Medium scale gravity waves in the ionospheric F-region and their possible origin in weather disturbances

This study results from a coordinated experiment involving ionospheric observations of Faraday rotation between a geostationary satellite and three ground based receivers at Aberystwyth and Bournemouth in the U.K. and Lannion, France, together with incoherent scatter observations at St. Santin-Nancay, France.Quasi-periodic variations of electron content observed simultaneously at the three stations are interpreted in terms of medium scale gravity waves travelling in the ionospheric F-region. Characteristics of these waves are derived by means of a cross-correlation technique.A reverse ray tracing computation, using data on the neutral atmosphere and neutral wind stratification from the incoherent scatter observations, has been used in an attempt to locate the sources of these waves.The results show that some of the waves are almost certainly generated above 100 km altitude, probably by auroral phenomena, while the others could be produced near ground level by meteorological sources. The reverse ray tracing indicates that the latter sources are in general located in a geographic area in the vicinity of a weather disturbance. A production mechanism for these waves is proposed involving ageostrophic perturbations of the neutral wind in a jet stream.     

Spurious contribution to cosmic ray scattering calculations

The quasi-linear theory for cosmic ray propagation is a well-known and widely accepted theory. In this paper, we discuss the different contributions to the pitch-angle Fokker–Planck coefficient from large and small scales for slab geometry using the damping model of dynamical turbulence. These examinations will give us a hint on the limitation range where the quasi-linear approximation is a good approximation.     

Acoustic damping of fast kink oscillations of coronal loops

The damping of fast kink oscillations of solar coronal loops attributable to the radiation of MHD waves into the surroundings is considered in the thin-tube approximation. The oscillation damping decrement is calculated both by using a new energy method and by solving the dispersion equation for magnetic-tube eigenmodes. The two approaches are in good agreement under appropriate assumptions. The damping is negligible if MHD waves are radiated perpendicular to the magnetic field. The low Q factor of the loop oscillations in active regions found with the TRACE space telescope is associated with the generation of running waves that propagate along magnetic field lines.     

Peculiarities of entropy and magnetosonic waves in optically thin cosmic plasma

The stability problem for small magnetohydrodynamic (MHD) perturbations in an optically thin, perfectly conducting uniform plasma with a cosmic abundance of elements is solved in the linear approximation. The electron heat conduction along the magnetic field and the proton heat conduction across the field are taken into account. We have shown for the first time that the entropy waves can grow exponentially, while the magnetosonic waves are damped in a wide range of physical conditions closest to the conditions in stellar coronae with the proper allowance for radiative losses. Slow magnetosonic waves are damped particularly rapidly. For the solar corona, the calculated damping decrement of slow magnetosonic waves agrees well with the averaged one in 11 quasi-periodic events observed from the TRACE satellite in extreme ultraviolet radiation. Other possible astrophysical applications of the results obtained are briefly discussed.     

Hook whistlers observed at low latitude ground station Varanasi

Employing the Haselgrove ray tracing equations and a diffusive equilibrium model of the ionosphere, the propagation characteristics of hook whistlers recorded at low-latitude ground station Varanasi (geomag. lat., 16°6′.N) are discussed. It is shown that the two traces of the hook whistlers are caused by the VLF waves radiated from the return stroke of a lightning discharge which after penetrating the ionosphere at two different entry points, propagated to the opposite hemisphere in the whistler mode and were received at 16 geomagnetic latitude. Further the crossing of ray paths for the same frequency leads to the explanation of the hook whistler. The lower and higher cut-off frequencies are explained in terms of their deviating away from the bunch of the recorded whistler waves and crossing of ray paths for the same frequency.     

Modified electron thermal waves

The linear and nonlinear properties of the modified electron thermal waves are studied. The waves are of acoustic nature and can exist without significant damping in a two-electron temperature plasma. Nonlinearly, they can form propagating localized regions with depletion of hot electrons. Practical applications of our results to space plasmas are discussed.     

