Solar Minimum has Arrived
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March 6, 2006: Every year in February, the students of Mrs. Phillips's 5th grade class in Bishop, California, celebrate Galileo's birthday (Feb. 15th) by repeating one of his discoveries. They prove that the sun spins.
It's simple. Step 1: Look at the sun. Galileo did this using a primitive telescope; Mrs. Phillips's students use the internet. Step 2: Sketch the sunspots. Step 3: Repeat daily. After only a few days, it's obvious that the sunspots are moving and sun is spinning, performing one complete turn every 27 days.
Right: A picture of the sun taken Feb. 10, 2006, by the Solar and Heliospheric Observatory (SOHO). 
For almost the entire month of February 2006 the sun was utterly blank. If Galileo had looked at the sun on his 442nd birthday, he would have been disappointed—no sunspots, no spin, no discovery.
What's going on? NASA solar physicist David Hathaway explains: "Solar minimum has arrived."
Sunspots come and go with an 11-year rhythm called the sunspot cycle. At the cycle's peak, solar maximum, the sun is continually peppered with spots, some as big as the planet Jupiter. But for every peak there is a valley, and during solar minimum months can go by without a single sunspot.
"That's where we are now—at minimum," says Hathaway.
Actually, we're at the beginning of the minimum. February 2006 was the first month in almost ten years with mostly no sunspots. For 21 of February's 28 days, the sun was blank. Hathaway expects this situation to continue for the rest of 2006.
Below: Sunspot counts from the time of Galileo through the end of 2005. In recent centuries, counts have gone up and down with an 11-year period. [More]
No sunspots means low solar activity. Sunspots are sources of solar flares and coronal mass ejections that can disrupt radio communications and even cause power outages on Earth during severe magnetic storms. These problems should subside during the year ahead. Auroras, a beautiful side-effect of magnetic storms, should subside, too. "Too bad," says Hathaway, who enjoys Northern Lights.
Galileo was lucky. The year 1610 was close to a maximum of the sunspot cycle, so when he projected an image of the sun through his spyglass, he immediately saw enormous spots. The spots themselves did not surprise him. Chinese astronomers looking at the sun naked-eye through clouds and mist had reported seeing sunspots as early as 28 BC. The reality of sunspots was uncontroversial, but the nature of sunspots was a mystery. Were they satellites of the sun? Dark clouds in the sun's atmosphere? Or something else? Galileo's daily sketches showed plainly that the sun was spinning and that sunspots were close to the surface of the spinning orb. Personally, Galileo thought sunspots might be clouds.
Now we know what they really are: great islands of magnetism. Sunspots appear when magnetic force-fields generated by the sun's inner dynamo poke through the surface. These fields block the flow of heat from below, cooling the sun in their vicinity. If you stuck a thermometer in a sunspot, it would register "only" a few thousand degrees Celsius. This makes it look dark compared to the surrounding inferno.
Sunspots are in a state of non-stop upheaval. Tangled lines of magnetism twist and stretch until the tension becomes too great and an explosion occurs—a flare. This link between flares and spots is why solar minimum is so quiet.
"But not absolutely quiet," adds Hathaway. "During solar minimum we can have occasional sunspots and solar flares." Indeed there was at least one monster spot and one X-class solar flare (the most powerful kind) during each of the last three minima in 1976, 1986 and 1996.
2006 will probably be the same—long stretches of quiet with occasional episodes of spots and flares. How long will this last? Stay tuned for the answer in our next story: "Solar Storm Warning."
A Brief History of Sunspots -- from Rice University's Galileo Project
Sunspot numbers -- counting sunspots is not as simple as it sounds
Look at the sun without hurting your eyes -- SOHO's realtime solar images page
Who's Afraid of a Solar Flare? -- (Science@NASA) Solar activity can be surprisingly good for astronauts.
Solar Myth -- (Science@NASA) With solar minimum near, the sun continues to be surprisingly active.