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

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 $[\delta t_{\mathrm{oi}}]$ 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 $1.6\Mm$.

Solar Physics; 15 pages, 6 figures

References in corpus (3)

• Acoustic tomography of solar convective flows and structures
• Linear Sensitivity of Helioseismic Travel Times to Local Flows
• High-resolution mapping of flows in the solar interior: Fully consistent OLA inversion of helioseismic travel times

Cited by in corpus (20)

• Seismic Sounding of Convection in the Sun
• Computational helioseismology in the frequency domain: acoustic waves in axisymmetric solar models with flows
• Spatially resolved vertical vorticity in solar supergranulation using helioseismology and local correlation tracking
• Comparison of solar surface flows inferred from time--distance helioseismology and coherent structure tracking using HMI/SDO observations
• Additional Evidence Supporting a Model of Shallow, High-Speed Supergranulation
• Testing Helioseismic-Holography Inversions for Supergranular Flows Using Synthetic Data
• Inversions for Average Supergranular Flows Using Finite-frequency Kernels
• Helioseismic Investigation of Modeled and Observed Supergranule Structure
• The Quest to Understand Supergranulation and Large-Scale Convection in the Sun
• Sensitivity kernels for time-distance helioseismology: efficient computation for spherically-symmetric solar models
• Analyzing supergranular power spectra using helioseismic normal-mode coupling
• Combined helioseismic inversions for 3D vector flows and sound-speed perturbations
• Recent Developments in Helioseismic Analysis Methods and Solar Data Assimilation
• Plasma flows and sound-speed perturbations in the average supergranule
• Iterative inversion of synthetic travel times successful at recovering sub-surface profiles of supergranular flows
• Validating Forward Modeling and Inversions of Helioseismic Holography Measurements
• Probing sunspots with two-skip time-distance helioseismology
• Observational characterisation of large-scale transport and horizontal turbulent diffusivity in the quiet Sun
• Validating time-distance helioseismic inversions for non-separable subsurface profiles of an average supergranule
• Statistics of the two-point cross-covariance function of solar oscillations