On the difference between low-ℓ p-mode frequencies seen in velocity and in intensity

Abstract

We compare the line profiles and frequencies of low ℓ=0, 1, 2 acoustic oscillations seen in observations in velocity (by the GOLF and GONG experiments) and in intensity (LOI instrument). Our study indicates that the systematic shift between the frequencies of low-ℓ pmodes in intensity and in velocity measurements recently discovered by Toutain and co-workers is merely an artifact of their reduction techniques. The results obtained agree perfectly with the theoretical expectation that solar oscillations are the global eigenmodes simultaneously visible in velocity and intensity with the frequencies and line profiles coinciding within the error bars.