Colloquium: The diamagnetic cavity at comets: have we finally solved its mysteries?
As comets travel through the solar system, the heat input from the Sun leads to sublimation of gases on the surface. As this neutral gas cloud expands, it is ionised and forms a plasma cloud around the comet nucleus.
As with all things in the solar system, comets are embedded in the solar wind. Cometary plasma physics as a field explores how the cometary plasma cloud interacts with the solar wind. While the best known result of this interaction is probably the bright blue tails that are visible through telescopes, the inner interaction region contains many mysteries that were largely unexplored until the Halley armada visited comet 1P/Halley in 1986. Since then our understanding of how energy and momentum are transferred from the solar wind to the cometary plasma has grown exponentially. This is in great part due to the extensive, two-year long measurements of ESA’s Rosetta mission at comet 67P/Churyumov-Gerasimenko.
In this talk I will explore recent results that help us understand the inner most region of the plasma environment: the diamagnetic cavity. Within this region the interplanetary magnetic field is expelled and a zero-magnetic-field region is created. While it was observed at both Halley and 67P, the fundamental differences between the two comets have made it difficult to explain the size of this region at 67P as well as the solar wind’s influence on its properties. Only recently have we started to understand these mysteries.