Quenching of the beam-plasma instability by 3-D spectra of large scale density fluctuations

A model is presented to explain the highly variable yet low level of Langmuir waves measured in situ by spacecraft when electron beams associated with Type III solar bursts are passing by; the low level of excited waves allows the propagation of such streams from the Sun to well past 1 AU without catastrophic energy losses. The model is based, first, on the existence of large scale density fluctuations that are able to efficiently diffuse small k beam unstable Langmuir waves in phase space, and, second, on the presence of a significantly isotropic nonthermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The strength of the model lies in its ability to predict various levels of Langmuir waves depending on the parameters. This feature is consistent with the high variability actually observed in the measurements

Modeling stretched solitary waves along magnetic field lines

International audienceA model is presented for a new type of fast solitary waves which is observed in downward current regions of the auroral zone. The three-dimensional, coherent structures are electrostatic, have a positive potential, and move along the magnetic field lines with speeds on the order of the electron drift. Their parallel potential profile is flattened and cannot fit to the Gaussian shape used in previous work. We develop a detailed BGK model which includes a flattened potential and an assumed cylindrical symmetry around a centric magnetic field line. The model envisions concentric shells of trapped electrons slowly drifting azimuthally while bouncing back and forth in the parallel direction. The electron dynamics is analysed in terms of three basic motions that occur on different time scales characterized by the cyclotron frequency We , the bounce frequency wb , and the azimuthal drift frequency wg. The ordering We >> wb >> wg is required. Self-consistent distribution functions are calculated in terms of approximate constants of motion. Constraints on the parameters characterizing the amplitude and shape of the stretched solitary wave are discussed

Evolution of a localized Langmuir packet in the solar wind and on auroral field lines

Langmuir emissions in space are reported to be clumpy and intermittent. The high-frequency wave power appears concentrated in spatial packets, whether amidst the solar wind or on auroral field lines. Due to the plasma motion relative to the spacecraft, determining the source for the wave free energy in the three-dimensional electron distribution function has always been difficult, since the unstable features pass by the detector in presumably too short time to be measured. The range of unstable phase velocities and growth rates have generally been estimated rather than determined by unequivocal measurements. The analysis of wave-particle interactions in a space environment has taken recently a new turn with the development of wave correlators on board rockets and satellites. Such instruments seek to identify correlations between the phase of the wave-field and the fluxes of energetic particles. The data interpretation is complex, however, it must be backed by a detailed theoretical understanding of the wave-particle interaction, including the phase relation for inhomogeneous packets. To this end Langmuir packets interacting with fast electrons can be studied in the appropriate regime by means of particle-in-cell simulations, provided that one succeeds in reducing the level of the fluctuations, enhancing the signal-to-noise ratio, and incorporating the appropriate boundary conditions. The first results of such simulations are presented here as a test and expansion of previous analysis

Approximate Analytic Solution for the Spatiotemporal Evolution of Wave Packets undergoing Arbitrary Dispersion

We apply expansion methods to obtain an approximate expression in terms of elementary functions for the space and time dependence of wave packets in a dispersive medium. The specific application to pulses in a cold plasma is considered in detail, and the explicit analytic formula that results is provided. When certain general initial conditions are satisfied, these expressions describe the packet evolution quite well. We conclude by employing the method to exhibit aspects of dispersive pulse propagation in a cold plasma, and suggest how predicted and experimental effects may be compared to improve the theoretical description of a medium's dispersive properties.Comment: 17 pages, 4 figures, RevTe

On deformation of electron holes in phase space

This Letter shows that for particularly shaped background particle distributions momentum exchange between phase space holes and the distribution causes acceleration of the holes along the magnetic field. In the particular case of a non-symmetric ring distribution (ring with loss cone) this acceleration is nonuniform in phase space being weaker at larger perpendicular velocities thus causing deformation of the hole in phase space.Comment: Original MS in EPL style, 1 Figur