Upper-hybrid Frequency Research Articles

A complete electrode model for plasma impedance probes

Plasma impedance probes measure the impedance spectrum of an antenna immersed in a plasma. The 1964 work of Balmain remains the standard method to interpret these data, using the peak in the magnitude at the upper-hybrid frequency to infer plasma electron density. The primary limitations of Balmain's model are the assumption of a homogenous plasma and a cylindrical dipole. This work presents a numerical model applicable to inhomogeneous plasma and arbitrary antenna geometry based on the cold, fluid approximation given by Balmain. This model solves Poisson's equation using the finite element method and accounts for the effects of the dipole using the plasma complete electrode model (PCEM). The PCEM is developed in this article and accounts for the voltage shunting effects of the dipole elements, the discrete current to the dipole, and the plasma sheath surrounding the dipole. The sheath is incorporated as a contact impedance between the dipole and the plasma in a manner analogous to the complete electrode model of electrical impedance tomography. The first portion of this paper presents the mathematical framework of the PCEM, starting from Maxwell's equations. The second part of the paper compares the output of this numerical method to Balmain's work and to data collected by an impedance probe in the Space Physics Simulation Chamber at the U.S. Naval Research Laboratory. The PCEM results agree with both the observed data and the prior modeling done by Balmain. An additional consequence of the numerical study is the observation that some second-order resonances not predicted by Balmain's model can be attributed to the presence of the plasma sheath.

The Hunt for Perpendicular Magnetic Field Measurements in Plasma

The one consistent technique for remotely estimating the magnetic field in plasma has been Faraday rotation. It is only sensitive to the portion of the vector parallel to the propagation path. We show how to remotely detect the portion of the vector that is perpendicular using a modified measurement. Isolating this electromagnetic propagation wave mode to measure the magnetic field enables us to (i) study more about the magnetic field vector in plasma, (ii) reduce error in total electron content measurements, and (iii) discover new magnetic field information from archived data sets. The Appleton–Hartree equation is used to verify a new approach to calculating the phase change to an electromagnetic wave propagating through a plasma at frequencies larger than the gyrofrequency, the cyclotron frequency, and the upper hybrid frequency. Focusing on the perpendicular propagation modes, the simplified equation for the integrated path effect from a perpendicular magnetic field is calculated. The direction of the perpendicular component is unknown, because the magnetic field is squared. Isolating the magnetic term in the equation with dual frequency waves is shown. We also show how to eliminate the magnetic field contribution to total electron content measurements with a similar approach. In combination with Faraday rotation, the degeneracy of the magnetic field vector direction is reduced to a cone configuration.

Juno Plasma Wave Observations at Europa

AbstractJuno passed by Europa at an altitude of 355 km on 29 September, day 272, 2022. As one of Juno's in situ science instruments, the Waves instrument obtained observations of plasma waves that are essential contributors to Europa's interaction with its environment. Juno observed chorus, a band at the upper hybrid frequency providing the local plasma density, and electrostatic solitary structures in the wake. In addition, impulses due to micron‐sized dust impacts on Juno were recorded with a local maximum very close to Europa. The peak electron density near Europa was ∼330 cm−3 while the surrounding magnetospheric density was in the range of 50–150 cm−3. There was a significant separation between the Europa flyby and Juno's crossing of Jupiter's magnetic equator, enabling a unique identification of effects associated with the moon as opposed to magnetospheric phenomena normally occurring at the magnetic equator near 10 Jovian radii.

