Upper-hybrid Waves Research Articles

Linear and Nonlinear Plasma Processes in Ionospheric HF Heating

Featured observations of high frequency (HF) heating experiments are first introduced; the uniqueness of each observation is presented; the likely cause and physical process of each observed phenomenon instigated by the HF heating are discussed. A special point in the observations, revealed through the ionograms, is the competition between the Langmuir parametric instability and upper hybrid parametric instability excited in the heating experiments and the impact of the natural cusp at foE (the peak plasma frequency of the ionospheric E region) on the competition. The ionograms also infer the generation of Langmuir and upper hybrid cavitons. Ray tracing theory is formulated. With and without the appearance of large-scale field-aligned density irregularities in the background ionosphere, ray trajectories of the ordinary mode (O-mode) and extraordinary mode (X-mode) sounding pulses are calculated numerically. The results explain the artificial Spread-F recorded by the digisondes in the heating experiments. Parametric instabilities, which are the directly relevant processes to achieve effective heating of the ionospheric F region, are formulated and analyzed. The threshold fields and growth rates of Langmuir and upper hybrid parametric instabilities are derived as the theoretical basis of many radar observations and electron-plasma wave interactions. Harmonic cyclotron resonance interaction processes between electrons and upper hybrid waves are introduced. Formulation and analysis are presented. The numerical results show that ultra-energetic electrons are generated. These electrons enhance airglow at 777.4 nm as well as cause ionization. Physical processes leading to the generation of artificial ionization layers are discussed. The nonlinear Schrodinger equation governing the nonlinear evolution of Langmuir waves and upper hybrid waves are derived and solved. The nonlinear periodic and solitary solutions of the equations are obtained. The localized Langmuir and upper hybrid waves generated by the HF heater form cavitons near the HF reflection layer and near the upper hybrid resonance layer, which induce bumps in the virtual height spread of the ionogram trace similar to that induced by the density cusp at E-F1 transition layer; the down-going Langmuir waves and upper hybrid waves evolve into nonlinear periodic waves propagating along the magnetic field, which backscatter incoherently the sounding pulses to cause downward virtual height spread.

On the frequency spectra of the electrostatic upper hybrid wave and linear Landau damping in a weakly magnetized relativistic electron plasma

On the basis of Vlasov-Kinetic model and using Maxwell-Juttner distribution function, dispersion relation of electrostatic wave in a collisionless homogeneous relativistic electron plasma is derived. The analysis of the oscillation spectra is carried out in a weak magnetic field limit ≥ Ω. Approximate analytic expression for the weakly damped mode is obtained in the subluminal limiting range. It is shown that the relativistic weakly damped mode can be estimated analytically in the range where the wave phase velocity v φ becomes greater than relativistic thermal velocity of the electron but less than speed of light. Results in the non-relativistic and the ultrarelativistic cases are also discussed.

On possibility of strong anomalous damping of an ordinary pump wave in the electron cyclotron resonance heating experiments due to the low-power-threshold parametric decay instability

We analyze the low power-threshold parametric decay instability of an ordinary wave in the first electron cyclotron harmonic frequency range, which leads to the excitation of a localized upper hybrid (UH) wave and a lower hybrid wave. We investigate its transition into the saturation mode due to both the pump wave depletion and the cascade of successive secondary decays. A set of equations describing this nonlinear frequency down-conversion of the primary UH wave is derived and then solved both analytically and numerically under the conditions typical of the off-axis O1-mode electron cyclotron resonance heating experiments in the FTU tokamak. The possibility of a strong anomalous absorption of the pump power in the case of even-step cascade of secondary decays is discovered.

Direct imaging of plasma waves using ultrafast electron microscopy.

A femtosecond plasma imaging modality based on a new development of ultrafast electron microscope is introduced. We investigated the laser-induced formation of high-temperature electron microplasmas and their subsequent non-equilibrium evolution. Based on a straightforward field imaging principle, we directly retrieve detailed information about the plasma dynamics, including plasma wave structures, particle densities, and temperatures. We discover that directly subjected to a strong magnetic field, the photo-generated microplasmas manifest in novel transient cyclotron echoes and form new wave states across a broad range of field strengths and different laser fluences. Intriguingly, the transient cyclotron waves morph into a higher frequency upper-hybrid wave mode with the dephasing of local cyclotron dynamics. The quantitative real-space characterizations of the non-equilibrium plasma systems demonstrate the feasibilities of a new microscope system in studying the plasma dynamics or transient electric fields with high spatiotemporal resolutions.

