行星际磁通量绳尺度的幂律谱分布

利用最小二乘法拟合了1995年1月至2001年9月Wind卫星观测到的行星际磁通量绳。根据拟合所得磁通量绳的直径,分析了行星际磁通量绳在这段时间内的发生率随磁通量绳直径D变化的关系,发现磁通量绳的发生率P(D)随直径D的变化可近似以幂律形式表示为:P(D)≈64D-0.768。行星际磁通量绳的发生率相对其直径的幂律分布表明所有行星际磁通量绳很可能是同一类现象且有共同的源,即它们都是太阳上日冕物质抛射的行星际对应物,只不过小尺度的磁通量绳对应较小的日冕物质抛射。最后,对行星际磁通量绳、日冕物质抛射和太阳耀斑的可能关系做了讨论。     

中尺度磁绳结构的行星际观测

由磁绳结构主导、平均尺度约二、三十个小时的行星际磁云是日冕物质抛射在行星际膨胀、传播的体现。最近，Moldwin等人报道在太阳风中还观测到一些尺度在几十分钟的小尺度磁绳结构，并认为太阳风中的磁绳结构在尺度分布上可能具有双峰特征，在全面检视了WIND卫星(1995年-2000年)和ACE卫星(1998年-2000年)的观测资料后，发现了在行星际太阳风中一些尺度为几个小时的中尺度磁绳结构，利用初步整理的其中28个中尺度磁绳结构事件，认为太阳风中的磁绳结构在尺度分布上可能是连续的，这对行星际太阳风中磁绳结构物理起源的研究可能提出重要的物理限制。     

通过建立自然坐标系确定磁云边界方法的探讨

通常卫星探测到的关于行星际空间磁场的数据都是在GSE坐标系(地心太阳黄道坐标系)里表示出来的．磁云是行星际日冕物质抛射事件的一个重要子集,它们的边界认证一直是磁云研究中感兴趣的问题之一．通过讨论坐标系转换确定磁云边界的方法,由磁云的磁通量管特征建立磁云自然坐标系,然后把行星际空间的磁场转换到磁云自然坐标系里．在磁云自然坐标系里磁云作为一个磁通量管的结构能够清晰地显示出来．结合磁云的等离子体特征就可以比较容易地把磁云和背景太阳风分辨出来,即确定了磁云的边界．     

基于太阳爆发灾变模型的数值实验

在Isenberg等人发展的灾变模型基础上根据接近真实的日冕环境,通过数值实验,对磁通量绳的平衡高度对光球磁场变化的响应开展了研究.利用NIRVANA程序进行了计算.日冕的等离子密度分布采用了一个半经验的模型,模拟中包含了物理耗散.考察了:磁通量绳的平衡位置及其演化特征;参考半径的变化对磁通量绳平衡位置的影响;磁通量绳内部平衡的性质以及在磁通量绳失去平衡之后一段时间内的动力学与运动学特征.结果表明:数值实验中得到的磁通量绳的平衡态位置与Isenberg等人的理论结果有微小的偏离,但是演化特征基本一致,在临界点处系统迅速失去平衡,向爆发态演化;参考半径的变化对磁通量绳平衡位置的影响与灾变模型给出的结果基本一致;磁通量绳在随着宏观磁结构演化的同时,还通过自身的调节达到内部平衡,当磁通量绳的内部和外部平衡都实现之后,系统整体也就达到了平衡状态;在爆发态下,磁通量绳的运动特征与Lin-Forbes模型和观测给出的结果一致,并且在通量绳的前方有快模激波出现;由于数值实验中包括了耗散,爆发过程中的磁能向其他形式能量的转换非常明显.     

日冕物质抛射的理想MHD模型研究

概括了日冕物质抛射的一些观测结果和它们与其它太阳活动现象的相关性。简要回顾了较早期日冕物质抛射的理论研究,着重介绍了最近研究得较多的理论机制,即能量储存机制,以及其中的磁通量绳突变模型与其它理论模型的MHD数值和解析研究以及相应的重要应用．     

