Space and Atmospheric Electricity Group, Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB, UK.
Sci Rep. 2017 Jun 23;7(1):4152. doi: 10.1038/s41598-017-04546-3.
Coronal mass ejections (CMEs) are episodic eruptions of solar plasma and magnetic flux that travel out through the solar system, driving extreme space weather. Interpretation of CME observations and their interaction with the solar wind typically assumes CMEs are coherent, almost solid-like objects. We show that supersonic radial propagation of CMEs away from the Sun results in geometric expansion of CME plasma parcels at a speed faster than the local wave speed. Thus information cannot propagate across the CME. Comparing our results with observed properties of over 400 CMEs, we show that CMEs cease to be coherent magnetohydrodynamic structures within 0.3 AU of the Sun. This suggests Earth-directed CMEs are less like billiard balls and more like dust clouds, with apparent coherence only due to similar initial conditions and quasi homogeneity of the medium through which they travel. The incoherence of CMEs suggests interpretation of CME observations requires accurate reconstruction of the ambient solar wind with which they interact, and that simple assumptions about the shape of the CMEs are likely to be invalid when significant spatial/temporal gradients in ambient solar wind conditions are present.
日冕物质抛射（CME）是太阳等离子体和磁通量的爆发性释放，它们会在太阳系中传播，引发极端空间天气。CME 观测的解释及其与太阳风的相互作用通常假设 CME 是连贯的，几乎是固态的物体。我们表明，CME 从太阳向外的超音速径向传播会导致 CME 等离子体包以比当地波速更快的速度进行几何膨胀。因此，信息无法在 CME 中传播。将我们的结果与超过 400 个 CME 的观测特性进行比较，我们表明 CME 在距太阳 0.3AU 内不再是连贯的磁流体动力学结构。这表明，朝向地球的 CME 不像是台球，而更像是尘云，只有在初始条件相似且它们所经过的介质具有准均匀性时，才会表现出明显的相干性。CME 的不连贯性表明，对 CME 观测的解释需要准确重建与它们相互作用的环境太阳风，并且当环境太阳风条件存在显著的空间/时间梯度时，关于 CME 形状的简单假设可能是无效的。
