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

The velocity distributions observed in space have too many fast particles, by Maxwell's standards. This ubiquitous property raises doubts about the va lidity of models based on a set of fluid equations whose closure requires t he distributions to be nearly Maxwellian. I discuss here two generic cases: bound structures and winds. Near rapidly rotating magnetised planets, part icles channelled along co-rotating magnetic field lines are acted on by the field-aligned component of the centrifugal force, which exceeds the gravit ational attraction beyond a few planetary radii. With dipolar magnetic fiel ds, 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 tha t the temperatures are rising with distance from the potential wells, if th e velocity distributions are not Maxwellian - in sharp contrast to classica l isothermal equilibrium; and the density profiles fall off less steeply th an a Gaussian - just as the velocity distributions fall off less steeply th an a Maxwellian. While these bound structures are shaped along closed magne tic field lines, winds can brow along open field fines. A suprathermal tail in the electron velocity distribution increases the electric field which e nsures the balance of ion and electron fluxes, and should thus increase the wind speed above the value predicted by classical hydrodynamic escape. (C) 2001 Elsevier Science Ltd. All rights reserved.