H-K Project

Active Region Evolution

We can tell from the Sunspot Cycle that evolution of active regions occurs. Unlike other effects related to activity, though, active region growth and decay is generally periodic, although on the Sun, new active regions tend to appear close to where previous active regions existed. These large active regions typically last for several weeks, sometimes months. However, smaller active regions might only last for a few days.

These data of the Sun's activity over six months show the formation and subsequent decay of a large active region. The sinusoidal variations are caused by the Sun's rotation as the active region is brought in and out of view every 25 days. The amplitude of the variation changes with a trapezoid-shaped pattern, showing the rise, peak, and fall of activity corresponding to the size of the active region. When there are more active regions on the Sun, each contributes a signal like this to the disk-integrated signal.
Activity of the Sun over several months
The activity of the Sun over several months showing the changing size of an active region as the Sun rotates.

Active region growth on the star 31 Aquilae (HD 182572).
These data from the HK Project show similar behavior for the star 31 Aquilae (G8IV). The rotation period is 41 days. The line overplotting the data shows the increasing amplitude of the rotational variation, indicating that the dominant active region is growing in size.

It is very difficult to monitor active region growth and decay because the variations are not periodic. A new active region can erupt at any time, and at any longitude. If more than one active region exists on the surface of the star at a particular time, the variations in the time series can become very complicated, making it difficult to accurately determine a rotation period. If there are several active regions on the surface at the same time, the combined rotational signals might cancel each other out, making it impossible to detect rotation!

Learning about the time scales of active region evolution for a larger sample of stars is important because it aids in addressing the problems with measuring stellar rotation, detecting and monitoring surface differential rotation, and in understanding the processes involved in forming large-scale activity on the surface.