Wave-particle instabilities driven by departures from local thermodynamic equilibrium have been conjectured to play a role in governing solar wind dynamics. We calculate the statistical variation of linear stability over a large subset of Helios I and II observations of the fast solar wind using a numerical evaluation of the Nyquist stability criterion, accounting for multiple sources of free energy associated with protons and helium including temperature anisotropies and relative drifts. We find that 88% of the surveyed intervals are linearly unstable. The median growth rate of the unstable modes is within an order of magnitude of the turbulent transfer rate, fast enough to potentially impact the turbulent scale-to-scale energy transfer. This rate does not significantly change with radial distance, though the nature of the unstable modes, and which ion components are responsible for driving the instabilities, does vary. The effect of ion-ion collisions on stability is found to be significant; collisionally young wind is much more unstable than collisionally old wind, with very different kinds of instabilities present in the two kinds of wind.
We discuss the solar wind electron temperatures T e as measured in the nascent solar wind by Parker Solar Probe during its first perihelion pass. The measurements have been obtained by fitting the high frequency part of Quasi-Thermal Noise spectra recorded on-board by the Radio Frequency Spectrometer (Pulupa et al. 2017). In addition we compare these measurements with those obtained by the electrostatic analyzer Halekas & al. (2019). These first electron observations show an anti-correlation between T e and the wind bulk speed V : this anti-correlation is most likely the remnant of the well-known mapping observed at 1 AU and beyond between the fast wind and its coronal hole sources, where electrons are observed to be cooler than in the quiet corona. We also revisit HELIOS electron temperature measurements and show, for the first time, that an in-situ (T e , V) anti-correlation is well observed at 0.3 AU but disappears as the wind expands, evolves and mixes with different electron temperature gradients for different wind speeds.
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