Upper-hybrid Waves Research Articles

Comment on “Terahertz wave generation by the upper hybrid wave” [Phys. Plasmas 18, 022304 (2011)

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Scattering and Transformation of Waves on Heavy Particles in Magnetized Plasma

The scattering and transformation of the waves propagating in magnetized plasma on a heavy stationary charged particle located at a plane plasma-vacuum boundary is considered. The scattering (transformation) occurs due to the nonlinear coupling of the incident wave with the polarization (shielding) cloud surrounding the particle. It is shown that the problem is reduced to the determination of the nonlinear (three index) dielectric tensor of magnetized plasma. The angular distribution and the cross section for scattering (transformation) of high-frequency ordinary and extraordinary waves and low-frequency upper-hybrid, low-hybrid, and magnetosonic waves are investigated within a cold plasma (hydrodynamic) model.

Theoretical constraints on the generation mechanism of auroral medium frequency burst radio emissions

[1] Auroral medium frequency (MF) burst is an impulsive auroral radio emission observed at ground level between ∼1.3 and 4.5 MHz for a few minutes at substorm onsets. Despite multiple suggested theories, the exact generation mechanism for MF burst remains unknown. The foremost theory suggests that MF burst originates from Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside or topside F region, which linearly mode convert to the electromagnetic LO mode, gaining access to ground level. Analysis of plasmas with two electron components implies the existence of additional normal modes, commonly referred to as the electron acoustic (EA) and electron cyclotron sound (ECS) modes, which have been suggested to potentially play a role in the generation of MF burst. Analytical calculations using Waves in Homogeneous, Anisotropic Multicomponent Plasmas, or WHAMP, and other dispersion-solving calculations are applied to applicable ionospheric normal modes (RX, LO, Langmuir or upper hybrid, EA, and ECS), compared to previous results, and applied to MF burst and the auroral plasma environment. Computation of resonant parallel energies suggests that, in the presence of an auroral electron distribution, instability of EA, ECS, LX, and Langmuir or upper hybrid modes is possible. The same analysis suggests that LO and RX modes are unlikely to be directly excited. Excitation of the EA or ECS mode in the frequency range applicable to MF burst is found to be more favorable for higher ionospheric plasma densities (fpe/fce > 2.36) and higher secondary electron temperatures (Th ≈ 100s of eV). Growth rate calculations suggest that growth of the EA or ECS mode is excitable by the auroral electron beam with higher growth rates for the ECS mode.

MF/HF auroral radio emissions emanating from the topside ionosphere

This is a brief report on the first observation of polarization features and the source region of MF/HF auroral radio emissions emanating from the topside ionosphere. These emissions are called “Terrestrial Hectometric Radiation (THR)” and regarded as a counterpart of auroral roar and MF burst which are observable from the ground. THR typically occurs in either or both of two frequency bands near 1.5–2.0 MHz and 3.0–4.0 MHz, whose polarization features correspond to the L‐O and R‐X mode. The R‐X mode, which has never been reported as auroral roar and MF burst, can be attributed to nonlinear coupling of two upper hybrid waves. The Akebono satellite observation shows THR emissions merge with upper hybrid waves in a frequency‐time diagram under the matching condition fUH ∼ 2fce. This observation suggests that plasma instability enhances the upper hybrid waves under this condition, and then they are converted into MF/HF auroral radio emissions.

Simulation of mode conversion process from upper-hybrid waves to LO-mode waves in the vicinity of the plasmapause

Abstract. In order to clarify the role of the mode conversion process in the generation mechanism of LO-mode waves in the equatorial region of the plasmasphere, we have investigated the linear mode conversion process among upper-hybrid-resonance (UHR)-mode, Z-mode and LO-mode waves by a numerical simulation solving Maxwell's equations and the equation of motion of a cold electron fluid. The wave coupling process occurring in the cold magnetized plasma are examined in detail. In order to give a realistic initial plasma condition in the numerical experiments, we use initial parameters inferred from observation data obtained around the generation region of LO-mode waves obtained by the Akebono satellite. A density gradient is estimated from the observed UHR frequency, and wave normal angles are estimated from the dispersion relation of cold plasma by comparing observed wave electric fields. Then, we perform numerical experiments of mode conversion processes using the density gradient of background plasma and the wave normal angle of incident upper hybrid mode waves determined from the observation results. We found that the characteristics of reproduced LO-mode waves in each simulation run are consistent with observations.

