Electrons in the Collisionally Young Solar Wind: Parker Solar Probe Observations

Abstract We report new observations by the Parker Solar Probe of unique electron velocity distribution functions (VDFs) in low-density solar wind streams at heliocentric distances of ∼10–20 R ⊙ . These VDFs have a significant core anisotropy and lack a clear energy spectral break point between the thermal core and suprathermal strahl populations, with a parallel cut through the anti-sunward distribution closely following a single-temperature Maxwellian function over up to ∼6 decades in distribution function. These characteristics correlate strongly with low collisional age parameters and high Knudsen numbers, and also to some degree with higher local and asymptotic bulk solar wind flow speeds and lower strahl temperatures. The observed VDFs may represent a unique characteristic of low-density solar wind streams, or alternatively they could represent a common occurrence in any sufficiently collisionally young stream. In either case, these observations have implications both for the detailed evolution of the electron VDF and the large-scale structure and acceleration of the solar wind.