Helioseismic rays trapped in a nonmagnetic acoustic cavity suffer a +90° phase jump at their lower (Lamb) turning point and −90° at the upper (acoustic cutoff) reflection point. That the two cancel allows helioseismologists to effectively assume that phase is locally continuous along a ray path joining two surface points. However, in strong surface magnetic field, as found in sunspots, it is shown – for an isothermal model with uniform magnetic field – that the phase jump for fast magnetoacoustic rays that penetrate the acoustic/Alfvénic equipartition level (c=a) is around −120°. Moreover, there are further negative phase jumps on the upgoing and downgoing legs at c=a that add to the net phase change. Neglecting these effects can lead to a misinterpretation of helioseismic data in terms of travel-time shifts.
KeywordsWaves, magnetohydrodynamic Helioseismology, theory
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