What is an active region on the Sun?
Active regions are areas with strong magnetic fields – as high as 1,000 or more times the average solar magnetic field strength – on the surface of the Sun. Active regions often produce sunspots, which are regions of plasma that appear darker than the surrounding photosphere due to their significantly cooler temperatures. Active regions and their associated sunspots appear and disappear on the surface of the Sun on the timescale of days to a few months. Though they may look small in pictures of the solar surface, they can actually be many tens of thousands of kilometers across. Active regions occur more frequently during the peak of the solar cycle, when the Sun is most magnetically active.
Why are active regions on the Sun of interest?
Active regions are of great scientific interest because they are associated with explosive magnetic events, like solar flares and coronal mass ejections (CMEs). The strong magnetic fields around active regions often lead to the release of large amounts of energy in the form of electromagnetic radiation – especially in X-ray and UV radiation – as well as the release of large amounts of energy in the form of particle radiation – including electrons, protons, and other high-energy solar energetic particles (SEPs). These solar storms, in addition to providing important insight into the connection between magnetic fields and the Sun’s outer layers, can also affect our daily lives here on Earth. High-energy radiation and particles can negatively impact satellites, the health of astronauts in space, disrupt radio communications on Earth, and even disrupt or significantly damage power grids. In 2003, a group of active regions produced a series of eruptions, including the most powerful solar flare ever observed and multiple CMEs, in what are now referred to as the “Halloween Storms of 2003.”
How do active regions on the Sun form?
Active regions on the Sun form when magnetic field loops emerge from the solar interior, out through the photosphere, and into the outer layers of the Sun. These magnetic loops often appear in pairs, with one loop “footpoint” corresponding to north polarity, and the other loop “footpoint” corresponding to south polarity. Active regions are composed of large quantities of these loop pairs. The formation, evolution, and eventual disappearance of active regions reflects the complex structure of the solar magnetic field and the influence of convective motions, differential rotation, and magnetic reconnection in the Sun.