Comets have their own 'Northern Lights', space agency says

A comet has been seen trailing its own aurora off Jupiter – the first time the phenomenon has been clearly identified.

The comet was monitored by the South-Western Research Institute (SwRI), using instruments on the European Space Agency's Rosetta aircraft.

Auroras are formed when charged particles from the Sun are drawn along the Earth's magnetic field lines to the north and south poles.

There they collide with atoms in the atmosphere to form the colourful display we recognise as the Northern Lights.

Auroras have been seen on other planets, moons and a star but this is the first time the phenomenon has been seen on a comet.

Charged particles were moved by solar winds towards with the gas and dust that surrounds the comet, which is called 67P/Churyumov-Gerasimenko.

The particles are not visible to the naked eye so researchers used instruments including a Alice far-ultraviolet (FUV) spectrograph and the Ion and Electron Sensor (IES) to make the discovery.

SwRI Vice President Dr. Jim Burch said: “Charged particles from the Sun streaming towards the comet in the solar wind interact with the gas surrounding the comet’s icy, dusty nucleus and create the auroras.

“The IES instrument detected the electrons that caused the aurora.

“I’ve been studying the Earth’s auroras for five decades,” Mr Burch added. “Finding auroras around 67P, which lacks a magnetic field, is surprising and fascinating."

The gas layer around the comet, called the “coma”, becomes excited by the solar particles and glows in ultraviolet light, which the Alice spectograph detected.

“Initially, we thought the ultraviolet emissions at comet 67P were phenomena known as ‘dayglow,’ a process caused by solar photons interacting with cometary gas,” said SwRI’s Dr. Joel Parker who leads the Alice spectrograph.

“We were amazed to discover that the UV emissions are aurora, driven not by photons, but by electrons in the solar wind that break apart water and other molecules in the coma and have been accelerated in the comet’s nearby environment.

"The resulting excited atoms make this distinctive light.”

In 2014, Rosetta became the first space mission to place a lander on a comet when the Philae lander successfully descended onto 67P/Churyumov-Gerasimenko.