The aureole effect

By Paolo Tanga

During transits of Venus, a bright arc has often been reported, seen around the circumference of Venus’ disk which is partially outside the solar limb. The first observer who seems to have recorded this bright “aureole” is the French astronomer Chappe d’Auteroche, back in 1761. This peculiar aspect of the planet has been observed in all the subsequent transits, with varying intensity and aspect. In fact, its luminosity does not appear uniform: during the planet’s ingress, it can show at first segments or spots that then join into a continuous bright arc when the planet is nearly seen entirely on the solar disk. At egress, the sequence is reversed but is not reproduced identically in general. A common, recurrent feature, however, is the presence of a long-lasting luminescence – an isolated spot – visible alone as a first sign of the aureole at the start of the transit (and the last one at the end). Only in the 20th century, when the rotational properties of the planet were known, the astronomers were able to verify that this spot coincides roughly with one of the planet’s poles.

Drawing by M. Frassati (Unione Astrofili Italiani) representing the aureole during the 2004 transit egress. 20 cm Schmidt Cassegrain telescope.

The brightness of the aureole is generally not much lower than the solar photosphere close to the limb, making it visible through a solar filter. Its visibility, however, depends upon the intensity of turbulence, which effectively disperse the tenuous light of the thin ring. For this reason, it can only be seen under good observing conditions, using an excellent telescope.

Aureole by Swedish 1-m Solar Telescope on La Palma

The following table gives an overview of the different reported aspects of the aureole effect.

As promptly inferred by the early observers, the aureole has to be ascribed to the presence of an atmosphere around Venus. In some sense, it can be considered as the first solid confirmation of its presence! The ring of light is caused by refraction – not scattering – of sun light in the dense upper atmosphere of Venus. The Cytherean atmospheric conditions determine the appearance of the aureole. The closer a ray of light passes to the Venus surface, the larger is its deviation. For this reason, an extended region of the solar photosphere contributes to the light flux creating the aureole. The deepest rays that reach the observer are those grazing the opaque Venus cloud deck, which blocks all the light passing at even lower altitude. The altitude of the clouds at different positions along the limb is thus related to the intensity of the aureole.

Animations of the transit egress, obtained in 2004 by A. and S. Rondi. The Sun is hidden behind the occulting screen of a Lyot coronagraph. The false colours enhance the subtle luminosity variations. The red half-circle indicates the position of Venus.

Numerous people observed and recorded the aureole effect during the 2004 transit of Venus, at both ingress and egress. Most of these observers saw the aureole when Venus was at least halfway onto the Sun. Belgian observer Tonny Vanmunster saw the aureole at ingress: “Shortly before the very black disk of Venus completely slided into the sun’s bright edge, it grew a small, easy to spot halo (aureole) of light around its dark edge. The halo lasted a few minutes and then vanished.” At egress the aureole was seen again: “Shortly after the start of egress, I switched back to visual observing, and immediately noticed Venus’ atmosphere again,” says Tonny. “My wife Kathleen also easily spotted it, although I didn’t mention where to look. Again, the halo disappeared after a few minutes, shortly before half of Venus’ disk had left the sun’s limb.”

Because Venus will pass in front of the northern half of the sun’s disk during the 2012 transit of Venus (see picture), the aureole will then form near the north pole of Venus. Be on the lookout for this remarkable phenomenon, that will allow you to contribute to our understanding of the upper atmosphere of Venus!

The aureole during the transit ingress, by L. Comolli (Osservatorio di Tradate, Unione Astrofili Italiani). The ring of light has been enhanced for ease of visualization.