Characterization of high-frequency waves in the Martian magnetosphere

Context. Various high-frequency waves in the vicinity of upper-hybrid and Langmuir frequencies are commonly observed in different space plasma environments. Such waves and fluctuations have been reported in the magnetosphere of the Earth, a planet with an intrinsic strong magnetic field. Mars has no intrinsic magnetic field and, instead, it possesses a weak induced magnetosphere, which is highly dynamic due to direct exposure to the solar wind. In the present paper, we investigate the presence of high-frequency plasma waves in the Martian plasma environment by making use of the high-resolution electric field data from the Mars Atmosphere and Volatile Evolution missioN (MAVEN) spacecraft. Aims. This study aims to provide conclusive observational evidence of the occurrence of high-frequency plasma waves around the electron plasma frequency in the Martian magnetosphere. We observe two distinct wave modes with frequency below and above the electron plasma frequency. The characteristics of these high-frequency waves are quantified and presented here. We discuss the generation of possible wave modes by taking into account the ambient plasma parameters in the region of observation. Methods. We have made use of the medium frequency (100 Hz–32 kHz) burst mode-calibrated electric field data from the Langmuir Probe and Waves instrument on board NASA’s MAVEN mission. Due to the weak magnetic field strength, the electron gyro-frequency is much lower than the electron plasma frequency, which implies that the upper-hybrid and Langmuir waves have comparable frequencies. A total of 19 wave events with wave activities around electron plasma frequency were identified by examining high-resolution spectrograms of the electric field. Results. These waves were observed around 5 LT when MAVEN crossed the magnetopause boundary and entered the magnetosheath region. These waves are either a broadband- or narrowband-type with distinguishable features in the frequency domain. The narrowband-type waves have spectral peak above the electron plasma frequency. However, in the case of broadband-type waves, the spectral peak always occurred below the electron plasma frequency. The broadband waves consistently show a periodic modulation of 8–14 ms. Conclusions. The high-frequency narrowband-type waves observed above the electron plasma frequency are believed to be associated with upper-hybrid or Langmuir waves. However, the physical mechanism responsible for the generation of broadband-type waves and the associated 8–14 ms modulation remain unexplained and further investigation is required.

Electrostatic upper-hybrid mode instability driven by a ring electron distribution

Quasi electrostatic fluctuations in the upper-hybrid frequency range are commonly detected in the planetary magnetospheric environment. The origin of such phenomena may relate to the instability driven by a loss-cone feature associated with the electrons populating the dipole-like magnetic field. The present paper carries out a one-dimensional electrostatic particle-in-cell simulation accompanied by a reduced quasilinear kinetic theoretical analysis to investigate the dynamics of the upper-hybrid mode instability driven by an initial ring electron distribution function, which is a form of loss-cone distribution. A favorable comparison is found between the two approaches, which shows that the reduced quasilinear theory, which is grounded in the concept of a model of the particle distribution function that is assumed to maintain a fixed mathematical form except that the macroscopic parameters that define the distribution are allowed to evolve in time, can be an effective tool in the study of plasma instabilities, especially if it is guided by and validated against the more rigorous simulation result.

Solved and unsolved riddles about low-latitude daytime valley region plasma waves and 150-km echoes

The Earth’s atmosphere near both the geographic and magnetic equators and at altitudes between 120 and 200 km is called the low-latitude valley region (LLVR) and is among the least understood regions of the ionosphere/thermosphere due to its complex interplay of neutral dynamics, electrodynamics, and photochemistry. Radar studies of the region have revealed puzzling daytime echoes scattered from between 130 and 170 km in altitude. The echoes are quasi-periodic and are observed in solar-zenith-angle dependent layers. Populations with two distinct types of spectral features are observed. A number of radars have shown scattering cross-sections with different seasonal and probing-frequency dependencies. The sources and configurations of the so-called 150-km echoes and the related irregularities have been long-standing riddles for which some solutions are finally starting to emerge as will be described in this review paper. Although the 150-km echoes were discovered in the early 1960s, their practical significance and implications were not broadly recognized until the early 1990s, and no compelling explanations of their generation mechanisms and observed features emerged until about a decade ago. Now, more rapid progress is being made thanks to a multi-disciplinary team effort described here and recent developments in kinetic simulations and theory: 18 of 27 riddles to be described in this paper stand solved (and a few more partially solved) at this point in time. The source of the irregularities is no longer a puzzle as compelling evidence has emerged from simulations and theory, presented since 2016 that they are being caused by photoelectrons driving an upper hybrid plasma instability process. Another resolved riddle concerns the persistent gaps observed between the 150-km scattering layers—we now understand that they are likely to be the result of enhanced thermal Landau damping of the upper hybrid instability process at upper hybrid frequencies matching the harmonics of the electron gyrofrequency. The remaining unsolved riddles, e.g., minute-scale variability, multi-frequency dependence, to name a few, are still being explored observationally and theoretically—they are most likely unidentified consequences of interplay between plasma physics, photochemistry, and lower atmospheric dynamic processes governing the LLVR.

