The Dynamics of the Magnetic-field-free Cavity around Comets

Abstract

The diamagnetic cavity is the innermost region of the magnetosphere of an active comet, from which the magnetic field is expelled by the outflowing matter. This phenomenon, first detected around comet 1P/Halley, was extensively studied recently by the Rosetta comet chaser mission. Rosetta observed a surprisingly large diamagnetic cavity around comet 67P/Churyumov-Gerasimenko and revealed an unforeseen structure, rich and highly dynamic. Here we present a simple (1+1)-dimensional analytic MHD model of the diamagnetic cavity, which for the first time explains the unexpected size and variability of the cavity. In this model, inward and outward moving time-dependent solutions emerge, featuring distinct differences, in accordance with observations. The plasma density is enhanced in the entire magnetized region. The density enhancement is more pronounced for weak comets, resulting in a stronger interaction and hence the larger than expected cavity. Space weather effects determine the asymptotic plasma speed, thus driving the variations of the plasma properties observed near the cavity.