对地日冕物质抛射研究

日冕物质抛射，作为太阳大气中频繁发生的极为壮观的活动现象，越来越受到太阳物理学家的关注。其中一类特殊的抛射事件－－对地日冕物质抛射，通常与大的地磁暴、行星际激波和高能粒子事件相伴生，具有强烈的地球物理效应，是影响空间天气的主要因素之一。概括了对地日冕物质抛射的研究现状，重点介绍了与对土日冕物质抛射事件相联系的光球向量磁场演化的观测研究成果，并由典型事件探讨了暗条爆发、耀五等剧烈太阳活动和对地日冕物质抛射之间的密切关系，提出了尚待解决的主要问题和进一步的研究方向。     

磁中子星巨耀发的观测和理论研究

软γ重复暴(soft gamma-ray repeater, SGR)被认为产生于磁中子星。发生在SGR上的巨耀发在短时标内释放出大于10^39J的巨大能量,被认为是宇宙中已知最强的能量释放过程之一,其剧烈程度仅次于超新星爆发和γ暴。详细介绍了几种磁球层理论模型,并重点介绍了以太阳爆发日冕物质抛射灾变理论为基础建立的磁中子星巨耀发的磁流体力学的半解析模型。在模型中,板块的转动或错位造成磁球层内磁场的扭缠,从而导致磁通量绳的形成和磁能缓慢的积累。当积累的能量超过阈值,系统就会失去平衡,然后产生爆发并释放能量。用该模型计算的SGR 1806-20, SGR 0526-66和SGR 1900+14这3次巨耀发的光变曲线都与观测基本相符。此外,有关磁中子星巨耀发的磁流体动力学的数值模拟工作也得到了全面的展开,通过求解各种初始条件和边界条件下的磁流体力学方程组,计算机的数值模拟可以得到磁中子星巨耀发过程中的磁场形态演化和内部精细结构。     

太阳光球磁亮点的基本特征研究及其对日冕加热的贡献

在太阳光球表面出现的磁亮点是目前观测手段能够分辨的最小磁结构,也被认为是日冕中的磁绳在光球足点运动的可靠示踪者。磁亮点的尺度约为100～300 km,寿命从几分钟到几十分钟。磁亮点被观测到不仅具有漩涡运动现象,还有很强的振荡现象。磁亮点是在磁通量管的对流坍缩过程中形成的,这已被观测和数值模拟所验证;磁亮点的运动导致其所在的磁通量管产生振荡,或者与其他磁通量管发生扭绞。理论上认为,这些振荡会以波的形式向色球和日冕传送能量,而磁通量管之间的扭绞会在色球和日冕中发生磁重联并释放能量,从而加热色球和日冕。为了解开日冕加热和色球加热等未解之谜,对磁亮点的研究显示出它特殊的重要性。对磁亮点的基本特征、形成原理、观测证据、光球磁亮点和太阳大气其他亮点之间的关系,以及磁亮点对日冕加热贡献等方面进行了介绍和讨论。     

磁云传播的动力学演化过程研究进展

磁云因其独特的磁场结构经常是重大灾害性空间天气的驱动源. 近来从磁云的边界层结构、环向通量、大尺度结构等方面关于磁云传播的动力学演化过程的研究取得了一些进展. 在磁云边界存在一个由于磁场重联而形成的边界层结构. 在磁云传播过程中, 这种发生在边界处的磁场重联可能会把磁云的磁场剥蚀掉, 进而引起其磁通量绳结构环向通量的减少以及不对称. 在磁云内部, 经常会观测到多个子通量绳结构. 这些特性各异的子通量绳可以通过磁场重联而合并, 进而引起磁云磁结构的改变. 关于磁云大尺度磁场拓扑位形的演化机制, 除了较早提出的交换重联外, 目前的研究表明在行星际空间中, 磁云边界处的重联过程也可以将磁云闭合或半开放的磁场线打开或断开. 尽管在相关研究中已经取得了较大进展, 但关于磁云传播的动力学演化过程还有许多问题尚不清楚. 在行星际小尺度磁通量绳边界也发现了边界层结构, 那么磁云是否会因剥蚀而成为小尺度通量绳? 磁云内子通量绳结构在相互作用中会不会引起某些不稳定性而导致整个通量绳系统的崩溃? 这些问题的解决还有待于进一步的理论、观测和数值模拟研究.     

