Sunstruck: Solar hurricanes rip comet’s tail

Comet 2P/Encke has been looping around the sun for thousands of years. But last April, just after the comet slipped inside Mercury’s orbit, magnetic hurricanes belching from the sun chopped off its ion tail. Spacecraft images of the event provide the first clear evidence of such a curtailment.

RIP-OFF. The ion tail of Comet 2P/Encke was severed on April 20, as documented in this image sequence taken by one of the two STEREO spacecraft. Circles mark the comet’s nucleus; lines, the tail. NASA

As a comet nears the sun, it typically sports two tails—a brilliant dust tail and a fainter ion tail. The latter comes about as the solar wind, a breeze of charged particles blowing from the sun, sweeps ionized gas molecules from the comet’s nucleus into a tail that stretches for millions of kilometers into space. The wind also carries along a magnetic field that it drapes over the comet.

The cutting of 2P/Encke’s tail was observed by one of a pair of spacecraft, called STEREO, that study the sun (SN: 3/3/07, p. 133). The images provide new insight into the magnetic interplay between comets and solar explosions, says Angelos Vourlidas of the Naval Research Laboratory in Washington, D.C. He and his colleagues describe the event in the Oct. 10 Astrophysical Journal Letters.

An amateur astronomer first discovered the cometary rip in a movie of STEREO images routinely posted online. He alerted Vourlidas and his colleagues that on April 20, the tail had been crunched, then severed. “Everyone was speechless,” recalls Vourlidas.

In replaying the movie, the team saw that a coronal mass ejection (CME)—a magnetized cloud of charged particles thrown out from the sun’s outer atmosphere—had swept past 2P/Encke just as the comet lost its ion tail. A few hours later, the tail had grown back, replenished by the comet’s nucleus.

Reviewing earlier data, the team made a second startling discovery, Vourlidas told Science News. The comet’s tail had also been severed the day before the April 20 sighting. A bright blob off to one edge of the STEREO images may have been the culprit. The blob might have been an earlier CME from the same region of the sun that caused the April 20 event. This suggests that interactions between CMEs and comets “are more common than we thought,” says Vourlidas.

The pressure exerted by the April 20 CME was too small to have severed the tail, the team calculates. The researchers therefore suggest that the rip stemmed from a collision between the magnetic field carried by the CME and the field draping the comet.

The team’s analysis indicates that the magnetic fields in the CME and the comet pointed in opposite directions on April 20. In this orientation, the fields readily merge, releasing a burst of energy that could have torn apart the ion tail.

Another spacecraft, the Solar and Heliospheric Observatory, had previously seen hints that CMEs disrupted the tails of two other comets. But the April 20 STEREO images of 2P/Encke provide the most compelling evidence yet that CMEs can disrupt comets, comments Casey Lisse of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

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