The stationary non-equilibrium plasma of cosmic-ray electrons and positrons

•The thermodynamic variables of the cosmic-ray electron–positron plasma are calculated.•Spectral fits to the AMS-02 and HESS GeV–TeV electron and positron fluxes are performed.•A semi-empirical phase-space reconstruction of the partial probability densities is carried out.•Partition function & entropy of a relativistic plasma in stationary non-equilibrium are derived.•The positron fraction extrapolated to TeV energies shows a broad peak & exponential decay.

The statistical properties of the two-component plasma of cosmic-ray electrons and positrons measured by the AMS-02 experiment on the International Space Station and the HESS array of imaging atmospheric Cherenkov telescopes are analyzed. Stationary non-equilibrium distributions defining the relativistic electron–positron plasma are derived semi-empirically by performing spectral fits to the flux data and reconstructing the spectral number densities of the electronic and positronic components in phase space. These distributions are relativistic power-law densities with exponential cutoff, admitting an extensive entropy variable and converging to the Maxwell–Boltzmann or Fermi–Dirac distributions in the non-relativistic limit. Cosmic-ray electrons and positrons constitute a classical (low-density high-temperature) plasma due to the low fugacity in the quantized partition function. The positron fraction is assembled from the flux densities inferred from least-squares fits to the electron and positron spectra and is subjected to test by comparing with the AMS-02 flux ratio measured in the GeV interval. The calculated positron fraction extends to TeV energies, predicting a broad spectral peak at about 1 TeV followed by exponential decay.