The rotation of the Sun varies with latitude because it is composed of gaseous plasma. The rate of rotation is observed to be fastest at the equator and decreases as latitude increases. AT the equator the Solar rotation period is 24.47 days which is the sidereal motion and a synodic rotation period of 26.24 days, which is the time for a fixed feature of the Sun to rotate to the same apparent position as viewed from Earth. The synodic period is longer because the Sun must rotate for a sidereal period plus an extra amount due to the orbital motion of the Earth around the Sun. Sometimes literature uses the definition of a Carrington rotation, a synodic rotation period of 27.26 days. This chosen period corresponds to a rotation at a latitude of 26 degrees, consistent with a typical latitude of sunspots and solar activity.
USING SUNSPOTS TO MEASURE ROTATION
Sunspots viewed from Earth appear to move from left to right across the face of the Sun. By tracking sunspots, the period of solar rotation can be defined. Christopher Scheiner in 1630 was the first to determine the differential solar rotation, noting the rotation was slower at higher latitudes.
Internal rotation in the Sun shows differential rotation in the outer convective region and almost uniform rotation in the central radiative region. The transition between these regions is called the tacholine. Helioseismology can be used to image the far side of the Sun as sunspots absorb helioseismic waves. This sunspot absorption causes a seismic deficit that can be imaged at the antipode of the sunspot. Helioseismology is the study of solar waves, not solar seismic activity as there is none!
CAUSES OF DIFFERENTIAL ROTATION
Differential rotation can be applied to any type of fluid body such as gaseous planets, stars and galaxies. Some causes might be:
Because of pre-stellar accretion phase, and the conservation of angular momentum, rotation is induced.
Differential rotation is caused by convection in stars. The movement of mass is due to steep temperature gradients from the core outwards. This mass carries a portion of the star's angular momentum, thus redistributing the angular velocity, possibly even far enough for the star to loose angular velocity in stellar winds.
Differential rotation depends on temperature differences in adjacent regions.