Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from Small (6∘) to Large (170∘) Spacecraft Separation Angles

Abstract

We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles (α sep≈6∘,43∘,89∘,127∘,and 170∘). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of α sep≈22∘ – 125∘, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of α sep≈6∘ – 127∘ we find a mean 3D misalignment angle of μ PF≈21∘ – 39∘ of stereoscopically triangulated loops with magnetic potential-field models, and μ FFF≈15∘ – 21∘ for a force-free field model, which is partly caused by stereoscopic uncertainties μ SE≈9∘. We predict optimum conditions for solar stereoscopy during the time intervals of 2012 – 2014, 2016 – 2017, and 2021 – 2023.