The Effects of Upper-Hybrid Waves on Energy Dissipation in the Electron Diffusion Region.

Using a two‐dimensional particle‐in‐cell simulation, we investigate the effects and roles of upper‐hybrid waves (UHW) near the electron diffusion region (EDR). The energy dissipation via the wave‐particle interaction in our simulation agrees with measured by magnetospheric multiscale (MMS) spacecraft. It means that UHW contributes to the local energy dissipation. As a result of wave‐particle interactions, plasma parameters which determine the larger‐scale energy dissipation in the EDR are changed. The y‐directional current decreases while the pressure tensor increases/decreases when the agyrotropic beam density is low/high, where ‐coordinates correspond the ‐boundary coordinates. Because the reconnection electric field comes from , our result implies that UHW plays an additional role in affecting larger‐scale energy dissipation in the EDR by changing plasma parameters. We provide a simple diagram that shows how the UHW activities change the profiles of plasma parameters near the EDR comparing cases with and without UHW.

Estimation of the Electron Density From Spacecraft Potential During High‐Frequency Electric Field Fluctuations

AbstractSpacecraft potential has often been used to infer electron density with much higher time resolution than is typically possible with plasma instruments. However, recently, two studies by Torkar et al. (2017, https://doi.org/10.1002/2017JA024724) and Graham, Vaivads, Khotyaintsev, Eriksson, et al. (2018, https://doi.org/10.1029/2018JA025874) have shown that external electric fields can also have an effect on the spacecraft potential by enhancing photoelectron escape from the surface. Consequently, should the electron density derived from the spacecraft potential be used during an event with a large electric field, the estimation would be contaminated and the user would see the effects of the electric field rather than density perturbations. The goal of this paper is to propose a method to remove the electric field effects to allow the density derived from spacecraft potential to be used even during large‐amplitude wave events such as Langmuir waves or upper hybrid waves.

Experimental Determination of Threshold Powers for the Onset of HF-Enhanced Plasma Lines and Artificial Ionization in the Lower F-Region Ionosphere

HF heating experiment, with linear power ramp from 50 to 92 dBW, was conducted to determine the HF threshold power levels for the onset of detectable plasma waves and the onset of artificial ionization inferred from HF-enhanced plasma lines (HFPLs). The threshold free-space electric field at 200-km altitude, where HFPLs were detected, is about 53 mV/m. However, the HF electric field near the reflection height is enhanced by a swelling factor ~ 3.76 and also conversion to a linear dipole pump mode ( $\surd 2$ ); the actual threshold field is about 281 mV/m that is consistent with theory. Artificial ionization in the lower region was detectable via UHF radar. Short wavelength upper-hybrid waves, which were excited parametrically by the HF heating wave at higher threshold, implement Doppler shifted harmonic-cyclotron resonance interaction, via finite Larmour radius effect, to effectively accelerate electrons. Monitoring the spectral power of the parametric decay instability (PDI) line in the HFPLs by radar was shown to determine the artificial ionization onset time more precisely than that by observing the start of a sharp downward trend in the altitude of the HFPLs; at this time, the HF free-space electric field slightly below 200-km altitude is about 550 mV/m. Langmuir cascade lines in the HFPLs are separated by intervals about double the ion-acoustic frequency (about 4–5 kHz) generated by the PDI. These lines, observed at lower power facilities, were not observed. The upper-hybrid OTSI and PDI excited at HighPower Active Auroral Research Program (HAARP) and the mode competition nonlinear-damping mechanism are suggested as the processes, suppressing cascade enhanced HFPLs.

On the substantial anomalous absorption regime at the extraordinary wave decay into trapped and propagating upper hybrid waves

We analyze the strong nonlinear regime of low power-threshold parametric decay instability (PDI) of a pump extraordinary wave. The PDI leads to the excitation of two upper hybrid (UH) waves only one of which is trapped in the vicinity of the local maximum of a non-monotonic density profile. The pump depletion and the multi-step secondary decays of the localized daughter UH wave are treated as the mechanisms leading to the instability saturation. It is shown that in the case of the even-step secondary cascade decay of trapped primary UH wave the pump wave depletion is responsible for the transition of a nonlinear phenomenon into a stationary regime, and the strong anomalous absorption is possible.

