Our Spotty Sun

Our Sun may be middle-aged, but that doesn’t stop it from having a teenager’s spots.

The photosphere, the surface layer of the Sun, is a balmy 5800 degrees Kelvin. It’s composed of convection cells called granules, which are columns of hot plasma that rise and descend as they heat up and cool down, creating convection currents on the surface. The Sun also has a magnetic field 100 times as strong as the Earth’s. At solar minimum, you can picture the field as like that of a giant bar magnet, with huge loops from pole to pole, but as the sun rotates and approaches solar maximum, odd things begin to happen.

The Sun spins on its axis every 25.4 days, but it doesn’t rotate evenly. It experiences differential rotation, where the equator spins faster than the poles. This twists up the magnetic field lines, making them tangled and complicated—and, in places, up to 10,000 times stronger than the rest of the field.

Since intense magnetic fields inhibit the convection of heat, these strong, twisted parts of the magnetic field create fascinating phenomena called sunspots. Typically a few thousand kilometres across, sunspots are dark “holes” in the Sun that appear near the equator during solar maximum. Really, they’re just patches that are much cooler than the rest of the Sun, making them appear much darker, almost black.

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Sunspots vary over a period of 11 years, with both their location and intensity changing periodically. Their variation can be beautifully described by a Butterfly Diagram, which shows that that sunspots first form at mid-latitudes (+/- 35 degrees of latitude), then widen and move progressively closer to the equator as the cycle wears on.

Every 11 years, the magnetic field of the Sun flips over so the north pole becomes the south, and vice versa, so the Sun’s activity varies according to a 22 year cycle—11 years to flip one way, another 11 to flip back to how it was originally.

Solar activity is directly correlated to the number of sunspots: as sunspots increase, the solar activity increases, and so does energy output from the sun. Some evidence suggests that the energy changes throughout the solar cycle affect Earth’s climate. No consensus has been reached yet, but it’s thought that the energy fluctuations don’t cause change in weather, only in global climate. Careful satellite measurements tell us that total solar energy varies 0.1% over an 11-year cycle, which changes Earth’s global tropospheric temperature by 0.5 to 1 degree Celsius.

One famous example is the Maunder Minimum, when from 1645 to 1715, the number of sunspots reduced radically. Over 30 years, only 50 were observed, as opposed to the 40,000 – 50,000 observed in that same time today. This prolonged sunspot minimum coincided with a “Little Ice Age”, where Europe and North America experienced particularly cold winters.

Image Credit: Wikimedia Commons