Helioseismic holography of simulated solar convection and prospects for the detection of small-scale subsurface flows

We perform helioseismic holography on realistic solar convection simulations and compare the observed travel-time perturbations with the expected travel times from the horizontal flows in the simulations computed from forward models under the assumption of the Born approximation. We demonstrate reasonable agreement between the observed and model travel times, which reinforces the validity of helioseismic holography in the detection of subsurface horizontal flows. An assessment is made of the uncertainty of the measured p-mode travel times from the rms of the residuals. From the variation of the signal-to-noise ratio with depth we conclude that the helioseismic detection of individual flow structures with spatial scales of supergranulation or smaller is not possible for depths below about 5 Mm below the surface over timescales of less than a day. The travel-time noise estimated from these simulations appears to be similar to noise in measurements made using solar observations. We therefore suggest that similar limitations exist regarding the detection of analogous subsurface flows in the Sun. A study of the depth dependence of the contribution to the travel-time perturbations for focus depths between 3 and 7 Mm is made, showing that approximately half of the observed signal originates within the first 2 Mm below the surface. A consequence of this is a rapid decrease ( and reversal in some cases) of the travel-time perturbations with depth due to the contribution to the measurements of oppositely directed surface flows in neighboring convective cells. This confirms an earlier interpretation of similar effects reported from observations of supergranulation.