How do you determine the size of a sunspot region?

The area of sunspot groups is expressed in "millionths of a solar hemisphere" (MH), with 1000MH corresponding to 3.043,7 million square kilometres. This means that a sunspot group which has an area coverage of 1870MH covers 0.187% of the earth-facing solar surface. Most sunspot groups cover an area about the size of the surface of the Earth (which equals almost 170MH) but the larger sunspot groups can easily reach 1000MH or even much more than that.

Sunspot region 11944 compared with Earth

Image: Sunspot region 11944 at a size of 1480MH as seen by the Solar Dynamics Observatory. The Earth has been added for scale.

Solar area conversion table

Millionths of Hemisphere Square Kilometres Square Degrees
10 30.4 million 0.2
100 304.3 million 2.1
250 760.9 million 5.2
600 1826.2 million 12.4
1200 3652.4 million 24.7

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Last X-flare:2017/09/10X8.2
Last M-flare:2017/09/09M1.1
Last geomagnetic storm:2017/10/15Kp5 (G1)
Number of spotless days in 2017:64
Current stretch spotless days:2

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