Heat flux dropouts in the solar wind and Coulomb scattering effects

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Abstract
Measurements of solar wind electrons at ISEE 3 located 0.01 AU upstream from the Earth (McComas et al., 1989) indicate periods of time when the flux of antisunward suprathermal electrons decreases suddenly, leaving the velocity distribution nearly isotropic and causing the solar wind heat flux to drop. These heat flux dropouts (HFDs) are usually found in regions of increased plasma density and decreased electron temperature, and they are associated with sector boundaries. It has been suggested that HFDs may be due either to disconnection from the Sun of the magnetic flux tube in which they are found, or to enhanced Coulomb scattering of halo electrons in transit from the Sun to the Earth. Using the vector electron spectrometer on ISEE 1, we have found eight intervals of greatly reduced heat flux which appear to be associated with HFDs at ISEE 3. Five of the eight events were delayed by an appropriate convection time and had approximately the same duration as the corresponding ISEE 3 event. The onset times for the other three intervals of reduced heat flux could not be determined in the ISEE 1 data. Other events from the ISEE 3 set were not observed because ISEE 1 either was not in the solar wind or was inside the foreshock, which obscured the HFD identification. Velocity distributions during HFDs at ISEE 1 show that the depletion of halo electrons traveling away from the Sun is most pronounced in the 100‐eV range, while there is essentially no depletion in the 1‐keV range, and that in four cases the magnitude of the halo depletion and its upper velocity limit both depend on the density increase in the HFD. These results are shown to be in agreement with the υ−3 dependence of the Coulomb collision frequency. Thus we conclude that Coulomb scattering effects play a substantial role in at least some heat flux dropout events.