DEPARTMENT 4 : Solar Physics
LYRA, the LYman-α RAdiometer
LYRA is an experiment that will embark together with SWAP in 2007 onboard PROBA2, a technologically oriented European Space Agency (ESA) micro-mission.
A consortium of one Swiss and five Belgian institutes (Royal Observatory of Belgium - Brussels, Davos Observatory, Material Research Institute - Diepenbeek, Verhaert - Antwerpen, Liège Space Center, Belgian Institute for Space Aeronomy - Brussels) will soon manufacture the instrument. Its scientific management is at the ROB. LYRA has been pre-selected by ESA in the fall of 2002.
The absolute measurements of the Sun brightness in the far ultraviolet (UV) are particularly difficult to achieve. It must be done from space with a special methodology dedicated to guarantee the successive calibrations and their reliability.
These measurements are to be even more consistent in time when the scientific study focuses on the longer term. This is for example the case with the analysis of the solar influence on the Earth climate; the far-UV plays indeed a dominant role in the chemistry of the mid-atmosphere (e.g. the ozone hole issue).
The knowledge of the evolution of the Sun’s emission is decisive to the understanding of our star itself. The UV variations along the 11-years solar cycle are exhibiting amplitudes from two to a hundred, depending on the spectral range [Figure 2], but on the considered decade too. These changes are currently known in a too piecemeal way to be confronted with the physical models. Finally, the disturbance and disruptions induced to the high technologies, such as the telecommunication systems, satellite orbits, GPS, etc., express also the solar far-UV impact.
LYRA will partly fill the gaps by monitoring the solar flux in four UV ranges with an as yet unsurpassed correctness. Two are relevant to aeronomy and space weather: Lyman-alpha (121.6 nm) and the Schumann-Runge continuum (175-205 nm). Two other ranges are additionally valuable for technology assessment and solar physics: 200-220 nm, 20-70 nm.
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