Dynamic structure factor and excitation spectrum of the one‐component plasma: The case of weak to moderate magnetization

AbstractMagnetized plasmas are well known to exhibit a rich spectrum of collective modes. Here, we focus on the density modes in dense or cold plasmas, where strong coupling effects alter the mode spectrum known from traditional weakly coupled plasmas. In particular, we study the dynamic structure factor (DSF) of the magnetized one‐component plasma with molecular dynamics simulations. Extending our previous results (H. Kählert and M. Bonitz, Phys. Rev. Research 2022, 4, 013197), it is shown that Bernstein modes can be observed in the weakly magnetized regime, where they are found below the upper hybrid frequency, provided the coupling strength is sufficiently low. We investigate the DSF for a variety of different wave numbers and plasma parameters and show that even small magnetization can give rise to a strong zero‐frequency mode perpendicular to the magnetic field and change the dispersion as well as the damping of the upper hybrid mode.

Juno Plasma Wave Observations at Ganymede.

The Juno Waves instrument measured plasma waves associated with Ganymede's magnetosphere during its flyby on 7 June, day 158, 2021. Three distinct regions were identified including a wake, and nightside and dayside regions in the magnetosphere distinguished by their electron densities and associated variability. The magnetosphere includes electron cyclotron harmonic emissions including a band at the upper hybrid frequency, as well as whistler-mode chorus and hiss. These waves likely interact with energetic electrons in Ganymede's magnetosphere by pitch angle scattering and/or accelerating the electrons. The wake is accentuated by low-frequency turbulence and electrostatic solitary waves. Radio emissions observed before and after the flyby likely have their source in Ganymede's magnetosphere.

Quasi-longitudinal propagation of nonlinear whistlers with steep electrostatic fluctuations

The quasi-longitudinal whistlers are recently reported in magnetized laboratory plasmas, i.e., at densities considerably higher than the space or magnetospheric plasmas. Given their oblique nature, these whistlers are known to be accompanied by density perturbations which undergo strong nonlinear steepening exclusively for their propagation close to resonant cone angle [Yoon et al., J. Geophys. Res. 119, 1851 (2014)]. This aspect is examined in the parameter regime of laboratory experiments where quasi-longitudinal whistler fluctuations are reported. A systematic study by a set of dedicated single mode numerical solution of the fully nonlinear model of quasi-longitudinally propagating whistlers is presented predominantly covering the high-density (low magnetic field) regime relevant to the laboratory whistler experiments. Following the recovery of existing computational results available for low-density cases, the computations in the newer regime are performed in the present study. The evolution recovered in both these regimes finds the sharp density structures or oscillations to be of resonant origin. While structures accompanying the whistlers' low-density resonant cone readily agree with the upper hybrid resonance frequency, the freshly covered high-density regime shows that the strong nonlinear nature of the whistler is capable of producing a modification in the resonant frequency, causing it to downshift from its linearly expected upper hybrid frequency.

Variation in the propagation characteristics of extraordinary mode with density, magnetic field and temperature in a relativistic plasma environment

Propagation characteristics of extraordinary mode (perpendicularly propagating electromagnetic wave which is partly transverse and partly longitudinal known as X-mode) are examined in a relativistic, weakly relativistic and non-relativistic plasma environment for various density to magnetic field ratio (). The dispersion relation for X-mode is derived by using the relativistic kinetic model and employing Maxwell-Juttner distribution function. The integrals involved in the calculation are evaluated by using numerical quadrature approach. The mass dependent quantities like the cyclotron frequency and the plasma frequency will change by introducing sufficiently high temperature for which . This will effect significantly the propagation characteristics of X-mode. As the relativistic effects increase, the absorption at cyclotron frequency reduces and the dispersion characteristics of plasma vanishes. With the increase in the density or decrease in the magnetic field, the upper hybrid frequency and the right cutoff frequency shifted to a higher value, altering the propagation region.

