Estimating the solar meridional circulation by normal mode decomposition

TL;DR

This paper applies Fourier-Hankel decomposition to solar Doppler data to estimate meridional flow velocities, confirming previous results and suggesting potential for measuring deep convection zone flows with further improvements.

Contribution

It demonstrates the use of Fourier-Hankel decomposition on SOHO data to estimate meridional circulation, confirming earlier findings and proposing a new technique for deep zone measurements.

Findings

Average meridional flow of 15 m/s in upper 20 Mm of the convection zone

Reproduces earlier results using the same method

Potential for measuring deep convection zone flows with improvements

Abstract

The objective of this article is to use Fourier-Hankel decomposition as suggested earlier by Braun & Fan (1998) to estimate the integrated horizontal meridional flow velocity as a function of mode penetration depth, and to find ways of potentially improve this technique. We use a time series of 43200 (30 days) consecutive full-disk Dopplergrams obtained by the MDI (Michelson Doppler Imager) instrument aboard the SOHO (Solar Heliospheric Observatory) spacecraft in April 1999. We find averaged meridional flow estimates of 15 m/s for modes with a penetration depth in the upper 20 Mm of the solar convection zone. This reproduces the results of the earlier investigations. Moreover we conclude that this method has the potential to become a new technique to measure the meridional circulation in the deep convection zone, if some improvements will be applied.