Solar-Radius Variations Over a Solar Cycle Observed with the Tokyo Photoelectric Meridian Circle

Abstract As the Sun shows the well-known magnetic activity of eleven-year period, many other interesting observational features of the Sun have been examined in connection with the solar cycle. The diameter of the Sun is one of these interesting observable quantities. The apparent radius of the Sun has been observed regularly since 1985 with the Tokyo Photoelectric Meridian Circle (Tokyo PMC) at Mitaka. Here, we show the average value of the solar radii observed by us. We also show that the annual-mean values of the observed radii take the smallest value near the solar-cycle maximum, in 1989 and 1990, and the largest ones around the solar-cycle minimum, in 1986 and 1994. The average peak-to-peak shift of the annual-mean solar radius for the solar cycle is ΔR⊙/R⊙ ~ 1.2 × 10−4, which is close to the relative change of the frequency of the 5-minute p-mode solar oscillation within one solar cycle.