Science Objectives and Design of Ionospheric Monitoring Instrument Ionospheric Anomaly Monitoring by Magnetometer And Plasmaprobe (IAMMAP) for the CAS500-3 Satellite

The Ionospheric Anomaly Monitoring by Magnetometer And Plasma-probe (IAMMAP) is one of the scientific instruments for the Compact Advanced Satellite 500-3 (CAS 500-3) which is planned to be launched by Korean Space Launch Vehicle in 2024. The main scientific objective of IAMMAP is to understand the complicated correlation between the equatorial electro-jet (EEJ) and the equatorial ionization anomaly (EIA) which play important roles in the dynamics of the ionospheric plasma in the dayside equator region. IAMMAP consists of an impedance probe (IP) for precise plasma measurement and magnetometers for EEJ current estimation. The designated sun-synchronous orbit along the quasi-meridional plane makes the instrument suitable for studying the EIA and EEJ. The newly-devised IP is expected to obtain the electron density of the ionosphere with unprecedented precision by measuring the upper-hybrid frequency (fUHR) of the ionospheric plasma, which is not affected by the satellite geometry, the spacecraft potential, or contamination unlike conventional Langmuir probes. A set of temperaturetolerant precision fluxgate magnetometers, called Adaptive In-phase MAGnetometer, is employed also for studying the complicated current system in the ionosphere and magnetosphere, which is particularly related with the EEJ caused by the potential difference along the zonal direction.

X-Wave Propagation Characteristics in a Collisional, Inhomogeneous Plasma Slab

Abstract: Normally incident electromagnetic wave on a collisional, inhomogeneous magnetoplasma slab has been treated as a multilayered system of homogeneous sub-cells within the transfer matrix method. For incident wave frequencies much above the ion cyclotron frequency, the extraordinary elliptically polarized wave mode is relevant for wave propagation normal to a dc-magnetic field. For a sinusoidal plasma density profile, the transmittance, reflectance and absorbance are plotted versus wave frequency normalized to electron cyclotron frequency for different ratios of electron cyclotron to electron plasma frequencies. For fixed and by varying the wave frequency, the curves of the transmittance and reflectance show two pass and two stop bands. When the ratio of cyclotron frequency to plasma frequency is increased, all bands shift to the region of low wave frequency, the pass bands become broader and the stop bands become narrower. The absorbance show three absorption bands; namely, two collisional absorption bands of evanescent waves at the X-wave cut-off frequencies and a resonance absorption band at the upper hybrid frequency. It has also been found that a homogeneous plasma slab overestimates the collisional absorption at cut-offs and has a broader absorption band of the upper hybrid resonance. Keywords: Waves in inhomogeneous magnetoplasma, Upper hybrid resonance, Reflectance, Absorbance, Transmittance.

Generation of a Directed Flux of Megawatt THz Radiation as a Result of Strong REB-Plasma Interaction in a Plasma Column

Experiments at the multimirror open trap as plasma emitter of THz radiation (GOL-PET) facility have shown that the injection of a relativistic electron beam (REB) with a kiloampere current into a magnetized plasma column is accompanied by the generation of a THz radiation flux. Our experiments have revealed three-generation mechanisms. The first mechanism is the scattering of upper hybrid waves on plasma density gradients, the second is the direct pumping of electromagnetic waves by an electron beam, and the last one is the merging of two upper hybrid waves into an electromagnetic one. Both theoretical and experimental studies have shown the possibility to achieve a megawatt power of the flux at the upper hybrid frequency in the presence of significant plasma density gradients. Resent experiments have shown that a strong density decrease at the end of the plasma column is necessary for high-efficiency emission of the generated flux along its axis. This THz radiation flux is emitted from the column into a vacuum chamber and then through an output window to the atmosphere. The measurement results of the spatial and angular properties of such a megawatt flux are described in our article.

