LASCO observations of the coronal rotation and morphology of tracers seen at solar maximum and comparison with solar minimum |
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| Authors: | Lewis DJ Simnett GM |
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| Institution: | 1.School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, U.K
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| Abstract: | We present data obtained from the Large Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory spacecraft (SOHO). We compare the rotation of the white-light corona as seen during a period approaching the maximum of the solar 11-year activity cycle with that observed in a previous study made at solar minimum (Lewis et al., 1999). We find no fundamental difference in the rotation characteristics and again find the white-light corona to be radially rigid. The rotation has been observed at altitudes from 2.5 R ⊙ to beyond 15 R ⊙ and as predicted in the previous study, the greater level of complexity in the coronal structures and their relatively rapid evolution has not allowed periods to be determined as accurately as at solar minimum. Our best estimate of the mean synodic rotation period during the period of study (7 March 1999 to 6 March 2000) is 27.5±0.3 days. This is consistent with the relatively small scale structures associated with the surface activity imposing their rotation signature on an otherwise axisymmetric background corona. The short-lived nature of the small scale coronal morphologies at this epoch has made a thorough analysis of the latitudinal variation difficult, although we again find some evidence for the white light corona's increased latitudinal rigidity when compared to the underlying photosphere. However, we again note how projection effects create difficulties in confirming the exact degree of rigidity in the corona at these altitudes and a very simple coronal model is used to highlight how the appearance of lower latitude features in projection can contaminate the coronal signal observed at other latitudes. We also note evidence for a sudden and apparently fundamental change to the global coronal morphology on the approach to solar maximum and suggest this may represent the time beyond which the classical solar dipole ceases to dominate the coronal field. |
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