ASPIICS (Association of Spacecraft for Polarimetry and Imaging Investigation of the Corona of the Sun)

The Coronagraph on PROBA-3 is called ASPIICS and is developed by a consortium of European Institutes and Industries from Belgium, Czech Republic, Greece, Italy, Ireland, Poland and Romania. The ASPIICS unprecedented field of view makes it uniquely suited for studies of the solar corona, as it will fill the crucial observational gap between the fields of view of Solar EUV imagers and conventional space coronagraphs.

The top-level scientific objectives of ASPIICS are in first place to understand the physical processes that govern the quiescent solar corona by answering the following questions:

• Nature of the solar corona on different scales

• Processes that contribute to the heating of the corona and the role of waves

• Processes that contribute to the solar wind acceleration.

Secondly, to understand the physical processes that lead to CMEs and determine space weather by answering the following questions:

• Nature of the coronal structures that form the CME

• How do CMEs erupt and accelerate in the low corona

• The connection between CMEs and active processes close to the solar surface

• Where and how can a CME drive a shock in the low corona.

In the early 21^st century, classical externally-occulted coronagraphs are 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 (R_sun). Formation flying offers an 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 ~100 m. Such an instrument, namely ASPIICS, has been selected in 2010 by ESA to fly on its PROBA-3 mission of formation flying demonstration. 

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.

The ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) instrumentation, proposed by the Laboratoire d'Astrophysique de Marseille (LAM), France, is composed of two platforms separated by about 145 m, and forming a giant externally-occulted coronagraph: one satellite (OSC) hosts the external occulter and the second one (CSC), the optical system. Andrei Zhukov of the Royal Observatory of Brussels (ROB) is the PI (Principal Investigator) of ASPIICS. A total of 12 institutes from 9 different countries are involved in the ASPIICS project, each supported by its national funding agency.

The ASPIICS instrumentation is developed by a consortium of European Institutes and Industries from Belgium, Czech Republic, France, Germany, Greece, Italy, Luxembourg, Russia, and the UK — referred to in the text as the ASPIICS consortium.

The science objectives of ASPIICS are to address the following questions:

• How is the corona heated? What is the role of waves? The origin of the plasma fluctuations (waves, turbulence and shocks) that are believed to heat the corona and accelerate the plasma is not yet entirely clear.

• How are the different components of the solar wind, slow and fast, accelerated? For some time, it is known that the fast solar wind originates from coronal holes; however, the respective roles of plumes and inter-plumes is disputed. The combined imaging and spectral diagnostics capabilities available on 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 several emission lines. Spatially resolved velocity profiles will be reconstructed and will shed new light on the above questions.

• To what degree do coronal inhomogeneities affect the heating and acceleration processes? An important aspect of solar wind investigations is to determine how the dominant physical processes vary between neighboring flux tubes in the inherently filamentary nature of the coronal plasma.

• How are CMEs accelerated? The most basic question involves the nature of the interaction between the CME plasma and the magnetic field that drives the eruption.

• What is the configuration of the magnetic field in the corona? The importance of obtaining physical measurements of the magnetic field direction in the corona (as contrasted with field directions inferred from the simple appearance of structures) is crucial in a magnetically dominated corona.

Methodology: The scientific objectives will be achieved thanks to broadband imaging of the corona at high spatial resolution and to diagnostic capabilities aimed at characterizing the local plasma (temperature, velocity). While imaging is straightforward, the second capability requires a specific device. Our preferred approach consists in analyzing the bi-dimensional distribution of the spectral profiles of several coronal emission lines (CEL) by a set of quasi concentric fringes generated by the Fabry-Perot etalon interferometer, a technique already implemented during solar eclipses.

To perform a precise line profile analysis far enough out in the corona, the strongest emission line available, the forbidden line of Fe XIV at 530.285 nm, is the prime choice. Additional emission lines will be included to better address different coronal regions as well as a broad spectral channel to image the white light corona and derive electron densities. The fringes have an instrumental profile of typically 0.02 nm, narrower than the width of the line (~0.1 nm for Fe XIV), so that the observed profiles are not significantly affected by the instrumental function and directly give the real profiles of the coronal emission line to a very good accuracy. The etalon will be mechanically tilted to displace the set of fringes and increase the resolution.

Design Specifications:

a) Telescope specifications

- Telescope effective focal length: 1150 mm
- Entrance aperture diameter: 50 mm
- Stray light/Sun disk light rejection: 5 x 108
- FOV: 1.02 R_sun ± 2.7/3 R_sun
- Spatial scale: 2.8 arcsec/pixel

b) Detector specifications

- Size: 2048 x 2048 pixels; 15 µm pixel
- Dynamic range: 16 bit
- Operating temperature < 60ºC

c) Optical specifications

- Bandpass 540–590 nm
- Narrow Bands at Fe XIV 530.3 nm and He I 587.6 nm

A full, more in detail description can be found at the EOPORTAL.ORG, from which parts of this page were taken over.