Angular momentum transport and dynamo action in the sun - Implications of recent oscillation measurements

Abstract

We consider the implications of a newly proposed picture of the Sun's internal rotation (Brown et al, Morrow) for the distribution and transport of angular momentum and for the solar dynamo. The new results,' derived from an analysis of solar acoustic oscillations, affect our understanding of how momentum is cycled in the Sun and provide clues as to how and where the solar dynamo is driven. The data imply that the only significant radial gradient of angular velocity exists in a transitional region between the bottom of the convection zone, which is rotating like the solar surface, and the top of the deep interior, which is rotating rigidly at a rate intermediate between the equatorial and polar rates at the surface. Thus the radial gradient must change sign at the latitude where the angular velocity of the surface matches that of the interior. These inferences suggest that the cycle of angular momentum that produces the observed latitudinal differential rotation in the convection zone may be coupled to layers of the interior beneath the convection zone. In particular, they support the idea of a time averaged balance of torques in the transitional zone such that the torques' caused by some physical source of stress at low latitudes, balance the torques exerted in the opposite direction at high latitudes. We show that a balance of torques is likely by employing a simple model which includes a constant eddy viscosity and transition layer depth along with differences in angular velocity across the region that are inferred from forward analysis of the oscillations data. Earlier estimates of solar surface rotation derived both from Doppler analysis of the plasma velocity and from the motions of magnetic features imply differences in angular velocity that also support a balance of torques and thus a balanced exchange of angular momentum between the convection zone and deep interior. We use a simple Cartesian model of a a 2-co dynamo in the transitional region with an appropriate range of velocity differences to demonstrate the plausibility of operating a dynamo there.

Coupling the Solar Dynamo and the Corona: Wind Properties, Mass, and Momentum Losses During an Activity Cycle

“…A (middle), and magnetic braking timescale δt sd = J /J (bottom) during the solar cycle. J is the Sun's angular momentum (Gilman et al 1989);J is the wind's angular momentum-loss rate. a chromosphere-corona transition region and a coronal heating flux dependent on the local magnetic field's strength.…”

“…ρ (r) r 4 dr ≈ 1.84 × 10 48 g cm 2 s −1 (12) (Gilman et al 1989;Stix 2002) using a seismically calibrated solar model for ρ (r) (a CESAM model;Morel 1997;Brun et al 2002).…”

The Solar-Stellar Connection

“…One can identify key angular momentum transport processes which are expected to act inside a stellar convection zone or radiative interior: Reynolds and Maxwell stresses, meridional circulation, viscous processes (usually negligible), large scale magnetic torque, gravity waves (see Gilman et al 1989;Zahn 1992;Pinsonneault 1997;Zahn 2004, 2005;Denissenkov et al 2010;Mathis 2011;Reiners and Mohanty 2012;Mathis 2013b;Gallet and Bouvier 2013;van Saders and Pinsonneault 2013, and references therein). The amplitude and role of each process varies as a function of time.…”

The Internal Solar Rotation from Helioseismology

“…2). By correcting for this it has been estimated that the actual width of the tachocline is less than approximately 0.05R (e.g., Corbard et al 1998;Charbonneau et al 1999 for the operation of a dynamo mechanism which may be responsible for the Ll-year solar magnetic cycle (e.g., Gilman, Morrow, & DeLuca 1989).…”