Impact of locally suppressed wave sources on helioseismic traveltimes

Abstract

Wave traveltime shifts in the vicinity of sunspots are typically interpreted as arising predominantly from magnetic fields, flows and local changes in sound speed. We show here that the suppression of granulation related wave sources in a sunspot can also contribute significantly to these shifts, and in some cases, an asymmetry between ingoing and outgoing wave traveltimes. The tight connection between the physical interpretation of traveltimes and source-distribution homogeneity is confirmed. Statistically significant traveltime shifts are recovered upon numerically simulating wave propagation in the presence of a localized decrease in source strength. We also demonstrate that these time shifts are relatively sensitive to the modal damping rates; thus we are only able to place bounds on the magnitude of the effect of suppressed sources. We see a systematic reduction of 10–15 s in p-mode mean traveltimes at short distances (∼6.2 Mm) that could be misinterpreted as arising from a shallow (thickness of 1.5 Mm) increase (∼4 per cent) in the sound speed. At larger travel distances (∼24 Mm), a 6–13 s difference between the ingoing and outgoing wave traveltimes is observed; this could mistakenly be inferred as being caused by flows.