On earth, the northern lights provide spectacular sky spectacles – colored luminous phenomena in the sky of high latitudes. However, as data from the Rosetta space probe has now revealed for the first time, such aurors also exist in comets. According to this, the high-energy electrons from the solar wind excite the gases in the shell of comet 67P / Churyumov-Gerasimenko. This creates a glow in the ultraviolet wave range of light. This underlines that polar lights can also appear around celestial bodies without their own magnetic field, as the researchers explain. At the same time, the mechanism behind the comet aurors is unique.
Aurora borealis arise when the charged, high-energy particles of the solar wind hit the earth’s magnetic field. They are accelerated along the magnetic field lines and directed into high latitudes. There they meet atoms and molecules of nitrogen and oxygen in the upper atmosphere and stimulate them. When they return to their basic state, these gas particles emit energy in the form of photons – they emit light and thus generate the greenish, reddish or whitish shining northern lights. The decisive factor for defining such luminous phenomena as aurora is that the gas particles are excited by fast electrons coming from outside. This is what distinguishes the northern lights from the so-called airglow, also called night sky lights. In this case, the high-energy UV light from the sun strikes electrons from the gas particles in the atmosphere and these photo-induced particles then stimulate the gas molecules to glow.
“Chury” lights up in the UV range
Astronomers have already observed a diffuse glow around some comets. According to spectral measurements, this radiation in the UV range comes from excited hydrogen and oxygen atoms in the coma, the gas envelope, the comet’s nucleus. The comet 67P / Churyumov-Gerasimenko, or Chury for short, also shows this glow in the UV range, as reported by researchers working with Marina Galand from Imperial College London. The European space probe Rosetta had registered this UV glow as it orbited the comet. “At first we thought that these ultraviolet emissions from the comet were due to airglow – the interaction of solar photons with the comet’s gas,” explains co-author Jim Burch of the Southwest Research Institute in San Antonio. The researchers have now checked whether this is actually the case with the aid of the measurement data from the entire instrument cluster of the space probe. “This enabled us to clearly identify how the ultraviolet emissions from Chury are formed,” says Galand.
To the surprise of the scientists, the data revealed that the UV glow of the comet’s coma is not due to electrons generated locally by solar radiation, as in the case of Airglow. “Our analysis of the Rosetta data has shown that solar wind electrons are the reason for the glow on Comet Chury and not photons, as previously assumed,” says Galand. According to this, the atoms of the water vapor in the gas envelope of the comet are excited by fast electrons from the solar wind. These are accelerated and then come from outside and hit the gas particles of the cometary coma. “Since this process is very energetic, the resulting glow is also energetic and therefore in the ultraviolet range, which is invisible to the human eye,” explains co-author Martin Rubin, co-author of the study from the University of Bern.
Unique shape of the aurora
The diffuse glow of Comet Chury thus fulfills the criteria of an aurora – and proves that polar lights are also possible with such celestial bodies. “Rosetta is the first mission to observe an aurora in the UV range on a comet,” comments Matt Taylor of the European World Agency ESA. “Aurors are basically exciting, but when you can observe something like this for the first time and study the details, it’s even more exciting.” As closer analyzes have shown, the electrons of the solar wind are only accelerated in the vicinity of the comet. The driving force for this are electrical fields that are created by the interaction of the charged cometary plasma with the solar wind. While these fields and the electrons in them are influenced by the terrestrial magnetic field and directed into certain areas, they remain widely distributed in the comet. As a result, the resulting UV aurora is also diffuse.
“The resulting glow is unique,” says Galand. “It is caused by a mix of processes that are observed on Earth, Mars, but also at the moons of Jupiter.” Although the Rosetta mission ended in 2016 and the space probe crashed on Comet Chury in a controlled manner underlines the current knowledge of how valuable your data is to this day. Because not all of the information that the probe’s measuring instruments transmitted to Earth during its mission has been evaluated.
Source: Marina Galand (Imperial College London) et al., Nature Astronomy, doi: 10.1038 / s41550-020-1171-7