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.
How a solar storm stirs up havoc for travellers and households
Radio communications and navigation
Electromagnetic disturbances in the atmosphere disrupt both conventional radio communications and satellite navigation systems, such as the Global Positioning System. Solar storms in October 2003 and December 2006 caused a serious loss of accuracy in GPS – and a superstorm could have dire consequences for anyone dependent on satnav.
Satellites
An estimated 800 operational satellites are in Earth orbit – and all suffer slow degradation from the impact of cosmic rays from outer space and charged particles from the Sun.
Their solar panels and internal electronics are vulnerable. A sudden solar storm can cause levels of damage within a few hours that would normally occur over months or even years. Electric utilities
A solar storm induces wild oscillations in the Earth’s magnetic field, which in turn cause large voltage fluctuations in electric power systems.
Many blackouts have been triggered in this way. The most extensive was the collapse of the Hydro-Quebec grid in Canada in March 1989, which left 6m people without power for nine hours.
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”.
“If we had a repeat of the Maunder Minimum it would be very exciting [for science] but that is not likely to happen,” says Nancy Crooker, research professor at Boston University. Like the majority of astronomers, she expects solar activity to pick up soon, leading to the next maximum around 2012.
There is no agreement, however, on how disturbed the Sun will be during the next maximum. According to some predictions, it will be relatively calm, with fewer solar storms than during the last few maxima (in 2001-02, 1990-91 and 1980-81). Others say that, despite the low starting point, the Sun could still build up to an exceptionally intense maximum over the next four years.
“Predicting the next maximum now is rather like forecasting next summer’s weather in the middle of winter,” says Jim Wild, a space scientist at Lancaster University.
The 11-year cycle is driven by the changing magnetism of the “solar dynamo”. The faster rotation of the Sun’s equatorial regions than the poles amplifies the magnetic field until it bursts through to the surface, causing sunspots and shooting billions of tonnes of solar material into space – a “coronal mass ejection” (see picture above – the black circles are sunspots). But there is no good model to explain the timing or intensity of these events.
“If I was the operator of a satellite system or an electricity grid, I would be happy for the Sun to remain quiet,” Dr Wild says. But he and other solar experts warn against complacency.
While there are far fewer coronal mass ejections during a solar minimum, they can nevertheless take place without warning at any time. Prof Crooker says a sudden solar storm can be even more damaging if it occurs during a quiet period, when the solar wind is weak, than during a solar maximum.
What astronomers believe was the most intense eruption from the Sun for several centuries took place in August 1859 during an otherwise fairly tranquil solar cycle. The aurora borealis or Northern Lights – the vivid photoelectric display triggered when solar particles hit Earth’s upper atmosphere – moved down from its normal polar haunts to put on a spectacular show as far as the tropics. Other effects included putting the world’s nascent telegraph network out of action for several hours.
If such a solar superstorm occurred now, it would cause tens of billions of dollars of damage to communications and navigational satellites and cause continent-wide electrical blackouts that might last for weeks, say Sten Odenwald and James Green, Nasa scientists who have analysed the 1859 event.
One certain effect of a quiet Sun is that more high-energy cosmic rays from elsewhere in the universe can beat their way through the weakened solar wind and reach Earth. Although these cosmic rays do not arrive in a sudden eruption, like a solar storm, their steady cumulative impact may shorten the lifetime of satellites.
This quiet period would not be a good time to launch a manned space mission beyond Earth orbit, to the Moon or Mars, adds Prof Crooker. Astronauts would face the harmful impact of increased cosmic radiation, which would outweigh the reduced likelihood of solar storms. Another effect of reduced solar activity is a cooling and thinning of the upper atmosphere. This too has an upside and a downside: the reduced drag will enable both satellites and space junk to stay aloft for longer.
The relationship between solar variability and climate remains a mystery, says Jamie Casford, a climate researcher at Durham University. While the Sun’s magnetic field and the solar wind change remarkably over the years – the Ulysses satellite measurements show that they are 20 to 30 per cent weaker now than at the last solar minimum in 1996-97 – the accompanying changes in the Sun’s total energy output are tiny.
When the Sun is very quiet, the amount of energy that reaches Earth is only 0.1 per cent less than when it is very active – a change too small to produce significant global cooling on its own. “I would say that solar variability does feed into the climate system but we really do not know what the mechanism is,” Dr Casford says.
There are several theories. A controversial one comes from Henrik Svensmark, a physicist at the Danish National Space Centre in Copenhagen; he believes increased cosmic radiation, hitting the atmosphere when the Sun is quiet, stimulates cloud formation – which cools the planet. Paul Mayewski, director of the University of Maine Climate Change Institute, says the primary impact of solar variability is on atmospheric circulation, which then affects temperature.
Although some people who are sceptical about the human influence on global warming like to emphasise the link between solar variability and climate, Prof Mayewski turns their argument on its head: “The fact that we are not in conditions like the little ice age today shows that the atmosphere is being perturbed by human activities,” he says.
If the Sun stays quiet for the next few years, it may temper the effects of man-made global warming for a while but most experts believe that rising levels of carbon dioxide in the atmosphere will eventually push temperatures higher again.
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