Reconnection and field line shrinkage in solar flares
Reconnection and field line shrinkage in solar flares
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DOI：
10.1086/176896
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发表时间：
1996-03-01
影响因子：
4.9
通讯作者：
Acton, LW
中科院分区：
文献类型：
--
作者：
Forbes, TG;Acton, LW
我们使用 Yohkoh 上的软 X 射线望远镜 (SXT) 拍摄的耀斑环图像来估计开放场线在重新连接形成闭环后所经历的高度下降。根据之前的实践，我们将这种减少称为场线收缩，并假设强度结构追踪出场线。在这项研究中，我们检查了肢体附近的两个长时间事件，这些事件具有随时间不断增长的耀斑环。通过将场线位于喇叭形环路系统的最外边缘时的高度与随后位于最内边缘时的高度进行比较来确定收缩率。我们发现，在一个耀斑中，磁力线比初始高度收缩了约 20%，而在另一个耀斑中，则收缩了约 32%。这些值与重新连接场的简单模型预测的收缩率在 5% 以内，该模型假设除了从环路顶部向上延伸的电流片之外，场在任何地方都是潜在的。模型密度沿视线的数值积分意味着观测结果与理论之间的大部分差异是由于以任意角度观察环形拱廊时发生的投影效应造成的。两个耀斑在环形顶部都有明亮区域，但在一个耀斑中，该区域的下部比环形的其余部分更冷、更密集，而在另一耀斑中则不然。考虑将明亮区域映射到环路的足点意味着冷区域是由环路最上部的重连接流出下游的热不稳定性形成的。另一个耀斑中缺乏凉爽、致密的区域可能是因为它是一个非常弱的事件，温度和密度太低而无法引发热不稳定性。
We use images of flare loops taken by the Soft X-ray Telescope (SXT) on Yohkoh to estimate the decrease in height that open field lines undergo after they have reconnected to form closed loops. Following previous practice, we refer to this decrease as field line shrinkage and assume that intensity structures trace out the field lines. For this study, we examine two long-duration events near the limb which have flare loops that continually grow with time. The shrinkage is determined by comparing the height of a field line when it lies at the outermost edge of the flare loop system with the height it has later on when it lies at the innermost edge. We find that the field lines shrink by about 20% of their initial height in one flare and by about 32% in the other. These values are within 5% of the shrinkage predicted by a simple model of the reconnecting field which assumes that the field is potential everywhere except for a current sheet extending upward from the top of the loops. Numerical integration of the model density along the line of sight implies that most of the discrepancy between the observations and the theory is due to projection effects which occur when an arcade of loops is viewed at an arbitrary angle.Both flares have bright regions at the top of the loops, but in one flare the lower part of the region is cooler and denser than the rest of the loop, while in the other flare it is not. Consideration of the mapping of the bright regions to the footpoint of the loops implies that the cool region is formed by a thermal instability downstream of a reconnection outflow in the uppermost part of the loop. The absence of a cool, dense region in the other flare may be caused by the fact that it is a very weak event with temperatures and densities too low to trigger a thermal instability.