Sunspots squeeze and stretch the day

MOST of us don't notice it, but not all days are the same length. Now it seems that sunspotsMovie Camera - dark regions that emerge on the sun's surface - may be partly responsible for the millisecond fluctuations in the time it takes Earth to rotate once on its own axis. The finding could help to steer spacecraft more accurately.

There are already explanations for why the exact length of a day varies. Changes in winds and ocean currents cause the Earth's spin to slow slightly or speed up to compensate, preserving the planet's total angular momentum. Meanwhile, shifts in how matter is distributed around the planet due to climate change may also affect the speed of Earth's spin.

The latest association, between sunspots, whose abundance rises and falls on an 11-year "solar cycle", and the Earth's spin rate, is perhaps the most bizarre yet.

Researchers have long observed that the spin rate fluctuates with the seasons, in response to shifting wind patterns. Now, a team led by Jean-Louis Le Mouël at the Paris Institute of Geophysics in France has found that this seasonal effect also grows and shrinks in an 11-year cycle, rather like sunspots. Seasons have a bigger effect on spin rate when sunspots are scarce, and a smaller effect when spots are abundant, according to an analysis of data from 1962 to 2009 (Geophysical Research Letters, vol 37, L15307).

The team suspects that this link between sunspot abundance and spin rate is due to sunspots somehow altering wind patterns on Earth. One way this might occur is through sunspot-mediated changes in the ultraviolet brightness of the sun. Since UV light heats the stratosphere, sunspots could plausibly alter wind patterns, says Steven Marcus of NASA's Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study.

He says researchers need to better pin down where and when wind changes occur and determine if they tie in with these UV fluctuations. "This is an intriguing result, but some major pieces of the puzzle are still missing," Marcus says.

Filling in those pieces could pay dividends by improving predictions of when and how the rotation rate will change. These are important when using Earth-based radio dishes to track spacecraft. A 1-millisecond error in the rotation period can skew calculations of spacecraft locations by thousands of kilometres at the distance of Mars, Marcus says, "an important difference when trying to land on or even orbit the planet".

via Sunspots squeeze and stretch the day - space - 27 August 2010 - New Scientist.