Sparks fly when the solar wind blows. Charged particles emitted by the sun seem to be able to influence the rate of lightning strikes here on Earth, hints a new analysis.
By all accounts, lightning should be impossible. Air is an electrical insulator, so getting it to conduct current requires an incredibly intense electrical field. The puzzle is that observations of thunderstorms show that their fields are much too weak for bolts to fly, says Christopher Scott at the University of Reading, UK.
One idea is that high-energy particles from outer space called cosmic rays might provide the necessary boost. When incoming particles collide with air molecules, they set electrons loose and ionise the air, creating a cascade that lets current flow.
This theory hasn’t been fully confirmed. But if the mechanism holds, Scott and his colleagues think they have found another extraterrestrial trigger: the sun. Charged particles emitted by the sun, known as the solar wind, blast Earth at regular intervals, with a peak every 27 days or so.
The solar wind has its own magnetic field, and during a peak this field deflects about 1 per cent of incoming cosmic rays from hitting Earth. At the same time, the number of solar particles reaching Earth increases by 6 per cent. If solar particles ionise the air in the same way as cosmic rays, the net effect should be an increase in lightning strikes during a peak in solar wind strength, says Scott.
To test this idea, the team compared data from the Advanced Composition Explorer (ACE) spacecraft, which measures the solar wind, with lightning rates seen between 2000 and 2005, as measured by monitoring stations for the UK Met Office. In the 40 days after a blast of solar wind, the UK saw an average of 422 lightning strikes. By contrast, there were 321 strikes on average in the 40 days before a blast.
The solar wind peaks regularly, and its cycles are measured by spacecraft like ACE. That means it should be possible to predict dangerous storms, says Scott. “Lightning is one of the major meteorological hazards. I think it is up there with flooding and mudslides for killing people worldwide,” he says.
That’s not to say solar particles will turn a calm day into lightning-fuelled misery, because storm conditions already have to be in place to spark lightning.
“The effects of solar wind are limited, if this paper is true, to making lightning happen more often, providing conditions are already conducive to lightning,” says Christopher Emersic at the University of Manchester, UK.
Scott also notes this point, but he adds that if a thunderstorm is coming, meteorologists might be able to use data on the solar wind to predict how severe a storm is going to be.
The result also doesn’t make it clear how lightning is created in the first place, says Joseph Dwyer at the Florida Institute of Technology in Melbourne. “The paper finds that conditions in outer space are somehow affecting the production of lightning in thunderstorms. This is a very puzzling result, since it is not obvious how the two are connected,” says Dwyer. “However, scientists still do not understand how lightning is initiated inside thunderstorms, so perhaps this work is providing a clue.”
Journal reference: Environmental Research Letters, DOI: 10.1088/1748-9326/9/5/055004
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