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1.
汪景琇 《中国天文和天体物理学报》1992,(1)
在以无力磁场的非势特征近似描述磁剪切位形时,无力因子α即可做为磁剪切的量度;而磁剪切的发展则为方程所解析描述.这里G=B/t×B,可称为磁剪切产生函数.该方程表明,磁剪切产生于局地的发电机作用,是磁场与磁流体运动相互作用的结果.相反极性磁结构的挤压,磁通量的浮现与下沉,都可以和剪切运动一样,有效地导致磁剪切的发展. 相似文献
2.
探讨了复杂磁结构上空日冕物现状与磁剪切的关系。结果表明在强磁场的磁中性线上方磁剪切会引起具有强电流和较强等离子体压力的低磁弧。这可解释Ｙｏｈｋｏｈ卫星的观测结果。 相似文献
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通过对活动区ＮＯＡＡ６８９１中三个Ｘ线耀斑前后的向量磁场分析,研究耀斑发生条件与耀斑发生前后横向磁场和磁剪切变化的关系。我们发现与Ｈａｇｙａｒｄ的耀斑发生条件不同的是,强的横向磁场和磁剪切不是活动区中耀斑发生的充要条件。我们的结果表明,活动区ＮＯＡＡ６８９１ １９９１年３０日的耀斑发生在横向磁场和磁剪切剧烈下降后。尽管１０月２７日的耀斑发生后横向磁场和磁剪切变化很大,但由于有新磁流浮现,造成磁中性 相似文献
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通过对活动区NOAA 6891中三个X级耀斑前后的向量磁场分析 ,研究耀斑发生条件与耀斑发生前后横向磁场和磁剪切变化的关系。我们发现与Hagyard的耀斑发生条件不同的是 ,强的横向磁场和磁剪切不是活动区中耀斑发生的充要条件。我们的结果表明 ,活动区NOAA 6891 1 991年 30日的耀斑发生在横向磁场和磁剪切剧烈下降后。尽管 1 0月 2 7日的耀斑发生后横向磁场和磁剪切变化很大 ,但由于有新磁流浮现 ,造成磁中性线的改变 ,使得横向磁场和磁剪切变化与耀斑发生的联系变得比较复杂。 相似文献
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我们在文[1]里对磁弧剪切作了数值解,得到了剪切速度ω和磁场B_z的分析解,但对二维速度(u,v)的振幅占δ′/ζ仅有只依赖于时间的近似解。本文在密度为常数条件下得到了磁弧剪切在线性演化阶段的较精确的解析解,比较了密度为常数和密度重力分层两种情形下的数值解,证实当β(=气压/磁压)很小(量级为10~(-2))时两者差别不大,因此本文结果近似可用于密度不为常数的实际太阳大气中的磁弧剪切动力学过程。解析解的主要结果是导出振幅δ′/ζ的高度依赖关系:随着时间增加,振幅δ′/ζ随高度下降越来越慢。这导致磁弧顶越升越高而脚根基本上不朝外移动,这样闭合的磁弧将有可能逐渐变为开场。 相似文献
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采用二维三分量理想磁流体力学模型,研究光球磁对消引起的日珥扰动．日珥下方光球表面的磁对消将磁通量向日珥传输,引起日珥内部磁通量和磁螺度增加．日珥的状态与所积累的磁通量（或磁螺度）有关．数值结果显示,如果日珥磁通的相对增量δＦ（或相应的磁螺度相对增量δＨ）较小,日珥只略微上升和膨胀,并不离开光球;而对于较大的δＦ或δＨ）;日珥将脱离光球,悬浮在低层日冕中,在其下方形成垂直电流片． 相似文献
8.
