What Phil said about sunspots is correct. However the very interesting questions not said are: What produces those very localized magnetic fields that create the sunspots? Why do sunspots appear in 11 year cycles?

In modeling low altitude spacecraft trajectories it is vital to know what the local atmospheric density is. The upper atmospheric density is strongly a function of two solar parameters:

10.7 CM SOLAR FLUX
GEOMAGNETIC DAILY INDEX

If you plot these two parameters as a function of time, you'll end up with something that looks like white noise in the short term but has a nice 11 year cycle over the long haul. Nobody understands why the Sun does this (people have theories of course). Understanding the Sun is vital in terms of spacecraft navigation and also for more prosaic things like radio communication and climate modeling. Oddly enough we spend relatively little on solar astronomy. People at NASA have been wanting to send a spacecraft up close to the sun for years but they can't get it funded (not sexy enough). One of the big contributers to solar research is/was the US Air Force. Several years ago the USAF funded several follow-on spacecraft to the SAO series (STP/78-1 was an SAO clone).

Gary