WASHINGTON - The sun has dialled back its furnace to the lowest levels seen in the space age, new measurements from a space probe show.
But don't worry -- it's too small a difference to change life on Earth, scientists say.
In fact, it means satellites can stay in orbit a little longer.
The solar wind -- a stream of charged particles ejected from the sun's upper atmosphere at 1.6 million kilometres per hour -- is significantly weaker, cooler and less dense than it has been in 50 years, according to new data from the NASA-European solar probe Ulysses.
And for the first time in about a century, the sun went for two months this summer without sunspots, said NASA solar physicist David Hathaway.
That record was broken Monday when a cluster of eight sunspots surfaced.
Sunspots are temporary regions of high magnetic activity that from Earth appear to be black splotches.
The cause for the sun's slight weakening seems to be a change in its magnetic flux, said Dave McComas of the Southwest Research Institute.
Why it's happening is a mystery, but it has fluctuated like this in the past.
Weaker solar winds mean less drag on satellites so they can stay in orbit a bit longer.
While that's good for satellites, it also means more space junk.
Normally the sun goes through an 11-year cycle of more, then fewer, sunspots and a similar cycle when it comes to solar wind strength.
But scientists said Tuesday the sun is in "a very prolonged minimum." Typically a solar minimum lasts about a year, but this low point has gone on since the summer of 2006.
It is "like turning down the heat on a stove," said McComas, a scientist who used the Ulysses solar probe to document a significantly weaker solar wind. The 17-year-old space probe, which circles the sun from a distance of about 542 million kilometres, has been studying the environment above and below the poles of the sun. It is just months away from shutting down because of freezing fuel.
Recently, the solar wind has been about 14 per cent cooler and 17 per cent less dense, according to a paper by McComas in the journal Geophysical Research Letters.
For the past 15 years or so, the sun's overall output seems to be lower than normal, even when it was at the maximum for its cycle about eight years ago, McComas said. It may be part of a centurylong trend, said Boston University space physicist Nancy Crooker.
Some people historically have connected sunspots to weather, such as the Old Farmer's Almanac. But solar scientists say there is no evidence to make any connection between solar activity and weather or long-term climate change.
