ASPIICS: a giant, white light and emission line coronagraph for the ESA proba-3 formation flight mission

Abstract

INTRODUCTIONClassical 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 useful observations of the white light corona inside typically 2-2.5 solar radii (Rsun). Formation flying offers and elegant solution to these limitations and allows conceiving 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 a distance of hundred meters [1,2]. Such an instrument ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire) has been selected by the European Space Agency (ESA) to fly on its PROBA-3 mission of formation flying demonstration which is presently in phase B (Fig. 1). The classical design of an externally-occulted coronagraph is adapted to the formation flying configuration allowing the detection of the very inner corona as close as ~0.04 solar radii from the solar limb. By tuning the position of the occulter spacecraft, it may even be possible to reach the chromosphere and the upper part of the spicules [3]. ASPIICS will perform (i) high spatial resolution imaging of the continuum K+F corona in photometric and polarimetric modes, (ii) high spatial resolution imaging of the E-corona in two coronal emission lines (CEL): Fe XIV and He I D3, and (iii) two-dimensional spectrophotometry of the Fe XIV emission line. ASPIICS will address the question of the coronal heating and 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 mapping 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 in an effort to determine how the different components of the solar wind, slow and fast are accelerated. With a possible launch in 2014, ASPIICS will observe the corona during the maximum of solar activity, insuring the detection of many Coronal Mass Ejections (CMEs). By rapidly alternating highresolution imaging and spectroscopy, CMEs will be thoroughly characterized.

Direct observations of a complex coronal web driving highly structured slow solar wind

“…Future and planned missions, including NASA's (the National Aeronautics and Space Administration's) in-development Polarimeter to Unify the Corona and Heliosphere mission 42 , will partially address this need for instruments operating from the Earth's perspective. Additional missions proposed and in development that will explore the middle corona specifically, such as ECCCO 43 , the Sun Coronal Ejection Tracker 44 and PROBA3/ASPIICS 45 , among others, will help to fully connect the global corona-heliosphere system. Combined with 3D information we can derive from the PSP 46 , the Solar Orbiter's Metis 47 and Extreme-Ultraviolet Imager 48 away from the Sun-Earth line and new spectral capabilities in several proposed missions, we can address these concerns to fully close these questions.…”

Stray light rejection in giant externally-occulted solar coronagraphs: experimental developments

“…The distance scale is in millimetres as measured on the detector, i.e., in the focal plane of the Televue objective, the origin being at the center of the occulters. Note that this scale is about ¾ of that of the first focal plane of ASPIICS, that is the conjugated plane of its external occulter [4]. The FWHM of the disk and cone are remarkably similar and amounts to 42 µm, while that of the knife-edge appears slightly narrower, 35 µm.…”

“…Therefore it is the defocused image of the unblocked outer ring of the fringe which is superposed on the corona. It is not yet clear whether an IO will be needed for ASPIICS (a radial compensating neutral filter that both reduces the gradient of the corona and the diffraction fringe has been baselined [4]) and a final decision on this question is precisely part of the present investigation. Whatever the final choice, the image of the fringe must be as sharp as possible to facilitate its blocking (if relevant) and limit its contamination of the corona, and further with the lowest possible intensity.…”

“…Whatever the final choice, the image of the fringe must be as sharp as possible to facilitate its blocking (if relevant) and limit its contamination of the corona, and further with the lowest possible intensity. The study and optimization of the occulting system must therefore involve at least the EO and the first objective of the coronagraph (equivalent to the Three-Mirror Anastigmat in the present ASPIICS optical design [4]). This represents a formidable challenge because of the involved dimensions.…”

Coronal Magnetic Fields Derived with Images Acquired during the 2017 August 21 Total Solar Eclipse

“…The method shown here is related to a fundamental property of coronal plasma that has been missed so far, and that I try to explain here based on empirical and theoretical reasoning. Because the method described here is simply based on the analysis of standard coronagraphic images acquired in visible light, it could be easily applied to convert into a "coronal magnetometer" every already existing or future coronagraph observing the inner corona, such as the groundbased Mauna Loa coronagraphs (Tomczyk et al 2022), and the space-based ASPIICS coronagraph on board PROBA-3 (Lamy et al 2017). In this paper, after general introductory considerations on the distributions of different energies in the solar corona (Section 2), I describe the input observational data and the theoretical model employed for the comparison between the measured and the simulated magnetic fields (Section 3).…”