The silent Sun’s uncertain course

The Sun has gone quiet, very quiet. The solar wind – which is comprised of electrically charged particles streaming out from the star – is weaker than at any time since scientists began accurate observations in the 1950s, and the number of sunspots in 2008 may be the lowest since the 19th century.

This year’s solar silence has surprised space physicists, who were expecting the Sun to have moved away from the minimum point of the 11-year solar cycle by now. “To see such a significant and consistent long-term reduction in the solar wind output is really remarkable,” says David McComas, a senior scientist on the Ulysses solar satellite mission, a joint project of the European and US space agencies.

Back on Earth, the Sun’s inactivity ought to represent good news for the companies that operate satellites, run power grids or make terrestrial radio systems, which are all vulnerable to damage and disruption from solar storms. In one interpretation of its long-term implications, however, the effects could be far from benign.

Experts are reluctant to predict the consequences for Earth and its inhabitants because there are so many complex interactions between the Sun’s output, the planet’s atmosphere and magnetic field, and cosmic radiation from outer space. Some climatologists say that, over a period of decades, a quieter Sun means a cooler Earth, although the relationship between solar activity and climate is particularly controversial.

To add to the uncertainty, no one knows how long the Sun is likely to stay quiet. One extreme would be a continued period of inactivity, with very few sunspots or solar storms, that could last for decades. The last such suspension of the 11-year solar cycle occurred between 1645 and 1715, a period known by historians of astronomy as the Maunder Minimum, which coincided with the coldest period of the past millennium, known as the “little ice age”.

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