Sun Storm May Have Caused Flare-Up of Rosetta's Comet
The ESA/NASA Solar and Heliospheric Observatory spacecraft captured this image of a coronal mass ejection erupting on the sun on Sept. 30, 2015.

Material from the sun may have caused Comet 67P/Churyumov-Gerasimenko to flare up nearly 100 times brighter than average in some parts of the visual spectrum, new research reports.

At about the same time that charged solar particles slammed into Comet 67P, the European Space Agency's (ESA) Rosetta spacecraft observed that the icy wanderer dramatically brightened. Initially, scientists assumed that unusual effect came from jets of material within the comet. However, newly released observations of 67P suggest that a burst of charged particles from the sun, known as a coronal mass ejection (CME), could have caused the change.

"The [brightening] was characterized by a substantial increase in the hydrogen, carbon and oxygen emission lines that increased by roughly 100 times their average brightness on the night of Oct. 5 and 6, 2015," John Noonan told Noonan, who just completed his undergraduate degree at the University of Colorado at Boulder, presented the research at the Division for Planetary Sciences meeting in Pasadena, California, in October. [Photos: Europe's Rosetta Comet Mission in Pictures]

After reading a report of a CME that hit 67P at the same time, Noonan realized that the increased emissions from water, carbon dioxide and molecular oxygen observed by Rosetta’s R-Alice instrument could all be explained by the collision of the comet with material jettisoned from the sun.

"This doesn't yet rule out that an outburst could have happened, but it looks possible that all of the emissions could have been caused by the CME impact," Noonan said.

A simulation reveals how the plasma of the solar wind should interact with Comet 67P/C-G.
A simulation reveals how the plasma of the solar wind should interact with Comet 67P/C-G.
Credit: Modelling and simulation: Technische Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt; Visualisation: Zuse-Institut Berlin

Rosetta entered orbit around Comet 67P in August 2014, making detailed observations until the probe deliberately crashed into the icy body at the end of its mission in September 2016. 

So Rosetta was tagging along when Comet 67P made its closest pass to the sun in August 2015. (Such "perihelion passages" occur once every 6.45 years — the time it takes the icy object to circle the sun.) 

As 67P neared the sun, newly warmed jets began to release gas from the surface, building up the cloud of debris around the nucleus known as the coma. Jets continued to spout throughout Rosetta's observations as different regions of the comet rotated into sunlight. Such spouts were initially credited with the extreme brightening that took place in October 2015.

In addition to warming the comet, the sun also interacted with it through its solar wind, the constant rush of charged particles streaming into space in all directions. Occasionally, the sun also blows off the collections of plasma and charged particles known as CMEs. When CMEs collide with Earth, they can interact with the planet’s magnetic field to create dazzling auroral displays; this interaction can also damage power grids and satellites

Niklas Edberg, a scientist on the Rosetta Plasma Consortium Ion and Electron Spectrometer instrument on the spacecraft, and his colleagues recently reported that RPC/IES observed a CME impact on Rosetta at the same time as the bizarre brightening. The ESA/NASA Solar and Heliospheric Observatory (SOHO) spacecraft detected the CME as it left the sun on Sept. 30, 2015. [The Sun's Wrath: Worst Solar Storms in History]

According to Edberg, the CME compressed the plasma material around the comet. Because Rosetta was orbiting within the coma, the probe hadn't sampled any material streaming from the solar wind since the previous April, and wasn’t expected to do so for several more months. When the CME slammed into the comet, however, the coma was compressed and Rosetta briefly tasted part of the solar wind once again.

"This suggests that the plasma environment had been compressed significantly, such that the solar wind ions could briefly reach the detector, and provides further evidence that these signatures in the cometary plasma environment are indeed caused by a solar wind event, such as a CME," Edberg and his team wrote in their study, which was published in the journal Monthly Notices of the Royal Astronomical Society in September 2016.

For Noonan, the realization that a CME had impacted the comet at the same time of its unusual brightening had an illuminating effect.

"I read this [Edberg et al.] paper and realized that the substantial increase in electron density could account for the increased emissions from the coma that R-Alice observed, and set about testing what the density of the coma's water, carbon dioxide and molecular oxygen components would have to be to match what we saw," Noonan said. 

Charged particles from the CME may have excited cometary material, causing it to release photons, he added. Some of the observed changes could be created only by interacting electrons, causing what Noonan called "unique fingerprints" that let the scientists know electrons were impacting the material. Of special importance was the transition of oxygen line in the spectra, a change that can only be caused by electrons.

"During the course of the CME, we saw this line increase in strength by roughly hundredfold," Noonan said.

The charged particles were unlikely to have come from the solar wind, which Noonan said would be blocked from ever penetrating this deep.

While CMEs have been observed around other comets, they have only been viewed remotely. From such great distances, only large-scale changes in the comets' comas and tails could be observed, Edberg said. Over the course of its two-year mission at Comet 67P, Rosetta's close orbit allowed it to observe other CMEs interacting with the comet, but Noonan said none were as noticeable as the event of Oct. 5-6, 2015. 

"Prior to Rosetta, these electron impact emissions had never been observed around a comet, and it was these emissions that gave away that the CME might be a factor in causing them," Noonan said.

He cautioned that it isn't a given that the influx of charged particles caused the bizarre brightening, which still could be caused by the jets of material.

"At this point, we are still working to understand exactly what was the cause to see if it was the CME, and outburst, or both, that caused the emission," Noonan said.

Given the timing of the impact, however, it is unlikely that the flare-up was the result of gas released by jets alone.

"There are more forces at play than just a higher density of gas," Noonan said.

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