SDO/HMI Observations Create New Challenge

Tuesday, February 11, 2014

HAO's Mausumi Dikpati proposes thermal forcing as the likely candidate for meridional circulation flow braking.

Accurate knowledge of the Sun’s meridional circulation profile, speed and its time variation from observations and theory is necessary for solar dynamo models and successful predictions of solar cycle features. It is even more necessary now, given that the advection-dominated dynamo models, assuming a single cell meridional circulation in each hemisphere, led to a high solar cycle 24 amplitude, which is unlikely to be validated.

Inferred meridional circulation pattern
Figure 1: Inferred meridional circulation pattern by Zhao et al

New helioseismic inversion methods and new data from HMI are providing the first pictures of meridional circulation below the photosphere. Using HMI data, Zhao et al. found evidence of a second meridional flow cell with depth (see Figure 1). Using SoHO/MDI data Schad et al. found as many as four meridional circulation cells with latitude in each hemisphere, all of which extend to the bottom of the convection zone. Surface Doppler data indicated two or sometimes just one cell with latitude in the photosphere3, and assuming that the flow goes all the way down to the bottom of the convection zone a two-celled flow pattern in the convection zone can be inferred.

Solar-like differential rotation pattern
Figure 2: (a) Solar-like differential rotation pattern as observationally inferred by Corbard et al.5 ; (b) meridional circulation pattern produced by this differential rotation and turbulent viscous Reynolds stresses.

Dikpati has shown that the radial and latitudinal Coriolis forces from the helioseismicly well-known differential rotation are very strong for the Sun, and the meridional circulation driven by these Coriolis forces contains two cells in latitude (see Figure 2), the cells going all the way to the bottom of the adiabatically stratified convection zone. For solar-type turbulent viscosity, the resulting poleward flow speed in the primary cell is two orders of magnitude larger than observed. These results indicate that there must be some other force within the convection zone that brakes the meridional circulation flow speed to produce a solar-like amplitude. Thermal forcing originating from a negative buoyancy and/or nonviscous turbulent stresses are the likely candidates for braking.

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