Aurora Cometalis: The Northern Lights of Comet 67P

Sept. 22, 2020: Imagine putting your thumb on a garden hose and sending a jet of water into the sky. At the apex of the stream, auroras form. It turns out, some comets can actually perform this trick.

In a paper published this week in Nature Astronomy, researchers described how comet 67P/Churyumov-Gerasimenko turns vaporous jets of water into auroras.

The European Space Agency’s Rosetta spacecraft observed the weird lights while it was orbiting Comet 67P in 2014-2016. At first researchers misunderstood what the glow was. It couldn’t be an aurora, could it? For one thing, the comet doesn’t even have a magnetic field–a key ingredient of geomagnetic storms. Also, the lights of Comet 67P are invisible to the human eye. They shine at far ultraviolet wavelengths, unlike the familiar red and green curtains that dance around Earth’s poles.

“Nevertheless, they are auroras,” says Marina Galand of Imperial College London, UK, lead author of the new study.

It took years for Galand and colleagues to figure out what was going on. Solving the mystery required data from five of Rosetta’s sensors plus a physics model to calculate how the solar wind interacts with the comet’s atmosphere.

They found that electric fields naturally occurring in the comet’s atmosphere can grab electrons from the solar wind and hurl them inward. Those electrons rush headlong into water molecules spewing out of the comet’s core. Debris from the collision–excited atoms of H and O–produce an ultraviolet glow: Aurora Cometalis.

Comet auroras respond to space weather much like Earth auroras do. Gusts of solar wind can rev them up, and a good coronal mass ejection (CME) can cause an outright auroral storm. Indeed, a CME that hit Comet 67P on Oct. 22, 2014, caused “a sharp intensification” of the UV brightness.

Galand says that “other comets should have these kind of auroras, too.” The basic physics is universal. 67P/Churyumov-Gerasimenko’s auroras are special only in the sense that the Rosetta spacecraft was there to observe them.

Thought experiment: Suppose you could see UV light. What would the auroras of Comet 67P look like from ground level? “They would be diffuse, but not uniform,” speculates Galand. “Some parts of the sky would be brighter than others–especially if a jet of water crossed your field of view!”

And here’s the strangest part of all: The auroras would descend all the way down to the comet’s surface. “So you would be surrounded by the light,” she says.

Yet another reason to visit a comet….

Solar Cycle 25 Has Begun

Sept. 15, 2020: Solar Cycle 25 is officially underway. NASA and NOAA made the announcement during a media teleconference earlier today. According to an international panel of experts, the sunspot number hit rock bottom in Dec. 2019, bringing an end to old Solar Cycle 24. Since then, sunspot counts have been slowly increasing, heralding new Solar Cycle 25.

“How quickly solar activity rises is an indicator on how strong the next solar cycle will be,” says Doug Biesecker of NOAA’s Space Weather Prediction Center, co-chair of the Solar Cycle 25 Prediction Panel. “Although we’ve seen a steady increase in sunspot activity this year, it is slow.”

The panel believes that new Solar Cycle 25 will be a weak one, peaking in 2025 at levels similar to old Solar Cycle 24. If their prediction is correct, Solar Cycle 25 (like Solar Cycle 24 before it) will be one of the weakest since record-keeping began in 1755.

“While we are not predicting a particularly active Solar Cycle 25, violent eruptions from the sun can occur at any time,” warns Biesecker. Indeed, even Solar Minimum can produce a superstorm, so Solar Cycle 25 should not be taken lightly despite the panel’s low expectations. Radio blackouts, power outages and severe geomagnetic storms are possible in the years ahead.

Above: Solar Cycle 25 auroras photographed on Sept. 14, 2020, by Jani Ylinampa in Finland.

For now, solar activity should remain generally low. Sunspot counts still have a long way to go before they reach levels typical of Solar Maximum. For the rest of 2020, periods of quiet will be occasionally interrupted by minor solar storms, with only a slight chance of big events.

On the bright side, the first Northern Lights of Solar Cycle 25 are dancing around the Arctic Circle right now, and the coming season for aurora watching promises to be the best in years. Stay tuned! Aurora alerts: SMS Text.