Polarimetric tomography applied to synthetic multi-spacecraft white-light images: observing coronal mass ejections in 3D

ABSTRACT A discrete tomography method has been developed that is able to reconstruct 3D coronal mass ejection (CME) density structure. We test the method by producing synthetic coronagraph imagery for three events using the CORona–HELiosphere (CORHEL) model. We combine images from different numbers of observing spacecraft and we perform the method separately using polarimetric and non-polarimetric reconstructions, as a means to test their relative effectiveness. We show that increasing the number of observing spacecraft consistently reduces the mean relative absolute error (MRAE) between the simulated and reconstructed density. Furthermore, the MRAE is generally lower when using polarimetric reconstructions compared to non-polarimetric reconstructions. Methods applied to localize the CME front work well for all spacecraft configurations, and are improved when using polarimetric, over non-polarimetric, reconstructions. The presence of a CME front can be identified with an accuracy of $(72\pm 9)\, \rm per\,cent$, $(70\pm 8)\, \rm per\,cent$, and $(52\pm 12)\, \rm per\,cent$ for CME1, CME2, and CME3 via polarimetric reconstructions using only three spacecraft at L1, L4, and L5. The radial position of the CME front can be constrained to a high level of precision when using polarimetric reconstructions using the same three spacecraft; $0.003\pm 0.004$ , $0.004\pm 0.005$ , and $0.005\pm 0.004$ au for CME1, CME2, and CME3, respectively. We expect that at least four spacecraft are required in order to derive accurate information about 3D CME structure. We find no strong evidence of improvement when including out-of-ecliptic observers, but that their inclusion increases the volume of space within which the inversion can be performed.