Abstract
The Rosetta mission provides an unprecedented possibility to study the interaction of comets with the solar wind. As the spacecraft accompanies comet 67P/Churyumov–Gerasimenko from its very low-activity stage through its perihelion phase, the physics of mass loading is witnessed for various activity levels of the nucleus. While observations at other comets provided snapshots of the interaction region and its various plasma boundaries, Rosetta observations allow a detailed study of the temporal evolution of the innermost cometary magnetosphere. Owing to the short passage time of the solar wind through the interaction region, plasma instabilities such as ring--beam and non-gyrotropic instabilities are of less importance during the early life of the magnetosphere. Large-amplitude ultra-low-frequency (ULF) waves, the ‘singing’ of the comet, is probably due to a modified ion Weibel instability. This instability drives a cross-field current of implanted cometary ions unstable. The initial pick-up of these ions causes a major deflection of the solar wind protons. Proton deflection, cross-field current and the instability induce a threefold structure of the innermost interaction region with the characteristic Mach cone and Whistler wings as stationary interaction signatures as well as the ULF waves representing the dynamic aspect of the interaction.This article is part of the themed issue ‘Cometary science after Rosetta’.
Highlights
The Rosetta mission provides an unprecedented possibility to study the interaction of comets with the solar wind
If the magnetic field is directed perpendicular to the solar wind flow, pick-up of the newborn ions is via the convectional electric field ECONV = −uSW × BIMF, which causes co-motion of the ions with the solar wind flow
Mass loading controls the interaction of comets with the solar wind during the strong-activity phase of a comet
Summary
Cometary tails are one of the most fascinating features of any night sky. Without the existence of plasma and dust tails, we would not know about the existence of comets at all. If the magnetic field aligns with the solar wind flow, E × B pick-up is unimportant In this case, the newborn ions represent a heavy-ion beam distribution in velocity space. Scattering of pick-up ions by solar wind fluctuations and the self-generated plasma waves/turbulence eventually causes the build-up of shell-like ion distributions [13,14,15]. Mass loading as anticipated by Biermann and co-workers [3] happens and causes deceleration of the flow as momentum and energy transfers from the solar wind to the cometary ions. The solar wind flow significantly decelerates and particle densities of both protons and cometary ions increase steadily.
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