Systematic Center-to-Limb Variation in Measured Helioseismic Travel Times and Its Effect on Inferences of Solar Interior Meridional Flows

TL;DR

This study identifies a systematic center-to-limb variation in helioseismic travel times affecting the measurement of solar interior meridional flows, proposing a correction method that significantly alters flow estimates.

Contribution

It reveals the existence of a systematic variation in helioseismic measurements and introduces a practical correction method to improve the accuracy of solar interior flow inferences.

Findings

Systematic center-to-limb variation affects travel-time measurements.

Subtracting east-west from north-south travel times mitigates this variation.

Corrected meridional flow speeds are about 10 m/s slower.

Abstract

We report on a systematic center-to-limb variation in measured helioseismic travel times, which must be taken into account for an accurate determination of solar interior meridional flows. The systematic variation, found in time-distance helioseismology analysis using SDO/HMI and SDO/AIA observations, is different in both travel-time magnitude and variation trend for different observables. It is not clear what causes this systematic effect. Subtracting the longitude-dependent east-west travel times, obtained along the equatorial area, from the latitude-dependent north-south travel times, obtained along the central meridian area, gives remarkably similar results for different observables. We suggest this as an effective procedure for removing the systematic center-to-limb variation. The subsurface meridional flows obtained from inversion of the corrected travel times are approximately 10 m/s slower than those obtained without removing the systematic effect. The detected center-to-limb variation may have important implications in the derivation of meridional flows in the deep interior, and needs a better understanding.