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

Parametric decay instabilities (PDIs) exciting daughter waves trapped inside a magnetized plasma with a non-monotonic density profile are investigated numerically. The investigation is motivated in particular by observations of low threshold PDI signatures during second harmonic electron cyclotron resonance heating experiments in magnetically confined fusion experiments. We use the particle-in-cell code EPOCH to study conversion of a fast X-mode pump wave into a combination of half frequency X-mode and electron Bernstein waves and identify two regimes where PDIs can excite trapped electrostatic waves. Above the second harmonic upper hybrid (UH) density, a PDI known also as a two plasmon decay (TPD) instability excites a pair of UH waves that we locate in frequency and wavenumber space. At lower densities, a PDI known as stimulated Raman scattering may produce one trapped and one returning X-mode daughter wave with a much slower growth rate than the TPD instability. In both cases, we show that the frequency separation of the daughter waves depends on the density in a predictable manner. With little loss from the decay region, the trapped daughter waves become unstable with respect to secondary parametric instabilities (PIs), leading to distinctly different phases of the UH spectrum. Unlike the primary instability, the secondary PIs are shown to depend on ion dynamics. Furthermore, we observe escaping waves near the 3/2 pump frequency resulting from tertiary PIs in agreement with recently proposed backscattering during magnetically confined fusion experiments.

Periodic Narrowband Radio Wave Emissions and Inward Plasma Transport at Saturn's Magnetosphere

Abstract The abrupt brightening of an Energetic Neutral Atom (ENA) blob or cloud has been interpreted as plasma injection in the Kronian magnetosphere (termed ENA injection herein). Morphologically, there appears to be two types of abrupt ENA cloud brightening: (1) the brightening of a large cloud usually seen at distances >10–12 R s (R s ∼ 60,268 km) in the midnight or postmidnight region; (2) the brightening of a smaller cloud usually seen at distances <10–12 R s around 21–03 magnetic local time. Among many radio waves observed at Saturn, type 2 ENA injections correlate best with the 5 kHz narrowband (NB) waves. Using Cassini Ion and Neutral Camera (INCA) and Radio and Plasma Wave Science (RPWS) data, we examine the periodicities of the type 2 ENA injections and the 5 kHz NB emissions as well as their cross-correlations, which have been previously used to measure the lag times or phase differences. Because correlational analysis can only establish linear relationships, we also use mutual information to establish linear and nonlinear relationships. On average, the peak of the 5 kHz NB emission lags those of the type 2 ENA injection by about a few minutes to 2 hr. The injection of hot plasma to the inner magnetosphere can lead to temperature anisotropy, which can generate electrostatic upper hybrid waves, which upon encountering the high-density gradient at the outer edge of the Enceladus density torus, can mode convert to Z mode and then to O mode. The 5 kHz NB waves commonly propagate in the O mode.

The Photoelectron‐Driven Upper Hybrid Instability as the Cause of 150‐km Echoes

Abstract150 kilometer echoes are strong, coherent echoes observed by equatorial radars looking close to perpendicular to Earth's magnetic field. Observations over a day show a distinct necklace pattern with echoes descending from 170 km at sunrise to 130 km at noon, before rising again and disappearing overnight. This paper shows that the upper hybrid instability will convert photoelectron energy into plasma wave energy through inverse Landau damping. Using parameters from a WACCM‐X simulation, the upper hybrid wave growth rates over a day show a nearly identical necklace pattern, with bands of positive growth rate following contours of the plasma frequency. Small gaps in altitude with no echoes are explained by thermal electrons Landau damping the instability where the upper hybrid frequency is a multiple of the gyrofrequency. This theory provides a mechanism that likely plays a crucial role in solving a long‐standing mystery on the origin of 150‐km echoes.

