Probing the Variation with Depth of the Solar Meridional Circulation using Legendre Function Decomposition
Abstract
The solar meridional circulation is a crucial component of magnetic flux transport and dynamo models. Despite decades of helioseismic study, no consensus exists regarding the variation of its properties with depth. It has become apparent that the main challenges consist of 1) overcoming realization noise with multi-year long datasets, and 2) the identification and robust removal of systematic center-to-limb effects. Here we apply the helioseismic methodology of Legendre Function Decomposition (LFD) to 7.5 years of Dopplergrams obtained by the Helioseismic and Magnetic Imager (HMI) as the basis of inferring the depth variation of the meridional flow between 20 and 60 degrees latitude in both hemispheres. The LFD method, first developed by Braun and Fan in 1998, probes subsurface flows through the Doppler-effect induced distortion of power spectra. The procedure is optimized for the detection of meridional flows and uses Legendre functions (of the first and second kind) to characterize poleward and equatorward wave propagation in spherical coordinates. For this study we have developed control procedures which assess and remove center-to-limb artifacts, using measurements obtained by applying the procedure to pseudo poles at the east and west limbs. Forward modeling is carried out to evaluate the consistency of the corrected LFD frequency shifts with various assumed models of the depth variation of the meridional circulation.
DB is supported by the NASA Heliophysics Division (awards 80NSSC18K0066 and 80NSSC18K0068) and by the Solar Terrestrial program of the National Science Foundation (award AGS-1623844).- Publication:
-
American Astronomical Society Meeting Abstracts #234
- Pub Date:
- June 2019
- Bibcode:
- 2019AAS...23431802B