Electrostatic Waves in Weakly Magnetized Quantum Plasmas

By using the quantum magnetohydrodynamic model, the electrostatic waves in weakly magnetized quantum plasmas are investigated. The electrons are treated as a quantum and magnetized species, while the ions are classical unmagnetized ones. The general dispersion relations are derived. It is shown that, both the high frequency electron waves (Langmuire wave and upper-hybrid wave) and the low frequency ion acoustic wave can propagate when the plasmas are cold.

Nonlinear VLF effects in the topside ionosphere

The Demeter satellite observed intense broadband lower‐ and upper‐hybrid electrostatic waves and plasma perturbations associated with high‐power whistler‐mode signals from the very‐low frequency (VLF) transmitter NWC. This paper shows that the Demeter observations can be explained by nonlinear interactions driven by VLF pump waves, thereby suggesting that nonlinear effects are responsible for energy losses of high‐power VLF signals in the ionosphere.

Excitation of an upper hybrid wave by two intense laser beams and particle acceleration

This Letter presents an investigation of the excitation of an upper hybrid wave (UHW) by cross focusing of two intense laser beams in a collisionless hot magnetoplasma, when relativistic and ponderomotive nonlinearities are operative. The electric vectors of the two beams are polarized along uniform static magnetic field and the beams propagate perpendicular to the static magnetic field. Analytical expressions for the beam width of the laser beams, electric vector and power of the excited UHW and energy gain have been obtained. The UHW generation at the difference frequency and particle acceleration has also been studied. The nonlinear coupling between intense laser beams and UHW is so strong that UHW gets excited and a large fraction of the laser beam energy gets transferred to UHW and this UHW accelerates electrons. It has been shown that the presence of a magnetic field affects significantly the power of the UHW and energy gain by the electron in the presence of the UHW.

Excitation of upper-hybrid waves by a gyrating relativistic electron beam in a magnetized dusty plasma cylinder

A gyrating relativistic electron beam propagating through a magnetized dusty plasma cylinder drives an upper-hybrid wave to instability via cyclotron interaction. Numerical calculations of the growth rate and unstable mode frequencies have been carried out for the typical parameters of dusty plasma experiments. It is found that as the density ratio of negatively charged dust grains to electrons increases, the unstable mode frequency of the upper-hybrid waves increases. Moreover, the growth rate of the instability also increases with the density ratio. The growth rate of the unstable mode has the largest value for the modes whose eigenfunctions peak at the location of the beam. The growth rate of the unstable mode scales as the one-third power of the beam density.

Stimulated Raman forward scattering of a laser in a plasma with transverse magnetic field

The effect of a transverse static magnetic field on stimulated Raman forward scattering (SRFS) of a laser in a plasma is studied. The x-mode excites an upper hybrid wave and two localized Stokes/anti-Stokes sidebands. The laser and the sideband exert a ponderomotive force on electrons driving the upper hybrid wave. The latter couples with the pump to drive the sidebands. The growth rate of SRFS monotonically increases by applying a static magnetic field. It also increases with the pump amplitude; however, the dependence is slower than linear.

Laboratory studies of spectral features of stimulated electromagnetic emission during the ionospheric heating experiments

We present the results of laboratory studies of the formation of a number of spectral components of stimulated electromagnetic emission, which are related to the excitation of small-scale irregularities in the heated ionosphere. In the laboratory experiment, the small-scale irregularity was formed as a result of thermal self-channeling of short-wavelength quasielectrostatic oscillations in a magnetoplasma. Using the method of probing waves, it is experimentally shown that the trapping and waveguide propagation in a small-scale plasma irregularity are exclusively due to Langmuir waves, whereas the upper-hybrid waves with anomalous dispersion are not trapped into the irregularity. It is found that satellites shifted by about 1–2 MHz from the carrier frequency (700 MHz under the experimental conditions) are formed in the Langmuir wave spectrum during the thermal self-channeling. Two mechanisms of generation of spectral satellites have been detected. The first (dynamic) mechanism is observed during the formation of a small-scale irregularity with rapidly increasing longitudinal size. In this case, one low-frequency satellite is excited in the trapped-wave spectrum. The mechanism of the formation of this satellite is apparently related to the Doppler shift of the frequency of the Langmuir waves trapped inside the irregularity. The second (stationary) mechanism is observed in the case of a developed irregularity where its shape is close to cylindrical. In this regime, the trapped-wave spectrum has two symmetric spectral satellites, namely, high- and low-frequency ones. It may be hypothesized that the generation of these satellites is due to scattering of trapped Langmuir waves from drift oscillations of the irregularity.

