Abstract
The solar corona is the outer layer of the Sun’s atmosphere. The brightness of the solar corona is a million times lower than that of the solar disk. The Earth’s sky brightness itself is high enough to cover the coronal signal during ground-based observations. For this reason, the study of sky characteristics plays a fundamental role in observing the solar corona. To date, the only place with the sky characteristics that allow continuous coronagraphic measurements from Earth is at the MLO (Mauna Loa Observatory; Hawaii, \({\approx }\,3400\mathrm{\,m}\) above sea level). This paper shows the results obtained as part of the “Extreme Solar Coronagraphy Antarctic Program Experiment” (ESCAPE) at Concordia Base, Antarctica (Dome C plateau-coord.: 75∘06′ S, 123∘20′ E, \({\approx }\,3300\mathrm{\,m}\) above sea level) during the 37th campaign and gives a summary of all the sky-brightness measurements obtained from this site (34th and 35th campaigns). Dome C is confirmed to be a coronagraphic site with a sky brightness of about \(1 \times 10^{-6} \mathrm{\,B_{\odot }}\). For the first time, it was also possible to detect a coronal signal and to compare it with what was measured by the COronal Solar Magnetism Observatory (COSMO) K-coronagraph (K-Cor) at the MLO. All these results were obtained by using a micropolarizer camera mounted within a coronagraph designed for Antarctic environments.
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Notes
PNRA 2021, cfr. https://www.pnra.aq/.
The AntarctiCor optical design is based on the externally occulted coronagraph ASPIICS (Association de Satellites pour l’Imagerie et l’Interferométrie de la Couronne Solaire) for the European Space Agency (ESA) formation-flying mission PROBA-3 (Project for On-Board Autonomy-3). More details can be found in Galy et al. (2015).
The PolarCam© is manufactured by 4-D Technology Corporation, AZ 85706, United States (cfr. https://4dtechnology.com/). The PolarCam model used during the observational campaigns is the U4 (Zecchino, 2017).
Examples of PolarCam demosaicization are presented in Liberatore et al. (2022).
A high temporal resolution is fundamental to be able to observe eventual fast/dynamic structures in the solar corona. As an example, Telloni et al. (2022) were able to obtain the first evidence of a switchback in the solar corona thanks (also) to a high temporal resolution.
It is important to notice that AntarctiCor has an aperture of \(5~\mathrm{cm}\) while K-Cor aperture is about \(20~\mathrm{cm}\) (Nelson et al., 2010). This means that has a signal-to-noise ratio four times higher! Even for this reason, the K-Cor telescope can more simply observe coronal signals at higher heliocentric heights.
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Acknowledgments
This paper has been possible thanks to the whole INAF & ESCAPE Project team and the Italian Piano Nazionale Ricerche Antartico (PNRA). The AntarctiCor - ESCAPE project is funded by the PNRA, grant N. 2015-AC3.02. The authors thank all persons who contributed in any way to the results reported in this article. Thanks also to Dr. Angelo Lupi (PI of BSRN project) for providing us with access to the images of the AstroConcordia all-sky camera. A particular acknowledgment to OPTEC S.p.A for the AntarctiCor telescope thermal and structural design and realization as well. The authors also thank ALTEC Company for providing logistic support during the many AntarctiCor calibration periods and the European Space Agency - ESA for its support to the PROBA-3/ASPIICS mission. The research was partially carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).
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Alessandro Liberatore took part in the different Antarctic missions, calibrations, and data analysis. Gerardo Capobianco, and Silvano Fineschi also took part in the different Antarctic missions and calibrations. Giuseppe Massone, Luca Zangrilli, and Gianalfredo Nicolini helped with the camera and telescope calibration before the different missions. Roberto Susino developed software for the telescope camera for easier and more efficient data processing.
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Liberatore, A., Capobianco, G., Fineschi, S. et al. Sky Brightness Evaluation and First Coronal Signal Detection from Concordia Base (Antarctica) with a Calibrated Micropolarizer Array Camera. Sol Phys 298, 115 (2023). https://doi.org/10.1007/s11207-023-02195-6
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DOI: https://doi.org/10.1007/s11207-023-02195-6