Polar aurorae on Uranus - first pictures taken from the Earth

The magnetic environment of Uranus, the seventh and next to last planet of the solar system, which orbits the Sun at a distance of 3 million kilometers, is even more complex and weird than we had imagined. The polar aurorae bear witness to this: they were photographed from the Earth for the first time in the ultra-violet domain, on the 16th and 29th of November last, by the Hubble space telescope. They are made up of bright points, which switch on and off on a time scale of a few minutes, on the visible face of the planet (the day side, illuminated by the Sun). A quarter of a century earlier, during its flyby of Uranus, the Voyager 2 probe had detected permanent aurorae on the night side, quite similar to their terrestrial cousins.

The new observations furnish information about the present configuration of Uranus’ magnetosphere, the cavity carved out of the solar wind by the planet’s magnetic field. A singular feature: the magnetic axis of the body is inclined at roughly 60° with respect to its north-south rotation axis, around which the planet turns with a period of almost 17 hours. Moreover, this latter lies almost in the ecliptic plane, which contains the orbits of the planets in the solar system. The results were obtained by an international team which includes five scientists from the Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique - LESIA1 - of the Observatoire de Paris (Observatoire de Paris/CNRS/Université Pierre et Marie Curie/Université Paris Diderot), as well as scientists from the Institut de Recherche en Astrophysique et Planétologie in Toulouse and the Institut de Planétologie et d’Astrophysique in Grenoble.

Polar aurorae are luminous phenomena which characterize planets with a magnetic field, like the Earth and the giant planets Jupiter, Saturn, Uranus and Neptune. As the name indicates, they are observed in the vicinity of the magnetic poles. They are created by the arrival in the upper atmosphere of bursts of particles originating in the magnetosphere, accelerated by complex processes and transported along the magnetic field lines.

This flux of charged particles carries tens of gigawatts of energy, which is then dissipated as radiation. The study of the aurorae is thus a means of probing the magnetosphere from a distance, without actually approaching it. On the Earth, the most intense aurorae signal the arrival of a solar wind perturbation. This can affect electric lines, the functioning of satellites, even the health of astronauts.

The magnetosphere of Uranus is very atypical. Its configuration changes with each rotation (period about 17h), and also with the seasons (a revolution around the Sun last 84 years). During the study, the scientists showed that on a time scale of a quarter of a century, there are significant differences in the aurorae observed by Voyager 2 at the beginning of local summer (the solstice) in the hemisphere illuminated by the Sun, and those photographed by Hubble at the beginning of the following autumn (the equinox). The morphology of the bright and variable points on the day side indicates that their origin is different, presumably due to a particularly dynamic interaction at that time between the magnetosphere and the solar wind.

In the 1980s, ultra-violet spectrometric observations made by the Voyager 1 and 2 missions showed that all the giant planets, both gas (Jupiter and Saturn) and icy (Uranus and Neptune), have aurorae, which are more or less sensitive to the solar wind. Then, in the following decade, during the 1990s, the Hubble telescope took over with intense ultra-violet imaging campaigns targeting the Jovian and Saturnian aurorae. It was more difficult to do this in the case of Uranus. The planet is twice as far as Saturn, half as large, and its aurorae are half as bright. In 1998 and 2005, two attempts to detect these aurorae with the Hubble telescope, the most powerful ultra-violet telescope in existence, failed.

A third attempt, in November 2011, was successful. The scientists were helped by a favourable celestial conjunction: an almost perfect alignment of the Sun, the Earth, Jupiter and Uranus. During this period, with solar activity on the increase after four years of calm and a peak expected around 2013, they watched out for a favourable window of opportunity. In September 2011, three eruptions took place on our star, and the scientists were able to compute and follow in real time their motion from planet to planet. The salvos shot by the Sun towards the Earth were first detected by NASA’s Stereo satellites. They were moving at a speed of 2 million km/h (500 km/s). Two days later, they had reached our planet, and NASA’s Wind satellite. Intense aurorae were registered by the NOAA2 monitoring satellites. Fifteen days later, as expected, the shock waves induced polar aurorae on Jupiter, recorded in the radio domain by Stereo. Two months later, in November, these perturbations were due at Uranus.
This innovative study - in different spectral domains and covering many planets - succeeded because of the exceptional alignment of the Sun, the Earth, Jupiter and Uranus. It highlights the growing importance of space meteorology, a young discipline which has enabled the effects of our luminary’s spasms to be followed from one end of the solar system to the other.

1. The LESIA is a department of the Observatoire de Paris and a laboratoire Observatoire de Paris/CNRS/Université Pierre et Marie Curie/Université Paris Diderot
2. National Oceanic and Atmospheric Administration NOAA

Collaboration

The following scientists contributed to this work at the Observatoire de Paris: Laurent Lamy, Renée Prangé, Philippe Zarka, Baptiste Cecconi and Jean Aboudarham at the
Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique LESIA. The work also involved Nicolas André from the Institut de Recherche Astrophysique et de Planétologie IRAP at Toulouse, and Mathieu Barthélémy from the Institut de Planétologie et d’Astrophysique at the Grenoble IPAG, as well as partners in the U.S at Michigan University, Boston University, the South-Western Research Institute, the University of Arizona (U.S.A.), and in the U.K, London University and its Imperial College, Leicester University. In France, this work was supported by the CNES.

Images, drawings

Reference

Earth-based detection of Uranus’ aurorae, published in the 14th of April 2012 issue of the journal Geophysical Research Letters of the American Geophysical Union.