对自引力磁均分和磁守恒两种模型的径向振荡稳定性的研究表明：在同时考虑自引力和磁场作用的情况下,吸积盘在磁均分和磁守恒模型中均存在着三种振荡模式,其中粘滞模式总是稳定的,磁声模式是不稳定的,中性模式在图中较长波段范围趋于稳定,在较短波段范围是不稳定的．这些结果有利于解释活动天体的Mark421和Pks2155—304的长周期光变现象．同时阐明了自引力在两种模型中对三种模式的影响有相同的趋势,而对磁场则分别起着相反的作用．这个结论表明磁均分吸积盘模型在解释光变现象时更为有利． 相似文献
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本文利用阳光卫星的软Ｘ 射线资料,日本国立天文台的Ｈα单色像和相对应的北京天文台怀柔站的活动区矢量磁场资料及美国基特峰天文台的全日面磁图,对发生在１９９４ 年５ 月１８ 日的软Ｘ 射线和Ｈα增亮事件进行了仔细的研究。研究表明增亮事件明显与新浮剪切磁流出现相关,Ｈα和软Ｘ 射线亮点的投影位置正好位于新浮小磁流环的三个足点之间,同时亮点也位于分界面与光球面的交线附近,故该增亮事件也许是太阳低层大气磁重联的观测证据 相似文献
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The finite element method was used to solve a partial differential equation (magnetostatic equation) for multipolar magnetic regions. It is found that the height of magnetic field lines above the magnetic neutral line of a central strong bipolar magnetic field decreases as the field lines' footpoints approach the neutral line and also with increased magnetic shear. Both the electric current density and plasma pressure in the sheared low-lying loops are high. We suggest that the sheared low-lying loops may store the energies of large coronal mass ejections (CMEs) and filament eruptions. In addition, it is found that a lower pressure area exists above the low-lying loops and that it is similar in morphology to a coronal cavity. Above the lower pressure area there is a higher pressure area, which may be the source of CMEs. In this area magnetic shear leads to magnetic reconnection, which may be the cause of high coronal temperature. 相似文献
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We study an active region coronal jet that evolved from southward of a major sunspot of NOAA AR12178 on 04 October 2014. This jet is associated with an onset of the GOES C1.4 flare. We use SDO/AIA, SDO/HMI, GONG \(H\upalpha\) and GOES data for analysing the observed event. We term this jet as a two-stage confined eruption of the plasma. In the first stage, some plasma erupts above the compact flaring region. In the second stage, this eruptive jet plasma and associated magnetic field lines interact with another set of distinct magnetic field lines present in its south-east direction. This creates an X-point region, where the second stage of the jet eruption is deflected above it on a curvilinear path into overlying corona. The lower part of the jet is followed by a cool surge eruption, which is visible only in \(H{\upalpha}\) emissions. The magnetic flux cancellation at the footpoint causes the triggering of C-class flare eruption. This flare energy release further triggers first stage of the coronal jet eruption. The second stage of the jet eruption is a consequence of an interaction of two distinct sets of magnetic field lines in the overlying corona. The first stage of the coronal jet and co-spatial but lagging cool surge may have common origin due to the reconnection generated heating pulses. This complex evolution of the coronal jet involves flare heating induced first stage plasma eruption, guiding of jet’s material above a junction of two distinct sets of field lines in the corona, and intra-relationship with cool surge. In effect, it imposes rigid constraints on the existing jet models. 相似文献
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本文对盔状冕流底部磁通量喷发的大气响应进行数值模拟。数值结果表明:在喷发磁场的作用下,原冕流中的物质将受到压缩并向上运动,形成高密度亮环结构,同时在环的前方出现快磁声波,在环的下方出现低密度暗区。据此我们认为,磁通量喷发可能是触发环形日冕物质喷射的一种重要机制。 相似文献
15.
G. W. Pneuman 《Solar physics》1980,65(2):369-385
The observed interrelationship between coronal transients and eruptive prominences is used as the basis for a theoretical MHD model of these events. The model begins with an equilibrium configuration consisting of a coronal loop or arcade with a filament lying underneath with its axis oriented perpendicular to the overlying field. The lifting of the filament from the solar surface produces an increase in magnetic pressure under the helmet which drives it outward. This increased pressure is associated with the internal field of the filament as well as the field beneath it. The underlying field could be that which produced the filament eruption or, alternatively, reconnected field lines formed by the inward collapse of the legs of the transient towards the neutral line beneath the rising prominence. We do not attempt to explain the filament eruption which may be due to internal forces in the prominence or, alternatively, from forces imposed from beneath as would be produced by emerging flux. In the latter case, the filament is passive and merely acts as a tracer for the more fundamental underlying process.It is shown that the outward force per unit mass produced by the driving magnetic field and the inward restoring forces in the overlying field due to magnetic tension and gravity all decrease with distance at the same rate - namely, as the inverse square of the distance from the solar center. Hence, the ratio of net outward to inward force is independent of radial distance from the Sun. A stability analysis shows that this situation is one of neutral stability.A mathematical model of this physical process is described in which the MHD equations in simplified form, neglecting gas pressure forces, are solved in time for the velocity, width, density, and magnetic field strength of the transient. The solutions show that the velocity increases sharply close to the Sun but quickly approaches a constant value. The width increases linearly with radial distance. Both of these results are in agreement with observations. An examination of the forces exerted on the legs of the transient shows that their motion should be horizontally inward.On leave from the High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colo., U.S.A. 相似文献
16.
