Periodic brightness fluctuations of the solar chromosphere at lambda=2.8 centimeters

The 5 minute velocity oscillations in the solar photosphere have been well documented in the literature but evidence for related radio brightness fluctuations is weak. At a frequency of 10.7 GHz the Stanford interferometer was used to look for periodic fluctuation in the chromosphere which might be connected to the velocity oscillations. The introduction of two-dimensional spectral analysis combined with an EW resolution of 17'' gave a favorable signal-to-noise ratio. The observed brightness fluctuations have a power spectrum which decays as frequency squared. No conspicuous peak was found near periods from 200 to 300 s. At the 99% level of confidence we cannot reject the hypothesis that the fluctuations are due to a random process in the solar atmosphere. A conservative upper limit for the amplitude of periodic oscillations is 1.5 Jy at a frequency of 4 mHz, which, under certain assumptions, corresponds to a ΔT< or approx. =20 K for an oscillator size of 10''. We also show that the power-law character of the spectrum guarantees the exustence of the so-called quasi-periodic radio oscillations