Sky Brightness evaluation at Concordia Station - Dome C, Antarctica for ground-based observations of the Solar Corona

s:introduction The Sun has an atmosphere divided into several layers. The outermost is called solar corona. It consists of plasma at very high temperatures (up to $10^{6}$ K) that extends millions of kilometres into outer space. The solar corona is the results of three different main contributions: the K-Corona, due to Thomson scattering by the free coronal electrons of the photospheric radiation; the F-Corona, due to diffusion of solar radiation by dust particles and E-Corona due to emission processes by coronal ions identified as \sayforbidden lines.

The brightness of each component decreases with a power law moving away from the Sun (Phillips, 1992; November and Koutchmy, 1996). Figure \ireffig:brightness_solar_corona_components shows that the brightness of the sky (\sayclear with haze) makes ground-based observations of the solar corona difficult. Total solar eclipses give the opportunity to observe the corona with a reduced sky brightness. However the short duration of these events (max 7.5 minutes), the possibility of adverse weather conditions and the frequent need of accessing remote locations for the observing sites, makes it difficult to carry out continuous and detailed coronal studies. A \saypure blue sky for ground-based coronagraphic observations is defined as $\approx 10^{-6}$ of the Sun’s disc brightness ($\mathrm{\,B/B_{\odot}}$). Ground-based observations of the solar corona were made possible with the development of the internally-occulted coronagraph by Lyot (Lyot, 1932).

Mauna Loa, in the Big Island of Hawai’i is a coronagraphic site hosting the Mauna Loa Solar Observatory (MLSO) operated by the US High Altitude Observatory. MLSO can carry out systematic observations of the solar corona thanks to a sky-brightness value of $\approx 1-5\times 10^{-6}\mathrm{\,B_{\odot}}$ for different wavelengths (Tomczyk and Elmore, 2015). One of the goals of the Extreme Solar Coronagraphy Antarctic Program Experiment (ESCAPE: Fineschi et al., 2019a) is the determination in Antarctica of a location with a \saycoronagraphic sky that would allow systematic ground-based observations of the solar corona. In the next section we will introduce the ESCAPE project instrumentation and goals (Sec. \irefs:ESCAPEProject) and we will show the results of sky-brightness measuraments (Sec. \irefs:Results) during the 34th (austral summer 2018/2019) and 35th (austral summer 2019/2020) Italian Expeditions in Antarctica, Concordia Station - Dome C plateau at $\approx 3233\mathrm{\,m}$ above sea level.

s:ESCAPEProject Antarctica offers a great opportunity for ground-based observations of the solar corona. Actually, the high altitude of the Antarctic plateau of Dome C ($\approx 3233\mathrm{\,m}$ a.s.l.), the high latitude (75° 06^′ S; 123° 20^′ E), the large amount of daily hours of observations during the Antarctic summer (Fig. \ireffig:sun24h) and the almost total absence of anthropic pollution, are necessary conditions for a low sky-brightness.

First attempts to characterise the sky-brightness at Dome C were perfomed in 2008 by the pioneering observations of J. Arnaud (Faurobert, M., Arnaud, J., and Vernisse, Y., 2012). One of the goals of the ESCAPE Project, at the Italian-French Station Concordia, is to quantitatively evaluate the sky brightness at the Dome C plateau. Within the ESCAPE project we developed an internally-occulted antarctic coronagraph (AntarctiCor) for observations of the K-corona polarized brightness ($pB$) generated by Thomson scattering of photospheric light of coronal free electrons for the determination of the coronal electron density (Van De Hulst, 1950).

In the following section, we will report the sky-brightness measurements. The coronal images are still under analysis. More details about ESCAPE and its science objectives can be found in Fineschi et al. (2019a). The evaluation of the sky-brightness was performed during the 34th and 35th Italian Expeditions in Antarctica by using the Antarctica solar Coronagraph, AntarctiCor (Fig. \ireffig:Montato).

s:Results During the Antarctic campaigns, systematic images of the sky were acquired in order to evaluate its brightness. The sky brightness in units of solar disc brightness ($B_{sky}\mathrm{\,[B_{\odot}]}$) was measured the first time during the 34th Campaign (austral summer 2018/2019) and during the full data acquisition during the 35th Campaign (austral summer 2019/2020) at almost regular intervals during all days.³³3The 36th Campaign (austral summer 2020/2021) was limited to logistical activities due to the COVID-19 pandemic. The $B_{sky}\mathrm{\,[B_{\odot}]}$ can be evaluated by considering the first Stokes parameter $I$ and performing a ratio between what is obtained pointing to the sky ($I_{sky}$) and what is obtained pointing to the Sun using a diffuser ($I_{diff}$). Both quantities must be normalized by the respective exposure time $t_{exp}^{i}$. Then, by considering the diffuser transmissivity $T_{diff}\approx 28\%$ (EdmundOptics, 2019) we obtain:

