ESCAPE: Extreme Solar Coronagraphy Antarctic Program Experiment

The Concordia station, located on the DOME-C Plateau in Antarctica at 3233 meters above sea level, is a site of potential interest for solar coronagraphy, or the study of the solar corona, the part of the outer solar "atmosphere" that extends from the photosphere into interplanetary space. The solar corona is observable from Earth even with the naked eye during total eclipses of the Sun, but for its continued observation, it requires special instruments, telescopes capable of simulating a total eclipse, called coronagraphs, this is because the solar corona is millions of times less bright than the Sun and from a direct observation, without occultation of the Sun, the telescope would be blinded and the corona not visible. But the use of coronagraphs alone, is not sufficient to guarantee observation of the solar corona. Crucial is the choice of observing site. High mountain conditions are required for the sky not to be so intense as to cover the solar corona. The ESCAPE project, installed a DOME-C coronagraph and made sky characterization measurements, verifying that throughout the observation period, the solar corona is visible and collected scientific data of the corona itself.

The FP6 European Network Program has indicated the significant potential of Dome-C for solar physics and in particular for observations of the solar corona. Observations of the solar corona, very close to the Sun's edge, normally possible only during total solar eclipses and which are expected from next-generation space coronagraphs (e.g., METIS on Solar Orbiter, PROBA-3 training mission), are possible from Earth are at sites where the sky brightness is very low. The state-of-the-art coronagraph technologies developed for space missions, namely the optics of the externally occulted ASPIICS coronagraph backup telescope for the PROBA-3 mission, enabled our team at the Turin Astrophysical Observatory (INAF-OATo) of the development of an internally occulted version of the ASPIICS coronagraph to be installed at the Concordia station at Dome-C.

The Antarctic Coronagraph (Antarctic Coronagraph), is equipped with a new CCD camera that, thanks to the micorarray of linear polarizers applied directly on each pixel, allows the measurement of linearly polarized K-corona emission. As part of the Extreme Solar Coronagraphy Antarctic Program Experiment - ESCAPE - (PNRA 2015-AC3.02), AntarctiCor is designed to perform 2D, spectroscopic and polarimetric observations of the broadband (585-595 nm) K-corona spectral emission and the FeXIV emission line (530.3 nm) by taking advantage of the unique features of the Concordia station for solar observations from Earth.

The linearly polarized K-corona emission provides information about the density of electrons in the solar corona. This is a critical physical parameter for studying the physics of the corona and the solar wind. In addition, interpretation through the Hanle effect of the linearly polarized FeXIV forbidden emission line and the direction of its linear polarization vector provides information about the direction of the coronal magnetic field, another critical physical parameter that determines the morphology and dynamics of the corona and solar wind.

The base of the corona is where the solar wind is accelerated and the heating of the solar corona up to a million degrees begins. Current space telescopes for observations of the corona, coronagraphs, cannot observe the area of the solar corona near the flap because of scattered light from the occulters used to create artificial eclipses by shielding the solar disk, which is more than a billion times brighter than the corona. Ground-based coronagraphic observations require a "pure blue sky" with a brightness of about 10-6 B⦿ or less.

Confirming that Dome-C has a low sky brightness (sky brightness " 10-6 B⦿ or lower), would make Concordia station one of the rare terrestrial locations for coronagraphic observations. The only other location from which systematic observations of the solar corona are made is from Mauna Loa, Hawaii, where the High Altitude Observatory /HAO), which houses the K-Cor- coronagraph, is located. An internally cloaked telescope installed at Concordia would allow systematic observations, during the Antarctic summer, near the solar flap.

The goals set for AntarctiCor's three summer campaigns (XXXIV, XXXV and XXXVII) Italian expedition to Antarctica) were:

Integration activities and pre-installation testing of the coronagraph:
Installation of AntarctiCor in the astronomical dome.
Scientific data acquisition, which can be divided into three types:
Observation of sky brightness (sky brightness);
Observation of the solar corona;
In situ radiometric calibration of the telescope.
All the objectives of the campaigns were achieved. Specifically with regard to sky brightness, the value measured during the previous campaigns of approximately 10-6 of the solar disk brightness (B⦿) was confirmed. Figure 6 shows the sky brightness measurement in B⦿ during the different campaigns.

Coronal measurements show very good measurements up to about 1.4 solar radii, from this heliocentric distance the stray-light/sky brightness becomes dominant over the corona. Coronal images of polarized brightness of the K-corona acquired by AntarctiCor are consistent with those obtained by the K-Corona from Hawaii.