Generalized Two Thermal Regime Approach: Bipoly Fluid Modeling

Abstract The isopoly bifluid approach assumes an isothermal evolution of the solar wind near the Sun up to the radial distance r iso , followed by a polytropic evolution constrained by the observed polytropic indices. This approach provides a more accurate model of the interplanetary fluid properties of the solar wind and their radial evolution than Parker’s isothermal and polytropic wind. In this article, we present an improvement of the isopoly approach by considering a generalized two thermal regime approach, embedding two distinct polytropic evolutions, the “bipoly” modeling. To demonstrate the capability of the approach, the models are fitted to both interplanetary and coronal observations, all classified by wind speed population in the spirit of M. Maksimovic et al. The set of observations used as constraints are coronal temperatures inferred from charge-state ratio observations from Solar Orbiter, and interplanetary measurements from Helios and Parker Solar Probe. The relaxation of the isothermal criteria in the near-Sun region permits one to significantly improve the fast wind acceleration for low-coronal-temperature conditions. In summary, the new model matches closely the observational constraints: the coronal temperature and the radial evolution of the wind plasma properties in the interplanetary medium, and this for all the wind speed populations.