天文

天王星の幻想的なオーロラと環　ハッブル捉える

天王星の幻想的なオーロラと環　ハッブル捉える
Ever since Voyager 2 beamed home spectacular images of the planets in the 1980s, planet-lovers have been hooked on extra-terrestrial aurorae. Aurorae are caused by streams of charged particles like electrons, that come from various origins such as solar winds, the planetary ionosphere, and moon volcanism. They become caught in powerful magnetic fields and are channelled into the upper atmosphere, where their interactions with gas particles, such as oxygen or nitrogen, set off spectacular bursts of light. The alien aurorae on Jupiter and Saturn are well-studied, but not much is known about the aurorae of the giant ice planet Uranus. In 2011, the NASA/ESA Hubble Space Telescope became the first Earth-based telescope to snap an image of the aurorae on Uranus. In 2012 and 2014 astronomers took a second look at the aurorae using the ultraviolet capabilities of the Space Telescope Imaging Spectrograph (STIS) installed on Hubble. They tracked the interplanetary shocks caused by two powerful bursts of solar wind travelling from the Sun to Uranus, then used Hubble to capture their effect on Uranus’ aurorae — and found themselves observing the most intense aurorae ever seen on the planet. By watching the aurorae over time, they collected the first direct evidence that these powerful shimmering regions rotate with the planet. They also re-discovered Uranus’ long-lost magnetic poles, which were lost shortly after their discovery by Voyager 2 in 1986 due to uncertainties in measurements and the featureless planet surface. This is a composite image of Uranus by Voyager 2 and two different observations made by Hubble — one for the ring and one for the aurorae.


　
この美しい画像は、太陽系第7惑星の「天王星」のボイジャー2号とハッブル宇宙望遠鏡の2回の観測結果を合成したもの。ハッブル宇宙望遠鏡の観測では片方で天王星の環を、もう片方で天王星のオーロラを捉えることに成功しています。
　
このようなオーロラは地球でもおなじみなだけでなく、木星や土星のオーロラも有名です。しかし天王星のものは、あまり研究が進められてきませんでした。しかし2011年のハッブル宇宙望遠鏡による観測を皮切りに、徐々にその姿が明らかになっていったのです。
　
さらにパリ天文台による2012年と2014年の観測では、オーロラが天王星の自転とともに移動していること、またボイジャー2号の観測以来わからなくなっていた磁極も観測することに成功しています。
　
Image Credit: NASA
■Hubble Spots Auroras on Uranus
https://www.nasa.gov/image-feature/goddard/2017/hubble-spots-auroras-on-uranus
■Auroras on Uranus Dazzle in New Hubble Telescope Views
http://www.space.com/36416-uranus-auroras-hubble-voyager-photos.html