Helioseismic Investigation of Modeled and Observed Supergranule Structure

TL;DR

This study compares the subsurface structure of an average supergranule derived from HMI data with a helioseismic flow model, revealing significant differences in depth extent and flow velocities, and highlighting limitations in current helioseismic techniques.

Contribution

It provides the first detailed comparison between observed supergranule structures and a helioseismic model, identifying key discrepancies and methodological limitations.

Findings

HMI supergranules are more extended in depth than models suggest.

Observed flow velocities are much smaller than model predictions.

Systematic inaccuracies in helioseismic techniques are confirmed.

Abstract

The subsurface structure of an "average" supergranule is derived from existing HMI pipeline time-distance data products and compared to the best helioseismic flow model detailed in Duvall and Hanasoge (2013). We find that significant differences exist between them. Unlike the shallow structure predicted by the model, the average HMI supergranule is very extended in depth, exhibiting horizontal outflow down to $7$--$10$~Mm, followed by a weak inflow reaching a depth of $\sim20$~Mm below the photosphere. The maximal velocities in the horizontal direction for the average supergranule are much smaller than the model, and its near-surface flow field RMS value is about an order of magnitude smaller than the often-quoted values of $\sim250-350$~$\rm{m\,s^{-1}}$ for supergranulation. Much of the overall HMI supergranule structure and its weak flow amplitudes can be explained by examining the HMI pipeline averaging kernels for the near-surface inversions, which are found to be very broad in depth, and nearly identical to one another in terms of sensitivity along the $z$-direction. We also show that forward-modeled travel times in the Born approximation using the model (derived from a ray theory approach) are inconsistent with measured travel times for an average supergranule at any distance. Our findings suggest systematic inaccuracies in the typical techniques used to study supergranulation, confirming some of the results in Duvall and Hanasoge (2013).