A Study of the Rotation of the Solar Corona

Abstract

Synoptic photoelectric observations of the coronal Fe xiv and Fe x emission lines at 530.3 nm and 637.4 nm, respectively, are analyzed to study the rotational behavior of the solar corona as a function of latitude, height, time and temperature between 1976 (1983 for Fe x) and 2001. An earlier similar analysis of the Fe xiv data at 1.15 R ⊙ over only one 11-year solar activity cycle (Sime, Fisher, and Altrock, 1989, Astrophys. J. 336, 454) found suggestions of solar-cycle variations in the differential (latitude-dependent) rotation. These results are tested over the longer epoch now available. In addition, the new Fe xiv 1.15 R ⊙ results are compared with those at 1.25 R ⊙ and with results from the Fe x line. I find that for long-term averages, both ions show a weakly-differential rotation period that may peak near 80° latitude and then decrease to the poles. However, this high-latitude peak may be due to sensing low-latitude streamers at higher latitudes. There is an indication that the Fe xiv rotation period may increase with height between 40° and 70° latitude. There is also some indication that Fe x may be rotating slower than Fe xiv in the mid-latitude range. This could indicate that structures with lower temperatures rotate at a slower rate. As found in the earlier study, there is very good evidence for solar-cycle-related variation in the rotation of Fe xiv. At latitudes up to about 60°, the rotation varies from essentially rigid (latitude-independent) near solar minimum to differential in the rising phase of the cycle at both 1.15 R ⊙ and 1.25 R ⊙. At latitudes above 60°, the rotation at 1.15 R ⊙ appears to be nearly rigid in the rising phase and strongly differential near solar minimum, almost exactly out of phase with the low-latitude variation.