Drag forces in the near and distant solar system

Abstract

Like the solar photons the solar wind particles induce a drag force onto the zodiacal dust grains in the heliosphere. For the distant solar wind with high Mach numbers the drag coefficient is a constant, but close to the Sun, where Mach numbers become small, the drag coefficient is a complicated function of the ion sound speed, density and temperature. We discuss the dynamics of dust particles due to this drag force and compare it with that in the distant solar wind. Especially in the near solar wind the eccentricity varies in a complicated way with the inclination of the orbits, also the semimajor axis decreases faster closer to the Sun. These variations are quite different in the distant solar wind. In addition, we apply an analogous mathematical formalism to the dust dynamics in the outer region of the heliosphere (>20 AU) where the neutral gas density becomes comparable or larger than that of the solar wind plasma. Here the neutral hydrogen gas induces a drag force onto the dust particles similar to the plasma Poynting-Robertson effect. But different to the radial solar wind, the velocity of the interstellar gas is mono-directional, and hence with respect to the inflow direction of the interstellar material this introduces an axial-symmetric force onto the dust particles. This force acts asymmetric in the orbit, and causes the eccentricity to increase fairly fast. The lifetime for dust grains in the Edgeworth-Kuiper Belt is no longer determined by the electromagnetic Poynting-Robertson lifetime, but by that of the neutral gas and is in the order of half a million years for a 10 μm sized particle.