Measuring Meridional Circulation in the Sun

TL;DR

This paper investigates methods to accurately measure the Sun's meridional circulation depth variation, addressing systematic errors in time-distance helioseismology and applying forward modeling to improve measurement reliability.

Contribution

It identifies key systematic errors affecting meridional flow measurements and explores solutions using ray theory-based forward models.

Findings

Systematic errors include apparent shrinking of the Sun and leakage of rotation signals.

Methods to understand and mitigate these errors are discussed.

Forward models help test the sensitivity of travel times to different flow models.

Abstract

Measuring the depth variation of the meridional flows is important for understanding the solar cycle, at least according to a number of dynamo models. While attempting to extend the early observations of \citet{giles_thesis} of time-distance measurements of flow, we have stumbled upon some systematic errors that can affect these measurements: 1) the additional distance traveled by radiation coming from points away from disk center causes an apparent `shrinking' Sun, that is an apparent flow towards the disk center and 2) in measurements away from the central longitude, the rotation signal can leak into meridional flow signals. Attempts to understand and overcome these systematic problems will be presented. Forward models based on ray theory have been applied in order to test the sensitivity of travel times to various models.