Solar Activity Responsible for Increased Lightning Strikes on Earth

They discovered that when streams of high-speed solar particles strike the Earth's atmosphere, the average number of lightning strikes increased by 32% for more than a month afterwards. The study is the first to implicate the solar wind – the stream of particles launched from the sun at over a million miles per hour – in triggering lightning, a phenomenon that has puzzled scientists.

Previous research had suggested the involvement of cosmic rays, highly energetic particles from deep space. In this scenario, the solar wind should protect Earth because it carries a magnetic field that was expected to deflect the cosmic rays, which would lower the rate of lightning strikes.

The new research shows the opposite effect. "We expected to see a decrease in lightning rates with the solar wind stream but what we actually saw was an enhancement. This caused us a bit of head-scratching," said Chris Scott from the University of Reading, who led the study published in the journal Environmental Research Letters.

The researchers used weather data from the UK Met Office's lightning detection system, covering the period between 2000 and 2005. They compared this with data from Nasa's Advanced Composition Explorer (ACE) and Geostationary Operational Environmental Satellites (GOES) spacecraft, which both measure the characteristics of the solar particle streams.

The results suggest that the solar wind helps accelerate particles into our atmosphere rather than deflecting them, but the exact mechanism remains unknown.

"The thing we need to do to prove this is to track the particles all the way through the atmosphere but that is quite challenging," said Scott. Balloons at different altitudes could be used to carry appropriate detectors.

The researchers restricted their data to any event that occurred within a radius of 500km from central England. This opens the question of whether the effect is global or whether different regions of Earth will respond in different ways to the solar wind.

Scott says he thinks they will. "We looked at Northern Europe because that's where the detection system is located, but lightning in Africa is much more intense than over Europe."

Lightning reaches temperatures of up to 27,000C, more than three times hotter than the surface of the sun. A 2008 study estimated that up to 24,000 people die every year from lightning strikes.

The hope is that the new findings could increase the accuracy of regional weather forecasts by linking solar wind events to the increased probability of lightning strikes. The solar wind streams return every 27 days as the sun rotates.

"We may be able to use data on the returning stream to say something about the severity of the expected lightning storms. It gives us a tantalising possibility for improving long-term weather forecasting," said Scott.