The Sun’s seismic radius, measured from the frequencies of f modes, is determined using both MDI and HMI data, covering a total of 21 years. It is found that the seismic radius is reduced by 1-2 km during the maxima, but the largest change of the radius happens at about 5 Mm beneath the surface.
Motivated by recent observations we have explored whether the flux-transport dynamo model can work with multi-cell meridional flow. We find that it can work when certain conditions are fulfilled.
Analysis of a large number of supergranules observed with HMI and simulations with a convectively stabilized solar model imply that the average supergranular cell has a peak upflow of 240 m s-1 at a depth of 2.3 Mm and a corresponding peak outward horizontal flow of 700 m s-1 at a depth of 1.6 Mm.
Solar meridional circulation, if mechanically driven and thermally
braked, contains two cells in latitude; generating two cells in depth,
recently observed by SDO/HMI, is a new challenge to theory.