particles due to reconnection and leakage of plasma outwards as a slow solar wind as the observable results. The presence of the almost-closed magnetic bottles of streamers that can store high-energy particles for significant times provides the birdcage for solar cosmic rays, the reconnection in the sheet feeds medium-energy protons into the corona for the large-scale storage needed for certain flare models, and the build-up of excess density sets the stage for coronal mass ejections.     

Signature of coronal holes and streamers in the interplanetary space

The properties of different solar wind streams depend on the large scale structure of the coronal magnetic field. We present average values and distributions of bulk parameters (density, velocity, temperature, mass flux, momentum, and kinetic and thermal energy, ratio of thermal and magnetic pressure, as well as the helium abundance) as observed on board the Prognoz 7 satellite in different types of the solar wind streams. Maximum mass flux is recorded in the streams emanating from the coronal streamers while maximum thermal and kinetic energy fluxes are observed in the streams from the coronal holes. The momentum fluxes are equal in both types of streams. The maximum ratio of thermal and magnetic pressure is observed in heliospheric current sheet. The helium abundance in streams from coronal holes is higher than in streams from streamers, and its dependences on density and mass flux are different in different types of the streams. Also, the dynamics of -particle velocity and temperature relative to protons in streams from coronal holes and streamers is discussed.     

Energization of Plasma Species by Intermittent Kinetic Alfvén Waves

We propose a new phase-mixing sweep model of coronal heating and solar wind acceleration based on dissipative properties of kinetic Alfvén waves (KAWs). The energy reservoir is provided by the intermittent ∼1 Hz MHD Alfvén waves excited at the coronal base by magnetic restructuring. These waves propagate upward along open magnetic field lines, phase-mix, and gradually develop short wavelengths across the magnetic field. Eventually, at 1.5–4 solar radii they are transformed into KAWs. We analyze several basic mechanisms for anisotropic energization of plasma species by KAWs and find them compatible with observations. In particular, UVCS (onboard SOHO) observations of intense cross-field ion energization at 1.5–4 solar radii can be naturally explained by non-adiabatic ion acceleration in the vicinity of demagnetizing KAW phases. The ion cyclotron motion is destroyed there by electric and magnetic fields of KAWs.     

Recent developments in thermal characteristics of surface dielectric barrier discharge plasma actuators driven by sinusoidal high-voltage power

Flow control using surface Dielectric Barrier Discharge(DBD) plasma actuators driven by a sinusoidal alternating-current power supply has gained significant attention from the aeronautic industry. The induced flow field of the plasma actuator, with the starting vortex in the wall jet,plays an important role in flow control. However, the energy consumed for producing the induced flow field is only a small fraction of the total energy utilized by the plasma actuator, and most of the total energy i...     

基于适航标准的飞机风挡加温控制方式研究

民用飞机适航条款要求在预定运行的环境条件下风挡不应结冰或结雾,防止影响机组人员的视界。因此应确保在整个飞行过程中风挡具有足够的热量以使其外表面高于冰点,内表面不低于座舱露点温度。以适航条款为基础对风挡加温系统的设计要求进行阐述,分析影响系统设计的主要因素,包括加温方式的选择、 防冰热载荷的计算和加温控制规律的确定等;利用 LMS.AMESim 软件建立风挡瞬态传热模型,计算某型飞机风挡在不同控制规律下其表面温度和加热功率的变化。结果表明：全功率加温式控制方式下风挡升温速率快,,能将玻璃始终保持在一个较高温度,有利于系统的防冰除雾性能：占空比式加温控制方式可大幅降低对玻璃的热冲击,但由于其加热功率波动非常频繁,会对上游供电设备造成较大影响。     

Modeling of Venus, Mars, and Titan

Increased computer capacity has made it possible to model the global plasma and neutral dynamics near Venus, Mars and Saturn??s moon Titan. The plasma interactions at Venus, Mars, and Titan are similar because each possess a substantial atmosphere but lacks a global internally generated magnetic field. In this article three self-consistent plasma models are described: the magnetohydrodynamic (MHD) model, the hybrid model and the fully kinetic plasma model. Chamberlain and Monte Carlo models of the Martian exosphere are also described. In particular, we describe the pros and cons of each model approach. Results from simulations are presented to demonstrate the ability of the models to capture the known plasma and neutral dynamics near the three objects.     

