Harmonic Electron Cyclotron Research Articles

Reconstruction of magnetic island electron temperature in mixed second and third harmonic electron cyclotron emission conditions.

We develop a method to use the mixed third and second harmonic electron cyclotron emission (ECE) signal in the DIII-D tokamak to reconstruct the electron temperature profile of a rotating magnetic island. The third harmonic ECE is removed by extracting the rotating-island-associated fluctuations in the mixed signal, and the extracted fluctuation is combined with the equilibrium temperature obtained from other diagnostics after correcting for the third harmonic reabsorption. The accuracy of the reconstruction is studied by considering a DIII-D shot where an unmixed signal from an island is available on the low field side of the plasma and a mixed signal from the same island is available from the high field side. It is found that the reconstruction method successfully reproduces the island shape and temperature perturbation magnitude without the distortion caused by third harmonic ECE mixing. However, the radial location of the reconstructed island is somewhat displaced relative to the location of the q = 2 surface in the axisymmetric equilibrium reconstruction, resulting in a corresponding inaccuracy in the absolute temperature of the island. It is conjectured that this may arise from an inaccuracy of the reconstructed axisymmetric equilibrium in this region.

Beam‐Driven Electron Cyclotron Harmonic and Electron Acoustic Waves as Seen in Particle‐In‐Cell Simulations

AbstractRecent study has demonstrated that electron cyclotron harmonic (ECH) waves can be excited by a low energy electron beam. Such waves propagate at moderately oblique wave normal angles (∼70°). The potential effects of beam‐driven ECH waves on electron dynamics in Earth's plasma sheet is not known. Using two‐dimensional Darwin particle‐in‐cell simulations with initial electron distributions that represent typical plasma conditions in the plasma sheet, we explore the excitation and saturation of such beam‐driven ECH waves. Both ECH and electron acoustic waves are excited in the simulation and propagate at oblique wave normal angles. Compared with the electron acoustic waves, ECH waves grow much faster and have more intense saturation amplitudes. Cold, stationary electrons are first accelerated by ECH waves through cyclotron resonance and then accelerated in the parallel direction by both the ECH and electron acoustic waves through Landau resonance. Beam electrons, on the other hand, are decelerated in the parallel direction and scattered to larger pitch angles. The relaxation of the electron beam and the continuous heating of the cold electrons contribute to ECH wave saturation and suppress the excitation of electron acoustic waves. When the ratio of plasma to electron cyclotron frequency ωpe/ωce increases, the ECH wave amplitude increases while the electron acoustic wave amplitude decreases. Our work reveals the importance of ECH and electron acoustic waves in reshaping sub‐thermal electron distributions and improves our understanding on the potential effects of wave‐particle interactions in trapping ionospheric electron outflows and forming anisotropic (field‐aligned) electron distributions in the plasma sheet.

Energetic particle activity in AD Leo: Detection of a solar-like type-IV burst

Context. AD Leo is a young and active M dwarf with high flaring rates across the X-ray-to-radio bands. Flares accelerate particles in the outer coronal layers and often impact exo-space weather. Wide-band radio dynamic spectra let us explore the evolution of particle acceleration activity across the corona. Identifying the emission features and modelling the mechanisms can provide insights into the possible physical scenarios driving the particle acceleration processes. Aims. We performed an 8 h monitoring of AD Leo across the 550-850 MHz band using upgraded-Giant Metrewave Radio Telescope (uGMRT). The possible flare and post-flare emission mechanisms are explored based on the evolution of flux density and polarisation. Methods. The python-based module, Visibility Averaged Dynamic spectrum (VISAD), was developed to obtain the visibility-averaged wide-band dynamic spectra. Direct imaging was also performed with different frequency-time averaging. Based on existing observational results on AD Leo and on solar active region models, radial profiles of electron density and magnetic fields were derived. Applying these models, we explored the possible emission mechanisms and magnetic field profile of the flaring active region. Results. The star displayed a high brightness temperature (TB≈ 1010−1011 K) throughout the observation. The emission was also nearly 100% left circularly polarised during bursts. The post-flare phase was characterised by a highly polarised (60–80%) solar-like type IV burst confined above 700 MHz. Conclusions. The flare emission favours a Z-mode or a higher harmonic X-mode electron cyclotron maser emission mechanism. The >700 MHz post-flare activity is consistent with a type-IV radio burst from flare-accelerated particles trapped in magnetic loops, which could be a coronal mass ejection (CME) signature. This is the first solar-like type-IV burst reported on a young active M dwarf belonging to a different age-related activity population (‘C’ branch) compared to the Sun (‘I’ branch). We also find that a multipole expansion model of the active region magnetic field better accounts for the observed radio emission than a solar-like active region profile.

