Vol. 618
In section 10. Planets and planetary systems

First observations of magnetic holes deep within the coma of a comet

by F. Plaschke, T. Karlsson, C. Götz, et al. A&A 618, A114


Magnetic holes, meaning regions that have depressed interplanetary magnetic field strength, have been known to exist in the solar wind for over 40 years, and also to penetrate into the magnetosheaths of Earth and Mercury. Using data from the Rosetta fluxgate magnetometer at comet 67P Churyumov-Gerasimenko, Plaschke et al. report here for the first time that magnetic holes exist in the complex, near-surface, ionized environment of a comet. This environment consists of the interacting populations of solar wind protons and heavy cometary ions, produced by the sublimation of surface ices followed by ionization of the expanding gases. The magnetic holes near the comets have common properties with magnetic holes in the solar wind and in the terrestrial magnetosheath. Remarkably, cometary magnetic holes are still observed at periods of large cometary activity, when solar wind protons are almost absent.