Contribution of the ageing effect to the observed asymmetry of interplanetary magnetic clouds
Contribution of the ageing effect to the observed asymmetry of interplanetary magnetic clouds
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DOI：
10.1051/0004-6361/202038077
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发表时间：
2020-05
影响因子：
6.5
通讯作者：
P. Démoulin;S. Dasso;V. Lanabere;M. Janvier;C. Noûs
中科院分区：
文献类型：
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作者：
P. Démoulin;S. Dasso;V. Lanabere;M. Janvier;C. Noûs
上下文在太阳爆发期间，大型磁性结构被发射到远离太阳的地方。它们被日冕和日光层成像仪观测为（行星际）日冕物质抛射（ICME或CME）。它们中的一小部分被原位观测为磁云（MC）。要用模型恰当地拟合这些结构，就需要更好地理解它们的演变。目标。当航天器轨道穿过磁结构时，在当地进行原位测量。这些观测是在不同的时间对等离子体的不同元素进行的，因此会受到磁位形膨胀的影响。这种老化效应意味着在MC的前部比后部测量到更强的磁场。这种不对称性经常出现在MC数据中。然而，问题是观察到的不对称性是否可以单独从膨胀中定量解释。方法.基于自相似展开，我们导出了一种由观测到的等离子体速度估算膨胀率的方法。接下来，我们校正了观测到的磁场和航天器轨道上沿着的空间坐标，以消除老化效应。这提供了与同时观察MC内部结构的情况相同的校正数据。结果我们将该方法应用于近地球（1995-2012）90个观测最好的MC。老化效应是所观察到的磁不对称性的主要来源，只有28%的MC。在校正老化效应后，不对称性几乎对称地分布在MC前部磁场较强的MC和MC后部的MC之间。结论.所提出的方法可以有效地消除内的MCs，更一般地说，ICMEs的原位数据的老化偏差。这使我们能够用空间坐标分析数据，例如在模型或遥感观测中。
Context. Large magnetic structures are launched away from the Sun during solar eruptions. They are observed as (interplanetary) coronal mass ejections (ICMEs or CMEs) with coronal and heliospheric imagers. A fraction of them are observed in situ as magnetic clouds (MCs). Fitting these structures properly with a model requires a better understanding of their evolution. Aims. In situ measurements are made locally when the spacecraft trajectory crosses the magnetic configuration. These observations are taken for different elements of plasma and at different times, and are therefore biased by the expansion of the magnetic configuration. This ageing effect means that stronger magnetic fields are measured at the front than at the rear of MCs. This asymmetry is often present in MC data. However, the question is whether the observed asymmetry can be explained quantitatively from the expansion alone. Methods. Based on self-similar expansion, we derived a method for estimating the expansion rate from the observed plasma velocity. We next corrected the observed magnetic field and the spatial coordinate along the spacecraft trajectory for the ageing effect. This provided corrected data as in the case when the MC internal structure were observed at the same time. Results. We apply the method to 90 best-observed MCs near Earth (1995–2012). The ageing effect is the main source of the observed magnetic asymmetry for only 28% of the MCs. After correcting for the ageing effect, the asymmetry is almost symmetrically distributed between MCs with a stronger magnetic field at the front and those at the rear of MCs. Conclusions. The proposed method can efficiently remove the ageing bias within in situ data of MCs, and more generally, of ICMEs. This allows us to analyse the data with a spatial coordinate, such as in models or remote-sensing observations.