More attentive lighting on lightning reveals needle-shaped structures

The accelerated electric charge in lightning produces electromagnetic radiation over a wide range of frequencies. For more than a century, lightning has been studied using radio frequency detection systems. And in recent years, a radio telescope called Low Frequency Array (LOFAR) has been trained in lightning. This telescope includes thousands of antennas spread across several countries in Europe and can observe the structure of lightning with unprecedented spatial resolution. Write in Nature, Hare et al.¹ present an analysis of the LOFAR observations and report the discovery of needles – structural features 10 to 100 meters long – that extend perpendicularly from initially positively charged lightning channels (see also Ref 2). This discovery could help to better understand the lightning and explain why the lightning is wobbling.

A lightning bolt is a giant electric shock. A laboratory analogue is the discharge of an electronic device called a capacitor through a resistor – a process that can be extremely efficient because the charge on the capacitor decreases exponentially with time. The discharge of a thunderstorm by lightning is significantly less efficient, in part because the charge relies on particles distributed in the space. An effective discharge would require the establishment of a conductive path for each particle charged into the storm. Since the air must be ionized to provide all these pathways, such a process would require an unrealizable amount of energy.

Lightning instead forms a bidirectional channel of ionized air that propagates from the initiation point with positively and negatively charged ends – known as positive and negative leaders respectively (Fig. 1a). The positive leader extends down into a negatively charged region of graupel (soft hail) particles, while the negative leader extends upward into a positively charged ice-crystal zone. The discharge of a storm is therefore much more complex than that of a capacitor.

A lightning discharge differs from an idealized capacitor discharge by another key aspect that concerns many needles: the electrical resistance of lightning channels is not constant and increases sharply with current. For example, the resistance per unit length of a channel under a current of 1 amp is about 300 times that of a channel under 100 A³. Hare and his colleagues emphasize the role of this 'negative differential resistance' in stopping the current – a dramatic reduction in current flow – in the positive leader.

The term "polarity asymmetry" refers to differences in the macroscopic behavior of objects with opposite attributes, such as positive and negative charges. Polarity asymmetry of lightning leaders is evident and can be attributed to the marked polarity asymmetry of charge carriers in ionized air⁴Free electrons are very mobile, whereas heavier positive ions are not. Lightning channels are powered by free electrons, with electronic convergence at the top of the positive leader and divergence from the negative leader. As a result, the negative leader is fast and energetic, emits a lot of radio frequency radiation and produces many free electrons. On the other hand, the positive leader progresses slowly and gently, emits little radio radiation and generates few free electrons. These latter features could make the positive leader more fragile, more prone to current cuts and more likely to display needles than the negative leader.

The needles identified by the hare et al. can now be described in the context of asymmetric polarity leaders that cover positively and negatively charged areas of a storm cloud in a single intracloud flash (Fig 1a). During the flash, the charge deposited along a guide produces a large radial electric field that moves the charge away from the guide. This discharge forms a conical structure called a corona sheath that expands outward until the radial electric field becomes smaller than a particular threshold. Smaller sheath radii are therefore associated with larger thresholds. The asymmetry of polarity in these thresholds allows the cladding volume around the positive leader to be about 10 times greater than that of the negative leader.⁵ (Fig. 1b).

The negative charge carried by the graupel particles is mobilized by the volumic discharge⁶ in the corona sheath of the positive leader. This charge moves to the region of the positively charged storm cloud, but accumulates near the tip of the positive leader (Fig. 1c). Compared to the rest of the leader, this region is the least prone to current cuts because its free electron population is the most recent. Therefore, while lightning on a large scale exhausts the overall electrostatic energy, the local concentration of negative charge (and electrostatic energy) is increased. The small negative leaders – the needles – are then launched perpendicular to the positive leader, and LOFAR measurements can resolve the speed of their radial progression to check their negative charge.

Hare et al. point out that the decreased flow of negative charge towards the positive end of the storm cloud represents a decreased current in the channel of lightning. The decrease in current is a prerequisite for the rampant instability leading to the power cut^4,7,8 which is not easily explained in the conceptual models of the structure of lightning⁹. In future work, it will be useful to establish the link between needle formation and the development of backtracking and discharges, called K-changes in the positive leader. These effects are recognized signatures of power outage and the formation of extra brushstrokes in the flash. It will also be important to establish the role of corona lightning sheath in the appearance of other developments of bidirectional leaders observed near positive leaders.^ten,11.