On the speed and shape of electron acoustic solitary waves

Electron acoustic solitary waves in a collisionless plasma consisting of a cold electron fluid and non-thermal hot electrons are investigated by a direct analysis of the field equations. The Sagdeev potential is obtained in terms of electron acoustic speed by simply solving an algebraic equation. It is found that the amplitude and width of the electron acoustic solitary waves as well as the parametric regime where the solitons can exist are very sensitive to the population of energetic non-thermal hot electrons. The soliton and double layer solutions are obtained as a small-amplitude approximation. The effect of non-thermal hot electrons is found to significantly change the properties of the electron acoustic solitary waves (EAWs). A comparison with the Viking Satellite observations in the day side auroral zone is also discussed.     

Properties of dispersive Alfvén waves: 3. Hydrodynamics (Very Low,Intermediate, and Low Density Plasmas)

The behavior of dispersive Alfvén waves (DAWs), including inertial and kinetic Alfvén waves, in astrophysical plasmas of very low, intermediate, and low pressure is investigated in the hydrodynamic approximation. New full solutions are obtained. Our results are analyzed and compared with those from the kinetic approach. It is shown that one general solution for the DAWs in plasmas of very low, intermediate, and low pressure can be obtained in the framework of the hydrodynamic approach, as opposed to the kinetic one. In the very low damping region, the kinetic and hydrodynamic solutions agree very well; but there are parameter regions where the solutions are essentially different. The influence of the astrophysical medium parameters on the DAW behavior and properties is analyzed. All main wave characteristics—the dispersion, damping, polarization, density perturbations, and charge density perturbations—are obtained, whose the consideration is very important for the observation and detection of these waves, as well as for a more correct understanding of their behavior and role in various astrophysical processes taking place in the cosmic medium.     

Properties of Ion-Cyclotron Waves in the Open Solar Corona

Remote observations of coronal holes have strongly suggested the resonant interactions of ion-cyclotron waves with ions as a principal mechanism for plasma heating and acceleration of the fast solar wind. In order to study these waves, a WKB (Wentzel?CKramers?CBrillouin) linear perturbation analysis is used in the frame work of a collisionless multi-fluid model where we consider in addition to protons a second ion component made of alpha particles. We consider a non-uniform background plasma describing a funnel region in the open coronal holes and we use the ray tracing Hamiltonian-type equations to compute the ray path of the waves and the spatial variation of their properties. At low frequency (smaller than the proton cyclotron frequency), the results showed a distinct behavior of the two ion-cyclotron modes found in our calculations, namely the first one propagates anisotropically guided along the magnetic field lines while the second one propagates isotropically with no preferred direction.     

Quasilongitudinal approximation for whistler-mode waves in the magnetospheric plasma

The range of applicability of an improved quasilongitudinal approximation for whistler-mode waves in the equatorial magnetosphere (4 L 6.6) is specified based on the direct comparison between numerical solutions of the hot electromagnetic dispersion equation with the corresponding analytical quasilongitudinal solutions. It is pointed out that this approximation can be used at frequencies ω less than but not close to the electron gyrofrequency Ω (ω 0.6 Ω) and wave normal angles θ less than but not close to the resonance cone angle θ_R. At ω = 0.8 Ω the analytical results deviate considerably from numerical ones due to the strong damping of the waves, and so the quasilongitudinal solution becomes no longer valid.     

Nonextensive dust acoustic waves in a charge varying dusty plasma

Our recent analysis on nonlinear nonextensive dust-acoustic waves (DA) [Amour and Tribeche in Phys. Plasmas 17:063702, 2010] is extended to include self-consistent nonadiabatic grain charge fluctuation. The appropriate nonextensive electron charging current is rederived based on the orbit-limited motion theory. Our results reveal that the amplitude, strength and nature of the nonlinear DA waves (solitons and shocks) are extremely sensitive to the degree of ion nonextensivity. Stronger is the electron correlation, more important is the charge variation induced nonlinear wave damping. The anomalous dissipation effects may prevail over that dispersion as the electrons evolve far away from their Maxwellian equilibrium. Our investigation may be of wide relevance to astronomers and space scientists working on interstellar dusty plasmas where nonthermal distributions are turning out to be a very common and characteristic feature.     

