The Sun is constantly expelling charged particles, and the solar wind carries those particles and our star’s magnetic field across interplanetary space. Those particles interact with planets, asteroids, and comets. On Earth, they form the aurorae, while around comets they create large magnetic-free regions.

This phenomenon was first observed by the Giotto spacecraft when it flew past Halley’s Comet three decades ago. Now, after many months, scientists have finally been able to observe this region around Comet 67P/Churyumov-Gerasimenko, for a very short time, during its closest approach to the Sun.

The new paper, published in the journal Astronomy and Astrophysics, presents the detection of a diamagnetic cavity, the technical term for the magnetic field-free bubble, obtained by Rosetta on July 26.

"We had almost given up on Rosetta finding the diamagnetic cavity, so we were astonished when we eventually found it," said Charlotte Götz, lead author of the research, in a statement. "We were able to detect the cavity, and on many occasions, because it is much bigger and dynamic than we had expected."

When the heat from the Sun liberates gas from the comet, it goes and forms the coma and the tail of the comet. The coma is made of charged particles, and this plasma interacts with the solar wind, stopping the magnetic field from reaching the nucleus and giving rise to the diamagnetic cavity.

^{Shown is a diagram of the diamagnetic cavity. ESA / C.Carreau}

The team thought the cavity would extend between 50 and 100 kilometers (30 and 60 miles) from the comet, making it difficult for Rosetta to detect, as at the time it was orbiting at a distance of 170 kilometers (105 miles).

But the boundary of the cavity is actually unstable. Oscillations of the plasma propagate through the coma and are amplified, extending well beyond the expected distance. This also made the duration of the cavity bubbles more sporadic, with the one recorded on July 26 lasting 25 minutes, and the longest recorded in November lasting about 40 minutes.

"Three decades ago, Giotto’s detection at Comet Halley was a great success, because it was the first confirmation of the existence of a diamagnetic cavity at a comet," said Matt Taylor, Rosetta project scientist at ESA.

"But that was only one measurement, while now we have seen the cavity at Rosetta’s comet come and go hundreds of times over many months. This is why Rosetta is there, living with the comet and studying it up close."