p ‐Mode Intensity‐Velocity Phase Differences and Convective Sources

Abstract

We study the origin of the solar p-mode intensity-velocity phase di†erences at high degree (l[100). Observations show phase di†erences that are very di†erent from those derived from linear theory alone. The theory predicts a smooth variation with frequency, dependent only on atmospheric parameters, while observations show large Ñuctuations across modal frequencies. We support previous suggestions that Ñuctuations in the intensity-velocity phase di†erences and line asymmetries in the intensity and velocity power spectra are produced by ““ contamination ÏÏ of the p-mode signal with noise correlated with the excitation sources. It is demonstrated that the qualitative shapes of the observed phase-di†erence 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 downÑows accompanied by darkening (lowered emergent intensity) and subsequent acoustic excitation. An upward velocity pulse follows after the wave excitation, which suggests overshoot of inÑowing material that Ðlls in the evacuated volume in the wake of the new downÑow.