ToBeOrNotToBe

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To be or not to be - the role of projection effects in EUV imaging

Introduction

Coronal mass ejections (CMEs) are often accompanied with activities low in the solar atmosphere including, e.g., extreme-ultraviolet (EUV) waves and coronal dimmings. The bright EUV waves (also known as EIT waves; see nugget 79) are large-scale disturbances propagating through the solar atmosphere most probably driven by the laterally expanding CME flanks. The dark dimming regions represent decreased emission in EUV and soft X-rays, most probably caused by the CME expansion, and are therefore interpreted as low-coronal footprints of CMEs (see e.g. nuggets 114 & 179). Two different types of dimming regions are observed: core or twin dimmings -- stationary, long-lived regions of strongly reduced EUV emission closely located to the eruption site covering opposite magnetic polarities. Secondary or remote dimmings -- more wide spread and less dark regions extending to significant distances away from the eruption site. Those rarefied areas could be formed due to the plasma evacuation behind the flux rope and the eruption of overlying fields. In this nugget we investigate a coronal dimming/EUV wave event using quadrature observations from SDO and STEREO-A and show how line-of-sight (LOS) integration/projection effects may change the interpretation of these faint coronal features.

Two eyes (satellites) are better than one

Observing the same object with two spacecraft (SC1, SC2) located at different positions in the heliosphere, is crucial for our understanding of how LOS integration in the corona (as it is optically thin) influences the observed intensities of remote-sensing instruments. This means that the intensity of each image pixel of SC1 can be interpreted as the sum over the intensities of all image pixels that lie along the projected LOS of SC2 and vice versa. By selecting regions of interest from SC1’s point of view and reconstructing their corresponding LOS for data of SC2, we are able to check which coronal structures contribute to SC1’s observed intensity. For our purpose those regions of interest are EUV wave signatures and coronal dimming regions that occurred on 6 September 2011 in association with a X-class flare/halo CME event. This event was ob- served on-disk by SDO/AIA and in quadrature by STEREO-A/EUVI, where the eruption site was located close to the Western limb.