Thursday, August 20, 2015

Spotting Sunspots

Sunspot AR 2403 (c) John Ashley
Sunspot AR 2403 visible through a smokey sunset on August 20th

We have a lovely sunspot (AR 2403) spinning nicely into view over the next several days. And all of the smoke over western Montana is actually making it a lot easier to see this dark region on a dimmed-down sun.

Our roiling sun is stratified into several layers. The photosphere is a thin, outer layer that's about 60 miles (100 km) thick where the sun's energy is released as light, and this is where sunspots occur. Sunspots are actually quite bright, but they appear dark compared to the boiling plasma surrounding them. At 3,800K degrees, sunspots are about 1,500K degrees cooler than the brighter plasma. Sunspots include two groups or clusters. One group has a negative charge while the other group is positive. The darkest part of a spot is where the magnetic field is strongest.

These spots are caused by interactions in the sun's magnetic field, but we still don't fully understand the process. Smaller spots last a few days while the larger ones can persist for months and many revolutions around the sun. Because the sun isn't solid, different regions rotate at different speeds. A point on the sun's equator makes a complete revolution in about 24 Earth days, while the polar regions spin slower and take about 30 Earth days. Sunspots are concentrated in two bands on either side of the solar equator, so a big group might circle the sun in 25 days or so.

Sunspots come and go, but their numbers aren't random. They wax and wane in 11-year cycles known as solar minimum and maximum. The most recent solar minimum occurred in 2008, so we should be slowly building towards a solar maximum in 2019. Maybe by then all of this smoke will have cleared.