Regulation of the solar wind electron heat flux from 1 to 5 AU: Ulysses observations

In this study we use observations from the three-dimensional electron spectrometer and magnetometer aboard the Ulysses spacecraft to examine the solar wind electron heat flux from 1.2 to 5.4 AU in the ecliptic plane. Throughout Ulysses' transit to 5.4 AU, the electron heat flux decreases more rapidly (~R^-3.0) than simple collisionless expansion along the local magnetic field and is smaller than expected for a thermal gradient heat flux, q_∥e(r)=-κ_∥∇_∥T_e(r). The radial gradients and magnitudes expected for a number of electron heat flux regulatory mechanisms are examined and compared to the observations. The best agreement is found for heat flux regulation by the whistler heat flux instability. The upper bound and radial scaling for the electron heat flux predicted for the whistler heat flux instability are consistent with the observations.