Plasma Physics at Comets – What can we learn from laboratory experiments?

Diamagnetic cavities at comets were predicted already in the 1960s [1], and then observed at comet lP/Halley by the ESA/Giotto spacecraft in 1986 [2]. Recently, the ESA/Rosetta spacecraft spent two years orbiting comet 67P/Churyumov-Gerasimenko and encountered the diamagnetic cavity of comet 67P more than 700 times [3, 4]. Most encounters lasted a few minutes, with the duration varying from a few seconds up to more than 30 minutes. As the spacecraft moved very slowly (~lms ^_1), it can be considered stationary with respect to the plasma. Therefore, the quick succession of detections indicates that the boundary of the diamagnetic cavity moved over the spacecraft. Figure 1 (left) shows three diamagnetic cavity signatures observed with the plasma instruments on Rosetta on 16 September 2015 when the comet was close to perihelion. Rosetta was in the diamagnetic cavity during the periods of nearly zero magnetic field (marked by the coloured regions). Outside the cavity, the plasma was often characterised by a series of asymmetric, steepened waves which are visible in the magnetic field, as well as in the plasma density [5]. Since all observations to date have been made using a single spacecraft, the shape of the diamagnetic cavity boundary cannot be well constrained by measurements. However, it has been suggested, based on wave observations, that bulges on the cavity boundary move past the spacecraft, causing the latter to quickly move in and out of the cavity [6].