The Earth has a magnetic field that functions as an in-built force-field against galactic cosmic rays, particles from space which prompt a chain-reaction of events in thunderclouds that trigger lightning bolts. 

The only thing more powerful than our magnetic field is that of the Sun, and it may be playing a part in the generation of lightning strikes on Earth by temporarily 'bending' the Earth's magnetic field and allowing more energetic particles to enter the upper atmosphere. Over a five year period, the UK experienced around 50% more lightning strikes and researchers found that correlates to when the Earth's magnetic field was skewed by the Sun's own magnetic field.

This insight could lead to a reliable lightning forecast system that could provide warnings of hazardous events many weeks in advance. To do so, weather forecasters would need to combine conventional forecasts with accurate predictions of the Sun's spiral-shaped magnetic field known as the heliospheric magnetic field (HMF), which is spewed out as the Sun rotates and is dragged through the solar system by the solar wind.

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Lead author of the research Dr Matt Owens said, "We've discovered that the Sun's powerful magnetic field is having a big influence on UK lightning rates. The Sun's magnetic field is like a bar magnet, so as the Sun rotates its magnetic field alternately points toward and away from the Earth, pulling the Earth's own magnetic field one way and then another." 

In their study, the researchers used satellite and Met Office data to show that between 2001 and 2006, the UK experienced a 50% increase in thunderstorms when the HMF pointed towards the Sun and away from Earth.

This change of direction can skew or 'bend' the Earth's own magnetic field and the researchers believe that this could expose some regions of the upper atmosphere to more galactic cosmic rays--tiny particles from across the Universe accelerated to close to the speed of light by exploding stars.

"From our results, we propose that galactic cosmic rays are channelled to different locations around the globe, which can trigger lightning in already charged-up thunderclouds. The changes to our magnetic field could also make thunderstorms more likely by acting like an extra battery in the atmospheric electric circuit, helping to further 'charge up' clouds," Dr Owens continued.

The results build on a previous study by University of Reading researchers, also published in Environmental Research Letters, which found an unexpected link between energetic particles from the Sun and lightning rates on Earth.

Professor Giles Harrison, head of Reading's Department of Meteorology and co-author of both studies, said: "This latest finding is an important step forward in our knowledge of how the weather on Earth is influenced by what goes on in space. The University of Reading's continuing success in this area shows that new insights follow from atmospheric and space scientists working together."

Dr Owens continued: "Scientists have been reliably predicting the solar magnetic field polarity since the 1970s by watching the surface of the Sun. We just never knew it had any implications on the weather on Earth. We now plan to combine regular weather forecasts, which predict when and where thunderclouds will form, with solar magnetic field predictions. This means a reliable lightning forecast could now be a genuine possibility."