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P. Drossart, B. Sicardy, F. Roques, T. Widemann (DESPA, Obs. Paris-Meudon), G.R. Gladstone, J.H. Waite (SWRI), M. Vincent (NMSU)
The observation of the occultation of star HIP9369 (K magnitude \approx6.5) by Jupiter was performed with the ISAAC IR spectrometer on VLT/ANTU telescope on 10/10/1999, between 2.31--2.46~\mum, with a resolving power of 1200. A repetition time of 1.7~sec, with an integration time of 1~sec, gives a S/N \geq 100 on the star spectrum before the occultation, which occured near 60 dg North (outside the auroral oval). The 2 arcsec slit of the imaging spectrometer was centered on the star during immersion and emersion, allowing us to disentangle the star and Jupiter spectrum accurately. First, the light curves of the occultation are obtained, by summing all the wavelengths in the window. Half levels are observed at 6:25:11 and 7:45:10 (UTC), and correspond to pressure levels of about 2~\mubar. Isothermal fits of the light curves at immersion and emersion give stratospheric temperatures of 145~K and 165~K respectively, with an accuracy of 18~K. The absorption of methane (due to the \nu3 + \nu4 vibration/rotation spectrum at 2.3~\mum) is observed, and the retrieval of the methane absorption vs the pressure level gives a direct determination of the vertical profile of methane at atmospheric levels close to the homopause. This profile is itself mainly constrained by the eddy diffusion coefficient (Gladstone et al., {\em Icarus}, 1996). A preliminary value of the eddy diffusion coefficient is found to be 1.5 106 cm-2s-1, within a factor of 2. This value is consistent with a previous determination from Voyager He spectra (Veverka et al., {\em Icarus}, 1995), but lower than a global average value retrieved from methane fluorescence at 3.3\mum observed from ISO/SWS (Drossart et al., ESA SP-427, 1999). If confirmed by further analysis, this difference could be due to a spatial variability of the eddy diffusion coefficient on Jupiter.\\
{\small{\em Based on observations collected at the European Southern Observatory, Chile (ESO No 64.S-0029)}}