Laurent Lamy

ScienceShot: Hubble Spots Auroras on Uranus

If you think Earth's auroras are fleeting, consider those on the gas giant Uranus. While auroral glows in the atmosphere above our planet can flicker for hours, those adorning the distant blue orb—or at least those seen on the planet's sunlit side—apparently last for only a couple of minutes. Researchers caught their first glimpse of the brief auroras from our planet's neighborhood with the Hubble Space Telescope in November 2011, 3 months after a strong gust in the solar wind raced past Earth on its way to Uranus. Previously, scientists had observed the planet's auroras only once, during a Voyager flyby in 1986. At that time, instruments had a much better view of the glows—which lasted longer, covered a larger area, and festooned the unlit side of Uranus—but other factors were different, as well: Then, the planet's rotational axis was pointed almost directly at the sun, whereas in 2011 the axis lay almost perpendicular to the flow of solar wind. Last year's ephemeral glows (bright spots in images; rings surrounding the planet lie over the planet's equator) appeared near the planet's north magnetic pole, the researchers report in a forthcoming issue of Geophysical Research Letters. As on Earth, interactions between the solar wind and the planet's magnetic field generate the auroras. The new observations should help scientists better understand Uranus's odd magnetic field, whose axis is both offset from the center of the planet and tilts at an angle of 60° from the rotational axis.

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