磁云传播的动力学演化过程研究进展

磁云因其独特的磁场结构经常是重大灾害性空间天气的驱动源.近来从磁云的边界层结构、环向通量、大尺度结构等方面关于磁云传播的动力学演化过程的研究取得了一些进展.在磁云边界存在一个由于磁场重联而形成的边界层结构.在磁云传播过程中,这种发生在边界处的磁场重联可能会把磁云的磁场剥蚀掉,进而引起其磁通量绳结构环向通量的减少以及不对称.在磁云内部,经常会观测到多个子通量绳结构.这些特性各异的子通量绳可以通过磁场重联而合并,进而引起磁云磁结构的改变.关于磁云大尺度磁场拓扑位形的演化机制,除了较早提出的交换重联外,目前的研究表明在行星际空间中,磁云边界处的重联过程也可以将磁云闭合或半开放的磁场线打开或断开.尽管在相关研究中已经取得了较大进展,但关于磁云传播的动力学演化过程还有许多问题尚不清楚.在行星际小尺度磁通量绳边界也发现了边界层结构,那么磁云是否会因剥蚀而成为小尺度通量绳?磁云内子通量绳结构在相互作用中会不会引起某些不稳定性而导致整个通量绳系统的崩溃?这些问题的解决还有待于进一步的理论、观测和数值模拟研究.     

Are CME-Related Dimmings Always a Simple Signature of Interplanetary Magnetic Cloud Footpoints?

Coronal dimmings are often present on both sides of erupting magnetic configurations. It has been suggested that dimmings mark the location of the footpoints of ejected flux ropes and, thus, their magnetic flux can be used as a proxy for the flux involved in the ejection. If so, this quantity can be compared to the flux in the associated interplanetary magnetic cloud to find clues about the origin of the ejected flux rope. In the context of this physical interpretation, we analyze the event, flare, and coronal mass ejection (CME) that occurred in active region 10486 on 28 October 2003. The CME on this day is associated with large-scale dimmings, located on either side of the main flaring region. We combine SOHO/Extreme Ultraviolet Imaging Telescope data and Michelson Doppler Imager magnetic maps to identify and measure the flux in the dimming regions. We model the associated cloud and compute its magnetic flux using in situ observations from the Magnetometer Instrument and the Solar Wind Electron Proton Alpha Monitor aboard the Advance Composition Explorer. We find that the magnetic fluxes of the dimmings and magnetic cloud are incompatible, in contrast to what has been found in previous studies. We conclude that, in certain cases, especially in large-scale events and eruptions that occur in regions that are not isolated from other flux concentrations, the interpretation of dimmings requires a deeper analysis of the global magnetic configuration, since at least a fraction of the dimmed regions is formed by reconnection between the erupting field and the surrounding magnetic structures.     

Progressive Transformation of a Flux Rope to an ICME

The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by interplanetary coronal mass ejections (ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic flux and helicity, which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled by neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 – 10 November 2004. This MC was embedded in an ICME. After determining an approximate orientation for the flux rope using the minimum variance method, we obtain a precise orientation of the cloud axis by relating its front and rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic flux between the inbound and outbound branches and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). The MC is also studied using dynamic models with isotropic expansion. We have found (6.2±1.5)×10²⁰ Mx for the axial flux and (78±18)×10²⁰ Mx for the azimuthal flux. Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed by an extended coherent magnetic region. These observations are interpreted by considering the existence of a previously larger flux rope, which partially reconnected with its environment in the front. We estimate that the reconnection process started close to the Sun. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed to the observed ICME (with a remnant flux rope in the front part).     

Coronal Magnetic Flux Rope Equilibria and Magnetic Helicity

1 INTRODUCTIONObservations show that the magnetic helicity of solar magnetic structures has a predominantsign in each hemisphere of the Sun, positive in the southern hemisphere and negative in thenorthern, regardless of the solar cycle (Rust, 1994). The magnetic helicity is strictly conservedin the frame of ideal MHD (WOltjer, 1958), and approximately conserved in the presence ofresistive dissipation and magnetic reconnection in a highly conductive plajsma (Taylor, 1974;Berger, 1984; H…     

Soft X-ray Solar Activities Associated with Interplanetary Magnetic Flux Ropes

We studied the soft X-ray solar events that could be associated with the interplanetary magnetic flux ropes observed by the WIND satellite during 1995 through 1998. The timings of the launches of the magnetic flux ropes from the Sun were estimated using flux rope speeds derived by the fitting of a cylindrical model. In the reasonable time window, soft X-ray solar signatures were found in approximately 70% of the flux ropes. Parameters (e.g., axis direction, strength of magnetic field, radius, and helicity) of the magnetic flux ropes obtained by the model fitting were compared with the characteristics of the corresponding soft X-ray events observed by Yohkoh. According to the result of the comparison, the magnetic flux ropes with strong magnetic fields or high speeds were observed in association with higher soft X-ray solar activities.     

