The sign of temperature inhomogeneities deduced from time-distance helioseismology

Abstract

Inhomogeneities in wave propagation conditions near and below the solar surface 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 increases the sound speed but also lengthens the path along which the wave travels 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 cases the net effect of a temperature enhancement is an {\it increase} of the travel times. We argue that the reduced travel times that are observed are caused by a combination of magnetic fields in the active region and {\it reduced} subsurface temperatures. Such a reduction may be related to the increased radiative energy loss from small magnetic flux tubes.