Large scale structure of planetary environments: the importance of not being Maxwellian

Abstract

The velocity distributions observed in space have too many fast particles, by Maxwell's standards. This ubiquitous property raises doubts about the validity of models based on a set of fluid equations whose closure requires the distributions to be nearly Maxwellian. I discuss here two generic cases: bound structures and winds. Near rapidly rotating magnetised planets, particles channelled along co-rotating magnetic field lines are acted on by the field-aligned component of the centrifugal force, which exceeds the gravitational attraction beyond a few planetary radii. With dipolar magnetic fields, this tends to trap particles near the equator and produce torus-shaped structures, whereas gravitational confinement occurs closer to the planet. These confining forces act as high-pass filters for particle speeds, so that the temperatures are rising with distance from the potential wells, if the velocity distributions are not Maxwellian — in sharp contrast to classical isothermal equilibrium; and the density profiles fall off less steeply than a Gaussian — just as the velocity distributions fall off less steeply than a Maxwellian. While these bound structures are shaped along closed magnetic field lines, winds can blow along open field lines. A suprathermal tail in the electron velocity distribution increases the electric field which ensures the balance of ion and electron fluxes, and should thus increase the wind speed above the value predicted by classical hydrodynamic escape.