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BY KEITH COOPER ASTRONOMY NOW Posted: 22 April, 2009
The lack of current solar activity came under the spotlight today at the European Week of Astronomy and Space Science at the University of Hertfordshire. "Activity is at an all time low since the start of the space age," says Professor Mike Lockwood of the University of Southampton and the Rutherford Appleton Laboratory. Lockwood presented a talk analysing current solar activity, putting it into both a historical and environmental context.
The Sun has a (roughly) eleven year cycle of activity, but the onset of the latest cycle, 24, has been a painfully prolonged affair, resulting in very few sunspots and flares. Between cycles, activity drops and the Sun's magnetic field reverses itself, producing new sunspots of opposite polarity to those that came before. The first sunspot of cycle 24 was spotted in January 2008 but the cycle has just never got going. What solar physicists describe as the 'onset' - when the new cycle blossoms eradicating all traces of the old cycle - just hasn't happened yet, with cycle 23's prolonged minimum still clinging on in the Sun's southern hemisphere. Comparing the Sun's magnetic flux ratio over several cycles indicates that deviations from the norm began to occur in the southern hemisphere five or six years ago, suggesting that the Sun's decline has a deep-rooted explanation. Perhaps we shouldn't be too surprised, says Lockwood. Yes, it is the deepest solar minima since records began, but reliable records from dedicated space observatories only go back to the 1970s. "For the space age this is an exceptional solar minimum," he says. There are other means to study solar activity prior to the space age, including sunspot counts and cosmic ray falls recorded in tree trunks, which fluctuate with the strength of the Sun's magnetic filed and solar wind. These indicate that there is a greater, underlying cycle of grand maxima and minima in solar activity, stretching over centuries. One famous grand minimum was the Maunder Minimum between 1645 and 1715, which coincided with a mini ice-age in Europe, and the River Thames freezing over. Lockwood says that we are actually still in a grand maximum, but not for long. "I predict that we'll fall out of grand maximum before 2020," he claims. Certainly, sunspots numbers have been dropping since 1985, the solar flux peaked in 1987, and the solar wind pressure has fallen, so this decline has been occurring for some time and we're only really beginning to notice. Would this be enough to counter rising temperatures and climate change here on Earth? "There is certainly no evidence yet that there is a turning off of global warming because of solar activity," says Lockwood. Global temperatures do vary slightly over relatively short timescales, and so far there has been no slowing down of rising temperatures beyond this expected short fluctuations and Lockwood is even sceptical that a grand minimum would make much difference against the onslaught of climate change. On the other hand, and perhaps quite worryingly, he says that the more violent solar activity - flares and coronal mass ejections - tends to occur when the Sun is quieter, almost as if it has been holding tension pent up inside, which suddenly erupts out. A big solar flare directed towards Earth could knock out hundreds of satellites and terrestrial power grids, which would have obvious consequences. Even when the Sun is in a quiet phase, its raw energy is still quite astonishing. |
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