A Poynting-Robertson-like drag at the Sun's surface

TL;DR

This paper proposes that a Poynting-Robertson-like drag at the Sun's surface causes angular momentum loss, explaining the observed slow-down of surface rotation compared to the interior.

Contribution

It introduces a novel mechanism of angular momentum loss at the Sun's surface analogous to Poynting-Robertson drag, supported by new helioseismology analysis.

Findings

Identifies a large velocity gradient at the top of the photosphere.

Suggests the photospheric torque accounts for the near-surface shear.

Shows the torque's magnitude is comparable to the angular momentum deficit.

Abstract

The Sun's internal rotation {\Omega}(r,{\Theta}) has previously been measured using helioseismology techniques and found to be a complex function of co-latitude, {\theta}, and radius, r. From helioseismology and observations of apparently "rooted" solar magnetic tracers we know that the surface rotates more slowly than much of the interior. The cause of this slow-down is not understood but it is important for understanding stellar rotation generally and any plausible theory of the solar interior. A new analysis using 5-min solar p-mode limb oscillations as a rotation "tracer" finds an even larger velocity gradient in a thin region at the top of the photosphere. This shear occurs where the solar atmosphere radiates energy and angular momentum. We suggest that the net effect of the photospheric angular momentum loss is similar to Poynting-Robertson "photon braking" on, for example, Sun-orbiting dust. The resultant photospheric torque is readily computed and, over the Sun's lifetime, is found to be comparable to the apparent angular momentum deficit in the near-surface shear layer.