Ion Heating and Acceleration During Magnetic Reconnection Relevant to the Corona

The heating and acceleration of ions during magnetic reconnection relevant to coronal heating and flares is explored via particle-in-cell (PIC) simulations and analytic modeling. We show that the dominant heating mechanism of sub-Alvénic ions during reconnection with a guide field, the case of greatest relevance to the corona, results from pickup behavior during the entry into reconnection exhausts, which produces effective thermal speeds of the order of the Alfvén velocity based on the reconnecting magnetic field. There is a mass-to-charge (M/Q) threshold for pickup behavior that favors the heating of high-M/Q ions. Ions below the threshold gain little energy beyond that associated with convective flow. PIC simulations with protons and alphas confirm the pickup threshold. The enhanced heating of high M/Q ions is consistent with observations of abundance enhancements of such ions in impulsive flares. In contrast to anti-parallel reconnection, the temperature increment during ion pickup is dominantly transverse, rather than parallel, to the local magnetic field. The simulations reveal the dominance of perpendicular heating, which is also consistent with observations. We suggest that the acceleration of ions to energies well above that associated with the Alfvén speed takes place during the interaction with many magnetic islands, which spontaneously develop during 3-D guide-field reconnection. The exploration of particle acceleration in a full 3-D multi-island system remains computationally intractable. Instead we explore ion acceleration in a multi-current layer system with low initial β. Ion energy gain takes place due to Fermi reflection in contracting and merging magnetic islands. Particle acceleration continues until the available magnetic free-energy is significantly depleted so that the pressure of energetic ions approaches that of the reconnecting field. Depending on the strength of the ambient guide field and in spite of the low initial plasma β, the dominance of parallel heating can cause significant regions of the plasma to exceed the marginal firehose condition.     

基于光滑粒子流体动力学方法的液滴蒸发问题数值模拟研究

基于光滑粒子流体动力学方法（Smoothed Particle Hydrodynamics，SPH），开展了SPH新算法在蒸发燃烧领域的研究。建立了适用于SPH方法的蒸发数值模型，推导了基于傅立叶热传导公式和菲克扩散定律的SPH离散方程；借鉴VOF方法（Volume of Fluid）的思想，提出了SPH粒子的液相质量分数的概念，以有效表征蒸发过程中的相变问题。采用SPH方法对高温环境中单个液滴的蒸发过程进行数值模拟，结果符合D2定律，与理论模型相一致；在强迫对流环境中，液滴的蒸发过程受到对流作用及表面张力的影响，蒸发速率加快；进一步对双液滴在静止、对流环境中的蒸发过程进行数值模拟研究。结果表明，液滴的间距、滴径对多个液滴的蒸发过程影响至关重要，液滴间距至少在两倍的液滴直径以上，相互之间的影响才可以近似忽略。通过本文研究，拓宽了SPH方法在蒸发相变领域的应用范围，研究结果也能够为进一步的燃烧问题研究奠定基础。     

Why space physics needs to go beyond the MHD box

Magnetohydrodynamic (MHD) theory has been used in space physics for more than forty years, yet many important questions about space plasmas remain unanswered. We still do not understand how the solar wind is accelerated, how mass, momentum and energy are transported into the magnetosphere and what mechanisms initiate substorms. Questions have been raised from the beginning of the space era whether MHD theory can describe correctly space plasmas that are collisionless and rarely in thermal equilibrium. Ideal MHD fluids do not induce electromotive force, hence they lose the capability to interact electromagnetically. No currents and magnetic fields are generated, rendering ideal MHD theory not very useful for space plasmas. Observations from the plasma sheet are used as examples to show how collisionless plasmas behave. Interpreting these observations using MHD and ideal MHD concepts can lead to misleading conclusions. Notably, the bursty bulk flows (BBF) with large mean velocities left(〈 v 〉≥400 km s right) that have been interpreted previously as E×B flows are shown to involve much more complicated physics. The sources of these nonvanishing 〈 v 〉 events, while still not known, are intimately related to mechanisms that create large phase space gradients that include beams and acceleration of ions to MeV energies. The distributions of these nonvanishing 〈 v 〉 events are associated with large amplitude variations of the magnetic field at frequencies up to and exceeding the local Larmor frequency where MHD theory is not valid. Understanding collisionless plasma dynamics such as substorms in the plasma sheet requires the self-consistency that only kinetic theory can provide. Kinetic modeling is still undergoing continual development with many studies limited to one and two dimensions, but there is urgent need to improve these models as more and more data show kinetic physics is fundamentally important. Only then will we be able to make progress and obtain a correct picture of how collisionless plasmas work in space.     

Soft X-ray observations of helmet structures from SXT/Yohkoh

We present soft X-ray observations of helmet structures in solar active regions obtained from SXT/Yohkoh. These helmet structures are observed to form in the flare decay phase and to be associated with active region loop interactions. Their morphology is similar to the much larger scale helmet streamers that appear in the outer corona as shown in optical images of solar eclipse. The observed X-ray helmet structures appear to be in quasi-equilibrium with lifetimes greater than the MHD time scale. Using the filter ratio method for the X-ray observations, we find that the cusp region has lower temperature and higher density than that in the stalk region above it. The plasma pressure in the cusp region is about the same or slightly higher than that in the stalk region.     

