ELECTRON HEAT FLUX IN THE SOLAR WIND: ARE WE OBSERVING THE COLLISIONAL LIMIT IN THE 1 AU DATA?

Using statistically significant data at 1 AU, it has recently been shown (Bale et al.) that in the solar wind, when the Knudsen number KT (the ratio between the electron mean free path and the electron temperature scale height) drops below about 0.3, the electron heat flux q intensity rapidly approaches the classical collisional Spitzer–Ha ̈rm limit. Using a fully kinetic model including the effect of Coulomb collisions and the expansion of the solar wind with heliocentric distance, we observe that the heat flux strength does indeed approach the collisional value for Knudsen numbers smaller than about 0.3 in very good agreement with the observations. However, closer inspection of the heat flux properties, such as its variation with the heliocentric distance and its dependence on the plasma parameters, shows that for Knudsen numbers between 0.02 and 0.3 the heat flux is not conveniently described by the Spitzer–Ha ̈rm formula. We conclude that even though observations at 1 AU seem to indicate that the electron heat flux intensity approaches the collisional limit when the Knudsen drops below ∼0.3, the collisional limit is not a generally valid closure for a Knudsen larger than 0.01. Moreover, the good agreement between the heat flux from our model and the heat flux from solar wind measurements in the high-Knudsen number regime seems to indicate that the heat flux at 1 AU is not constrained by electromagnetic instabilities as both wave–particle and wave–wave interactions are neglected in our calculations.