The Shape of the Solar Tachocline

TL;DR

This study uses extensive helioseismic data and flexible modeling to reveal the complex, non-uniform shape of the solar tachocline, showing it bulges at mid latitudes and varies with latitude, challenging previous simple models.

Contribution

It introduces a more flexible modeling approach to determine the solar tachocline's shape, revealing its deviations from simple prolate or oblate forms and its complex latitudinal structure.

Findings

Tachocline bulges at mid latitudes.

Center of tachocline varies with latitude.

High-latitude shape is uncertain and model-dependent.

Abstract

Early helioseismic results have shown that the tachocline has a prolate shape. However, the models used in those studies constrained the tachocline to be either prolate or oblate. We use helioseismic data obtained from long time series (2304 and 4608 days) to determine the shape of the solar tachocline. Like previous work, we use forward modeling methods for this work; however, we allow more flexibility for the shape of the tachocline. We find that the tachocline does indeed deviate from a simple prolate structure and bulges out at mid latitudes. The center of the tachocline lies in the radiative zone at low latitudes, in the convection zone at intermediate latitudes, and back in the radiative zone at high latitudes. The high-latitude ($ > 60^\circ$) behavior is, however, uncertain and model dependent. Models that allow more variation of the shape indicate that the tachocline at high latitudes is almost coincident with the base of the convection zone.