Numerical Study of EUV Wave Phenomenon on 2009 February 13

Combining the observations of STEREO satellites with the method of three-dimensional magnetohydrodynamic (MHD) numerical simulation, adopt- ing the magnetic ?eld data of the Wilcox Solar Observatory (WSO) and the model of potential ?eld source surface to build up the initial magnetic ?eld in solar corona, and adding a time-varying disturbance of pressure to the active re- gion on the solar surface, the study on the event of coronal mass ejection (CME) and extreme-ultraviolet (EUV) wave happened at 05:35 UT of 2009 February 13 has been performed. It is judged from the images of COR1/STEREO-A that the front speed of this CME is about 350 km·s⁻¹, and the angular width is about 60°. By analyzing the running difference images of EUVI/STEREO-B at 195 ?A, it is found that the bright toroidal wavefront is spreading toward all directions around the active region, and behind the bright toroidal wavefront is a coronal dimming area. The positions of the wavefront in four directions are taken to perform linear ?ttings, it is known that the EUV wave speed is 247 km·s⁻¹, and the EUV wave speed obtained from the numerical simulation is 245 km·s⁻¹. After the IDL visualization program has been carried out for the calculated result, the structures of the bright loop and dimming area can be seen clearly. The numerical simulation is consistent with the satellite observation, which shows that the observed EUV wave may belong to the fast magnetosonic wave.     

Modeling a Maunder minimum

We introduce on/off intermittency into a mean field dynamo model by imposing stochastic fluctuations in either the alpha effect or through the inclusion of a fluctuating electromotive force. Sufficiently strong small scale fluctuations with time scales of the order of 0.3–3 years can produce long term variations in the system on time scales of the order of hundreds of years. However, global suppression of magnetic activity in both hemispheres at once was not observed. The variation of the magnetic field does not resemble that of the sunspot number, but is more reminiscent of the ¹⁰Be record. The interpretation of our results focuses attention on the connection between the level of magnetic activity and the sunspot number, an issue that must be elucidated if long term solar effects are to be well understood. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Discovery of drifting high-frequency quasi-periodic oscillations in global simulations of magnetic boundary layers

We report on the numerical discovery of quasi-periodic oscillations (QPOs) associated with accretion through a non-axisymmetric magnetic boundary layer in the unstable regime, when two ordered equatorial streams form and rotate synchronously at approximately the angular velocity of the inner disc. The streams hit the star's surface producing hotspots. Rotation of the spots leads to high-frequency QPOs. We performed a number of simulation runs for different magnetospheric sizes from small to tiny, and observed a definite correlation between the inner disc radius and the QPO frequency: the frequency is higher when the magnetosphere is smaller. In the stable regime, a small magnetosphere forms and accretion through the usual funnel streams is observed, and the frequency of the star is expected to dominate the light curve. We performed exploratory investigations of the case in which the magnetosphere becomes negligibly small and the disc interacts with the star through an equatorial belt. We also performed investigation of somewhat larger magnetospheres where one or two ordered tongues may dominate over other chaotic tongues. In application to millisecond pulsars, we obtain QPO frequencies in the range of 350–990 Hz for one spot. The frequency associated with rotation of one spot may dominate if spots are not identical or antipodal. If the spots are similar and antipodal, then the frequencies are twice as high. We show that variation of the accretion rate leads to drift of the QPO peak.     

Recent Progress of Hemispheric Coupling of Solar Activity Cycle

The hemispheric coupling phenomenon of solar activity cycle was discovered as early as the mid-20th century, and it is one of the most common topic in the long-term spatio-temporal evolution of the Sun, while the observational features and physical mechanism of hemispheric coupling have not been completely understood. The theoretical model of solar magnetohydrodynamics driven by this phenomenon is helpful in studying the basic information of the spatio-temporal evolution of solar activity cycle, and is also of great value to the short- and medium-term forecast of solar activity as well as the space weather. Here, we first give the discovery and observational history of the hemispheric coupling of solar activity. And then, the basic observational features of hemispheric coupling at different spatio-temporal scales and their possible mechanism are summarized. Finally, we give a discussion about the important unresolved issues and development trends in this important field.     

Implications of mean field accretion disc theory for vorticity and magnetic field growth

In addition to the scalar Shakura–Sunyaev α _ss turbulent viscosity transport term used in simple analytic accretion disc modelling, a pseudo-scalar transport term also arises. The essence of this term can be captured even in simple models for which vertical averaging is interpreted as integration over a half-thickness and each hemisphere is separately studied. The additional term highlights a complementarity between mean field magnetic dynamo theory and accretion disc theory treated as a mean field theory. Such pseudo-scalar terms have been studied, and can lead to large-scale magnetic field and vorticity growth. Here it is shown that vorticity can grow even in the simplest azimuthal and half-height integrated disc model, for which mean quantities depend only on radius. The simplest vorticity growth solutions seem to have scales and vortex survival times consistent with those required for facilitating planet formation. In addition, it is shown that, when the magnetic back-reaction is included to lowest order, the pseudo-scalar driving the magnetic field growth and that driving the vorticity growth will behave differently with respect to shearing and non-shearing flows: the former pseudo‐scalar can more easily reverse sign in the two cases.     

