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The energetic composition of radio lobes in the FR II galaxies $-$ estimatedby comparing their radio luminosities with the powers required to inflatecavities in the external medium $-$ seems to exclude the possibility of theirenergetic domination by protons. Furthermore, if the jets were dominated by thekinetic energy of cold protons, it would be difficult to efficiently accelerateleptons in the jets' terminal shocks. Assuming that the relative energycontents of leptons, protons and magnetic fields are preserved across theshocks, the above implies that the large-scale jets should also beenergetically dominated by leptons: $P_{\rm e,j} \gtrsim P_{\rm p,j}$. On theother hand, previous studies of small-scale jets in blazars and radio coressuggest a pair content (number of electrons and positrons per proton) of theorder of $n_{\rm e}/n_{\rm p} \sim 20$. Assuming further that the particlecomposition of jets does not evolve beyond the blazar scales, we show that thisimplies an average random Lorentz factor of leptons in large-scale jets of$\bar\gamma_{\rm e,j} \gtrsim 70(1+\chi_{\rm p})(20n_{\rm p}/n_{\rm e})$, andthat the protons should be mildly relativistic with $\chi_{\rm p} \equiv(\epsilon_{\rm p} + p_{\rm p})/\rho_{\rm p} c^2 \lesssim 2$, $p_{\rm p}$ thepressure of protons, $\epsilon_{\rm p}$ the internal energy density of protons,and $\rho_{\rm p} c^2$ the rest-mass energy density of protons. We derive thenecessary conditions for loading the inner jets by electron-positron pairs andproton-electron plasma, and provide arguments that heating of leptons in jetsis dominated by magnetic reconnection.

On the significance of relativistically hot pairs in the jets of FR II radio galaxies