阿尔文波动作为行星际日冕物质抛射内部相干性媒介的作用：1 au处多航天器测量研究
On the Role of Alfvenic Fluctuations as Mediators of Coherence within Interplanetary Coronal Mass Ejections: Investigation of Multi-Spacecraft Measurements at 1 au
AI总结：
本研究利用ACE与Wind在1 au处对10个ICME的多点观测，分析阿尔文波动与磁场相干性，发现AFs广泛存在却伴随较低磁场相关，质疑其作为相干媒介的作用。
摘要：
行星际日冕物质抛射（ICMEs）若能够以集体方式响应外部扰动，就被定义为“相干的”。这意味着信息必须能够在ICME结构之间传播；若情况并非如此，则单点原位测量不能被视为ICME整体性质的指示。在此，我们研究阿尔文波动（AFs）作为ICME相干性媒介的作用。我们考察了ACE和Wind航天器在1 au处、纵向间隔为0.5°–0.7°时观测到的10个ICME的多点磁场与等离子体测量。对于每个事件，我们根据波动的剩余能量和交叉螺旋度分析阿尔文性，并根据Wind与ACE之间的磁场相关性分析相干性。我们发现，分别约有65%和90%的ICME鞘区和磁抛射物（MEs）呈现覆盖至少20%结构的扩展AFs。交叉螺旋度表明，太阳起源和行星际起源的AFs可能在1 au处的ICME群体中共存。AFs主要集中在激波下游和MEs后部。鞘区内磁场相关性较差，而对于大多数磁场分量，相关性从MEs的前部到后部逐渐降低。AFs还与较低的磁场相关性相关。这表明，要么ICME相干性并非由阿尔文波介导，意味着相干尺度可能小于此前预测；要么磁场相关性并不是衡量相干性的指标。
英文摘要：
Interplanetary coronal mass ejections (ICMEs) are defined as ''coherent'' if they are capable of responding to external perturbations in a collective manner. This implies that information must be able to propagate across ICME structures, and if this is not the case, single-point in-situ measurements cannot be considered as indicative of global ICME properties. Here, we investigate the role of Alfvenic fluctuations (AFs) as mediators of ICME coherence. We consider multi-point magnetic field and plasma measurements of 10 ICMEs observed by the ACE and Wind spacecraft at 1 au at longitudinal separations of 0.5°-0.7°. For each event, we analyze the Alfvenicity in terms of the residual energy and cross helicity of fluctuations, and the coherence in terms of the magnetic correlation between Wind and ACE. We find that ~65% and 90% of ICME sheaths and magnetic ejecta (MEs), respectively, present extended AFs covering at least 20% of the structure. Cross helicity suggests AFs of solar and interplanetary origin may co-exist in the ICME population at 1 au. AFs are mainly concentrated downstream of shocks and in the back of MEs. The magnetic field is poorly correlated within sheaths, while the correlation decreases from the front to the back of the MEs for most magnetic field components. AFs are also associated with lower magnetic field correlations. This suggests either that ICME coherence is not mediated by Alfven waves, implying that the coherence scale may be smaller than previously predicted, or that the magnetic field correlation is not a measure of coherence.
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