Radial Profile of the Polytropic Index of Solar Wind Plasma in the Heliosphere

Abstract

Abstract We combine different measurements of the polytropic index of the proton plasma in the heliosphere: (i) near-adiabatic index in the inner heliosphere ∼1 au, (ii) subadiabatic indices in the outer heliosphere ∼20–40 au, and (iii) near-zero indices in the inner heliosheath. These observations are unified by a single theoretical model of the polytropic index throughout its radial extent in the heliosphere; the corresponding fitting reveals the decreasing trend of the polytropic index with increasing heliocentric distance R. We anticipate that with increasing R, (i) the Debye length and mean-free-path decreases; (ii) the Landau damping is less effective, transferring thus less wave energy to particles; and (iii) the collisionality degree increases, indicating that the proton plasma in the inner heliosheath might be collisionless.