### 10.3 Angular momentum variations

Given estimates of both density and rotation as functions of depth and latitude, one can calculate the
solar angular momentum locally or globally. Of course, such calculations will reflect, and in
some cases enhance, any errors in the input data, and should therefore be approached with
caution.
Komm et al. (2003) investigated the angular momentum variation based on the inversions of GONG
and MDI data used by Howe et al. (2000b,a) and found variations reflecting the torsional oscillation well
into the convection zone and 1.3 year variations close to the tachocline. Because the density increases
steeply with decreasing radius, variations at greater depths will be more strongly seen in the angular
momentum than in the rotation rate, but it should be remembered that no new information has been added
to the data.

Lanza (2007) approached the problem from the other direction, considering the role of angular
momentum transport in the modeling of the torsional oscillation.

Antia et al. (2008) investigated temporal variations of the solar kinetic energy, angular momentum and
higher-order gravitational multipole moments as derived from helioseismic inferences of the internal rotation
rate; they found variations on the time scale of the solar cycle (but not the 1.3 year cycle), with some
discrepancies between MDI and GONG results. They also speculate that the kinetic-energy changes might
contribute to the observed irradiance variations during the solar cycle; however, it is not clear that such a
contribution is needed, as the usual view is that the solar-cycle variation in irradiance can be modeled
simply from the effects of sunspots and plage on the surface, as discussed, for example, by Jones
et al. (2008).