Image caption Solar storms may be more powerful at times of middling solar activity

Within decades, solar storms are likely to become more disruptive to planes and spacecraft, say researchers at Reading University.

The work, published in Geophysical Research Letters, predicts that once the Sun shifts towards an era of lower solar activity, more hazardous radiation will reach Earth.

The team says the Sun is currently at a grand solar maximum.

This phase began in the 1920s - and has lasted throughout the space age.

Mike Lockwood, professor of space environment physics at Reading, said: "All the evidence suggests that the Sun will shortly exit from a grand solar maximum that has persisted since before the start of the space age.

"In a grand solar maximum, the peaks of the 11-year sunspot cycle are larger and the average number of solar flares and associated events such as coronal mass ejections are greater.

"On the other hand in a grand solar minimum there are almost no sunspots for several decades. The last time this happened was during the Maunder Minimum, between about 1650 and 1700."

The research indicates that most radiation hits the Earth during periods of middling solar activity. Increased radiation is a particular problem for aviation and communications - technology that did not exist the last time the sun cycle ended its grand maximum.

Ice core data

The research is based on evidence from ice cores and tree trunks going back 10,000 years. The team measured levels of nitrates and cosmogenic isotopes which enter our atmosphere and are deposited in ice and organic material.

Professor Lockwood told BBC News: "You can tell by the concentration of nitrates in ice sheets that there has been a solar event. What we showed was that they all cropped up at more middling activity than we have been used to.

"We used this data to say that an unfortunate combination of solar conditions is coming our way in the next few decades.

"It's just a question of how much worse the radiation gets and how long it lasts."

The most disruptive radiation is from solar energetic particles, which are carried away from the Sun by coronal mass ejections, or solar storms, which explode from the Sun's surface.

The evidence seems to indicate that although there are fewer solar storms once the Sun leaves its grand maximum, they are more powerful, faster and therefore carry more particles.

A decline in solar activity also allows more radiation from other parts of the galaxy to enter the Solar System.

In a separate study, a team at Stanford University in California, say they have a developed a technique that could give advance warning of the formation of sunspots before they become visible on the Sun's surface. Sunspots are areas of high magnetic activity. They are significant because these are the areas where solar storms or coronal mass ejections erupt.

Stathis Ilondis and colleagues, writing in the Journal Science, say their findings will help give advance warning of solar storms and the resulting radiation which disrupt communications and transport on Earth.

Image caption The Stanford researchers used a technique called helioseismology

The Stanford team used a novel technique called helioseismology, which is based on analysis of vibrations on the solar surface. The team discovered that these acoustic signals causing the vibrations moved faster in regions where sunspots were forming up to 65,000km (40,000m) deep.

The resulting sunspots appeared on the surface between one and two days after the differences in vibrations were detected.

Mr Ilondis told BBC News: "It's an early warning for emerging sunspots. This is our main finding."

"We can also predict the size and strength of the sunspot. And if it is a large sunspot then it is more probable to produce some big space weather events like some strong flares."