Interaction of suprathermal solar wind electron fluxes with sheared whistler waves: fan instability

Abstract

Abstract. Several in situ measurements performed in the solar wind evidenced that solar type III radio bursts were sometimes associated with locally excited Langmuir waves, highenergy electron fluxes and low-frequency electrostatic and electromagnetic waves; moreover, in some cases, the simultaneous identification of energetic electron fluxes, Langmuir and whistler waves was performed. This paper shows how whistlers can be excited in the disturbed solar wind through the so-called "fan instability" by interacting with energetic electrons at the anomalous Doppler resonance. This instability process, which is driven by the anisotropy in the energetic electron velocity distribution along the ambient magnetic field, does not require any positive slope in the suprathermal electron tail and thus can account for physical situations where plateaued reduced electron velocity distributions were observed in solar wind plasmas in association with Langmuir and whistler waves. Owing to linear calculations of growth rates, we show that for disturbed solar wind conditions (that is, when suprathermal particle fluxes propagate along the ambient magnetic field), the fan instability can excite VLF waves (whistlers and lower hybrid waves) with characteristics close to those observed in space experiments.

Interaction of suprathermal electron fluxes with lower hybrid waves

“…In general, for instabilities involving energetic electrons, the thermal bulk electrons are nonresonant and do not participate directly to the wave-particle interaction, even if they influence on the waves through the dielectric tensor; the tail involves the suprathermal electrons which can be in cyclotron resonance with the waves. Thus, in the numerical simulations ͑see below͒, in order to point out clearly the wave excitation mechanisms at work, the bulk electrons are generally not represented as macroparticles ͑however, they can be introduced in order to take into account Landau damping, 35 for example͒.…”

“…18,19 Its threshold is overcome if the number of electrons giving energy to the wave at the anomalous cyclotron resonance exceeds the number of electrons taking energy from the wave at the Landau and the normal cyclotron resonances. [20][21][22][23][24] This instability can also play an essential role in space plasmas, where energetic electron fluxes are ubiquitous, as it will be shown below by studying the linear and the nonlinear stages of the fan instability for one and several waves; in particular, such wave-particle interaction process can modify drastically the parallel velocity distribution and give rise to bumps in the electron tail.…”

Nonturbulent stabilization of ion fluxes by the fan instability

“…Then, the anisotropic character of the parallel velocity distribution is the cause of the resonant wave generation at the anomalous cyclotron resonances. Note that fan instability due to electron distributions [20][21][22][23][24][25][26][27] physics as well as in tokamak plasmas [28], where suprathermal tails of electrons can be created during the current drive [29,30]. Some authors [31] explain the perpendicular heating of ion beams observed above the auroral terrestrial acceleration region by the excitation of ion acoustic waves generated by the ion distributions owing to the fan instability; performing calculations in the frame of the quasilinear theory of weak turbulence, they obtain an estimate of the heating rate, which is consistent with satellite observations [9,19] and that they compare with numerical simulations performed in 1D geometry, all waves propagating in the same quasiperpendicular direction (see also [32]).…”

Stabilization of the fan instability: Electron flux relaxation

“…[1][2][3] The dynamical evolution of the amplitudes of discrete wave spectra interacting with anisotropic electron distribution functions was explained owing to numerical simulations. The main attention was paid to the detailed study of the physical mechanisms at work when the amplitudes of two or a discrete set of a few waves with initially close resonant velocities evolve so that the waves' resonance widths merge in the course of a complex wave-particle interaction process.…”