On the possible changes of the solar differential rotation during the activity cycle determined using microwave low-brightness-temperature regions and H alpha filaments as tracers

The solar rotation rate obtained using the microwave Low-brightness-Temperature Regions (LTRs) as tracers in the heliographic range +/- 55 degrees from the years 1979-1980, 1981-1982, 1987-1988, and 1989-1991 varied from 3% to 4% in medium latitudes, and below 1% at the equator. Using Her filaments as tracers at higher latitudes from the years 1979, 1980, 1982, 1984, and 1987, the solar rotation rate variation was between 2% and 8%. This represents an upper limit on the rotation rate variation during the solar activity cycle. Such changes could be caused by short-lived, large-scale velocity patterns on the solar surface. The Sun revealed a higher rotation rate on the average during the maxima of the solar activity cycles 21 and 22, i.e., in the periods 1979-1980 and 1989-1991, respectively, which differs from the rotation rates (lower on the average) in some years, 1981-1982 and 1987-1988, between the activity maximum and minimum (LTR data). Simultaneous comparison of rotation rates from LTRs and H alpha filament tracings was possible in very limited time intervals and latitude bands only, and no systematic relationship was found, although the rotation rates determined by LTRs were mostly smaller than the rotation rates determined by H alpha filaments. The errors obtained by applying different fitting procedures of the LTR data were analyzed, as well as the influence of the height correction. Finally, the north-south asymmetry in the rotation rate investigated by LTRs indicates that the southern solar hemisphere rotated slower in the periods under consideration, the difference being about 1%. The reliability of all obtained results is discussed and a comparison with other related studies was performed.