Sunspots are dark regions on the Sun’s surface. The number of sunspots varies with an approximately 11-year cycle and is a used as an indicator of the level of solar activity that drives space weather.
When Galileo pointed his telescope at the Sun and observed dark regions instead of a uniform disk, it triggered an upheaval in thinking about the Sun. We now know that sunspots are regions of strong magnetic fields that inhibit the heat convection from the Sun’s interior resulting in regions (4,500 K) that are cooler than the surrounding areas (6,000 K) and so appear darker. The strong magnetic fields within sunspot groups lead to solar flares and eruptions on the Sun producing “space weather,” which is a hazard to modern technology. The number of sunspots provides a good indicator of the level of solar activity and the space weather hazard to a variety of systems. Regular drawings of sunspots have been made for many years and used to produce a daily sunspot number...
- Van der Linden, R., and Vanlommel, P., (eds.), Sunspot Bulletin, Solar Influenes Data Analysis Center, http://sidc.oma.be.