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).

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