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Particle-in-cell Simulations of the Whistler Heat-flux Instability in Solar Wind Conditions

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In collision-poor plasmas from space, e.g., solar wind or stellar outflows, the heat flux carried by the strahl orbeaming electrons is expected to be regulated by the self-generated instabilities. Recently, simultaneous field andparticle observations have indeed revealed enhanced whistler-like fluctuations in the presence of counter-beamingpopulations of electrons, connecting these fluctuations to the whistler heat-flux instability (WHFI). This instabilityis predicted only for limited conditions of electron beam-plasmas, and has not yet been captured in numericalsimulations. In this Letter we report the first simulations of WHFI in particle-in-cell setups, realistic for the solarwind conditions, and without temperature gradients or anisotropies to trigger the instability in the initiation phase.The velocity distributions have a complex reaction to the enhanced whistler fluctuations conditioning the instabilitysaturation by a decrease of the relative drifts combined with induced (effective) temperature anisotropies (heatingthe core electrons and pitch-angle and energy scattering the strahl). These results are in good agreement with arecent quasilinear approach, and support therefore a largely accepted belief that WHFI saturates at moderateamplitudes. In the anti-sunward direction the strahl becomes skewed with a pitch-angle distribution decreasing inwidth as electron energy increases, which seems to be characteristic of self-generated whistlers and not to smallscaleturbulence.
Tijdschrift: Astrophysical journal letters
ISSN: 2041-8205
Volume: 882
Jaar van publicatie:2019