Does the spacecraft trajectory strongly affect the detection of magnetic clouds
Does the spacecraft trajectory strongly affect the detection of magnetic clouds
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DOI：
10.1051/0004-6361/201220535
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发表时间：
2012-11
期刊：
影响因子：
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通讯作者：
P. Démoulin;S. Dasso;M. Janvier
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文献类型：
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作者：
P. Démoulin;S. Dasso;M. Janvier
磁云 (MC) 是行星际日冕物质抛射 (ICME) 的一个子集，可在其中检测到磁通绳。没有检测到磁通绳的 MC 和 ICME 之间的差异是固有的还是由于观察偏差造成的？由于航天器与 MC 轨迹没有关系，因此 MC 的频率分布与航天器到 MC 轴的距离的关系预计将大致平坦。然而，Lepping 和 Wu (2010) 证实它是估计影响参数的强递减函数。对于较大的影响参数，磁通绳是否更容易未被检测到？为了回答上述问题，我们探讨了磁绳模型的参数空间，特别是长径比、边界形状和电流分布。通过将圆形线性无力场拟合到沿模拟交叉点计算的磁场，将所提出的模型作为 MC 进行分析。我们发现，磁通绳内扭转的分布、由于场旋转角度或幅度过低而导致的未检测仅对 MC 与冲击参数的预期频率分布产生微弱影响。然而，随着磁通绳横截面在与交叉轨迹正交的方向上更加伸长，估计的影响参数越来越偏向较低的值。观察到的 MC 分布是磁通绳横截面平均扁平化 2 至 3 倍的自然结果，具体取决于磁扭曲分布。然而，1 AU 下较快的 MC（V>550 km/s）呈现出 MC 与冲击参数的几乎均匀分布，与较慢的磁绳相比，这与圆形磁绳一致。我们的结论是，大多数非 MC ICME 要么在其磁通绳外部或腿部区域附近遇到，要么它们不包含任何磁通绳。
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) where a magnetic flux rope is detected. Is the difference between MCs and ICMEs without detected flux rope intrinsic or rather due to an observational bias? As the spacecraft has no relationship with the MC trajectory, the frequency distribution of MCs versus the spacecraft distance to the MCs axis is expected to be approximately flat. However, Lepping and Wu (2010) confirmed that it is a strongly decreasing function of the estimated impact parameter. Is a flux rope more frequently undetected for larger impact parameter? In order to answer the questions above, we explore the parameter space of flux rope models, especially the aspect ratio, boundary shape, and current distribution. The proposed models are analyzed as MCs by fitting a circular linear force-free field to the magnetic field computed along simulated crossings. We find that the distribution of the twist within the flux rope, the non-detection due to too low field rotation angle or magnitude are only weakly affecting the expected frequency distribution of MCs versus impact parameter. However, the estimated impact parameter is increasingly biased to lower values as the flux-rope cross section is more elongated orthogonally to the crossing trajectory. The observed distribution of MCs is a natural consequence of a flux-rope cross section flattened in average by a factor 2 to 3 depending on the magnetic twist profile. However, the faster MCs at 1 AU, with V>550 km/s, present an almost uniform distribution of MCs vs. impact parameter, which is consistent with round shaped flux ropes, in contrast with the slower ones. We conclude that either most of the non-MC ICMEs are encountered outside their flux rope or near the leg region, or they do not contain any.