Modulated Upper‐Hybrid Waves Coincident With Lower‐Hybrid Waves in the Cusp

AbstractDuring the Twin Rockets to Investigate Cusp Electrodynamics (TRICE‐2) High‐Flyer rocket's passage through the cusp the high frequency (HF) radio wave receiver observed three intervals of banded upper‐hybrid (UH) waves. The bands begin at the UH frequency (1.2–1.3 MHz), descending to as low as 1.1 MHz, with amplitudes of hundreds of mV/m. The spacing of the bands are 4.5–6 kHz and the number of bands ranges from 3 to 10. Simultaneously, the very low frequency (VLF) radio wave receiver observed lower‐hybrid (LH) waves with amplitudes ranging from 1 to 10 mV/m and frequencies of 4.5–6 kHz. Slight variations of the spacings of the bands in the UH waves were closely correlated with variations in the LH peak frequencies. Two possible wave‐wave interactions are explored to explain this phenomenon: decay of an UH wave into a lower frequency UH wave and a LH wave, and coalescence of independent UH waves and LH waves that spawn UH waves. Using a dispersion relation calculator with electron and ion distribution functions based off those observed by the particle instruments suggests that UH waves, and to a lesser degree LH waves, can be excited by linear instabilities. Kinematic analysis of the waves dispersion relations and the wave matching conditions show that wave‐wave interactions linking UH and LH modes are possible through either decay or coalescence. This analysis along with comparisons of the energy densities of the waves, and the ratio of their occupation numbers suggest that the decay process is more likely than coalescence.

Conditions for Topside Ion Line Enhancements

AbstractEnhanced ion line spectra as a response to magnetic field‐aligned high frequency (HF) pumping of the overdense polar ionosphere with left‐handed circular polarization, can be observed at the top and bottomside F‐region ionosphere under certain conditions. The European Incoherent Scatter (EISCAT) UHF radar was directed in magnetic zenith on October 18th and 19th, 2017 while stepping the pump frequency of the EISCAT Heating facility both upward and downward across the double resonance frequency of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 2‐min‐on, 5‐min‐30‐s‐off pump cycle. We present observations of two separate cases of topside HF‐enhanced ion lines (THFIL). THFIL simultaneous to bottomside HFIL (BHFIL), and conditioned by the relative proximity of the HF pump frequency to the bottomside double resonance frequency, consistent with previous observations (Rexer et al., 2018, https://doi.org/10.1029/2018JA025822) were observed for HF pulses on 19th October. Recurring THFIL appearing after BHFIL have faded and conditioned by the relative proximity to the double resonance frequency on the topside ionosphere, were observed on 18th October. The THFIL are consistent with transionospheric propagation through artificial radio windows by L mode guiding of the pump wave, facilitated by large scale density ducts present in the plasma.

Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles

Non-monotonic plasma density structures such as blobs and magnetic islands give rise to trapped upper hybrid (UH) waves. Trapped UH waves which satisfy Bohr–Sommerfeld quantization can be thought of as eigenmodes of a cavity. Using fully kinetic particle-in-cell simulations, we verify the existence of these UH eigenmodes and demonstrate their significance as only eigenfrequencies become unstable to three-wave interactions. The eigenmodes can be excited through parametric decay instabilities (PDIs) of an X-mode pump wave at approximately twice the UH frequency, as could be the case for a gyrotron beam traversing a blob in a magnetically confined fusion plasma. We derive a closed expression for the wavenumber of UH waves, which is accurate both close to the UH layer and to the electron cyclotron resonance. This allows for fast analysis of eigenmodes in a non-monotonic structure. An expression for the amplification of PDI daughter waves in an inhomogeneous plasma is extended to a decay region where the first several derivatives vanish. From the amplification in a convective PDI, we estimate the growth rate of the absolute PDI involving the trapped waves. We show that the excitation of eigenmodes through PDIs in our simulations are indeed absolute rather than convective due to the trapping of the daughter waves. Additionally, we show that only eigenmodes get excited through the PDIs, and that we are able to predict the growth rates of the daughter waves and how they scale with the pump wave intensity. This is evidence supporting a fundamental assumption of analytical theory describing low threshold strong scattering observed in magnetically confined fusion experiments during second harmonic electron cyclotron resonance heating (ECRH). Such low threshold instabilities can degrade ECRH performance but also offer novel uses for ion heating or as diagnostics.

