LYRA, the Lyman-Alpha Radiometer, is a solar EUV/VUV (extreme ultraviolet/vacuum ultraviolet) radiometer (Hochedez et al. 2006) that will embark in early 2009 on-board PROBA2, the Project for On-Board Autonomy 2, an ESA (the European Space Agency) micro-mission. It will monitor the solar flux in four passbands relevant to solar physics and space weather. LYRA has been designed by the Royal Observatory of Belgium (ROB), with Dr. J.-F. Hochedez as principal investigator, and built by the Physikalisch-Meteorologisches Observatorium, Davos (PMOD), with Dr. W. Schmutz as lead co-investigator. LYRA benefited from additional important contributions from its international partnership: CSL, Liege and IMOMEC, Diepenbeek both in Belgium, MPS, Lindau, Germany, and NIMS, Tsukuba, Japan. The four LYRA passbands are:

Royal Observatory of Belgium; Belgian Institute for Space Aeronomy, Brussels, Belgium e-mail: [email protected]

LYRA will benefit from pioneering wide bandgap detectors based on diamond. These sensors make the instrument radiation hard and solarblind: They minimize the use of additional filters needed to block the unwanted visible light but also attenuate seriously the desired UV radiation. This enhances the detector's effective area, and therefore increases the accuracy, the acquisition frequency, or an optimal combination of both. The PROBA2 heliosynchronous orbit generates brief eclipses three months per year. Our main purpose is to see whether those eclipses may be used to study the high atmosphere composition by the solar occultation method. Chemical species addressed are thermospheric N2, O, O2 and mesospheric O2 and O3 in the winter hemisphere. LYRA's high acquisition frequency (up to 100 Hz) and signal-to-noise ratio enable a very favorable vertical sampling, which is unfortunately counterbalanced by the extent and inhomogeneity of the solar source. For example, for a punctual light source, we would expect an altitude resolution of 3 km for a 1 Hz acquisition cadence, while the limitation introduced by the Sun extension is ~25 km. In this paper, we present the forward modeling and the chosen inversion method. The latter takes the above hurdles into account by dividing the Sun surface into parcels and analyzing the contribution of each parcel independently. We analyze the potential offered by this method and illustrate it with our first results, obtained from simulated data.

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