The aureole of Venus: light refraction in the mesosphere during the solar transit of June 6, 2004

When the disk of Venus is in proximity of the solar limb, the atmosphere of the planet is revealed by a thin arc of light known as "aureole" or "Lomonosov's arc". The refraction of sunlight is responsible for this phenomena, which has shown a certain degree of variability from one transit to the other. The event of 2004 was the first one observed with modern electronic devices, so it was possible to derive a photometry of the arc. Our results indicate the presence of latitude-dependent signatures, as for example a bright polar spot. A simple refraction model reproducing the observations is capable of providing useful physical parameters concerning the mesosphere of the planet. A better focused observational campaign for the transit of 2012 is proposed to provide data of higher quality.