Solar wind weakens as sun dials down its furnace

WASHINGTON -- The sun has dialed 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 said Tuesday. 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 million miles 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 337 million miles, 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% cooler and 17% less dense, according to a paper by McComas in the journal Geophysical Research Letters.