Chromospheric and Prominence Physics with the ASPIICS Formation Flyers 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. ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire) is a mission proposed to ESA in the framework of the PROBA-3 program of formation flyers which is presently under study to exploit this technique for coronal observations. ASPIICS is composed of a single coronagraph which performs high spatial resolution imaging of the corona as well as 2-dimensional spectroscopy of several emission lines (in particular the forbidden line of FeXIV at 530.285 nm) from the coronal base out to 3 Rs. The classical design of an externally occulted coronagraph is adapted to the detection of the very inner corona, and the addition of a Fabry-Perot interferometer. By tuning the position of the occulter spacecraft, it will be possible to attain the chromosphere and the upper part of the spicules. Filtergrams on the helium D3 line or even better, the hydrogen H-beta line (which is optically thin contrary to H-alpha) will give access to the "cold corona", and could allow to measure the chromospheric prolateness.