Solar Semi-diameter Variation Observed with the Tokyo Photoelectric Meridian Circle for the Years from 1985 to 1994

Yoshizawa M.

National Astronomical Observatory, Mitaka, Tokyo 181, Japan

The semi-diameter of the sun has been observed regularly since 1985 with the Tokyo Photoelectric Meridian Circle (Tokyo PMC) at Mitaka. At each observation a set of pinholes of 2.5 arcsec in diameter scan the solar limb several times, and photon countrates through the pinholes per 50 millisec of time were measured at the wavelength lambda0 = 550 nm. We define the solar limb as the positions where the time series of the photon countrates take zero after taking convolution with a numerical filter which simulates the discrete second derivatives. The semi-diameter of the sun of one day was determined by using about 40 limb positions observed over six parts of the solar limb. The observed apparent semi-diameters were reduced to the values corresponding to the solar semi-diameter seen at 1 AU distance. The planetary ephemeris DE200 was used to derive the Earth-sun distance of each day.

The average value of the solar semi-diameter is calculated to be 959.83 (± 0.01) arcsec based on 755 observations made during nine years from 1985 to 1994. From the power-spectral-density analysis a significant peak with 0.05 arcsec amplitude is found at the period of about 910 days. A longer-period variation (about 3040 days) with 0.04 arcsec amplitude is notified too, which is probably the component corresponding to the global 11 yr solar activity cycle. No other semi-diameter variations with period longer than 130 days are found to have amplitude larger than 0.04 arcsec.

The annual mean values of the observed semi-diameters take the minimum value at the epoch around 1989 to 1990, which is close to the maximum phase of the 22nd solar cycle. On the other hand, around the solar cycle minimum, 1986 and 1994, the annual solar semi-diameter take the largest values. The peak to peak difference of the annual solar semi-diameters is slightly less than 0.2 arcsec, which is close to the value to be expected if the frequency change of about 0.3·10E-6 Hz of the 5 minutes p-mode solar oscillation during one solar cycle is caused only by the change of the cavity size.