Subsurface Supergranular Vertical Flows as Measured Using Large Distance Separations in Time-Distance Helioseismology

TL;DR

This study measures vertical flows in solar supergranulation using large-distance time-distance helioseismology, revealing that vertical flows increase with depth and are detectable through improved filtering techniques.

Contribution

It introduces more effective filters for large-distance helioseismic measurements and demonstrates that vertical flows in supergranulation extend deeper than previously observed.

Findings

Vertical flows peak at 2.3 Mm depth with 240 m/s.

Travel time difference is constant at 5.1 seconds across distances.

Vertical flow increases with depth, requiring deeper flow models.

Abstract

As large--distance rays (say, 10\,-\,$24 ^\circ$) approach the solar surface approximately vertically, travel times measured from surface pairs for these large separations are mostly sensitive to vertical flows, at least for shallow flows within a few Mm of the solar surface. All previous analyses of supergranulation have used smaller separations and have been hampered by the difficulty of separating the horizontal and vertical flow components. We find that the large separation travel times associated with supergranulation cannot be studied using the standard phase-speed filters of time-distance helioseismology. These filters, whose use is based upon a refractive model of the perturbations, reduce the resultant travel time signal by at least an order of magnitude at some distances. More effective filters are derived. Modeling suggests that the center--annulus travel time difference $[\delta t_{\rm{oi}}]$ in the separation range $\Delta=10$\,-\,$24 ^\circ$ is insensitive to the horizontally diverging flow from the centers of the supergranules and should lead to a constant signal from the vertical flow. Our measurement of this quantity, $5.1 \pm 0.1\secs$, is constant over the distance range. This magnitude of signal cannot be caused by the level of upflow at cell centers seen at the photosphere of $10\ms$ extended in depth. It requires the vertical flow to increase with depth. A simple Gaussian model of the increase with depth implies a peak upward flow of $240\ms$ at a depth of $2.3\Mm$ and a peak horizontal flow of $700\ms$ at a depth of $1.6\Mm$.