April 13, 2012

Aurorae on Uranus detected by Hubble

Among all the known planetary magnetospheres, these gaps carved in the solar wind by magnetized planets (especially the Earth and giant planets), the exotic case of Uranus had been very little studied because of its remote location (19.2 AU away from the Sun, i.e. almost 3 billions km).


Its magnetosphere could only be analysed once when discovered by the Voyager 2 probe which flew by the planet in January 1986, nine years after being launched from Earth. At the same time, Voyager 2 performed an unexpected discovery: unlike the other planets, the Uranus' magnetic field is very inclined (~60º) compared to its rotation axis which almost tips in the plan of the ecliptic. This produces an asymmetric magnetosphere, unique in the solar system, the configuration of which varies greatly during the planetary rotation (~17h) and its revolution around the Sun (86 years).

If Voyager 2 flew by Uranus at the solstice, the planet was at the time at the equinox, a radically different and unknown situation.

The planetary aurorae are the only way to remotely study a magnetosphere. Indeed, a magnetosphere acts like a giant particle accelerator which are guided along the magnetic field's lines to the auroral regions (around the magnetic poles), where the injected energy is dissipated as intensive luminous radiation.

In the 1980's, the UV spectrometers of the Voyager 1 and 2 probes identified aurorae around the 4 giant planets (gaseous and icy planets), including Uranus.

Since the 1990's, the Hubble Space Telescope took over with thousands of images of Jupiter and Saturn's intensive aurorae but it failed trying to re-detect the Uranus ones (two attempts in 1998 and 2005). It is a more difficult task because, compared with Saturn, Uranus is twice smaller, twice farer and its aurorae are twice less bright. Hubble still remains the most powerful operating UV telescope, a third attempt was launched in 2011.

However, instead of randomly observing, the scientists chose to take advantage of the chock waves in the solar wind, an event known to "induce" the planetary aurorae when occurring. It is this characteristic which gave birth to space meteorology. In addition, by choosing a late 2011 observation, they also benefited from a quasi planetary alignment of the Earth, Jupiter and Uranus so they could observed the responses of those three planets to the same interplanetary passage.

In early September 2011, the Sun was emitting a series of coronal mass ejections which spread at approximately 500 km/s. Two days later, they arrived on Earth, two weeks later on Jupiter and after a two-month journey on Uranus where the Hubble's observations started in November. The result of the multi-spectral and multi-planet observation campaign was press-released in the GRL article.

In addition to the Earth and Jupiter's responses, this technique allowed the scientists to positively re-detect the aurorae of Uranus fifteen years later, to obtain the first images and above all, to provide the first information on its magnetosphere at the equinox. Moreover, the authors of the article confirmed these observations by re-analysing the Hubble's previous observations in 1998, when auroral signatures were already hidden. These emissions are weak, transitional (variably by a few minutes) and different from the other known planetary aurorae. Their morphologies (hotspots and rings on the day side in 2011 and 1998) were in contrast to the one observed by Voyager 2 (continuous emissions on the night side, similar to the Earth's) and indicated variations of the magnetic configuration within a quarter of an Uranus' revolution time (solstice to equinox).

These results opened a new rich sphere of study when the Uranus' system is the subject of a renewed interest. But in the absence of a selected mission, and even without the journey duration to reach the planet (estimated to 12-15 years), the remote observations of telescopes like Hubble remain the only way to study this magnetosphere.

Article references

Lamy, L., R. Prangé, K. C. Hansen, J. T. Clarke, P. Zarka, B. Cecconi, J. Aboudarham, N. André, G. Branduardi-Raymont, R. Gladstone, M. Barthélémy, N. Achilleos, P. Guio, M. K. Dougherty, H. Melin, S. W. H. Cowley, T. S. Stallard, J. D. Nichols, and G. Ballester (2012), Earth-based detection of Uranus’ aurorae, Geophys. Res. Let., in presse (acceepted on march 2012).

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Update: 04/13/2012

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