Carrington Rotations

The PROBA2 team is proud to present fully automated and daily updated Carrington rotation movies. A Carrington Rotation is a period of time chosen to represent one rotation of the Sun, allowing the comparison of features such as sunspot groups or active regions (a description of active regions can be found here). A period of 27.28 days was chosen to represent a single rotation that largely resembles the recurrence time of features near the equator.

 

The rotation period was initially conceived by Richard Carrington, when he determined the rotation rate of low latitude sunspots. Carrington determined a rotation rate of 25.38 days relative to background stars (sidereal rotation period). However, due to the Earths orbit, this is perceived as 27.28 days from the Earth. Each rotation of the Sun is assigned a Carrington Rotation Number, starting from 09-Nov-1853. That number has now reached 2194 at time of writing (13-Jun-2017); More information can be found here.

When observing the Sun for a prolonged period of time, it soon becomes evident that features on its surface, and in its outer atmosphere do not rotate at the same rate. This is because the Sun is not a solid body, but a big ball of magnetised plasma, whose rotation is variable with position and height in the solar atmosphere. One of the most striking observations is that of ‘differential’ rotation, where features on the solar surface and in the solar atmosphere are observed to rotate faster at the equator (rotation period = 25.4 days) when compared to those closer to the poles (rotation period = 36 days). This is evident in observations of sunspots, which have been used as tracers for measuring solar motion for many years.