On the temperature of the solar wind
  Solar wind provides an example of a weakly collisional plasma expanding from
a thermal source in the presence of spatially diverging magnetic field lines.
Observations show that in the inner heliosphere, the electron temperature
declines with the distance approximately as $T_{e}(r)\sim r^{-0.3} \dots
r^{-0.7}$, which is significantly slower than the adiabatic expansion law $
\sim r^{-4/3}$. Motivated by such observations, we propose a kinetic theory
that addresses the non-adiabatic evolution of a nearly collisionless plasma
expanding from a central thermal source. We concentrate on the dynamics of
energetic electrons propagating along a radially diverging magnetic flux tube.
Due to the conservation of their magnetic moments, the electrons form a beam
collimated along the magnetic field lines. Due to weak energy exchange with the
background plasma, the beam population slowly loses its energy and heats the
background plasma. We propose that no matter how weak the collisions are, at
large enough distances from the source a universal regime of expansion is
established where the electron temperature declines as $T_e(r)\propto
r^{-2/5}$. This is close to the observed scaling of the solar wind temperature
in the inner heliosphere. Our first-principle kinetic derivation may thus
provide an explanation for the slower-than-adiabatic temperature decline in the
solar wind. More broadly, it may be useful for describing magnetized winds from
G-type stars.
