The Electron Heat flux in the Solar Wind: Collisionless or Collisional Dominated?

Abstract:

Recent measurements by the WIND spacecraft at 1 AU (Bale et al., ApJL 769, L22) show that, when the Knudsen number (the ratio between the electron mean free path and the electron temperature scale height) drops below about 0.3, the electron heat flux appears strongly correlate with the classical electron heat flux expected for a collisional dominated plasma, while at higher values the heat flux is limited to a fraction of its free-streaming value. Using a fully kinetic model including the effect of Coulomb collisions, the expansion of the solar wind, as well as the self consistent large scale electric field needed to preserve quasi neutrality and zero current, we indeed observe the electron heat flux in very good agreement with observations. However, closer inspection of the heat flux properties, such as its dependence on the thermodynamic quantities and its variation with the distance, shows that for Knudsen number between 0.02 and 0.3 the heat flux is still not conveniently described by the Spitzer-Härm formula. Moreover, the good agreement between our model - that does not include wave-particle interactions - and the heat flux in the solar wind in the high Knudsen regime suggests that at 1AU the heat flux intensity is not constrained by electromagnetic instabilities, wave-particle and wave-wave interactions.