Protostellar evolution during time-dependent anisotropic collapse

The formation and collapse of a protostar involves the simultaneous infall and outflow of material in the presence of magnetic fields, self-gravity and rotation. We use self-similar techniques to self-consistently model the anisotropic collapse and outflow by using a set of angle-separated self-similar equations. The outflow is quite strong in our model, with the velocity increasing in proportion to radius, and material formally escaping to infinity in the finite time is required for the central singularity to develop.
Analytically tractable collapse models have been limited mainly to spherically symmetric collapse, with neither magnetic field nor rotation. Other analyses usually employ extensive numerical simulations, or either perturbative or quasistatic techniques. Our model is unique as an exact solution to the non-stationary equations of self-gravitating magnetohydrodynamics (MHD), which features co-existing regions of infall and outflow.
The velocity and magnetic topology of our model is quadrupolar, although dipolar solutions may also exist. We provide a qualitative model for the origin and subsequent evolution of such a state. However, a central singularity forms at late times, and we expect the late-time behaviour to be dominated by the singularity, rather than depend on the details of its initial state. Our solution may, therefore, have the character of an attractor among a much more general class of self-similarity.     

Simulations of mixed-morphology supernova remnants with anisotropic thermal conduction

We explore the role of anisotropic thermal conduction on the evolution of supernova remnants (SNRs) through interstellar media with a range of densities via numerical simulations. We find that a remnant expanding in a dense environment can produce centre-bright hard X-ray emission within 20 kyr, and centre-bright soft X-ray emission within 60 kyr of the supernova event. In a more tenuous environment, the appearance of a centre-bright structure in hard X-rays is delayed until about 60 kyr. The soft X-ray emission from such a remnant may not become centre bright during its observable lifetime. This can explain the observations that show that mixed-morphology SNRs preferentially occur close to denser, molecular environments. Remnants expanding into denser environments tend to be smaller, making it easier for thermal conduction to make large changes in the temperatures of their hot gas bubbles. We show that the lower temperatures make it very favourable to use high-stage ions as diagnostics of the hot gas bubbles in SNRs. In particular, the distribution of O  viii transitions from shell bright at early epochs to centre bright at later epochs in the evolution of an SNR expanding in a dense interstellar medium when the physics of thermal conduction is included.     

电场驱动的日冕数值模拟中时变边界条件研究

时变边界条件的处理对于数据驱动的日冕太阳风建模研究十分重要.本文基于特征边界条件,将松弛型无反射边界处理应用到三维磁流体力学(Magnetohydrodynamics,MHD)方程中,发展出一种自洽的、以切向电场作为数据驱动的时变边界条件,并将其应用于时变日冕太阳风的模拟中.为验证该时变边界条件的有效性,本文连续模拟研究了2187—2189三个卡林顿周的日冕太阳风结构.通过将模拟结果与SOHO、STEREO等卫星的观测结果进行对比研究,发现基于该时变边界条件的日冕太阳风的模拟可以较好地反映出日冕太阳风的大尺度结构及其演化过程,且模拟得到的太阳风参数与卫星观测结果较为吻合.说明本文所发展的时变边界条件可以有效地模拟数据驱动的时变日冕太阳风.

     

行星际速度增幅扰动的演化

采用二维理想MHD模型，分别在日球赤道面（二维二分量模型）和日球子午面（二维 三分量模型）内研究太阳风中纯速度增幅扰动的演化. 结果表明，该扰动在向外传播的过程 中逐渐演化为双重激波对，即由4个激波组成的激波系统. 该4个激波按离太阳由近及远依次 为后向快激波、后向慢激波、前向慢激波和前向快激波. 双重激波对在子午面内相对扰动源 中心法线基本对称，而在赤道面内则不对称：扰动源中心法线西侧双重激波对结构更为明显 ，所跨经度范围宽于东侧. 初步分析表明，行星际磁场的螺旋结构是产生日球赤道面内双重 激波对结构东西不对称性的主要原因.     

不考虑磁层顶磁重联的全球三维MHD模型

本文介绍了一个新的磁层全球三维MHD模型,该模型可通过把IMF和地球磁场分开处理的方法“关闭”磁层顶的磁重联,从而可直观地显示不同IMF条件下磁层顶的IMF与地磁场的反平行区域,即磁重联最可能发生的地方,结果表示,IMFBx分量对磁层顶磁重联有重要影响。由于可关闭磁重联,该模型还可有效地研究有无磁重联时,太阳风对磁层位形（如晨-昏不对称性）、粒子输运等重要问题的影响,有助于揭示磁层物理现象的基本特性。