A new approach for the global helioseismic investigation of the solar meridional flow

Helioseismic investigations of acoustic waves propagating through the Sun have given astonishing insights about the structure of the solar interior and its rotation in recent years. But still there are many open questions, especially concerning dynamical processes in the interior that are related to the Sun’s 11-year activity cycle. In particular, the meridional flow is of importance. The flow is involved in the transport of magnetic flux and the generation of the solar poloidal magnetic field, and consequently a fundamental element of solar dynamo theories. Near the surface, it is found to be a slow large-scale flow directed poleward on both hemispheres. Especially its penetration depth and velocity in the interior are critical values for predictions on the solar cycle length, but only marginally known.

In this thesis we develop and investigate the theoretical framework and a thereon based new analysis method for the global helioseismic inference of the meridional flow at large depths. The method bases on the perturbation of eigenfunctions of resonant acoustic waves propagating through the solar interior due to the advection of solar plasma by the flow. We relate these perturbations to the observable acoustic wave field at the solar surface by introduction of a new quantity, the amplitude ratio, which can be determined by a measure from cross-spectral analysis. We show that this measure is sensitive to mode perturbations due to the meridional flow as well as solar rotation and spatial observation artifacts. The analysis method is tested on simulated data and applied to spatio-temporally resolved Doppler velocity measurements from the Michelson Doppler Imager (MDI) instrument covering about six years of observations from 2004–2010. The results present a meridional flow that can be described by a superposition of at least two flow components, one with two large poleward directed flow cells and one small-scale component with eight flow cells in latitude. The small-scale flow component permeates the complete convection zone down to 195Mm depth and exhibits layer of counterflow cells starting at about 100Mm depth. Near the solar surface, our results are in agreement with flow measurements obtained from local helioseismic techniques that typically reach depths of about 20Mm. In summary, the here presented global helioseismic analysis method is able infer the meridional flow at depths much deeper than current used helioseismic techniques. Our flow measurements provide first global helioseismic evidence of a deep penetrating meridional flow consisting of multiple cells in latitude and depth as favored by some dynamo models. Therefore, our results help to extent the present prevalent helioseismic picture of the meridional flow and to constrain models of the solar dynamo.