Sensitivity of Time-Distance Helioseismic Measurements to Spatial Variation of Oscillation Amplitudes. I. Observations and a Numerical Model

It is well known that the observed amplitude of solar oscillations is lower in sunspots than in quiet regions of the Sun. We show that this local reduction in oscillation amplitudes, combined with the phase-speed filtering procedure in time-distance helioseismic analyses, could be a source of systematic errors in the range of 5%-40% in the measured travel-time anomalies of acoustic waves around sunspots. Removing these travel-time artifacts is important for correctly inferring the subsurface structure of sunspots. We suggest an empirical correction procedure and illustrate its usage for a small sunspot. This work uses data from SOHO MDI.