Solar filaments as tracers of subsurface processes

Solar filaments are discussed in terms of two contrasting paradigms. The standard paradigm is that filaments are formed by condensation of coronal plasma into magnetic fields that are twisted or dimpled as a consequence of motions of the fields’ sources in the photosphere. According to a new paradigm, filaments form in rising, twisted flux ropes and are a necessary intermediate stage in the transfer to interplanetary space of dynamo-generated magnetic flux. It is argued that the accumulation of magnetic helicity in filaments and their coronal surroundings leads to filament eruptions and coronal mass ejections. These ejections relieve the Sun of the flux generated by the dynamo and make way for the flux of the next cycle.     

Are There Different Populations of Flux Ropes in the Solar Wind?

Flux ropes are twisted magnetic structures that can be detected by in-situ measurements in the solar wind. However, different properties of detected flux ropes suggest different types of flux-rope populations. As such, are there different populations of flux ropes? The answer is positive and is the result of the analysis of four lists of flux ropes, including magnetic clouds (MCs), observed at 1 AU. The in-situ data for the four lists were fitted with the same cylindrical force-free field model, which provides an estimate of the local flux-rope parameters such as its radius and orientation. Since the flux-rope distributions have a broad dynamic range, we went beyond a simple histogram analysis by developing a partition technique that uniformly distributes the statistical fluctuations across the radius range. By doing so, we found that small flux ropes with radius R<0.1 AU have a steep power-law distribution in contrast to the larger flux ropes (identified as MCs), which have a Gaussian-like distribution. Next, from four CME catalogs, we estimated the expected flux-rope frequency per year at 1 AU. We found that the predicted numbers are similar to the frequencies of MCs observed in-situ. However, we also found that small flux ropes are at least ten times too abundant to correspond to CMEs, even to narrow ones. Investigating the different possible scenarios for the origin of these small flux ropes, we conclude that these twisted structures can be formed by blowout jets in the low corona or in coronal streamers.     

Coronal Flux Rope Equilibria in Closed Magnetic Fields

Using a 2.5-dimensional ideal MHD model in Cartesian coordinates,we investigate the equilibrium properties of coronal magnetic flux ropes in background magnetic fields that are completely closed.The background fields are produced by a dipole,a quadrupole,and an octapole,respectively,located below the photosphere at the same depth.A magnetic flux rope is then launched from below the photo-sphere,and its magnetic properties,i.e,the annular magnetic fluxφp and the axial magnetic fluxφz,are controlled by a single emergence parameter.The whole sys-tem eventually evolves into equilibrium,and the resultant flux rope is characterized by three geometrical parameters:the height of the rope axis,the half-width of the rope,and the length of the vertical current sheet below the rope.It is found that the geometrical parameters increase monotonically and continuously with increasing φp and φz:no catastrophe occurs.Moreover,there exists a steep segment in the profiles of the geometrical parameters versus either φp or φz,and the faster the background field decays with height,the larger both the gradient and the growth amplitude within the steep segment will be.     

Comparison study of magnetic flux ropes in the ionospheres of Venus, Mars and Titan

Magnetic flux ropes are created in the ionosphere of Venus and Mars during the interaction of the solar wind with their ionospheres and also at Titan during the interaction of the Saturnian magnetospheric plasma flow with Titan’s ionosphere. The flux ropes at Venus and Mars were extensively studied from Pioneer Venus Orbiter and Mars Global Surveyor observations respectively during solar maximum. Based on the statistical properties of the observed flux ropes at Venus and Mars, the formation of a flux rope in the ionosphere is thought first to arise near the boundary between the magnetic barrier and the ionosphere and later to sink into the lower ionosphere. Venus flux ropes are also observed during solar minimum by Venus Express and the observations of developing and mature flux ropes are consistent with the proposed mechanism. With the knowledge of flux rope structure in the Venus ionosphere, the twisted fields in the lower ionosphere of Titan from Cassini observations are studied and are found to resemble the Venus flux ropes.