Irregular rotation of the main sunspot in active region Hale 17 570 of 5-13 April 1981

The irregular rotation of the main sunspot in the active region Hale 17 570 was investigated on 5 to 13 April 1981. The characteristics of this motion indicate that damped oscillations were involved. The maximum amplitude of the rotation of the sunspot was 111.4deg. The angular velocity of the rotational motion reached its maximum (ω=3.5deg h-1) on 9 April 1981 at 6.7 h UT. The largest angular acceleration (Δω/Δt=0.22deg h-2) was observed at 04.9 h UT on 8 April 1981. Coefficients A and b for the exponential damping function y=+-A exp(-bt) were estimated: A=123deg, b=0.02047, time t0 being 06.8 h UT on 8 April 1981 and the time t being given in hours. No correlation was found between the characteristics of the irregular rotation and flare activity. (author)