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

TL;DR

This study investigates how projection effects influence the measurement of differential rotation in the solar UV corona, revealing that at higher latitudes, observed rigidity may be overestimated due to line-of-sight effects.

Contribution

The paper introduces a simple geometrical model to quantify how projection effects can bias measurements of coronal differential rotation, especially at higher latitudes.

Findings

Projection effects cause underestimation of differential rotation at high latitudes.

At solar maximum, the observed UV corona's rigidity is an upper limit due to projection.

Projection effects are negligible at solar minimum and near the equator.

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 Ang. 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.