Proc. 27th Meeting Polish Astron. Soc.,
Poznań, September 12–15, 1995
(eds.: M.J. Sarna, P.B. Marks), 65 – 66.

Toruń Radio Astronomy Observatory, Nicolaus Copernicus University, ul. Gagarina 11, PL-87-100 Toruń, Poland

Single frequency solar radio patrol observations at 127 MHz have been carried out at the Toruń station since autumn of 1958. The collected measurements of the daily mean flux densities were recently corrected for various factors to form a uniform data base (Gawrońska et al. 1994).

The base (updated with the most recent results) have the following statistical parameters:

Data span (Oct. 29, 1958 – Dec. 31, 1994) 13 213 days Number of daily means 10 348 Number of missed data 2 865 or 21.7 % Mean value of flux density 19.94 s.f.u. Median value 5.1 s.f.u. Modal (most frequent) value 3.6 s.f.u.

We have subjected the whole 36-year series to the classical Fourier analysis to look for possible periodicities. As communicated earlier (Gawrońska and Borkowski 1995), at the low frequency end of the obtained spectrum, clearly seen are peaks at periods of 48 years and 10.2 years. Recent years brought discoveries of number of periodicities in various indices of solar activity ranging from 24 to 155 days. Our spectrum does show number of peaks in the mentioned range of periods, some of which quite closely agree with known periodicities. We have checked their continuity throughout our data using a sliding window for the Fourier analysis.

From our analysis it follows that (see Fig. 1):

Figure 1:  Fourier power spectrum (standard deviation units) of the daily flux densities of the Sun at 127 MHz in the period Oct. 4, 1969 to January 8, 1984.

(1)	In the 22 cycle of sunspot activity, besides the slowly varying component (27 days), there exists a component at 24 days which has been discovered over 1988 to 1991 (thus it is not seen in Fig. 1).
(2)	51.7-day period is present in the years 1958 – 1985 (in May of 1985 starts an almost 2-year gap in our data) and is especially pronounced in the years 1980 – 1982 and to a lesser extent in 1966 – 1968. This oscillation may correspond to the one found in the radio emission at 10 cm wavelength by Pap et al. (1990).
(3)	80.6-day period is detected up to 1989 and is more pronounced just after the maxima of the sunspot cycles No 20 and 21, especially in the years 1970 – 1974 and 1980 – 1989. 78-day and 84-day periods were observed in the radio flux at 10 cm, in sunspot area and in flare activity (Bai and Sturrock 1991).
(4)	103.3-day period is seen during the 20th and 21st sunspot cycle, and especially in the years 1969 – 1976. This agrees well with Bai and Sturrock's (1991) 103-day period that exists in the rate of large flares.
(5)	142.3-day periodicity exists over the years 1973 to 1992 with clear enhancements in the years 1973 to 1976 and 1981 to 1983.

Some authors observe regular fluctuations with periods in the range 150 to 157 days in X– and optical flares data as well as in other indices of solar activity (e.g. Pap et al. 1990). They are very weak in our spectra and generally they do not exceed the limit of 3 σ (standard deviations). The above listed five peaks in the Fourier spectrum are at the level of about 6 – 7 σ.

    REFERENCES

Bai T., 1992, Astrophysical Journal Letters, 388, L69 – L72.

Bai T., Sturrock P.A. 1991, Nature, 350, 141 – 142.

Gawrońska G., Borkowski K.M., 1995, in: Radio Emission from the Stars and the Sun, eds. A.R. Taylor and J.M. Paredes, (in print) [Astron. Soc. Pacific Conf. Series, 93, 397 – 399].

Gawrońska G., Borkowski K.M., Usowicz J.B., 1994, Acta Astronomica, 44, 141 – 164.

Pap J., Tobiska W.K., Bouwer S.D. 1990, Solar Physics, 129, 165 – 189.

Schove D.J. (ed.), 1983, Sunspot cycles, Hutchinson Ross Publ. Co., Stroudsburg (Penn., USA), p. 378.