Interplanetary shock induced intensification of electron cyclotron harmonic waves in the Earth’s inner magnetosphere

Electron cyclotron harmonic (ECH) waves are electrostatic emissions frequently observed in the Earth’s magnetosphere. By precipitating magnetospheric hot electrons into the ionosphere, ECH waves play a critical role in the formation of diffuse aurora. Previous research has extensively investigated the strong dependence of ECH waves on the geomagnetic activities. In this study, we present the first report of the prompt response of ECH waves to an interplanetary shock on the basis of WIND and Van Allen Probes observations. Our observations and analyses demonstrate that the interplanetary shock compression can increase >0.1 keV hot electron fluxes in the dayside inner magnetosphere, consequently leading to the prompt intensification of ECH waves by promoting the wave instability. These findings expand our comprehension of the impacts of solar wind disturbances on magnetospheric plasma waves and offer fresh insights into solar wind-magnetosphere-ionosphere coupling.

Electron cyclotron harmonic waves in Jovian magnetosphere as seen by Juno

Electron cyclotron harmonic (ECH) waves along with whistler mode waves are suggested to be responsible for causing the persistent diffuse aurora in Jupiter. In this work, for the first time we systematically analyze the ECH waves in the Jovian inner magnetosphere, which was surveyed by Juno during the later orbits (>25). We find that in the Jovian inner magnetosphere, ECH waves occur in two specific regions—one equatorial and the other off-equatorial, just outside the Io torus. Equatorial ECH waves have higher intensity compared to their off-equatorial counterpart. We also notice an overlap between the region of mid-latitude hot injections and the region of off-equatorial ECH wave occurrence. Finally, we show an event to describe the complex nature of ECH wave growth/damping varying with particle density structures of the injection region at mid-latitude.

Conjugate Observation of Whistler Mode Chorus, ECH Waves and Dayside Diffuse Aurora by MMS and Ground‐Based Yellow River Station

AbstractDayside diffuse auroras (DDAs) originate from the precipitation of energetic electrons in the central plasma sheet, induced by wave‐particle interactions. This study examines a specific case characterized by the detection of whistler mode chorus and electron cyclotron harmonic (ECH) waves using the Magnetospheric Multiscale (MMS) satellites, along with the conjugate observation of DDA at the Yellow River (YR) station. ECH waves are observed shortly after the satellite enters the magnetosphere from the magnetosheath and persist throughout the magnetosphere region. Chorus waves are also observed within the magnetosphere, and probably generated near the B minima, followed by the propagation along magnetic field lines. Analysis of MMS particle data reveals an increase in low‐energy electrons and scattering of high‐energy electrons during periods of enhanced whistler mode chorus waves. These precipitated electrons manifest as DDA phenomena. The observed evidence demonstrates the simultaneous occurrence of ECH, chorus waves, electron pitch angle scattering, and DDA. Moreover, the concurrent presence of these waves in the outer magnetosphere during the dayside raises important considerations regarding the relative roles of different plasma waves, particle energization, precipitation dynamics, and the overall development of DDA phenomena.

