Measuring vortical flows in the solar interior

Abstract

This thesis focuses on observations of the effects of rotation on solar convection at the length scales of supergranulation and larger (>30 Mm). Rotation drives vortical flows through the Coriolis force and causes anisotropic velocity correlations that are believed to influence the large-scale solar dynamics. We obtain horizontal flows using photospheric Doppler velocity and continuum intensity images from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) spacecraft via the techniques of time-distance helioseismology (TD) and local correlation tracking (LCT) of granules. In time-distance helioseismology, the local vertical vorticity can be measured by taking the difference between wave travel times measured in the anti-clockwise and clockwise directions along a closed contour. The agreement between the TD and LCT methods is excellent up to +/-60° latitude, provided that a center-to-limb correction is applied. Averaging over longitude, one finds that there is a small but significant correlation between the horizontal divergence and the vertical vorticity component of supergranular flows away from the solar equator. By comparison to a noise model, we find that the TD technique can be used to probe the vertical vorticity of flows on spatial scales larger than about 15 Mm, thus including supergranules and also giant cells. We also find that the vertical vorticity signal is much easier to measure using SDO/HMI observations than previous observations. The impact of the Sun's rotation on supergranulation is studied in detail by making spatial maps of the vertical vorticity of the flows associated with the average supergranule. The average supergranule is constructed by co-aligning thousands of individual supergranules in a given latitude band. For the first time, we are able to spatially resolve vorticity associated with inflows and outflow regions. In the northern hemisphere, outflows are on average associated with a clockwise circulation. The signal vanishes at the equator and has opposite sign in the southern hemisphere. Inflow and outflow regions have vertical vorticity of opposite sign, as expected from predictions based on the effects of the Coriolis force. The peak of the vertical vorticity in the average supergranular outflow region is rather broad and weak (full width at half maximum, FWHM, of 13 Mm and peak value of 4 x 10^{-6}/s clockwise at 40° latitude) compared to the average inflow region (8 Mm FWHM and peak value of 8 x 10^{-6}/s anti-clockwise). Furthermore, we study the magnetic field around the average supergranule (in the inflow regions) at the equator using SDO/HMI observations. We discover an anisotropy in the average magnetic field strength, which is larger in the west (prograde) than in the east by about 10%. This surprising result adds to the mystery of solar supergranulation. Whether it is connected to other supergranular properties, such as pattern superrotation or wavelike properties, is unclear.