P‐Mode Intensity‐Velocity Phase Differences and Convective Sources

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We study the origin of the solar p-mode intensity-velocity phase differences at high degree (l > 100). Observations show phase differences that are very different from those derived from linear theory alone. The theory predicts a smooth variation with frequency, dependent only on atmospheric parameters, while observations show large fluctuations across modal frequencies. We support previous suggestions that fluctuations in the intensity-velocity phase differences and line asymmetries in the intensity and velocity power spectra are produced by "contamination" of the p-mode signal with noise correlated with the excitation sources. It is demonstrated that the qualitative shapes of the observed phase-difference and power spectra can be realized only if both temperature (intensity) and velocity (Doppler shift) observations contain correlated noise. Moreover, the details of the observed spectra allow only a limited choice of noise parameters and constrain well the convective process responsible for p-mode excitation. The inferred correlated noise signals are consistent with the (visible) formation of convective downflows accompanied by darkening (lowered emergent intensity) and subsequent acoustic excitation. An upward velocity pulse follows after the wave excitation, which suggests overshoot of inflowing material that fills in the evacuated volume in the wake of the new downflow.