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Papers by Alexander Volokitin

Fluids, 2019

The random density fluctuations observed in the solar wind plasma crucially influence on the Lang... more The random density fluctuations observed in the solar wind plasma crucially influence on the Langmuir wave turbulence generated by energetic electron beams ejected during solar bursts. Those are powerful phenomena consisting of a chain of successive processes leading ultimately to strong electromagnetic emissions. The small-scale processes governing the interactions between the waves, the beams and the inhomogeneous plasmas need to be studied to explain such macroscopic phenomena. Moreover, the complexity induced by the plasma irregularities requires to find new approaches and modelling. Therefore theoretical and numerical tools were built to describe the Langmuir wave turbulence and the beam’s dynamics in inhomogeneous plasmas, in the form of a self-consistent Hamiltonian model including a fluid description for the plasma and a kinetic approach for the beam. On this basis, numerical simulations were performed in order to shed light on the impact of the density fluctuations on the b...

The European Physical Journal D, 2014

The kinetic theory of plasmas, based on the Vlasov-Poisson system of equations, can efficiently s... more The kinetic theory of plasmas, based on the Vlasov-Poisson system of equations, can efficiently solve only some aspects of the extremely large panel of problems involving wave-particle and wave-wave interaction processes in plasmas. Therefore the dynamics of charged particles and waves has been modeled by other approaches as, for example, Hamiltonian models describing the self-consistent wave-particle and wave-wave interactions in homogeneous or inhomogeneous magnetized plasmas. Various physical problems could be efficiently studied by such methods, concerning nonlinear and turbulent stages of different instabilities of electron or ion distributions, wave packets’ saturation and particles fluxes’ relaxation processes, particle trapping and detrapping mechanisms by waves, wave-particle interactions at multiple resonances, quasilinear diffusion processes of particles in waves, wave turbulence in randomly inhomogeneous plasmas, acceleration of particles, wave focusing, scattering, reflection and decay, etc. In particular, the aim of the paper, after a brief description of such Hamiltonian models, is to present the most recent simulation results obtained when studying Langmuir turbulence in the presence of electron beams propagating in inhomogeneous plasmas as the solar wind, where random density fluctuations with average levels up to several percents of the background plasma density have been measured.

The Astrophysical Journal, 2015

<p>Recent spacecraft observations of plasma injections reveal abundance of small-sc... more <p>Recent spacecraft observations of plasma injections reveal abundance of small-scale nonlinear magnetic structures – sub-ion magnetic holes. These structures contribute to magnetosphere-ionosphere coupling and likely responsible for energetic electron scattering. Sub-ion magnetic holes propagate in plasma of two electron components with very different temperatures. Properties of such holes resemble properties of classical magnetosonic solitary waves propagating across the ambient magnetic field, but observations suggest that these holes do not disturb background ions. This study aims to generalize the linear theory of magnetosonic waves by including two electron components. In analog to the electron acoustic mode, cold electrons can act as ions for the generation of magnetosonic mode waves. This unstable electron magnetosonic mode can explain all properties of sub-ion holes in observations. We suggest that sub-ion holes can form during the nonlinear evolution this electron magnetosonic mode. We consider an adiabatic model for investigation of such nonlinear evolution and electron dynamical response to evolving hole electromagnetic field. This model describes slow formation of sub-ion magnetic holes from low-amplitude limit. The adiabatic electron response to such formation can include both electron colling and heating, for populations with different pitch-angles.</p><p>The work was supported by the Russian Scientific Foundation, project 19-12-00313.</p>

Journal of Geophysical Research, 2001

Results from experiments of neutral xenon gas releases in the ionosphere are presented. The relea... more Results from experiments of neutral xenon gas releases in the ionosphere are presented. The releases were made from the scientific satellite APEX, which had various plasma and field diagnostics. An enhancement of HF wave activity over a broad band (0.1-10 MHz) is shown to occur during releases made in the sunlight at high pitch angles to the magnetic field. No changes of plasma density were detected, but electron temperature measurements indicate electron energy enhancements during the releases. Theoretical calculations based on the conditions of the APEX releases show that chargeexchange collisions and elastic scattering of the injected Xe neutrals by the ambient plasma can lead to an ion beam instability that may account for the observed wave and electron energy enhancements. The interaction is akin to the critical ionization velocity effect. We calculate that because of the low injection density, this turbulence-fueled ionization should have a low yield (Ͻ1%), which may explain the lack of detectable plasma density enhancement.

<p&amp... more <p>A new method to calculate semi-analytically the radiation efficiency of electromagnetic waves emitted at specific frequencies by electrostatic wave turbulence in solar plasmas with random density fluctuations is presented. It is applied to the case of electromagnetic emissions radiated at the fundamental plasma frequency ω<sub>p</sub> by beam-driven Langmuir wave turbulence during Type III solar bursts. It is supposed that the main radiation mechanism is the linear conversion of electrostatic to electromagnetic waves on the background plasma density fluctuations, at constant frequency. Due to the presence of such inhomogeneities, the rates of electromagnetic radiation are modified compared to the case of uniform plasmas. Results show that the radiation efficiency of Langmuir wave turbulence into electromagnetic emissions at ω<sub>p</sub> is nearly constant asymptotically, the electromagnetic energy density growing linearly with time, and is proportional to the average level of density fluctuations. Comparisons with another analytical method developed by the authors and with space observations are satisfactory.</p>

