DIFFERENTIAL ROTATION OF THE ULTRAVIOLET CORONA AT SOLAR MAXIMUM

Abstract

Synoptic observations of the O VI 1032 A spectral line from the UltraViolet Coronagraph Spectrometer (UVCS) telescope on board the Solar and Heliospheric Observatory (SOHO) have been analyzed in order to establish the rotational characteristics of the solar corona in the time interval from 1999 March 18 to 2002 December 31, corresponding to the maximum phase of solar cycle 23. By using autocorrelation analysis techniques, we determined the latitude and time dependence of the coronal rotation rate at a heliocentric distance of 1.6 R ☉ from the solar equator up to about 15° from the poles. Although the equatorial rotation rate is initially consistent with the coronal synodic rotation period (~27.5 days) inferred in a previous study by Giordano & Mancuso around solar minimum, a systematic and substantial acceleration is observed to occur during the second part of the year 2000, with the equatorial coronal synodic rotation period settling to an average value of 25.7 days in the time interval extending from 2001 August to 2002 April, corresponding to a ~7% increase in coronal rotation rate. It is shown that the coronal magnetic structures rotate much faster at all latitudes, and less differentially, than the underlying small-scale magnetic structures linked to the photospheric plasma. The rotation rate of sunspots is however compatible, at least within ~20° from the solar equator, with the one estimated in the middle corona.