Nonlinear Viscous Damping of Surface AlfvÉn Waves in Polar Coronal Holes

We quantitatively re-examine the nonlinear viscous damping of surface Alfvén waves in polar coronal holes, using recently reported observational data on electron density and temperature and the magnetic field spreading near the edges. It is found that in the nonlinear regime the viscous damping of surface Alfvén waves becomes a viable mechanism of solar coronal plasma heating when strong spreading of magnetic field is taken into account. Our estimations confirm that coronal heating is more pronounced in the nonlinear case than in the linear one in presence of magnetic field spreading.     

Damping of electrostatic noise by warm auroral electrons

Cyclotron damping by warm electrons limits the amplitude of high frequency electrostatic waves propagating in discrete auroral arcs. The effect of this damping on whistler VLF hissupper hybrid noise and Bernstein modes is examined by calculating temporal growth rates and power flux intensities of amplified noise produced by precipitating electrons. The auroral electrons are described by a realistic distribution function. The effect of varying ionospheric conditions is also considered. Whistler mode noise is found to be less sensitive to the warm electron model than the upper hybrid mode. Bernstein modes are rapidly absorbed by the ionospheric and warm electrons. The difference in the peak power flux of the whistler and upper hybrid modes is indicative of the local value of the ratio of electron plasma frequency to electron gyrofrequency. For peak upper hybrid noise to exceed peak whistler noisethis ratio should be greater than 1. Ionospheric electron temperature has little effect on the spectrum, and intense narrow beams in the distribution function should be most effective at producing high noise levels for a given warm electron model.     

Specific features of VLF wave propagation in the earth’s inner magnetosphere

The ray trajectories of waves in the very low frequency (VLF) range in the case of nonducted propagation in the earth’s inner magnetosphere are studied as functions of location of their source region, frequency, and initial angle between the vector of wave normal and intensity vector of external magnetic field. Simulation is performed on the basis of geometric ray tracing approach in multicomponent plasma. The parameters of the magnetospheric medium were calculated using a diffusion model of the concentration distribution of plasma components and the International Geomagnetic Reference Field (IGRF) model. It is shown that the magnetospheric wave reflection can occur if the lower hybrid resonance frequency is greater than its own wave frequency (ω _LHF > ω), i.e., at the latitudes λ ≈ 50°. The simulation results confirm that the quasi-longitudinal approximation cannot be used to describe the magnetospheric whistler propagation. We present simulations of propagation of chorus-type wave magnetospheric emissions that were performed using realistic wave distributions over initial parameters. In particular, we present distributions of chorus waves over directions of wave vector as functions of geomagnetic latitude; these distributions are required to study the particle scattering and acceleration processes in the radiation belts. Our results well agree with CLUSTER satellite measurements.     

SUPRACENTER: Locating fireball terminal bursts in the atmosphere using seismic arrivals

