DEPARTMENT 4 : Solar Physics
USET, The Uccle Solar Equatorial Table
The solar observations in the optical domain are carried out on a daily basis from the site of the Observatory itself, on the Uccle plateau.
The equatorial table provides a platform that tracks the apparent diurnal motion of the Sun and allows the installation of various instruments. Currently, 3 refracting telescopes are mounted in parallel.
The visual telescope projects a 25-cm solar image on a white screen and is used for the daily sunspot drawings. The latter are cooler and darker features of the photosphere, the lower layer of the solar atmosphere that produces the visible (and blinding!) solar light. Those sunspots mark the emergence sites of intense and variable magnetic fields generated inside the Sun and responsible for multiple eruptive phenomena, known as the solar activity.
Uccle is one of the base stations of a worldwide network managed by the SIDC (Solar Influences Data analysis Center), which compiles the international sunspot index. This index, often called Wolf Number, is the oldest solar activity index and in this respect, it keeps all its importance for the long-term studies (influence on Earth climate, etc.). In spite of the prevailing variable and humid weather of Belgium, solar observations are collected in Uccle on more than 250 days per year.
In 2002, two digital imaging systems were mounted on the other two telescopes:
(1) The white-light camera allows the monitoring of sunspots and also of the faculae (brighter but low-contrast regions). This can be done quantitatively, contrary to the visual method. For instance, positions, areas and contrasts can be accurately measured in order to better understand the small variations of the global solar luminosity.
(2) The H-&alpha camera is dedicated to the monitoring of chromospheric activity and prominences, those veils of dense gas suspended in the coronal magnetic fields. This wavelength is the most appropriate for the detection of solar flares, those intense brightenings often associated with jets of particles, which have an influence on the Earth environment.
The USET images as well as the digitized version of the Uccle sunspot drawings are also distributed in near real-time through the SIDC web site in support of daily solar monitoring and forecasting services operating worldwide.
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