The sign of temperature inhomogeneities deduced from time-distance helioseismology

Inhomogeneities in wave propagation conditions near and below the solar sur face have been detected by means of time-distance helioseismology. Here we calculate the effect of temperature inhomogeneities on the travel times of sound waves. A temperature increase, e.g., in active regions, not only incr eases the sound speed but also lengthens the path along which the wave trav els because the expansion of the heated layers shifts the upper turning of the waves upward. Using a ray-tracing approximation we find that in many ca ses the net effect of a temperature enhancement is an increase of the trave l times. We argue that the reduced travel times that are observed are cause d by a combination of magnetic fields in the active region and reduced subs urface temperatures. Such a reduction may be related to the increased radia tive energy loss from small magnetic flux tubes.