Fluids, 2019

The random density fluctuations observed in the solar wind plasma crucially influence on the Lang... more The random density fluctuations observed in the solar wind plasma crucially influence on the Langmuir wave turbulence generated by energetic electron beams ejected during solar bursts. Those are powerful phenomena consisting of a chain of successive processes leading ultimately to strong electromagnetic emissions. The small-scale processes governing the interactions between the waves, the beams and the inhomogeneous plasmas need to be studied to explain such macroscopic phenomena. Moreover, the complexity induced by the plasma irregularities requires to find new approaches and modelling. Therefore theoretical and numerical tools were built to describe the Langmuir wave turbulence and the beam’s dynamics in inhomogeneous plasmas, in the form of a self-consistent Hamiltonian model including a fluid description for the plasma and a kinetic approach for the beam. On this basis, numerical simulations were performed in order to shed light on the impact of the density fluctuations on the b...

The European Physical Journal D, 2014

The kinetic theory of plasmas, based on the Vlasov-Poisson system of equations, can efficiently s... more The kinetic theory of plasmas, based on the Vlasov-Poisson system of equations, can efficiently solve only some aspects of the extremely large panel of problems involving wave-particle and wave-wave interaction processes in plasmas. Therefore the dynamics of charged particles and waves has been modeled by other approaches as, for example, Hamiltonian models describing the self-consistent wave-particle and wave-wave interactions in homogeneous or inhomogeneous magnetized plasmas. Various physical problems could be efficiently studied by such methods, concerning nonlinear and turbulent stages of different instabilities of electron or ion distributions, wave packets’ saturation and particles fluxes’ relaxation processes, particle trapping and detrapping mechanisms by waves, wave-particle interactions at multiple resonances, quasilinear diffusion processes of particles in waves, wave turbulence in randomly inhomogeneous plasmas, acceleration of particles, wave focusing, scattering, reflection and decay, etc. In particular, the aim of the paper, after a brief description of such Hamiltonian models, is to present the most recent simulation results obtained when studying Langmuir turbulence in the presence of electron beams propagating in inhomogeneous plasmas as the solar wind, where random density fluctuations with average levels up to several percents of the background plasma density have been measured.

The Astrophysical Journal, 2015

<p>Recent spacecraft observations of plasma injections reveal abundance of small-sc... more <p>Recent spacecraft observations of plasma injections reveal abundance of small-scale nonlinear magnetic structures – sub-ion magnetic holes. These structures contribute to magnetosphere-ionosphere coupling and likely responsible for energetic electron scattering. Sub-ion magnetic holes propagate in plasma of two electron components with very different temperatures. Properties of such holes resemble properties of classical magnetosonic solitary waves propagating across the ambient magnetic field, but observations suggest that these holes do not disturb background ions. This study aims to generalize the linear theory of magnetosonic waves by including two electron components. In analog to the electron acoustic mode, cold electrons can act as ions for the generation of magnetosonic mode waves. This unstable electron magnetosonic mode can explain all properties of sub-ion holes in observations. We suggest that sub-ion holes can form during the nonlinear evolution this electron magnetosonic mode. We consider an adiabatic model for investigation of such nonlinear evolution and electron dynamical response to evolving hole electromagnetic field. This model describes slow formation of sub-ion magnetic holes from low-amplitude limit. The adiabatic electron response to such formation can include both electron colling and heating, for populations with different pitch-angles.</p><p>The work was supported by the Russian Scientific Foundation, project 19-12-00313.</p>

Journal of Geophysical Research, 2001

Results from experiments of neutral xenon gas releases in the ionosphere are presented. The relea... more Results from experiments of neutral xenon gas releases in the ionosphere are presented. The releases were made from the scientific satellite APEX, which had various plasma and field diagnostics. An enhancement of HF wave activity over a broad band (0.1-10 MHz) is shown to occur during releases made in the sunlight at high pitch angles to the magnetic field. No changes of plasma density were detected, but electron temperature measurements indicate electron energy enhancements during the releases. Theoretical calculations based on the conditions of the APEX releases show that chargeexchange collisions and elastic scattering of the injected Xe neutrals by the ambient plasma can lead to an ion beam instability that may account for the observed wave and electron energy enhancements. The interaction is akin to the critical ionization velocity effect. We calculate that because of the low injection density, this turbulence-fueled ionization should have a low yield (Ͻ1%), which may explain the lack of detectable plasma density enhancement.

<p&amp... more <p>A new method to calculate semi-analytically the radiation efficiency of electromagnetic waves emitted at specific frequencies by electrostatic wave turbulence in solar plasmas with random density fluctuations is presented. It is applied to the case of electromagnetic emissions radiated at the fundamental plasma frequency ω<sub>p</sub> by beam-driven Langmuir wave turbulence during Type III solar bursts. It is supposed that the main radiation mechanism is the linear conversion of electrostatic to electromagnetic waves on the background plasma density fluctuations, at constant frequency. Due to the presence of such inhomogeneities, the rates of electromagnetic radiation are modified compared to the case of uniform plasmas. Results show that the radiation efficiency of Langmuir wave turbulence into electromagnetic emissions at ω<sub>p</sub> is nearly constant asymptotically, the electromagnetic energy density growing linearly with time, and is proportional to the average level of density fluctuations. Comparisons with another analytical method developed by the authors and with space observations are satisfactory.</p>