A long time ago, I studied comets during my years in graduate school. We studied cometary plasma tails, and found a lot of interesting things. But, we didn’t know at the time that a comet can have an aurora. That all changed with the in-depth study of Comet 67P/Churyumov-Gerasimenko. The Rosetta mission studied that comet in great detail, including emissions from and around the nucleus. Among those emissions: far-ultraviolet light that is generated when electrically charged particles from the Sun interact with the gassy coma surrounding a cometary nucleus.
From Dayglow to Cometary Aurorae
Initially, scientists interpreted the far-UV emissions as part of the “dayglow” surrounding the comet. When they re-analyzed the data, they found the emissions. The solar wind remains the culprit, just as it plays a role in forming the plasma tail of a comet. Charged particles in the solar wind interact with the gases in the coma. That actually causes water and other molecules to break apart and the resulting atoms give off far-ultraviolet light. And, that’s what forms the aurora around the comet.
Implications of Cometary Aurorae
Of course, the formation of such emissions at a comet tells scientists something about the solar wind, its particle loads and intensities. Studying what happens at a comet can give better insight into changes in the solar wind over time. It’s particularly important for understanding space weather, which is caused by the solar wind and its interactions with planetary magnetospheres. Space weather is a natural phenomenon and can affect satellites and astronauts in orbit around Earth. It would definitely have an effect on missions beyond Earth, too.
Back when we were studying comets and their plasma tails, we depended on a solar-orbiting spacecraft called Ulysses. It gave us information about the solar wind as it left the Sun. That allowed us to follow the effects of the solar wind on the comets we were studying. In particular, it helps shape the plasma tail, and disturbances in the solar wind definitely showed up at the comets some hours or days later.
New Data, New Ways of Seeing Comets
The Rosetta spacecraft is now “one” with the comet it studied. Its data adds a whole new dimension to what we know about comets and their interactions with the solar wind. Up until this discovery, I am not sure people thought about cometary aurorae. But, then again, back in the 1990s, we never thought comets could have x-ray emissions. Those were discovered by the Rœntgen X-ray Satellite and Rossi X-ray Timing Explorer at comet C/Hyakutake 1996 B2 and are also thought to be due to an interaction between the comet and the solar magnetic field entrained in the solar wind. This recent discovery is another in a long line of unexpected surprises about comets.