Plasma environment of comet Wirtanen during its low-activity stage

During the ROSETTA mission to comet Wirtanen the spacecraft will accompany the cometary nucleus on its flight towards the Sun on distances between about 3 and 1 AU. This offers the unique opportunity to study the development of the cometary activity over great ranges. The plasma environment of comet Wirtanen is mainly studied during its stage of low activity before a bow shock and a contact surface are built up. This will happen if the production rate is in the range of about 1023–1026 s−1. In two-dimensional collisionless bi-ion fluid simulations it is shown that the interaction of the solar wind (SW) with the strongly localized heavy ion source leads to the periodic formation of bi-ion discontinuities in regions where the cometary density is comparable to the SW density. The new type of plasma discontinuity results from the “resonance” between the heavy ion motion and a bi-ion plasma mode of very low phase velocity. The results are also characteristic for weak SW mass loading by other sources, e.g. AMPTE releases and possibly Phobos and Deimos or even Pluto.