High up in the clear blue noontime sky, the sun appears to be much the same day-in, day-out, year after year.
But astronomers have long known that this is not true. The sun does change. Properly-filtered telescopes reveal a fiery disk often speckled with dark sunspots. Sunspots are strongly magnetized, and they crackle with solar flares—magnetic explosions that illuminate Earth with flashes of X-rays and extreme ultraviolet radiation. The sun is a seething mass of activity.
Until it’s not. Every 11 years or so, sunspots fade away, bringing a period of relative calm.
“This is called solar minimum,” says Dean Pesnell of NASA’s Goddard Space Flight Center in Greenbelt, MD. “And it’s a regular part of the sunspot cycle.”
The sun is heading toward solar minimum now. Sunspot counts were relatively high in 2014, and now they are sliding toward a low point expected in 2019-2020.
While intense activity such as sunspots and solar flares subside during solar minimum, that doesn’t mean the sun becomes dull. Solar activity simply changes form.
For instance, says Pesnell, “during solar minimum we can see the development of long-lived coronal holes.”
Coronal holes are vast regions in the sun’s atmosphere where the sun’s magnetic field opens up and allows streams of solar particles to escape the sun as the fast solar wind.
Pesnell says “We see these holes throughout the solar cycle, but during solar minimum, they can last for a long time – six months or more.” Streams of solar wind flowing from coronal holes can cause space weather effects near Earth when they hit Earth’s magnetic field. These effects can include temporary disturbances of the Earth’s magnetosphere, called geomagnetic storms, auroras, and disruptions to communications and navigation systems.
During solar minimum, the effects of Earth’s upper atmosphere on satellites in low Earth orbit changes too.
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