Effects of toroidally-distributed-divertor biasing on scrape-off-layer (SOL) current drive, divertor particle flux and fast electron confinement in the QUEST spherical tokamak

A novel divertor biasing by four biasing plates distributed toroidally (TDDB) on the upper divertor target plate is applied to low density tokamak plasmas started-up by 28 GHz 2nd harmonic electron cyclotron current drive (ECCD) in a quasi-double null configuration of the QUEST spherical tokamak (ST). In the ST plasmas of line averaged electron density < ne > ∼0.7–1 × 1018 m−3, about 20%–40% of the current Ibias driven by ∼85 V sawtooth bias voltage reaches the lower divertor plate along the biased scrape-off-layer (SOL) flux tube as the SOL current ISOL . The fact ISOL is noticeably lower than Ibias indicates an appreciable leakage of parallel current from the biased SOL flux tube. The leakage currents in the toroidal and radial directions are confirmed by detection of them using the unbiased plates in the TDDB experiments. From the ion saturation current density profile obtained by a divertor Langmuir probe array, the fall-off lengths of divertor particle flux are estimated together with the strike line position. Total particle flux to the upper divertor, evaluated by the integrated ion saturation current density profile is reduced by up to 45% during positive biasing of the TDDB, depending on the position of the strike line to the biased plate. In addition, the TDDB also induces a noticeable loss of fast electrons produced by ECCD, leading to an ∼2% reduction in the maximum toroidal current of the ST plasma compared to a shot without the TDDB. Reduction of the divertor particle flux and enhancement of the fast electron losses are thought to be dominantly caused by E ×B drift induced by the TDDB. In the present experimental conditions, the effects of magnetic perturbations produced by the SOL currents on the fast electron losses can be neglected because of a too small SOL current.

Plasmapause Surface Waves Triggered by Substorms

AbstractIn this study, we present in‐situ measurement on board Van Allen Probes showing that surface wave can cause oscillations of the plasmapause and modulate hiss and electron cyclotron harmonic (ECH) waves. Plasmapause surface wave (PSW) in Ps6 band was observed during an intense substorm on 16 July 2017, accompanied with quasi‐periodic emissions of hiss (positively correlated with plasma density) and ECH waves (anticorrelated with density). Phase relation between magnetic field and velocity perturbations indicate that the PSW was an eigenmode between southern and northern ionosphere. Measurements from ground‐based magnetometers suggest that such PSW propagates sunward at the dusk flank and were excited around the expansion phase of an intense substorm. Additional cases of PSWs are also presented to demonstrate that such waves are commonly observed near plasmapause and are likely to be related to the substorm activity.

Statistical Analysis of Concurrent Electron Cyclotron Harmonic and Whistler Waves Driven by ULF Waves

AbstractPlasma sheet electron precipitation to the diffuse aurora is one of the primary mechanisms of magnetospheric energy transport to the Earth’s atmosphere. Such precipitation is provided by electron resonant interactions with electron cyclotron harmonics (ECH) and whistler‐mode waves in the near‐Earth plasma sheet. ECH and whistler‐mode wave generation is traditionally attributed to two main sources of free energy: the loss‐cone anisotropy and the temperature anisotropy due to transverse electron heating during earthward convection or plasma sheet injections. This paper considers a third free energy source for ECH and whistler‐mode wave generation—the electron distribution modulation by compressional ultra‐low‐frequency (ULF) waves. We conduct statistics of ECH and whistler‐mode wave bursts associated with ULF waves and show that: (a) such bursts are mostly observed on the dawn flank, (b) 83% of events show a correlation between ECH and whistler‐mode bursts, and the rest 17% show anti‐correlation, (c) ECH and whistler‐mode wave intensities are comparable with those observed around the night‐side injection region. We investigate the main characteristics of ECH and whistler‐mode waves modulated by ULF waves and provide an empirical model for these characteristics. This model can be directly incorporated into simulation frameworks, so as to include these ULF‐modulated wave bursts in simulations of the magnetosphere‐ionosphere coupling and radiation belt dynamics.

