Power Spectrum of Differential Refraction and Comparison with Solar Diameter Fluctuation Measurements

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Scientific paper

Two different observations are used to estimate the level of atmospheric noise present in solar diameter measurements. First, a direct measurement of the fluctuation of the angular distance between two stars separated by about one solar diameter. Second, a differential measurement of fluctuation of air mass along the two lines of sight to opposite solar limbs, which allows a determination of the power spectrum of these fluctuations in the frequency range 0.15-3.2 mHz. Our results, together with other direct stellar measurements, are used to estimate the r.m.s. atmospheric noise in solar diameter measurements to be 60 milli arc s in this frequency range, with a variance of a factor of 2 on this value. The shape of the atmospheric noise power spectrum in this frequency range is well fit by a ν-1 function.
The broad shape of the power spectrum of apparent solar diameter fluctuations published by Brown et al. (1978) is interpreted as a sum of aliased high frequency noise, atmospheric noise consistent with the present results and the solar five-minute oscillation. Our conclusion is that the published power spectrum does not clearly prove the presence of long-period solar pulsation modes because of the noise introduced by atmospheric fluctuations.

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