Solar Wind Electron Pressure Gradients, Suprathermal Spectral Hardness, and Strahl Localization Organized by Single-point Measurements of 0.1 nV m−1 Ambipolar E ∥

Abstract

A new, fast technique to measure the solar wind’s ambipolar E ∥ routinely with 10% precision and accuracy is demonstrated using 4 yr of 1 au electron data from the Wind 3DP experiment. The 3DP electron instrument duty cycle determines E ∥ ≃ 0.1 nV m−1 from a single spectrum over much shorter time intervals than those requiring radial transits for pressure profiles. The measured weak electric field is invariably strong (in the dimensionless sense of Dreicer), with a modal value of , and positively correlated with solar wind speed, while E ∥ decreases with increasing wind speed. These observations establish across all solar wind conditions the nearly equal accelerations provided by E ∥ and coulomb drags on thermal electrons, a central hypothesis of the Steady Electron Runaway Model (SERM) for the solar wind. Filtered E ∥ observations successfully recover previously reported 1 au bulk speed dependence of electron temperature gradients. The filter screens for unstructured spherically symmetric solar wind (USSSW) conditions of solar wind theory. Outside USSSW conditions much shorter scaled pressure gradients (of both signs) and stronger ∣E ∥∣ are observed predominantly in corotating regimes. Consistent with modeling by Dreicer and SERM, the observed spectral hardness of electrons at suprathermal energies is positively correlated with increasing local values of across the 4 yr data set. Virtually all strahl electrons, crucial to the electron heat flux, are shown to be confined within the local closed coulomb separatrix of each spectrum as determined using the locally measured value of .