The internal rotation of the Sun is intimately related to the processes that drive the activity cycle. Brown et al. (1989) stated that, “Knowledge of the internal rotation of the Sun with latitude, radius, and time is essential for a complete understanding of the evolution and the present properties of the Sun,” and this remains true today.
The Sun rotates on its axis approximately once every 27 days; however, the rotation is not uniform, being substantially slower near the poles than at the equator. This superficial aspect of the solar differential rotation was well known from sunspot observations as early as the 17th century. However it is only within the last 30 years that it has become possible to observe the rotation profile in the solar interior, and mostly within the most recent solar cycle that its subtle temporal variations have become evident. Helioseismology – the study of the waves that propagate within the Sun and the inference from their properties of the solar interior structure and dynamics – is the most important tool we have to measure this internal rotation.
In this review, we start by introducing some of the basic concepts of helioseismology (Section 2) and the inversion problem (Section 3) as it applies to the internal solar rotation. Next, after a brief historical overview (Section 4) of the observations, we consider what we have learned from helioseismology about the rotation profile and its variation with depth.
We consider first the time-invariant part of the solar rotation profile. The main features of interest are (Figure 1):
We will consider each of these in turn, working outwards from the core to the surface, and then discuss the time-varying part of the rotation – the torsional oscillation (Section 9) and the possible variations at the base of the convection zone (Section 10). We attempt to place the observations in the context of models; however, this is a review from an observer’s point of view, and an exhaustive examination of the models themselves is beyond its scope.
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