PROBING THE SOLAR SURFACE: THE OBLATENESS AND ASTROPHYSICAL CONSEQUENCES

Based on historical records, the Sun's dimensions are temporally dependent. Until the recent past, varying dimensions were keenly disputed. Recent accurate observations have removed the doubt, whether from direct limb observations or from helioseismology f-modes analysis. A shrinking or an expanding shape is ultimately linked to solar activity, as even a small variation in the solar radius causes variations in gravitational energy. Based on accurate space- and ground-based observations, we will argue that the oblateness of the Sun is time dependent. Indeed, considering the first two shape coefficients, we can interpret such a temporal variation as a change in the relative importance of the hexadecapolar term, i.e., at the time of high activity, only the dipolar moment c 2 has a significant effect, but at the time of low activity, c 4 is predominant; this results in a decrease of the total value of the oblateness. The combination of the two terms leads to a solar oblateness varying along with solar activity. More studies are needed to get accurate measurements from space, which will provide us with the unique opportunity to study detailed changes of global solar properties

Solar polar brightening and radius at 100 and 230 GHz observed by ALMA

Polar brightening of the Sun at radio frequencies has been studied for almost fifty years and yet a disagreement persists between solar atmospheric models and observations. Some observations reported brightening values much smaller than the expected values obtained from the models, with discrepancies being particularly large at millimeter wavelengths. New clues to calibrate the atmospheric models can be obtained with the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) radio interferometer. In this work, we analyzed the lower limit of the polar brightening observed at 100 and 230 GHz by ALMA, during its Science Verification period, 2015 December 16-20. We find that the average polar intensity is higher than the disk intensity at 100 and 230 GHz, with larger brightness intensities at the South pole in eight of the nine maps analyzed. The observational results were compared with calculations of the millimetric limb brightnening emission for two semi-empirical atmospheric models, FAL- C (Fontenla et al. 1993) and SSC (Selhorst et al. 2005a). Both models presented larger limb intensities than the averaged observed values. The intensities obtained with the SSC model were closer to the observations, with polar brightenings of 10.5% and 17.8% at 100 and 230 GHz, respectively. This discrepancy may be due to the presence of chromospheric features (like spicules) at regions close to the limb

Cartography of the Sun and the Stars

International audienc

The environments of the Sun and the Stars

International audienc

The environments of the Sun and the Stars

International audienc