Experimental tests of the generation mechanism of auroral medium frequency burst radio emissions

[1] Medium frequency (MF) burst is an impulsive auroral radio emission at 1.3–4.5 MHz commonly detected by ground-based instruments for a few minutes at substorm onsets. It is thought to arise from mode conversion radiation. The Dartmouth College MF radio interferometer at Toolik Field Station, Alaska (68.51 invariant latitude), measured spectra, amplitudes, and directions of arrival (DOA) of 47 MF burst events during 2006– 2007 and 49 events during 2007–2008. Statistical analysis of these events shows that they come predominantly from the south and east of Toolik, as expected because propagation conditions are more favorable poleward and westward of the active auroral arcs than equatorward or eastward during premidnight (westward moving) substorm onset activity. Case studies of a selected MF burst event on 20 November 2007 show that motions of the radio emissions qualitatively track the motions of auroral arcs simultaneously observed with all-sky camera. Case studies of DOA data of selected MF burst events on 31 January and 20 November 2007 show that higher-frequency components of MF burst arrive at higher elevation angles than lower-frequency components. Statistical studies confirm this trend. Ray-tracing analysis shows that this trend implies that sources of the higherfrequency components of the MF burst are at higher altitudes than those of the lowerfrequency components. The analysis also shows that the MF burst comes from the bottomside F region ionosphere. These observations are consistent with a mechanism of MF burst emission whereby the emissions originate from mode conversion of Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside F region.

Artificial E-region field-aligned plasma irregularities generated at pump frequencies near the second electron gyroharmonic

Abstract. E region ionospheric modification experiments have been performed at HAARP using pump frequencies about 50 kHz above and below the second electron gyroharmonic frequency. Artificial E region field-aligned plasma density irregularities (FAIs) were created and observed using the imaging coherent scatter radar near Homer, Alaska. Echoes from FAIs generated with pump frequencies above and below 2Ωe did not appear to differ significantly in experiments conducted on summer afternoons in 2008, and the resonance instability seemed to be at work in either case. We argue that upper hybrid wave trapping and resonance instability at pump frequencies below the second electron gyroharmonic frequency are permitted theoretically when the effects of finite parallel wavenumbers are considered. Echoes from a sporadic E layer were observed to be somewhat weaker when the pump frequency was 50 kHz below the second electron gyroharmonic frequency. This may indicate that finite parallel wavenumbers are inconsistent with wave trapping in thin sporadic E ionization layers.

Resonant excitation of the upper hybrid wave by relativistic cross focusing of two laser beams

AbstractThis article presents the resonant excitation of the upper hybrid wave (UHW) by cross focusing of two high power laser beams in a collisionless hot magnetoplasma; taking into account the relativistic nonlinearity. The electric vectors of the two beams are polarized along uniform static magnetic field and the beams propagate perpendicular to the static magnetic field. The resonant excitation of the UHW occurs when the frequency difference (FD) of the two laser beams and difference of their propagation vector satisfy the dispersion relation corresponding to the UHW. It has been observed that the power associated with the excited UHW, which depends on the background electron concentration, magnetic field and the intensity of the two laser beams, becomes drastically modified with the distance of propagation. The effect of the excited UHW at the FD on the acceleration of electrons has also been discussed. The amplitude of the UHW, excited by two high power laser beams and the electron energy are also calculated. This study is relevant in heating of plasma near the upper hybrid frequency as well as electron acceleration. The results are presented for typical laser plasma parameters.

Source locations of narrowband radio emissions detected at Saturn

Since Cassini's arrival at Saturn in 2004, the Radio and Plasma Wave Science instrument has detected numerous narrowband (NB) radio emission events. These emissions, mostly detected around 5 and 20 kHz, usually occur periodically for several days after intensifications of Saturn kilometric radiation. We present calculations based on an electron density profile of Saturn's plasma torus and a dipole magnetic field model showing that the NB emissions originate from the northern and southern edges of Saturn's plasma torus at L shells ∼ 8 to 10 for 5‐kHz NB and L ∼ 4 to 7 for 20‐kHz NB. In many cases, Cassini passes through the source region of the 20‐kHz NB, as indicated by intense electrostatic upper hybrid (ESUH) waves in close proximity to electromagnetic emissions on spectrograms. The positions of the spacecraft when intense ESUH waves are observed agree with the model predictions of the NB source locations. Source locations determined by goniopolarimetric (also known as direction‐finding) analysis of the NB emissions also support the above results, although sometimes the directions of arrival point toward the region interior to Saturn's plasma torus. A polarization reversal technique is applied to localize the NB emissions observed during spacecraft rotation, on the basis of the fact that the source is within the antenna plane when the apparent circular polarization degree switches sign. The NB emissions are found to be L‐O mode polarized, which is consistent with the prediction of linear/nonlinear mode conversion theory. It is also found that sometimes right‐hand polarized NB emissions are generated at second harmonic frequencies of the 20‐kHz NB; in which case, wave‐wave interactions between oppositely propagating ESUH waves may play an important role in the mode conversion process.

