Formation flyers applied to solar coronal observations: the ASPICS mission

Classical externally-occulted coronagraphs are presently limited in their performances by the distance between the external occulter and the front objective. The diffraction fringe from the occulter and the vignetted pupil which degrades the spatial resolution prevent observing the inner corona inside typically 2-2.5 solar radii. Formation flyers open new perspectives and allow to conceive giant, externally-occulted coronagraphs using a two-component space system with the external occulter on one spacecraft and the optical instrument on the other spacecraft at approximately 100 m from the first one. ASPICS (Association de Satellites Pour l’Imagerie Coronographique Solaire) is a mission proposed to CNES in the framework of their demonstration program of formation flyers which is presently under study to exploit this technique for coronal observations. In the baseline concept, ASPICS includes three coronagraphs operating in three spectral domains: the visible continuum (K-corona brightness), the HI Lyman alpha emission line at 121.6 nm, and the HeII emission line at 30.4 nm. Their unvignetted fields of view extend from 1.1 to 3.2 solar radii with a typical spatial resolution of 3 arcsec. In order to connect coronal activity to photospheric events, ASPICS further includes two disk imagers. The first one is devoted to the HI Lyman alpha emission line. The second one is a multi-channel instrument similar to SOHO/EIT and devoted to the HeII (30.4 nm), FeIX/X (17.1 nm) and FeXII (19.5 nm) emission lines. Two concepts of the space system are under consideration: a symmetric configuration where the disk imagers and the external occulter are on one spacecraft and the coronagraphs on the other, an asymmetric configuration where the external occulter is on one spacecraft and the scientific instruments are regrouped on the other one.