Coronal Rotation at Solar Minimum from UV Observations

Abstract

The observations of the UVCS SOHO instrument from 1996 May to 1997 May have been analyzed to reconstruct intensity time series of the O VI 1032 Å and H I Lyα 1216 Å spectral lines at different coronal heliolatitudes from 1.5 to 3.0 R_⊙ from Sun center. At solar minimum, some features persist for several rotations, thus allowing analysis of the UV emission as time series modulated at the period of the solar rotation. We find evidence of coronal differential rotation, which significantly differs from that of the photospheric plasma. The estimated equatorial synodic rotation period of the corona at 1.5 R_⊙ is 27.48 +/- 0.10 days. The study of the latitudinal variation shows that the UV corona decelerates toward the photospheric rates from the equator up to the poleward boundary of the midlatitude streamers, reaching a peak of 28.16 +/- 0.20 days around +30° from the equator at 1.5 R_⊙, while a less evident peak is observed in the northern hemisphere. This result suggests a real north-south rotational asymmetry as a consequence of different activity and weak coupling between the magnetic fields of the two hemispheres. The study of the radial rotation profiles shows that the corona is rotating almost rigidly with height, but we find an abrupt increase by about half a day between 2.3 and 2.5 R_⊙. The larger gradients of the rotation rates are localized at the boundaries between open and closed field lines, suggesting that in these regions the differential rotation might be a source of magnetic stress and, consequently, of energy release.