The Intensity–Velocity Phase Spectra of Evanescent Oscillations and Acoustic Sources
There are three major issues in modeling solar evanescent oscillations: the variation of the intensity [I]–velocity [V] phase difference of p-modes close to the base of photosphere; the existence of a plateau of negative I–V phase differences below and between the ridges of the low-frequency p-modes; the explanation of the I–V cross-spectra of the evanescent oscillations. We present new interpretations for the first two issues, based on modeling intensity fluctuations taking steep temperature gradients, opacity, and non-adiabatic cooling into account.
We also discuss consequences of our model for the explanation of power spectra and cross-power spectra of p-modes. In particular, we present evidence that the acoustic sources that generate evanescent waves produce a coherent background that explains the plateau–interridge regime of negative I–V phase difference.
KeywordsHelioseismology, direct modeling Waves, modes Velocity fields, photosphere
This work is dedicated to F.-L. Deubner, a pioneer of helioseismology and our master in this field. We thank the anonymous referee for making a number of very useful comments. One of the authors (GS) acknowledges financial support from the Italian Space Agency (ASI) and from the PRIN-INAF 2010 Asteroseismology: looking inside the stars with space- and ground-based observations. SDO/HMI data courtesy of NASA/SDO and the HMI science team.
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