Kelvin‐Helmholtz instabilities at the magnetic cavity boundary of comet 67P/Churyumov‐Gerasimenko

Abstract

We investigate the plasma environment of comet 67P/Churyumov‐Gerasimenko, the target of the European Space Agency's Rosetta mission. Rosetta will rendezvous with the comet in 2014 at almost 3.5 AU and follow it all the way to and past perihelion at 1.3 AU. During its journey towards the inner solar system the comet's environment will significantly change. The interaction of the solar wind with a well developed neutral coma leads to the formation of an upstream bow shock and, closer to the comet, the inner shock separating the solar wind, with cometary pick‐up ions mass‐loaded, from the inner cometary ions which are dragged outward through abundant collisions and charge exchange with the expanding neutral gas. As a consequence the interplanetary magnetic field is prevented from penetrating the innermost region of the comet, the so‐called magnetic cavity. We use our magnetohydrodynamics model BATSRUS (Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme) to simulate the solar wind – comet interaction. The model includes photoionization, ion‐electron recombination, and charge exchange. Under certain conditions our model predicts an unstable plasma flow at the inner shock. We show that the plasma shear flow around the magnetic cavity can lead to Kelvin‐Helmholtz instabilities. We investigate the onset of this phenomenon with change of heliocentric distance and furthermore show that a previously stable magnetic cavity boundary can become unstable when the neutral gas is predominately released from the dayside of the comet.