Catastrophe of coronal magnetic rope embedded in a partly open multipolar background magnetic field is studied by using a 2-dimensional, 3-component ideal MHD model in spherical coordinates. The background field is composed of three closed bipolar fields of a coronal streamer and an open field with an equatorial current sheet. The magnetic rope lies below the central bipolar field, and it is characterized by its annular and axial magnetic fluxes. For a given annual flux, there is a critical value of the axial flux, and for a given axial flux, there is a critical value of annual flux such that, below the critical value, the magnetic rope is attached to the solar surface and the system stays in equilibrium, but when the critical value is exceeded, the magnetic rope breaks free and erupts upward. This implies that catastrophe can occur in a coronal magnetic rope embedded in a partly open multipolar background magnetic field. Our computation gives a threshold value of magnetic energy that is about 15% greater than the energy of the partly open magnetic field (the central bipolar field open and the fields on either side closed). The excess energy may serve as source for solar explosions such as coronal mass ejections. 相似文献
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A numerical simulation is performed to investigate the prominence formation in a magnetic arcade by photospheric shearing motions. A two-and-a-half-dimensional magnetohydrodynamic (MHD) code is used, in which the gravitational force, radiative cooling, thermal conduction and a simplified form of coronal heating are included. It is found that a footpoint shear induces an expansion of the magnetic arcade and cooling of the plasma in it. Simultaneously the denser material from the lower part of the arcade is pulled up by the expanding field lines. A local enhancement of radiative cooling is thus effected, which leads to the onset of thermal instability and the condensation of coronal plasma. The condensed material grows vertically to form a sheet-like structure making dips on field lines, leading to the formation of the Kippenhahn- Schlüter type prominence. The mass of the prominence is found to be supplied not only by the condensation of the material in the vicinity but also by the siphon-type upflows. The upward growth of the vertical sheet-structure of the prominence is saturated at a certain stage and the newly condensed material is found to slide down from above the prominence along magnetic field lines. This drainage of material leads to the formation of an arc-shaped cavity of low density and low pressure around the prominence. The problem of force and heat balance is addressed and the prominence is found to be not in a static equilibrium but in a dynamic interaction with its environment. 相似文献
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Photospheric motion shears or twists solar magnetic fields to increase magnetic energy in the corona, because this process may change a current-free state of a coronal field to force-free states which carry electric current. This paper analyzes both linear and nonlinear two-dimensional force-free magnetic field models and derives relations of magnetic energy buildup with photospheric velocity field. When realistic data of solar magnetic field (B
0 103 G) and photospheric velocity field (v
max 1 km s–1) are used, it is found that 3–4 hours are needed to create an amount of free magnetic energy which is of the order of the current-free field energy. Furthermore, the paper studies situations in which finite magnetic diffusivities in photospheric plasma are introduced. The shearing motion increases coronal magnetic energy, while the photospheric diffusion reduces the energy. The variation of magnetic energy in the coronal region, then, depends on which process dominates. 相似文献
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W. J. Wagner 《Solar physics》1988,115(1):81-92
We have studied chromospheric mass injection into an overlying coronal dipole magnetic field using a 2-D ideal magnetohydrodynamic (MHD) numerical model. The results indicate that such injection can produce magnetic field deformation conducive to active region prominences - namely, Kippenhahn-Schlüter (K-S) type configurations for stable support of injected plasma. We show the optimum conditions for such dynamical formation of K-S-type field configurations.Observations show that an active region prominence formation is preceded by the accumulation of absorptive strands above a neutral line. We hypothesize that an absorptive strand is formed by a chromospheric asymmetric mass injection into the overlying coronal magnetic field and that a necessary condition for the accumulation of the strands is that the mass injection forms a K-S-type field configuration. The results of our numerical simulation of the injection dynamics support our hypothesis. To form a K-S-type magnetic field configuration, we find that a narrow range of injection density, velocity, and magnetic field strength must be used; spicule-like, asymmetric mass injection seems favorable.The limited parameter range that exists for the formation of K-S-type magnetic field configurations in asymmetric injections may explain why active region prominences do not form everywhere on every neutral line. 相似文献