Influence of projection effects on the observed differential rotation rate in the UV corona

Abstract

Following previous investigations by Giordano and Mancuso [1] and Mancuso and Giordano [2,3] on the differential rotation of the solar corona as obtained through the analysis of the intensity time series of the O VI 1032 Å spectral line observed by the UVCS/SOHO telescope during solar cycle 23, we analysed the possible influence of projection effects of extended coronal structures on the observed differential rotation rate in the ultraviolet corona. Through a simple geometrical model, we found that, especially at higher latitudes, the differential rotation may be less rigid than observed, since features at higher latitudes could be actually linked to much lower coronal structures due to projection effects. At solar maximum, the latitudinal rigidity of the UV corona, with respect to the differential rotating photosphere, has thus to be considered as an upper limit of the possible rigidity. At solar minimum and near the equatorial region throughout the solar cycle, projection effects are negligible.

Differential rotation of the solar corona: A new data-adaptive multiwavelength approach

“…However, east-west asymmetry has been seldom reported also because, while it is certainly difficult to justify and interpret from a physical point of view, it does foster suspicions on the reliability of the data at hand. This apparent east-west asymmetry, already noticed by Giordano & Mancuso (2008) and also mentioned by Lewis et al (1999) in their analysis of white-light data related to the same solar minimum period, can be tentatively attributed to a viewing effect arising from the inclination of the magnetic field axis with respect to the rotation one (e.g., Mancuso & Garzelli 2007) mixed with projection effects at higher latitudes (Mancuso & Giordano 2013). As a matter of fact, this is a major unsolved problem in the interpretation of our results as evinced from the GLS periodogram analysis of the singular intensity time series, and it does cast legitimate doubts on the robustness of the results obtained by Giordano & Mancuso (2008).…”

“…To overcome this problem, the data at different angles from the center of the slit were obtained by interpolating the spectra taken with the slit positioned at different heights, to avoid possible effects on the radial dependence of the coronal rotation rate. Finally, we only considered data in a range between 40 • and 140 • colatitude in order to avoid problems related to projection effects at higher latitudes (see discussion in Mancuso & Giordano 2013).…”