Sunspot

A sunspot is a temporary phenomenon on the Sun’s photosphere that appears as a dark patch because it is cooler than the surrounding surface. Sunspots form in regions where intense magnetic fields emerge from the solar interior and inhibit convective heat transport, producing a locally reduced surface temperature. They typically occur in groups within active regions and are indicators of strong magnetic activity on the Sun.

A sunspot has a central dark core called the umbra, surrounded by a lighter region called the penumbra. Umbrae are about 3,000–4,500 kelvin, while penumbrae are closer to the photospheric temperature of about 5,500 kelvin. Sizes range from a few thousand to over 100,000 kilometers in diameter, and the magnetic field strength within sunspots can reach several thousand gauss. Sunspots can last from days to months before dissipating.

Sunspot activity follows an approximately 11-year cycle, with the number and size of spots peaking at solar maximum and declining toward minimum. The Sun’s global magnetic field also reverses polarity with each cycle. Latitudinally, sunspots tend to appear at mid-latitudes at the start of a cycle and migrate toward the equator as the cycle progresses, a pattern sometimes described by a butterfly diagram. Large sunspot groups are often associated with solar flares and coronal mass ejections, which can affect space weather around Earth and other planets.

Sunspots are observed in white light and through spectral lines; studying their magnetic properties uses the Zeeman effect and spectropolarimetry, yielding measurements of field strength and orientation. Historical records of sunspots contribute to studies of solar variability and potential climate connections.