Possible transition from modest to strong anomalous absorption in X2-mode electron cyclotron resonance heating experiments in toroidal devices

We analyze both analytically and numerically, two cases of low-power-threshold two-upper hybrid-plasmon parametric decay instability saturation when the odd- or even-step cascade of secondary decays is possible. We have found that giant anomalous absorption (more than 80%) of pump power leads to its depletion in the regime when the instability is saturated due to the even-step cascade of decays of primary upper hybrid waves. This can make the power deposition profile different from that predicted by linear theory and provides an alternative explanation for the evident broadening observed under electron cyclotron resonance heating in toroidal devices.

Electromagnetic radiation from upper-hybrid wave turbulence in inhomogeneous solar plasmas

Type III solar radio bursts have been commonly observed in the solar wind and coronal plasmas. Electron beams accelerated in the Sun’s atmosphere generate weakly magnetized Langmuir (upper-hybrid) wave turbulence producing in turn electromagnetic emissions at the electron plasma frequency ωp and its harmonic 2ωp, via successive processes involving interactions between the background plasma, the waves and the beam particles. The impact of the random density fluctuations inherent to the background plasma on these processes requires the development of novel approaches and models. Owing to a new theoretical modeling, the radiation efficiency of electromagnetic waves radiated at ωp from a plasma source with random density fluctuations and developed upper-hybrid wave turbulence is calculated analytically and numerically. It is shown that the maximum radiation efficiency at frequency ωp scales as the average level of density fluctuations and as the ratio (c/vT)–2 (where vT is the thermal velocity). These scaling laws are found owing to two different and novel methods of determination of the electromagnetic radiation by turbulent inhomogeneous plasma sources through waves’ transformations on randomly fluctuating density irregularities. These results contribute significantly to the understanding of the processes involved in the generation of Type III solar radio bursts. Their presentation is preceded by a summary of previous studies on the dynamics of beam-driven Langmuir turbulence in inhomogeneous solar plasmas.

Nonlinear Upper Hybrid Waves Generated in Ionospheric HF Heating Experiments at HAARP

Excitation of nonlinear upper hybrid waves by O-mode HF heater in the ionospheric heating experiments was explored via HAARP digisonde operated in a fast mode. The observations are manifested by a bump in the virtual spread, which expands the ionogram echoes upward as much as 140 km and is located below the upper hybrid resonance frequency. The bump is similar to that, occurring in daytime ionograms, caused by the cusp at the E-F2 layer transition, indicating that there is a small ledge in the density profile similar to E-F2 layer transitions. The ionograms also show that the virtual height spread, which exceeds 50 km over a significant frequency range below the upper hybrid resonance frequency, is downward, rather than upward as observed in a natural Spread-F situation. It indicates that density irregularities along the geomagnetic field, rather than field-aligned, were generated. Theory shows that the upper hybrid waves excited by the parametric instabilities evolve into nonlinear periodic and solitary waves that enhance downward virtual height spread and generate density cavity to explain the experimental observations.

Tokamak research at the Ioffe Institute

Recent research at three small tokamaks with different parameters located at the Ioffe Institute—the spherical tokamak Globus-M, the large aspect ratio tokamak FT-2 and the compact tokamak TUMAN-3M—are reviewed. This overview covers energy confinement (Globus-M and FT-2), L–H transition (TUMAN-3M and FT-2), Alfvén waves (Globus-M and TUMAN-3M), ion cyclotron emission (TUMAN-3M), major plasma discharge disruption (Globus-M) and scrape-off layer (Globus-M) studies. A full-f global gyrokinetic modeling benchmark using synthetic diagnostics in FT-2 is described. Anomalous absorption and emission in electron cyclotron resonance heating experiments due to the parametric excitation of localized upper hybrid waves are analyzed theoretically. Progress in the development of the neutral particle analysis, gamma-ray spectrometry and divertor Thomson scattering combined with laser-induced fluorescence diagnostics for ITER is discussed. The status of the new Globus-M2 spherical tokamak is reported.