Abstract— Terminal bursts and fragmentations of meteoritic fireballs in the atmosphere may now be accurately located in four dimensions (three spatial + temporal) using seismic arrival times of their acoustic waves recorded by seismometer, camera, microphone, and/or infrasound stations on the ground. A computer program, SUPRACENTER, calculates travel times by ray tracing through realistic atmospheres (that include winds) and locates source positions by minimization of travel time residuals. This is analogous to earthquake hypocenter location in the solid Earth but is done through a variably moving medium. Inclusion of realistic atmospheric ray tracing has removed the need for the simplifying assumption of an isotropic atmosphere or an approximation to account for “wind drift.” This “drift” is on the order of several km when strong, unidirectional winds are present in the atmosphere at the time of a fireball's occurrence. SUPRACENTER‐derived locations of three seismically recorded fireballs: 1) the October 9, 1997 El Paso superbolide; 2) the January 25, 1989 Mt. Adams fireball; and 3) the May 6, 2000 Morávka fireball (with its associated meteorite fall), are consistent with (and, probably, an improvement upon) the locations derived from eyewitness, photographic, and video observations from the respective individual events. If direct acoustic seismic arrivals can be quickly identified for a fireball event, terminal burst locations (and, potentially, trajectory geometry and velocity information) can be quickly derived, aiding any meteorite recovery efforts during the early days after the fall. Potentially, seismic records may yield enough trajectory information to assist in the derivation of orbits for entering projectiles.     

Radiation from long pulse train electron beams in space plasmas

A previous study of electromagnetic radiation from a finite train of electron pulses is extended to an infinite train of such pulses. The electrons are assumed to follow an idealized helical path through a space plasma in such a manner as to retain their respective position within the beam. This leads to radiation by coherent spontaneous emission. The waves of interest in this region are the whistler slow (compressional) and fast (torsional) Alfvén waves. Although a general theory is developed, analysis is then restricted to two approximations, the short and long electron beam. Formulas for the radiation per unit solid angle from the short beam are presented as a function of both propagation and ray angles, electron beam pulse width and separation and beam current, voltage, and pitch angle. Similar formulas for the total power radiated from the long beam are derived as a function of frequency, propagation angle, and ray angle. Predictions of the power radiated are presented for representative examples as determined by the long beam theory.     

Effect of electron temperature on small-amplitude electron acoustic solitary waves in non-planar geometry

Effects of electron temperature on the propagation of electron acoustic solitary waves in plasma with stationary ions, cold and superthermal hot electrons is investigated in non-planar geometry employing reductive perturbation method. Modified Korteweg–de Vries equation is derived in the small amplitude approximation limit. The analytical and numerical calculations of the KdV equation reveal that the phase velocity of the electron acoustic waves increases as one goes from planar to non planar geometry. It is shown that the electron temperature ratio changes the width and amplitude of the solitary waves and when electron temperature is not taken into account,our results completely agree with the results of Javidan & Pakzad (2012). It is found that at small values of \(\tau \), solitary wave structures behave differently in cylindrical (\(\text {m} = 1\)), spherical (\(\text {m} = 2\)) and planar geometry (\(\text {m} = 0\)) but looks similar at large values of \(\tau \). These results may be useful to understand the solitary wave characteristics in laboratory and space environments where the plasma have multiple temperature electrons.     

Nonlinear excitation of small-scale Alfvén waves by fast waves and plasma heating in the solar atmosphere

We study a nonlinear mechanism for the excitation of kinetic Alfvén waves (KAWs) by fast magneto-acoustic waves (FWs) in the solar atmosphere. Our focus is on the excitation of KAWs that have very small wavelengths in the direction perpendicular to the background magnetic field. Because of their small perpendicular length scales, these waves are very efficient in the energy exchange with plasmas and other waves. We show that the nonlinear coupling of the energy of the finite-amplitude FWs to the small-scale KAWs can be much faster than other dissipation mechanisms for fast wave, such as electron viscous damping, Landau damping, and modulational instability. The nonlinear damping of the FWs due to decay FW = KAW + KAW places a limit on the amplitude of the magnetic field in the fast waves in the solar corona and solar-wind at the level B/B ₀∼10⁻². In turn, the nonlinearly excited small-scale KAWs undergo strong dissipation due to resistive or Landau damping and can provide coronal and solar-wind heating. The transient coronal heating observed by Yohkoh and SOHO may be produced by the kinetic Alfvén waves that are excited by parametric decay of fast waves propagating from the reconnection sites.