Chromospheric and coronal heating mechanisms II

We review the mechanisms which are thought to provide steady heating of chromospheres and coronae. It appears now fairly well established that nonmagnetic chromospheric regions of latetype stars are heated by shock dissipation of acoustic waves which are generated in the stellar surface convection zones. In the case of late-type giants there is additional heating by shocks from pulsational waves. For slowly rotating stars, which have weak or no magnetic fields, these two are the dominant chromospheric heating mechanisms.Except for F-stars, the chromospheric heating of rapidly rotating late-type stars is dominated by magnetic heating either through MHD wave dissipation (AC mechanisms) or through magnetic field dissipation (DC mechanisms). The MHD wave and magnetic field energy comes from fluid motions in the stellar convection zones. Waves are also generated by reconnective events at chromospheric and coronal heights. The high-frequency part of the motion spectrum leads to AC heating, the low frequency part to DC heating. The coronae are almost exclusively heated by magnetic mechanisms. It is not possible to say at the moment whether AC or DC mechanisms are dominant, although presently the DC mechanisms (e.g., nanoflares) appear to be the more important. Only a more detailed study of the formation of and the dissipation in small-scale structures can answer this question.The X-ray emission in early-type stars shows the presence of coronal structures which are very different from those in late-type stars. This emission apparently arises in the hot post-shock regions of gas blobs which are accelerated in the stellar wind by the intense radiation field of these stars.     

A Comparative Study of the Influence of the Active Young Sun on the Early Atmospheres of Earth, Venus, and Mars

Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation, heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by IR-radiating molecules like CO₂, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen and high Jeans’ escape rates for heavier species like H₂, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO₂, N₂ and H₂O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount of ∼250 bar of O⁺ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial ocean (TO (1 TO ∼1.39×10²⁴ g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and compare our findings with the results of preceding studies.     

Coronal heating by dissipation of magnetic structure

Coronal loops are heated by the release of stored magnetic energy and by the dissipation of MHD waves. Both of these processes rely on the presence of internal structure in the loop. Tangled or sheared fields dissipate wave energy more efficiently than smooth fields. Also, a highly structured field contains a large reservoir of free magnetic energy which can be released in small reconnection events (microflares and nanoflares). The typical amount of internal structure in a loop depends on the balance between input at the photosphere and dissipation. This paper describes measures of magnetic structure, how these measures relate to the magnetic energy, and how photospheric motions affect the structure of a loop.The magnetic energy released during a reconnection event. can be estimated if one knows the equilibrium energy before and after the event. For a loop with highly tangled field lines, a direct solution of the equilibrium equations may be difficult. However, lower bounds can be placed on the energy of the equilibrium field, given a measure of the tangling known as the crossing number. These bounds lead to an estimate of the buildup of energy in a coronal loop caused by random photospheric motions. Parker's topological dissipation model can plausibly supply the 10⁷ erg cm^–2 s^–1 needed to heat the active region corona. The heating rate can be greatly enhanced by fragmentation of flux tubes, for example by the breakup of photospheric footpoints and the formation of new footpoints.     

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.     

Simulation of temperature distribution and discharge crater of SiCp/Al composites in a single-pulsed arc discharge

SiC_p/Al composites are difficult-to-cut materials. In recent years, electrical arc discharge machining has been developed to improve the machinability of these materials. However, there is a big challenge to build a satisfactory heat transfer model of SiC_p/Al composites in the arc machining. This is not only because of the material property difference between the reinforcement and matrix material but also because of the micro-dimension SiC reinforcements. This paper established a new heat conduction simulation model considering the SiC particle-Al matrix interface and the phase change effects in a single-pulsed arc discharge of SiC_p/Al composites. A novel SiC particle-Al matrix cell geometric model was designed firstly. Then, the temperature distribution at a different depth from the workpiece surface was analyzed, the influence of sic volume fraction on temperature field was studied, and the contribution of the interface thermal resistance and latent heat were explained. To demonstrate the validity of the new numerical model, comparisons and verifications were employed. Finally, the method of improving the model was proposed and the machining mechanism of arc discharge of SiC_p/Al matrix materials was discussed. It was found that high temperature is prone to concentrate on the surface layers of the workpiece especially when the SiC fraction is high, also, the temperature fluctuates respectively at the evaporation point of aluminum and SiC, and the SiC-Al resistance has less influence on temperature distribution compared to latent heat, etc. The model build in this work improves the simulation accuracy observably compared to the previous model, and the simulation work will help to acquire a detailed mechanism of material removal of SiC_p/Al composites in the arc discharge machining.