A discussion on the mode conversion from purely perpendicular upper-hybrid mode waves to LO mode waves in an inhomogeneous plasma

We consider the equatorial region of the magnetosphere, where the magnetic field is perpendicular (or near to perpendicular) to the density gradient and mode conversion process from UH to LO-mode waves or reverse process are expected. We review and study the mode conversion from UH (upper hybrid) to LO (left hand polarized ordinary) mode waves by a spatially two dimensional plasma fluid code. Several simulations with different initial wave vectors under the same background plasma condition have been performed. We focus on the conversion efficiency from the UH-mode waves with purely perpendicular wave normal angle to the LO-mode waves, since one of the source of generation UH wave can be Bernstein mode as the purely perpendicular electrostatic waves. For this special case, the UH wave normal is kept in perpendicular direction with respect to the magnetic field, and difficult to be in matching direction for conversion to LO mode. Simulation results show that the mode conversion efficiency in this particular case is very weak, since two branches of Z-mode wave and LO mode wave in the dispersion relation are disconnected. We present a discussion to show that for this case (purely perpendicular propagation) a special angle (except 90°), between the magnetic field and the density gradient is necessary for occurrence of efficient mode conversion. For the case (purely perpendicular propagation and the magnetic field perpendicular to the density gradient), the mode conversion just occurred via the tunneling effect, where a steepness of the inhomogeneity plays an essential role.

The Role of Upper Hybrid Waves in the Magnetotail Reconnection Electron Diffusion Region

Abstract Plasma waves are believed to play an important role during magnetic reconnection. However, the specific role of upper hybrid (UH) waves in the electron diffusion region (EDR) remains elusive, owing to the absence of high-resolution measurements. We analyze one EDR event in the magnetotail on 2017 July 11 observed by the Magnetospheric Multiscale (MMS) mission. To the best of our knowledge, this is the first time that intense UH waves have been observed in the EDR by MMS. The agyrotropic crescent-shaped electron distributions could result in the observed UH waves. Concomitant with the observations of UH waves, the agyrotropy parameter of the electrons decreases, implying that the UH waves could effectively scatter the electrons in the EDR. The good accordance of positive energy conversion ( , likely dissipation) and the observed UH waves indicates that UH waves may contribute to the energy conversion from the magnetic fields to the plasma particles during magnetic reconnection.

Effects of wave trapping and ducting inside plasma density depletions are studied in a laboratory magnetoplasma

A laboratory experiment creates a pulsed magnetoplasma and demonstrates that oblique propagation of upper hybrid waves is necessary to understand ionospheric heating experiments.

High-Frequency Wave Generation in Magnetotail Reconnection: Nonlinear Harmonics of Upper Hybrid Waves.

MMS3 spacecraft passed the vicinity of the electron diffusion region of magnetotail reconnection on 3 July 2017, observing discrepancies between perpendicular electron bulk velocities and E→×B→ drift, and agyrotropic electron crescent distributions. Analyzing linear wave dispersions, Burch et al. (2019, https://doi.org/10.1029/2019GL082471) showed the electron crescent generates high‐frequency waves. We investigate harmonics of upper‐hybrid (UH) waves using both observation and particle‐in‐cell (PIC) simulation, and the generation of electromagnetic radiation from PIC simulation. Harmonics of UH are linearly polarized and propagate along the perpendicular direction to the ambient magnetic field. Compared with two‐dimensional PIC simulation and nonlinear kinetic theory, we show that the nonlinear beam‐plasma interaction between the agyrotropic electrons and the core electrons generates harmonics of UH. Moreover, PIC simulation shows that agyrotropic electron beam can lead to electromagnetic (EM) radiation at the plasma frequency and harmonics.

Ducting of upper-hybrid waves by density depletions in a magnetoplasma with weak spatial dispersion

The effects of wave trapping and ducting inside a plasma density depletion are studied in a large laboratory magnetoplasma in the upper-hybrid (UH) range of frequencies. A field-aligned density depletion is generated via localized rf plasma heating and subsequent plasma thermal diffusion. Test UH waves are emitted and detected by small-size electric monopole antennas. For a given set of experimental parameters, propagation of UH waves can be considered as collisionless. At the same time, wave surface topology in the UH range is determined by weak spatial dispersion, which is conditioned by the thermal motion of electrons and the ambient magnetic field. For various density depletion depths and diameters, ducting of UH waves belonging to different characteristic parts of the “dumbbell-shaped” wave surface can be observed. Particularly, quasiparallel Langmuir (L-mode) waves and oblique resonance cone (X-mode) waves can be ducted in different regimes. A qualitative explanation of ducting regimes is given based on ray tracing analysis.