The stellar thermal wind as a consequence of oblateness

TL;DR

This paper derives a new stellar thermal wind equation based on oblateness, revealing that the thermal wind's aspherical temperature anomaly in the Sun is smaller than previously thought, impacting helioseismic measurements.

Contribution

It introduces a generalized stellar thermal wind equation incorporating full oblateness and clarifies its implications for solar asphericity analysis.

Findings

Baroclinic anomaly is 3-60 times smaller than barotropic anomaly.

Full oblateness consideration reduces the expected thermal wind anomaly.

Potential to constrain the solar tachocline depth via helioseismic measurements.

Abstract

In many rotating fluids, the lowest-order force balance is between gravity, pressure, and rotational acceleration ('GPR' balance). Terrestrial GPR balance takes the form of geostrophy and hydrostasy, which together yield the terrestrial thermal wind equation. By contrast, stellar GPR balance is an oblateness equation, which determines the departures of the thermal variables from spherical symmetry; its curl yields the 'stellar thermal wind equation.' In this sense, the stellar thermal wind should be viewed not as a consequence of geostrophy, but of baroclinicity in the oblateness. Here we treat the full stellar oblateness, including the thermal wind, using pressure coordinates. We derive the generalised stellar thermal wind equation and identify the parameter regime for which it holds. In the case of the Sun, not considering the full oblateness has resulted in conflicting calculations of the theoretical aspherical temperature anomaly. We provide new calculation here and find that the baroclinic anomaly is ~3-60 times smaller than the barotropic anomaly. Thus, the anomaly from the thermal wind may not be measurable helioseismically; but if measurement were possible, this would potentially yield a new way to bracket the depth of the solar tachocline.