An analysis of relationships between latitudinal fine structures of the photospheric plasma differential rotation and solar activity shows that sunspot activity seems to be lower (as measured by the number and extension of sunspot groups) at latitudes where minima of angular velocity appear.
I. Introduction
It is well known (see, for example, Godoli, 1978) that latitudinal fine structures of the photospheric plasma differential rotation have been found, amongst others, by Solonsky (1972) and by Deubner and Vazquez (1975). Solonsky (1972) found a minimum of angular velocity for photospheric lines at a latitude of about 25 ~ The observations were made in the period between May-September 1968 at the vertical solar telescope of the Leningrad University (Laboratory of Solar Physics): 800 lines of different optical depths were examined. Data for the solar northern and southern hemispheres were averaged. Solonsky found that the amplitude of the minimum grows with the increasing depth in the solar atmosphere. It is worthwhile to remember that Solonsky found also that angular velocities became lower as the depth increased.
On the other hand, Deubner and Vazquez (1975) found a minimum of angular velocity at a latitude of about 6 ~ Their observations were made on October 16 and 17, 1974. They used the Doppler compensator of the Capri magnetograph recording simultaneously on two independent channels the position of the lines CI 538.0 nm and Fe I 536.5 nm. We notice that angular velocities determined by Deubner and Vazquez refer only to small longitude intervals near the eastern and western limb and are given separately for the four quadrants NE, NW, SE, SW and that the angular velocity minimum is present in each of these quadrants.
According to Deubner and Vazquez (1975) the latitudes of the angular velocity minimum found by Solonsky (25 ~ and by themselves (6 ~ closely coincide with the corresponding actual latitude of the sunspot zones during the periods of spectrographic observations. However, from Figure 1, we see that the positions of the Solonsky and Deubner-Solar Physics 83 (1983) 339-347. 0038-0938/83/0832-0339501.35.