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arXiv 1402.4703astro-ph.SR
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活动区尺度通量浮现的数值模拟:从斑点形成到衰变

Numerical simulations of active region scale flux emergence: From spot formation to decay

M. Rempel, M. C. M Cheung

摘要:

我们提出了数值模拟的活动区域尺度通量出现的时间跨度长达6天。通量出现是由底部边界条件驱动的,该边界条件将具有$1.7 \times 10^{22}$ Mx通量的磁场的半环面平流到计算域中。模拟结果表明,即使在没有扭曲的情况下,磁通量也能够通过对流区上部的$15.5$ Mm上升,并进入光球层形成光斑。我们发现,斑点的形成是敏感的持续性的上升流在底部边界的脚点,即持续的上升流将防止斑点的形成。此外,环向流的存在(在通过对流区的通量绳上升期间,根据角动量守恒,预计这种流进入逆行方向)导致这对斑点之间的显着不对称性,与太阳上的领先斑点相对应的斑点更具轴对称性和相干性,但也相对于后续斑点延迟形成。斑点形成相变直接转变为衰减阶段。次表层的流动使磁场破碎,并导致光球层下几乎没有磁场的等离子体侵入。当这些侵入物到达光球层时,光斑就会碎裂。斑点衰减的时间尺度与模拟域中存在的最长对流时间尺度相当。我们发现,从一个模拟点在衰减阶段的前两天的通量的分散是一致的湍流扩散的自相似衰变。

英文摘要:

We present numerical simulations of active region scale flux emergence covering a time span of up to 6 days. Flux emergence is driven by a bottom boundary condition that advects a semi-torus of magnetic field with $1.7 \times 10^{22}$ Mx flux into the computational domain. The simulations show that, even in the absense of twist, the magnetic flux is able the rise through the upper $15.5$ Mm of the convection zone and emerge into the photosphere to form spots. We find that spot formation is sensitive to the persistence of upflows at the bottom boundary footpoints, i.e. a continuing upflow would prevent spot formation. In addition, the presence of a torus-aligned flow (such flow into the retrograde direction is expected from angular momentum conservation during the rise of flux ropes through the convection zone) leads to a significant asymmetry between the pair of spots, with the spot corresponding to the leading spot on the Sun being more axisymmetric and coherent, but also forming with a delay relative to the following spot. The spot formation phase transitions directly into a decay phase. Subsurface flows fragment the magnetic field and lead to intrusions of almost field free plasma underneath the photosphere. When such intrusions reach photospheric layers, the spot fragments. The time scale for spot decay is comparable to the longest convective time scales present in the simulation domain. We find that the dispersal of flux from a simulated spot in the first two days of the decay phase is consistent with self-similar decay by turbulent diffusion.

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