Astronomy Journal Entry

Sun Is Unusually Quiet (April 25, 2009)

There's not much sunspot activity on the solar surface these days. In fact, it's been that way for awhile. Through this week, there have been 100 sunspot-free days in 2009, or 88% of the time. Last year, there were 266 sunspot-free days (73% of the time), fewer than any year since 1913.

The Sun goes through an activity cycle that lasts roughly eleven years. The last peak of the cycle was in 2001–2 and the next is expected in 2012–13. The cycle has been in an unusually long low-activity period for the past five years. Since 2004, there's been a total of 609 sunspot-free days. That's way above the average of 485 days for a typical solar minimum. Scientists aren't sure why the Sun's been stuck in the activity cycle's "valley" for so long.

Sunspots are planet-sized areas on the solar surface where the Sun's magnetic fields are distorted. They're linked to solar flares and high-energy particle streams called coronal mass ejections. These solar storms can disrupt satellites and trigger geomagnetic storm's in Earth's atmosphere.

Solar wind, the constant stream of charged particle's blowing from the Sun's upper atmosphere, has also slowed. Solar wind pressure has dropped about 20 percent since the mid-1990s. It's weaker than it's ever been since scientists began measuring it in the 1960s. The drop means more cosmic rays reach the inner solar system, increasing the radiation risk to astronauts. It also means fewer geomagnetic storms and auroras in Earth's atmosphere.