ECMI resonance in AKR revisited: Hyperbolic resonance, harmonics, and wave–wave interaction

Recapitulation of the resonance condition for the fundamental and higher electron cyclotron harmonics in the electron cyclotron maser instability (ECMI) enables radiation below and confirms the possibility of radiation in a narrow band above harmonics n &amp;gt; 1. Near n = 1 resonance on the confined lower X-mode branch, amplification is supported by the decrease in phase and group speeds. Confined slow large-amplitude quasi-electrostatic X-modes non-linearly modulate the plasma to form cavitons until self-trapped inside them at a further increasing wavenumber. They undergo wave–wave interaction, enabling escape into free space in the second harmonic band below n = 2. At a sufficiently large parallel wavenumber (oblique propagation), the fundamental resonance n = 1 is hyperbolic, a possibility so far missed but vital for an effective ECMI in the upward current region. Here, the resonance hyperbola favorably fits the loss-cone boundary, the presumably important ECMI upward-current source-electron distribution, to stimulate ECMI growth at available auroral electron energies.

Global Occurrence of Higher‐Bands ECH Waves in Radiation Belts Based on a Novel Noise Reduction Algorithm (NORA)

AbstractElectrostatic electron cyclotron harmonic (ECH) waves which can efficiently produce scattering loss of electrons, are frequently observed in the first band. Here we present new results on the global occurrence of higher‐band ECH waves in the radiation belts from Van Allen Probes data. We develop a novel noise reduction algorithm which can be widely applied to various plasma waves to better identify naturally occurring waves during periods of instrument interference. The occurrence rate of ECH waves in higher harmonic bands is comparable to that in the first harmonic band, except in the noon‐to‐dusk sector. As the level of geomagnetic activity increases, ECH waves in all bands can be observed at lower L‐shells. The occurrence rates of weak (moderate, strong) waves in all four bands increase 3–4 (4–6, 5–10) times during disturbed periods compared to those during quiet periods. The current results may provide better understanding of ECH‐driven magnetosphere‐ionosphere coupling.

Electron distribution-controlled electron cyclotron harmonic wave generation in the magnetosphere

Electron cyclotron harmonic (ECH) waves, contributing significantly to magnetospheric dynamics, usually appear as a series of harmonics between the multiples of electron gyrofrequency. Previous studies have demonstrated that ECH waves are the electron Bernstein mode excited by the electron loss cone distribution. To investigate how the electron distribution controls the frequency and wave normal angle of excited ECH waves, we derive a concise analytic formula for ECH growth rates using the bi-Maxwellian distribution. Parametric studies show that the expansion of the loss cone size could effectively enlarge ECH growth rates and move the peak growth rates to a smaller wave number k (higher frequency) region, while the deepening of the loss cone can only increase the growth rates. It is also found that the growth rate pattern in the k space exhibits different trends on the two sides of 1.5 fce because the dominant resonance orders on the two sides are different. This study further develops our understanding of the generation mechanism of ECH waves.

8.56-GHz quasi-optical launcher system with incident-mode selectivity on the QUEST spherical tokamak

An 8.56-GHz quasi-optical launcher system with incident-mode selectivity was developed for non-inductive electron cyclotron (EC) plasma ramp-up and long-pulse sustainment, and effective bulk-electron heating on the Q-shu University experiment with steady-state spherical tokamak (QUEST). A novel polarization-selective system was proposed to conduct fundamental and second harmonic EC heating and current drive experiments with ordinary and extra-ordinary mode waves, respectively. Selectivity of the incident modes or polarizations was confirmed using the whole developed system at a low power level. A large-sized mirror launcher was developed to inject the beam whose size was as small as possible near the QUEST major radius. The designed mirror performance was checked using a three-dimensional electromagnetic-wave simulator and a low-power test. Ultimately, 0.15-m sized beams for both the two polarizations were successfully attained near the QUEST major radius, even in low-frequency applications with long propagation.

