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The infant bow shock
The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko for two years, and this enabled observations of how the plasma boundaries developed when the comet approached the Sun and the outgassing from the comet's nucleus increased. A particularly interesting discovery was the infant bow shock, which was observed by Rosetta. Bow shocks exists at all the planets and at comets. Rosetta observed it about 50 times closer to the comet’s nucleus than anticipated, because at 2-2.5 AU from the Sun it was being formed. A bow shock in this state of development had never been seen before anywhere in the solar system.

Read more about the infant bow shock in the ESA news story or the original publication.

Waves in the diamagnetic cavity
When a comet gets close to the Sun a diamagnetic cavity develops around the nucleus. In this cavity the the magnetic field is much weaker than in the surrounding plasma (less than 1 nT in the diamagnetic cavity of comet 67P, when outside it can be 40 nT). One of the observations Rosetta made was that ion acoustic waves, at frequencies up to 1.5 kHz, appeared when the spacecraft entered the cavity and disappeared as soon as it left.

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Updated 2018-12-17