Auchère, F. ; Berghmans, D. ; Rochus, P. ; Long, D. ; Harra, L. ; Schüehle, U. ; Halain, J.-P. ; Parenti, S. ; Teriaca, L. ; Smith, P. ; Aznar Cuadrado, R. ; Stegen, K. ; Verbeeck, C. ; Kraaikamp, E. ; EUI Team, X.

Talk presented at 8th Solar Orbiter Workhop, Belfast (UK) on 2022-09-12

Abstract: At 3.8°, the field of view (FOV) of the Full Sun Imager (FSI) on Solar Orbiter is by far wider than that of any previous solar EUV imager. Depending on the distance of the probe to the Sun along its orbit, this corresponds to 14 to 4 solar radii, to be compared to the 3.5 Rs of STEREO/EUVI or Proba2/SWAP. This very large field of view opens up a new discovery space into a region largely unexplored in the EUV. Since it was expected that stray-light would dominate beyond 2 Rs, a moveable occulting disk can be inserted in the optical path to block light rays up to 0.78° off the optical axis. On March 21 2021, at 0.51 AU, FSI acquired deep exposures at 17.4 and 30.4 nm with the occulting disk in place. The data reveals solar structures extending up to 5 Rs which, to our knowledge, is the furthest ever recorded at these wavelengths. We present a comparison of the measured signal fall-off as a function of distance to Sun-center with a model of coronal emission taking into account collisional excitation and resonant scattering. The presence of resonant scattering off disk at 17.4 nm is predicted, but is so far not confirmed by observations. If confirmed, this resonantly scattered component can have implications on plasma diagnostics that assume line formation by collisional excitation.

Keyword(s): Sun ; corona ; EUI
Note: Oral contribution O61 of the conference abstract book