Phase Difference of Electron Cyclotron Harmonic (ECH) Waves Observed by Using the Interferometry Observation Mode of the Arase Satellite

Phase Difference of Electron Cyclotron Harmonic (ECH) Waves Observed by Using the Interferometry Observation Mode of the Arase Satellite

Electron cyclotron drift instability and anomalous transport: two-fluid moment theory and modeling

In the presence of a strong electric field perpendicular to the magnetic field, the electron cross-field (E × B) flow relative to the unmagnetized ions can cause the so-called electron cyclotron drift instability (ECDI) due to resonances of the ion acoustic mode and the electron cyclotron harmonics. This occurs in, for example, collisionless shock ramps in space, and in E × B discharge devices such as Hall thrusters. A prominent feature of ECDI is its capability to induce an electron flow parallel to the background E field at a speed greatly exceeding predictions by classical collision theory. Such anomalous transport is important due to its role in particle thermalization at space shocks, and in causing plasma flows towards the walls of E × B devices, leading to unfavorable erosion and performance degradation, etc. The development of ECDI and anomalous transport is often considered requiring a fully kinetic treatment. In this work, however, we demonstrate that a reduced variant of this instability, and more importantly, the associated anomalous transport, can be treated self-consistently in a collisionless two-fluid framework without any adjustable collision parameter. By treating both electron and ion species on an equal footing, the free energy due to the inter-species velocity shear allows the growth of an anomalous electron flow parallel to the background E field. We will first present linear analyses of the instability in the two-fluid five- and ten-moment models, and compare them against the fully-kinetic theory. At low temperatures, the two-fluid models predict the fastest-growing mode in good agreement with the kinetic result. Also, by including more moments, secondary (and possibly higher) unstable branches can be recovered. The dependence of the instability on ion-to-electron mass ratio, plasma temperature, and background B field strength is also thoroughly explored. We then carry out direct numerical simulations of the cross-field setup using the five-moment model. The development of the instability, as well as the anomalous transport, is confirmed and in excellent agreement with theoretical predictions. The force balance properties are also studied using the five-moment simulation data. This work casts new insights into the nature of ECDI and the associated anomalous transport and demonstrates the potential of the two-fluid moment model in efficient modeling of E × B plasmas.

Parametric Sensitivity of Electron Scattering Effects by Electrostatic Electron Cyclotron Harmonic Waves

AbstractIt is well known that electron cyclotron harmonic (ECH) waves can effectively precipitate hundreds of eV to tens of keV electrons into the upper atmosphere, resulting in the production of diffuse aurora. To deepen our outstanding of the contributions of ECH waves to diffuse aurora, in this study we perform a detailed analysis of the sensitivity of ECH wave‐induced electron scattering to ambient magnetic field intensity, total electron density and density ratio between hot and cold electrons. Our results show that for various parameter sets, ECH waves can cause the precipitation loss of diffuse auroral electrons at energies of 100 eV to several keV and are capable of accelerating 100 eV to keV electrons at intermediate pitch angle, therefore contributing to the formation of butterfly pitch angle distribution. We also find that the variations of background parameters significantly change the magnitude of diffusion coefficients and resonant pitch angle range of &lt; ∼1 keV electrons, while the scattering efficiency of &gt;∼1 keV electrons is almost unaffected. For &lt;∼1 keV electrons, an increase of magnetic field intensity or density ratio between hot and cold electrons weakens the electron scattering efficiency and narrows the resonant region to lower pitch angles, while an increase of total electron density generally plays an opposite role. In addition, the momentum diffusion can be stronger than pitch angle diffusion at large pitch angles for &lt;∼1 keV electrons, while it can be negligible at higher energies.

Harmonic Electron Cyclotron Maser Emission along the Coronal Loop

Efficient radiation at second and/or higher harmonics of Ωce has been suggested to circumvent the escaping difficulty of the electron cyclotron maser emission mechanism when it is applied to solar radio bursts, such as spikes. In our earlier study, we developed a three-step numerical scheme to connect the dynamics of energetic electrons within a large-scale coronal loop structure with the microscale kinetic instability energized by the obtained nonthermal velocity distribution and found that direct and efficient harmonic X-mode (X2 for short) emission can be achieved due to the strip-like features of the distribution. That study only considered the radiation from the loop top at a specific time. Here we present the emission properties along the loop at different locations and timings. We found that, in accordance with our earlier results, few to several strip-like features can appear in all cases, and the first two strips play the major role in exciting X2 and Z (i.e., the slow extraordinary mode) that propagate quasi-perpendicularly. For the four sections along the loop, significant excitation of X2 is observed from the upper two sections, and the strongest emission is from the top section. In addition, significant excitation of Z is observed for all loop sections, while there is no significant emission of the fundamental X mode. The study provides new insight into coherent maser emission along the coronal loop structure during solar flares.

