Lightning flashes have distinctive zig-zag shapes and physicists have long wondered why. Now, John Lowke and Endre Szili at the University of South Australia have done calculations that could explain this behaviour.
The duo created a model that describes the unusual propagation of “lightning leaders” – channels of ionized air – that connect thunderclouds to the ground. They propose that the zig-zag steps are associated with highly excited, metastable oxygen atoms – which make it far easier for electrical current to flow through the air.
Lightning appears to propagate in a series of steps that involve leaders, which are tens of metres long and originate from thunderclouds. A leader will light up for about 1 µs as current flows, creating a step. Then the channel will darken for tens of microseconds, followed by the formation of the next luminous step at the end of the previous leader – sometimes with branching occurring. This process repeats to create a familiar jagged lightning-bolt shape. A curious aspect of this process is that once a step has lit up and darkened, it does not light up again – despite being part of the conducting column.
This stepping is known to be responsible for the distinctive zig-zag patterns found in lightning streaks, but there are several unanswered questions about the physics behind this phenomenon. In particular, the nature of the dark yet conductive columns connecting leaders to thunderclouds has largely remained a mystery.
Singlet delta oxygen
In their study, Lowke and Szili calculate that the stepping behaviour could be connected to an accumulation of highly excited oxygen molecules called “singlet delta metastable oxygen”. These molecules have a radiative lifetime of roughly one hour, and cause electrons to detach from negative oxygen ions – enhancing the conductivity of the air surrounding them.
The duo suggests that time between successive steps corresponds to the time required for sufficient concentrations of the metastable molecules to accumulate at leader tips. This increases the electric field at the tip, making further ionization possible in the next step. In addition, the researchers propose that high concentrations of singlet delta oxygen should endure in earlier steps, allowing these steps to maintain their electrical conductivity, even without a sustaining electric field.
Lowke and Szili hope that a better understanding of this process could lead to new techniques and stronger regulations for protecting buildings from lightning strikes. This could minimize the economic and environmental damage caused by lightning while reducing the threat to life and limb.
The research is described in Journal of Physics D: Applied Physics.
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