Decay of Turbulent Upper-hybrid Waves in Weakly Magnetized Solar Wind Plasmas

Abstract Large-scale and long-term two-dimensional particle-in-cell simulations of high resolution are performed for the first time to study the dynamics of electrostatic decay of upper-hybrid wave turbulence generated by electron beams into Langmuir/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> -mode ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">L</mml:mi> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> ) waves in weakly to moderately magnetized plasmas, in conditions relevant to type III solar radio bursts. Simulations use parameters characteristic of beam–plasma interactions between ∼0.1 and 1 au. The impact of plasma magnetic field on decay is shown, and magnetic properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">L</mml:mi> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> waves are determined. During their energy transport through k wavevector scales, waves undergo several decay cascades, acquiring increasing magnetic energy until they reach electromagnetic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> -mode dispersion below the plasma frequency. Whereas the impact of magnetic field on decaying waves of large k = ∣ k ∣ is weak, important differences with respect to the unmagnetized plasma case manifest at small k -scales, where a boundary layer delimiting a spectral domain free of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">L</mml:mi> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> energy is revealed. It prevents decayed waves from reaching the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> -mode cutoff frequency and a high level of left-handed polarization, and it modifies the conditions for the appearance of modulational instabilities and strong turbulence phenomena at k ∼ 0. Ordinary <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">O</mml:mi> </mml:math> -mode waves are generated jointly with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> -mode waves at comparable energy levels, via electromagnetic decay, whereas <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">X</mml:mi> </mml:math> -mode emissions are much weaker in most cases. These results provide support for the interpretation of observations by satellites such as Parker Solar Probe and Solar Orbiter, and they supply a solid basis for tackling the more complex problem of dynamics of upper-hybrid wave turbulence in magnetized plasmas where random density fluctuations cannot be neglected.