In particular, in our case, the acquired $B_{sky}[B_{\odot}]$ frame was divided into four different regions (Area in Fig. \ireffig:frame_4division) and the final brightness was obtained by averaging them:

A so-called \saypure blue sky (i.e. $B_{sky}\approx 10^{6}\mathrm{\,B_{\odot}}$) is necessary to carry out ground-based observations of the corona (Fracastoro, 1948; Fracastoro and Righini, 1949; Elmore, 2007).
Figure \ireffig:brightness_solar_corona_components shows the sky brightness, in $\mathrm{\,B_{\odot}}$ units, measurements obtained during the 35th Italian Antarctic Expedition (2019-2020), at Concordia Station - Dome C.

From the images acquired during the 34th Campaign on January 08, 2019 (pointing at RA: 22° 42’ 32.3”, Dec: -22°  11’ 22”) and January 09, 2019 (pointing at RA: 22° 38’ 33.3”, Dec: -22° 18’ 19”), we obtain that:

During the 35th campaign it was possible to perform more systematic sky-brightness measurements. A summary of what obtained is shown in Fig. \ireffig:SkyBrightness_Bsun. Averaging over the different values we obtain:

During these measurements, Dome C showed the characteristics of a \saycoronagraphic sky. Sometimes, the presence of clouds, high wind or excessive suspended ice in the atmosphere made it impossible to perform good observations. On the other hand, during this campaign, we evaluated a percentage of good-weather days around the 80%!

During the same mission, we evaluated the sky brightness for different right ascensions (RA) and declinations (DEC) for a fixed day (see Fig. \ireffig:SkyBrightness_Bsun_CROX). The values of measured $B_{sky}$ are consistent with what obtained in Fig. \ireffig:SkyBrightness_Bsun and, as expected, it is possible to see a decrease of sky-brightness moving away from the Sun. The closest measurements to the solar limb are at about 0.7°, e.g. for the declination, the closest measurement was at 1° from Sun center (Fig. \ireffig:SkyBrightness_Bsun_CROX, up), so $1-0.26=0.74$° from the solar limb (where 0.26° is the solar radii). We cannot really go much closer than 1° from Sun center due to the instrument field of view along the $x$ and $y$ axis. The instrument FoV is $\pm 0.6$° (Table \ireftab:AntarctiCor_features) and we need that the Sun is completely out to take sky brightness measuraments.

These results show that the Antarctic sky at Dome C, where Concordia Station is located, has periods when it can be considered to have a \saycoronagraphic sky (e.g. Fracastoro 1948; Fracastoro and Righini 1949; Elmore 2007 and Fig. \ireffig:brightness_solar_corona_components).

s:Conclusion In the present work, we presented the first results from the ESCAPE project. In particular, after a description of the project and its main goal, we described the instrumentation used during the Antarctic missions and its characteristics. We described the internally-occulted coronagraph AntarctiCor and its innovative detector with arrays of micropolariser for linear-polarisation imaging. Then, we described how we evaluated the sky brightness $\bar{B}_{sky}$ at Concordia Station (Dome C plateau - Antarctica, coord. 75° 06^′ S; 123° 20^′ E) at an altitude of $\approx 3300\mathrm{\,m}$ above sea level. The $\bar{B}_{sky}$ value was obtained during two different missions. In the first one (34th Italian expedition to Antarctica - austral summer 2018/19), a sky brightness of $\approx 1.0\times 10^{-6}\mathrm{\,B_{\odot}}$ was measured. Due to a logistic problem, just few acquisitions during the mission were possible (from 18 January 2019 to 19 January 2019). These sky brightness values have been obtained for a fixed distance from the Sun. subsequently, during the 35th mission, we performed more systematic measurements (from 1 January 2020 to 22 January 2020). In addition, we performed measurements of the sky brightness not only for a fixed distance from the Sun, but for different declination and different right ascension values. In this second campaign we obtained that $\bar{B}_{sky}\approx 7.0\times 10^{-7}\mathrm{\,B_{\odot}}$. Both results quantitatively demonstrate, for the first time, the quality of the Dome C site for coronagraphic observations. We can conclude that the Antarctic sky at Concordia Station shows the characteristics of a \saycoronagraphic sky (i.e., $B_{sky}<10^{6}\mathrm{\,B_{\odot}}$). This holds the promises for Concordia Station to host a permanent coronagraph observatory for continuous studies of the solar corona, during the Austral summers.

The authors declare that there are no conflicts of interest.

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.