In this paper we propose a new method for measuring rotation rate and
clearly establish that: a) the active corona rotation rate
varies with respect to the 11-y sunspot cycle, b) there is a relationship
between rotation rate and magnetic flux, c) rotation grows first and then
the magnetic field emerges, d) activity increases the
rotation rate. The sidereal rotation rate of the active corona varies
between
and
respectively,
for activity maxima
and "minima". Nevertheless 10.7 cm rates of maxima and of "minima" include
the sunspot radiation rates. Lastly, active regions in the photosphere and in
the corona averaged over the whole cycle reproduce the
Carrington Rotation Rate. On the other hand, the frequencies over the whole
observation time, given in Fig. 3, show different frequencies
grouped round the Carrington Rate.
The rotation figures given in this paper have larger amplitudes than those of other authors. This is due to the high temporal resolution of the data and to the narrow windows used for computation of power spectra. Even if sunspots and 10.7 cm coronal flux data are disk observations, a direct comparison cannot be done. In case of sunspots, the measurements give differential rotation rates over long periods including cycles of activity. Data from analyses of 10.7 cm flux are frequencies distributed with respect to time, which cannot be averaged. According to Gilman & Howard (1984) and to Gupta et al. (1999), at the minimum of activity, sunspot rotation is faster than during the rest of the cycle, whereas coronal rotation is slower at that time, as was pointed out for all coronal observations. This remains unexplained.
Antonucci & Dodero (1977) showed the simultaneous presence of coronal structures with differential rotation and with rigid rotation in the same period. It is also well known that coronal holes show either rigid or differential rotation. This assertion is supported by the detection of Pivot Points (Mouradian et al. 1987), which designate regions of the solar photosphere in rigid rotation, alongside regions in differential rotation. Stenflo's (1977) sentence, "There is a coexistence on the sun of regions with rigid and differential-rotation properties" is here proved for the corona too. Observations revealed the simultaneous presence of a large number of rates, so that the smoothed differential rotation cannot be considert as a local rule.
Acknowledgements
The 10.7 cm Solar Flux data were provided by the National Research Council of Canada and the authors are in dept to Dr. K. Tapping. We are grateful for clarifying discussions to Dr. D. Heristchi. The authors thank the referee, Dr. H. Wöhl, for his valuable remarks and comments.
Copyright ESO 2002