Acceleration of polytropic solar wind: Parker Solar Probe observation and one-dimensional model

Abstract

The acceleration of the solar coronal plasma to supersonic speeds is one of the most fundamental yet unresolved problems in heliophysics. Despite the success of Parker's pioneering theory on an isothermal solar corona, the realistic solar wind is observed to be non-isothermal, and the decay of its temperature with radial distance usually can be fitted to a polytropic model. In this work, we use Parker Solar Probe data from the first nine encounters to estimate the polytropic index of solar wind protons. The estimated polytropic index varies roughly between 1.25 and 1.5 and depends strongly on solar wind speed, faster solar wind on average displaying a smaller polytropic index. We comprehensively analyze the 1D spherically symmetric solar wind model with the polytropic index γ∈[1,5/3]. We derive a closed algebraic equation set for transonic stellar flows, that is, flows that pass the sound point smoothly. We show that an accelerating wind solution only exists in the parameter space bounded by C0/Cg<1 and (C0/Cg)2>2(γ−1), where C0 and Cg are the surface sound speed and one half of the escape velocity of the star, and no stellar wind exists for γ>3/2. With realistic solar coronal temperatures, the observed solar wind with γ≳1.25 cannot be explained by the simple polytropic model. We show that mechanisms such as strong heating in the lower corona that leads to a thick isothermal layer around the Sun and large-amplitude Alfvén wave pressure are necessary to remove the constraint in γ and accelerate the solar wind to high speeds.