Quasilinear approach of the cumulative whistler instability in fast solar wind: Constraints of electron temperature anisotropy

Abstract

Context. Solar outflows are a considerable source of free energy that accumulates in multiple forms such as beaming (or drifting) components, or temperature anisotropies, or both. However, kinetic anisotropies of plasma particles do not grow indefinitely and particle-particle collisions are not efficient enough to explain the observed limits of these anisotropies. Instead, self-generated wave instabilities can efficiently act to constrain kinetic anisotropies, but the existing approaches are simplified and do not provide satisfactory explanations. Thus, small deviations from isotropy shown by the electron temperature (T) in fast solar winds are not explained yet. Aims. This paper provides an advanced quasilinear description of the whistler instability driven by the anisotropic electrons in conditions typical for the fast solar winds. The enhanced whistler-like fluctuations may constrain the upper limits of temperature anisotropy A ≡ T⊥/T∥ > 1, where ⊥, ∥ are defined with respect to the magnetic field direction. Methods. We studied self-generated whistler instabilities, cumulatively driven by the temperature anisotropy and the relative (counter)drift of electron populations, for example, core and halo electrons. Recent studies have shown that quasi-stable states are not bounded by linear instability thresholds but an extended quasilinear approach is necessary to describe these quasi-stable states in this case. Results. Marginal conditions of stability are obtained from a quasilinear theory of cumulative whistler instability and approach the quasi-stable states of electron populations reported by the observations. The instability saturation is determined by the relaxation of both the temperature anisotropy and relative drift of electron populations.