Narrowband Spikes Observed during the 2013 November 7 Flare

Abstract Narrowband spikes have been observed in solar flares for several decades. However, their exact origin is still discussed. To contribute to understanding of these spikes, we analyze the narrowband spikes observed in the 800–2000 MHz range during the impulsive phase of the 2013 November 7 flare. In the radio spectrum, the spikes started with typical broadband clouds of spikes, and then their distribution in frequencies changed into unique, very narrow bands having noninteger frequency ratios. We successfully fitted frequencies of these narrow spike bands by those, calculating dispersion branches and growth rates of the Bernstein modes. For comparison, we also analyzed the model where the narrow bands of spikes are generated at the upper-hybrid frequencies. Using both models, we estimated the plasma density and magnetic field in spike sources. Then, the models are discussed, and arguments in favor of the model with the Bernstein modes are presented. Analyzing frequency profiles of this spike event by the Fourier method, we found the power-law spectra with the power-law indices varying in the −0.8 to −2.75 interval. Because at some times this power-law index was close to the Kolmogorov spectral index (−5/3), we propose that the spikes are generated through the Bernstein modes in turbulent plasma reconnection outflows or directly in the turbulent magnetic reconnection of solar flares.

HIGHER RADIAL MODES OF AZIMUTHAL SURFACE WAVES ABOVE THE UPPER-HYBRID FREQUENCY IN CYLINDRICAL WAVEGUIDES PARTIALLY FILLED BY PLASMA

Azimuthal surface waves (ASWs) are known to be eigen waves of cylindrical metal waveguides partially filled by magnetoactive plasma. Zeroth radial modes were under study earlier. Their dispersion properties are known to be significantly influenced by the plasma column properties: its particle density, external axial static magnetic field, geometric dimensions, – rather than properties of the dielectric layer which separates the plasma column from the metal wall. Application of higher order ASWs in the low-frequency range was shown earlier to make it possible to get advantage of exciting ASWs with higher frequency than in the case of zeroth order ASWs without no change in the waveguide design. The present study generalises those investigation for the case of the waves above the upperhybrid frequency.

PIC Simulations of Microinstabilities and Waves at Near-Sun Solar Wind Perpendicular Shocks: Predictions for Parker Solar Probe and Solar Orbiter

Abstract Microinstabilities and waves excited at moderate-Mach-number perpendicular shocks in the near-Sun solar wind are investigated by full particle-in-cell simulations. By analyzing the dispersion relation of fluctuating field components directly issued from the shock simulation, we obtain key findings concerning wave excitations at the shock front: (1) at the leading edge of the foot, two types of electrostatic (ES) waves are observed. The relative drift of the reflected ions versus the electrons triggers an electron cyclotron drift instability (ECDI) that excites the first ES wave. Because the bulk velocity of gyro-reflected ions shifts to the direction of the shock front, the resulting ES wave propagates oblique to the shock normal. Immediately, a fraction of incident electrons are accelerated by this ES wave and a ring-like velocity distribution is generated. They can couple with the hot Maxwellian core and excite the second ES wave around the upper hybrid frequency. (2) From the middle of the foot all the way to the ramp, electrons can couple with both incident and reflected ions. ES waves excited by ECDI in different directions propagate across each other. Electromagnetic (EM) waves (X mode) emitted toward upstream are observed in both regions. They are probably induced by a small fraction of relativistic electrons. Results shed new insight on the mechanism for the occurrence of ES wave excitations and possible EM wave emissions at young coronal mass ejection–driven shocks in the near-Sun solar wind.

Parametric Interaction of High-Power Laser Radiation with Plasma in a Strong Magnetic Field

The article deals with interaction of an extraordinary laser wave of high amplitude with inhomogeneous plasma in a strong magnetic field in the vicinity of twice the upper-hybrid frequency. The analysis is carried out by means of numerical simulation based on the particle-in-cell (PIC) method. It is demonstrated that in this case, parametric resonance leads to a substantial heating of electrons. The results of analysis of spectra of the longitudinal field suggest that this kind of heating occurs due to nonlinear interaction of the upper-hybrid plasmons driven by the laser wave with electrostatic modes similar to the Bernstein ones in the linear approximation. The dependence of average energy of electrons gained in the course of heating on their initial temperature is investigated.