Helios observational constraints on solar wind expansion

Abstract

Helios particle and magnetic field observations between 0.3 and 1 AU are used to determine plasma parameters that characterize the bulk and internal energy state of the solar wind. Quantities expected to be conserved in a time‐stationary flow with local spherical symmetry in the ecliptic plane are actually found to be invariant within measurement uncertainties. These are the total mass, energy, and angular momentum fluxes for the anisotropic solar wind plasma composited by electrons, protons, and alphas. Although individual species have nonthermal velocity distributions, the total plasma pressure is almost isotropic (p⊥/p∥ = 0.9). The total heat flux divided by the mass flux Qr/ρur is markedly smaller than thermal speeds squared υ∥,⊥² = p∥,⊥/ρ. By this reason an appropriately defined polytropic index γ is found to be almost 5/3 and rather insensitive to heliocentric distance and flow speed. This index γ does not include terms due to wave turbulence or external heat sources but is solely based on the total particle heat flux. These observational findings indicate that the heat flux beyond 0.3 AU is observationally too small to cause a strong departure from adiabaticity. The solar wind expansion may be conceived in terms of a “single particle” moving in the binding gravitational potential and in the accelerating thermal (enthalpy and heat fluxes) and magnetorotational (azimuthal kinetic energy and Poynting flux) potentials. The radial profiles of these potentials are derived from observations.