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.