Physics textbooks depict the Earth as having a nice straight and symmetrical magnetic field, but in fact it is usually skewed, with its poles up to 5° out of line. These kinks are caused by the pull of the solar magnetic field, which fluctuates with sunspot number and the strength of the solar wind. Sometimes the solar magnetic field points towards the Sun and other times it points away.

“Typically the solar field shifts every 10 to 15 days, meaning that the Earth's field is usually bent in one direction or the other,” said Mathew Owens of the University of Reading, UK.

By studying lightning data gathered by the UK’s Meteorological Office and plotting it against satellite measurements of the solar magnetic field, Owens and his colleagues were able to assess the influence that the solar magnetic field has on electrical storms here on Earth.

The team found that from 2001 to 2006 lightning rates over the UK increased by an average of 40–60% when the solar magnetic field was pointing towards the Sun. Thunderstorm activity still occurred on days when the Earth was in an away sector (with the solar field pointing towards Earth), but was less frequent. For example, during July there was an average of 40 lightning flashes on “away sector” days, but around 90 flashes on “towards sector” days.

Exactly how the solar magnetic field causes these variations in thunderstorm activity isn't precisely clear, but Owens and his colleagues suspect that the changes in the shape of the Earth's magnetic field affect the amount of energetic charged particles (cosmic rays) that are channelled into the Earth's atmosphere from outer Space.

“Each time the Earth's field bends it exposes different locations to different particle intensities, changing local atmospheric ionisation, which makes it harder or easier to trigger lightning,” said Owens, whose findings are published in Environmental Research Letters (ERL).

Because geomagnetic variations are biggest at high latitudes the effect is likely to be greatest in higher latitude countries. But other factors are important too. “It may be that areas with lower atmospheric ionization from ground-level sources such as radon are more sensitive to cosmic-ray variation,” said Owens. “And equatorial regions generally have stronger and more frequent convection so there is more opportunity for solar modulation to act, even if that modulation is generally weaker.”

Next the researchers hope to see how the relationship between solar field and lightning holds for other countries, as well as assessing it over time using historical records going back 150 years. Ultimately Owens and his colleagues hope that their findings could be used to improve lightning forecasts, by incorporating solar magnetic field information into numerical weather predictions.

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