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| Paper: |
Modeling the Sun's Global Meridional Circulation |
| Volume: |
478, Fifty Years of Seismology of the Sun and Stars |
| Page: |
283 |
| Authors: |
Dikpati, M. |
| Abstract: |
Meridional circulation is an important ingredient for solar dynamo
models, and hence knowing its speed, its profile in latitude and depth and its
time variation is crucial for understanding the dynamo. Observations provide
us with knowledge about its speed and profile at the surface, more accurately
at low to mid-latiudes. Consensus has not been reached regarding what is
happening in polar regions and at greater depths. The theory of meridional
circulation with latitude and depth can give us guidance as to what may be
happening in these regions. A hierarchy of complexity of approaches to
develop this theory is possible. We have built a global hydro-dynamical model of
meridional circulation that includes Coriolis forces from differential
rotation, turbulent Reynolds stresses, pressure
forces, and provision for thermodynamics. By specifying differential rotations
motivated by observations, we find that the steady-state solutions from this
model include the following patterns: i) one long cell with poleward surface
flow and an equatorward return flow at the base of the convection zone when
there is no density increase with depth; ii) a primary flow-cell with poleward
surface flow at low to mid-latitudes, together with a second, high latitude,
reversed equatorward flow cell in the case of a solar-like density increase
with depth and a solar-like differential rotation. However, for solar-like
turbulent viscosity, the meridional flow speeds are much larger than observed,
implying that an additional physical mechanism is needed that works against
the meridional flow. The most likely candidate is a negative buoyancy force
arising from small departures of the radial temperature gradient from
the adiabatic gradient. There are several possible mechanisms for producing
such an effect, but none can be defined well from available observations. |
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