Power transport efficiency during O-X-B 2nd harmonic electron cyclotron heating in a helicon linear plasma device 1 1This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The publisher acknowledges the US government license to provide public access under the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

The principal objective of this work is to report on the power coupled to a tungsten target in the prototype-material plasma exposure experiment device during oblique injection of a microwave beam (<70 kW at 28 GHz) into a high-power (∼100 kW at 13.56 MHz) over-dense () deuterium helicon plasma column. The experimental setup, electron heating system, electron heating scheme, and IR thermographic diagnostic for quantifying the power transport is described in detail. It is demonstrated that the power transported to the target can be effectively controlled by adjusting the magnetic field profile. Using this method, heat fluxes up to 22 MW m−2 and power transport efficiencies in the range of 17%–20% have been achieved using 70 kW of microwave power. It is observed that most of the heat flux is confined to a narrow region at the plasma periphery. Ray-tracing calculations are presented which indicate that the power is coupled to the plasma electrons via an O-X-B mode conversion process. Calculations indicate that the microwave power is absorbed in a single pass at the plasma periphery via collisions and in the over-dense region via 2nd harmonic cyclotron resonance of the electron Bernstein wave. The impact of these results is discussed in the context of MPEX.

Simultaneous Observation of Whistler Waves and Electron Cyclotron Harmonic Waves in the Separatrix Region of Magnetopause Reconnection

AbstractA variety of plasma waves have been observed in the separatrix region during magnetic reconnection, and their roles on reconnection and exciting mechanisms are still controversial. In this paper, we report the whistler wave with a very narrow frequency band just above half of the electron cyclotron frequency in the separatrix region. This whistler wave is accompanied with the electron cyclotron harmonic (ECH) waves. The ECH waves have frequencies between the first and fifth harmonic of electron cyclotron frequency . The wave normal angle of the whistler waves is ∼20°–∼40°, meaning the quasi‐parallel propagation, while the ECH waves propagate perpendicular to the background magnetic field with . In the separatrix region, the electrons have a loss‐cone distribution while these two kinds of waves are observed. Based on the linear theory, such an electron distribution is unstable for both whistler waves and ECH waves. It indicates that these two kinds of waves are generated by the electron loss‐cone instability.

Observation of second harmonic electron cyclotron resonance heating and current-drive transition during non-inductive plasma start-up experiment in QUEST

Noninductive plasma current start-up using 2nd harmonic electron cyclotron resonance heating (ECRH) with oblique radio frequency (RF) injection is demonstrated in a Q-shu University experiment with steady-state spherical tokamak. A strong transition was observed in the heating and plasma current ramp-up. The initial bulk electron heating regime exhibits T ebulk ∼ 140 eV and no hard x-ray (HXR) emission with a low I p of ∼15 kA; it abruptly transitions to a regime that exhibits a low T ebulk of ∼10 eV and a strong HXR emission with a high I p of ∼50 kA. This behavior is distinctly different from that observed in previous fundamental ECRH experiments. The mechanism of the heating and current drive transition are investigated considering wave power absorption and plasma power balance. The results indicate that the transition is caused by the favorable heating of tail electrons where the RF power absorption at the 2nd harmonic increases nearly linearly with T etail, while the power transfer from the tail electrons to the bulk electrons decreases with 1/T etail 0.5. This causes a rapid transition to a state with high T etail while reducing T ebulk towards colder ion temperature. The understanding of the transition mechanism helps to consider plasma current start-up using 2nd harmonic ECRH for tokamak reactors such as JT-60 SA and ITER.