Plasma and field dynamics: some consequences from in-situ observations in solar wind and magnetospheres

Abstract

Because matter is most often in its plasma state, a strong coupling generally exists in the universe between flow dynamics and electromagnetic field. This is the reason why magnetic field plays such an important role in many astrophysical contexts, especially in sun and stars. Ideal MagnetoHydroDynamics can, to a certain extent, allow describing the magnetic field effects, but this elementary theory is unfortunately not of infinite validity: the understanding of phenomena as crucial as reconnection, dynamo effect, or plasma heating and acceleration for instance lies out of the scope of ideal MHD. For media sufficiently dense and for phenomena of sufficiently large scale with respect to the collision scales, resistive (and/or viscous) MHD has to be considered, which does not introduce much larger complexity. But for more dilute plasmas, or for shorter scale phenomena, resistive MHD is not valid either, and any interpretation must ground on more refined theories: Hall! -MHD, bi-fluid, multi-fluid, or fully kinetic. When applied, all these different theories do lead to quite different consequences. Because the observations from sun and stars can only rely on remote sensing techniques, it is generally quasi-impossible to discriminate between the different situations and decide which kind of theory is justified for interpreting correctly the phenomena. On the contrary, in the solar wind and the magnetospheres, in-situ measurements, from various experiments onboard many spacecraft, had provided a wealth of experimental results, often ruling out the pre-existing simplest interpretations and shedding some light on the reality of some phenomena like collisionless reconnection. The paper will try to display a few of such examples where solar wind and magnetosphere play a role of plasma laboratory for universal phenomena.