Local helioseismology techniques enable us to probe the subsurface magnetic fields and flows by observing p-modes. Active regions on the surface of the Sun have strong magnetic fields and techniques such as time-distance helioseismology can provide useful diagnostic for probing these structures. We investigate the effect of uniform horizontal magnetic field upon the travel time of acoustic waves. It is shown that such fields can lower the upper turning point of p-modes and hence affect their travel time.
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