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arXiv 1801.04355astro-ph.SR
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与日冕物质抛射有关的激波阵面的密度压缩比

The density compression ratio of shock fronts associated with coronal mass ejections

Ryun-Young Kwon, Angelos Vourlidas

摘要:

提出了一种新的方法来提取白光日冕图像中观测到的与日冕物质抛射(CME)有关的激波阵面的三维电子密度分布和密度压缩比。我们证明了快速晕日冕物质抛射($\sim$ 2000 km s $^{-1}$)的两个例子的方法观察到2011年3月7日和2014年2月25日。我们的方法使用椭球模型来推导三维(3D)的几何形状和运动学的前线。鞘层的密度分布采用具有四个自由参数的双高斯函数模型,电子分布在前面后面的薄壳层内。模型化的密度沿视线进行积分,以与COR 2-A中观察到的亮度进行比较,并且使用$χ^2$方法来获得高斯分布的最佳参数。上游密度是从事件前图像中的亮度反演和经验模型中获得的。导出了密度比和Alfvénic马赫数。我们发现密度压缩在CME头部附近达到最大值,并在较大的位置角处减小。这种行为与机头处的驱动激波和CME侧翼处的自由传播激波一致。有趣的是,我们发现超临界区在大面积的冲击和持续时间(几十分钟)比过去的报告。因此,日冕物质抛射冲击能够在扩展的空间和时间尺度上加速日冕中的高能粒子,并且可能是这些粒子在内日球层中广泛纵向分布的原因。我们的研究结果还表明,多视点日冕观测和正演模拟的权力,在远程推导关键的冲击属性,否则无法访问的制度。

英文摘要:

We present a new method to extract the three-dimensional electron density profile and density compression ratio of shock fronts associated with Coronal Mass Ejections (CMEs) observed in white light coronagraph images. We demonstrate the method with two examples of fast halo CMEs ($\sim$ 2000 km s$^{-1}$) observed on 2011 March 7 and 2014 February 25. Our method uses the ellipsoid model to derive the three-dimensional (3D) geometry and kinematics of the fronts. The density profiles of the sheaths are modeled with double-Gaussian functions with four free parameters and the electrons are distributed within thin shells behind the front. The modeled densities are integrated along the lines of sight to be compared with the observed brightness in COR2-A, and a $χ^2$ approach is used to obtain the optimal parameters for the Gaussian profiles. The upstream densities are obtained from both the inversion of the brightness in a pre-event image and an empirical model. Then the density ratio and Alfvénic Mach number are derived. We find that the density compression peaks around the CME nose, and decreases at larger position angles. The behavior is consistent with a driven shock at the nose and a freely-propagating shock wave at the CME flanks. Interestingly, we find that the supercritical region extends over a large area of the shock and last longer (several tens of minutes) than past reports. It follows that CME shocks are capable of accelerating energetic particles in the corona over extended spatial and temporal scales and are likely responsible for the wide longitudinal distribution of these particles in the inner heliosphere. Our results also demonstrate the power of multi-viewpoint coronagraphic observations and forward modeling in remotely deriving key shock properties in an otherwise inaccessible regime.

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