ASPIICS, a giant externally occulted coronagraph for the PROBA-3 formation flying mission

International audienceFormation flying opens new perspectives for coronal physics, 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. 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 flying which is presently in phase A, 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 from the coronal base out to 3 R⊙. The selected lines allow to address different coronal regions: the forbidden line of Fe XIV at 530.285 nm (coronal matter), Fe IX/X at 637.4 nm (coronal holes), HeI at 587.6 nm (cold matter). An additional broad spectral channel will image the white light corona so as to derive electron densities. The classical design of an externally occulted coronagraph is adapted to the detection of the very inner corona as close as 1.01 R⊙ and the addition of a Fabry-Perot interferometer using a so-called " étalon". This paper is dedicated to the description of the optical design and its critical components: the entrance optics and the Fabry-Pérot interferometer. ASPIICS will address the question of coronal heating and of the role of waves by characterizing propagating fluctuations (waves and turbulence) in the solar wind acceleration region and by looking for oscillations in the intensity and Doppler shift of spectral lines. The combined imaging and spectral diagnostics capabilities available with ASPIICS will allow to map the velocity field of the corona both in the sky plane (directly on the images) and along the line-of-sight by measuring the Doppler shifts of emission lines. We will attempt to determine how the different components of the solar wind, slow and fast are accelerated. ASPIICS will observe the corona during the maximum of solar activity, insuring the detection of many Coronal Mass Ejections (CMEs). By rapidly alternating high resolution imaging and spectroscopy, CMEs will be thoroughly characterized. In addition, ASPIICS will attempt to characterize the topology of the magnetic field in the corona