Sunspots are dark regions on the Sun. They are regions of intense magnetic field that are intimately associated with solar flares and coronal mass ejections. Sunspots appear dark on the sun’s surface because they are cooler (3700 K) than the rest of the solar surface (6000 K). Sunspots are cooler because their intense magnetic fields inhibit the rise of heat from the solar interior.
Sunspots can appear alone, or in close connection to other sunspots. Sunspots that are clearly connected to each other are grouped into active regions with an official number designated by NOAA. Active regions are classified according to their size and complexity with a scale known as the modified Zurich scale.
Though sunspots appear darker than the rest of the sun to the naked eye, when the active regions they appear in are examined in ultraviolet or x-rays wavelengths, then sunspots and the associated active regions appear considerably brighter than the other areas of the Sun. The irradiance these other wavelengths show actually originates at higher levels in the solar atmosphere, but in the same region of the sun.
The energy that sunspots lack in heat is made up instead by the energy of the magnetic field. The magnetic fields rise above the surface and remain strong, while the rest of the sun has weak overlying magnetic fields. The strong magnetic fields form into loops that confine solar plasma and heat it to extreme temperatures in excess of 1 million K. At these temperatures, we can’t see the plasma in visible light, but it becomes the dominant feature in ultraviolet and x-rays. Changes in these magnetic fields are the main source of most of space weather, like solar flares (Radio Blackouts) and fast coronal mass ejections (CMEs).
Sunspots change continuously, but individual spots may persist for only a few hours or for many weeks and even months. The total number of sunspots has long been known to vary with an 11-year period known as the solar cycle. The peak of the solar cycle is known as solar maximum and the valley of the cycle is known as solar minimum.
At solar minimum there can be many days in a row with absolutely no sunspots visible, while at solar maximum hundreds of spots may be visible at any time. Solar cycles are numbered with solar cycle 1 beginning in 1755 and the most recent solar cycle, cycle 24, began in December, 2008. Counting of sunspot numbers is a bit tricky, because not only does each sunspot count as 1, but each active region counts as 10. Because a lone sunspot is also considered an active region, it is counted as 11, so on a given day, the sunspot number can be 0, 11, or any number greater than 11.