KVI-Center for Advanced Radiation Technology, University of Groningen, Groningen, The Netherlands.
Inter University Institute for High Energies, Vrije Universiteit Brussels, Brussels, Belgium.
Nature. 2019 Apr;568(7752):360-363. doi: 10.1038/s41586-019-1086-6. Epub 2019 Apr 17.
Lightning is a dangerous yet poorly understood natural phenomenon. Lightning forms a network of plasma channels propagating away from the initiation point with both positively and negatively charged ends-called positive and negative leaders. Negative leaders propagate in discrete steps, emitting copious radio pulses in the 30-300-megahertz frequency band that can be remotely sensed and imaged with high spatial and temporal resolution. Positive leaders propagate more continuously and thus emit very little high-frequency radiation. Radio emission from positive leaders has nevertheless been mapped, and exhibits a pattern that is different from that of negative leaders. Furthermore, it has been inferred that positive leaders can become transiently disconnected from negative leaders, which may lead to current pulses that both reconnect positive leaders to negative leaders and cause multiple cloud-to-ground lightning events. The disconnection process is thought to be due to negative differential resistance, but this does not explain why the disconnections form primarily on positive leaders, or why the current in cloud-to-ground lightning never goes to zero. Indeed, it is still not understood how positive leaders emit radio-frequency radiation or why they behave differently from negative leaders. Here we report three-dimensional radio interferometric observations of lightning over the Netherlands with unprecedented spatiotemporal resolution. We find small plasma structures-which we call 'needles'-that are the dominant source of radio emission from the positive leaders. These structures appear to drain charge from the leader, and are probably the reason why positive leaders disconnect from negative ones, and why cloud-to-ground lightning connects to the ground multiple times.
闪电是一种危险但尚未被充分了解的自然现象。闪电形成一个等离子体通道网络，从起始点向两端传播，两端分别带正电荷和负电荷，称为正先导和负先导。负先导以不连续的步骤传播，在 30-300 兆赫的频段发出大量无线电脉冲，这些脉冲可以远程感应和以高时空分辨率成像。正先导传播得更连续，因此发出的高频辐射非常少。尽管如此，人们已经对正先导的无线电发射进行了映射，并且发现其发射模式与负先导不同。此外，据推断，正先导可以与负先导暂时断开连接，这可能导致电流脉冲重新连接正先导和负先导，并引发多次云地闪电事件。断开连接的过程被认为是由于负微分电阻，但这并不能解释为什么断开连接主要发生在正先导上，或者为什么云地闪电中的电流从不归零。事实上，人们仍然不了解正先导如何发射射频辐射，或者为什么它们的行为与负先导不同。在这里，我们报告了在荷兰用前所未有的时空分辨率进行的三维无线电干涉测量闪电观测。我们发现了小的等离子体结构——我们称之为“针”——这是正先导无线电发射的主要源。这些结构似乎从先导中排出电荷，这可能是正先导与负先导断开连接的原因，也是云地闪电多次连接到地面的原因。
