title: Ion-pressure equations derived from measurements in space.
abstract: Cluster measurements at the bow shock, the magnetosheath, and the magnetospheric boundary layer are used to derive ion-pressure equations for hot anisotropic plasmas. It is demonstrated that both perpendicular and parallel ion pressures are well approximated by polybaric expressions is proportional to N(gamma)B(kappa), where N is the plasma density, B is the magnetic field, gamma is in the range 0.5 to 2, and kappa is between -2 and 0. The parameters derived from observations are distinctively different from those predicted by double-adiabatic theory and are shown to hold for pressure variations over 4 orders of magnitude and for a range of plasma beta (ratio of kinetic/magnetic pressures) between 10(-4) and 10. The results are relevant for simulations and theories of astrophysical, solar, interplanetary, and magnetospheric processes based on MHD equations.
authors.affiliation: Swedish Institute of Space Physics, SE-751 21 Uppsala, Sweden. k.stasiewicz@irfu.se
authors.forename: K
authors.initials: K
authors.lastname: Stasiewicz