Two-dimensional PIC simulations of ion beam instabilities in Supernova-driven plasma flows

Supernova remnant blast shells can reach the flow speed vs = 0.1c and shocks form at its front. Instabilities driven by shock-reflected ion beams heat the plasma in the foreshock, which may inject particles into diffusive acceleration. The ion beams can have the speed vb ≈ vs. For vb ≪ vs the Buneman or upper-hybrid instabilities dominate, while for vb ≫ vs the filamentation and mixed modes grow faster. Here the relevant waves for vb ≈ vs are examined and how they interact nonlinearly with the particles. The collision of two plasma clouds at the speed vs is modelled with particle-in-cell simulations, which convect with them magnetic fields oriented perpendicular to their flow velocity vector. One simulation models equally dense clouds and the other one uses a density ratio of 2. Both simulations show upper-hybrid waves that are planar over large spatial intervals and that accelerate electrons to ∼10 keV. The symmetric collision yields only short oscillatory wave pulses, while the asymmetric collision also produces large-scale electric fields, probably through a magnetic pressure gradient. The large-scale fields destroy the electron phase space holes and they accelerate the ions, which facilitates the formation of a precursor shock.

Dynamics of the excitation of an upper hybrid wave by a rippled laser beam in magnetoplasma

This paper presents the effects of a laser spike (superimposed on an intense laser beam) and a static magnetic field on the excitation of the upper hybrid wave (UHW) in a hot collisionless magnetoplasma, taking into account the relativistic nonlinearity. The laser beam is propagating perpendicular to the static magnetic field and has its electric vector polarized along the direction of the static magnetic field (ordinary mode). Analytical expressions for the growth rate of the ripple, the beam width of the rippled laser beam, and the UHW have been obtained. It is found that the coupling among the main laser beam, ripple, and UHW is strong. The ripple gets focused when the initial power of the laser beam is greater than the critical power for focusing. It has been shown that the presence of a laser spike affects significantly the growth rate and the dynamics of the UHW. In addition, it has been seen that the effect of changing the strength of the static magnetic field on the nonlinear coupling and on the dynamics of the excitation of the UHW is significant. The results are presented for typical laser plasma parameters.

Laser beam filamentation and stochastic electron heating at upper hybrid layer

This paper presents an investigation of the filamentation (single hot spot) of an ultrahigh-power laser beam in homogeneous plasma. Upper hybrid wave (UHW) coupling in these filaments has been studied. We have discussed two extreme scenarios: (1) The laser beam has ultrahigh power so that relativistic and ponderomotive nonlinearities are operative; and (2) the laser beam power is moderate, therefore only ponderomotive nonlinearity dominates. At ultrahigh laser powers, relativistic and ponderomotive nonlinearities lead to filamentation of the laser beam. In these filamentary regions, the UHW gets coupled to the laser beam, and a large fraction of the pump (laser beam) energy gets transferred to UHW and this excited UHW can accelerate the electrons. In the second case, nonlinear coupling between the laser beam and the upper hybrid wave leads to the localization of the UHW. Electrons interacting with the localized fields of the UHW demonstrate chaotic motion. The simulation result confirms the presence of chaotic fields, and interaction of these fields with electrons leads to velocity space diffusion, which is accompanied by particle heating. Using the Fokker–Planck equation, the heating of electrons has been estimated. The effect of the change of background magnetic field strength on heating has also been discussed.

Excitation of an upper hybrid wave by a high power laser beam in plasma

AbstractIn the present investigation, the excitation of an upper hybrid wave (UHW) in a hot collisionless magneto-plasma by a relativistic laser beam propagating perpendicular to the static magnetic field and having its electric vector polarized along the direction of the static magnetic field (ordinary mode) is presented. Due to nonuniform intensity distribution of pump laser, the background electron concentration is modified. The amplitude of the UHW, which depends on the background electron concentration, is thus nonlinearly coupled with the laser beam. The effect of nonlinear coupling between the pump laser and UHW is studied. The effect of the relativistic electron mass nonlinearity and the relativistic self-focusing of the pump laser on the excitation of the UHW have been incorporated. The dynamics of the excitation of the UHW in different power domains of the laser beam is accordingly modified. It has been seen that the effect of changing the strength of the static magnetic field on the nonlinear coupling and the dynamics of the excitation of the UHW is significant. The focusing behavior of the UHW may find its relevance in the heating of plasmas near the upper hybrid resonance.

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

Abstract In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3°). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and twoMF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.