The existence of non-resonant gyro lines and their detectability by Thomson scatter radars

Both linear and nonlinear interactions between oblique whistler, electrostatic, quasi‐upper hybrid mode waves and an electron beam are studied by linear analyses and electromagnetic particle simulations. In addition to a background cold plasma, we assumed a hot electron beam drifting along a static magnetic field (Bo). Growth rates of the oblique whistler, oblique electrostatic, and quasi‐upper hybrid instabilities were first calculated. We found that there are four kinds of unstable mode waves for parallel and oblique propagations. They are the electromagnetic whistler mode wave (WW1), the electrostatic whistler mode wave (WW2), the electrostatic mode wave (ESW), and the quasi‐upper hybrid mode wave (UHW). When the angle θ between the wave vector k and Bo is small enough (≤10°), the electrostatic instability is dominant compared with the whistler mode instability. When θ is around 30°, the growth rates of whistler (both electrostatic and electromagnetic) mode waves and electrostatic mode waves are of the same order. When θ increases to 60°, the WW2 mode will be the most unstable mode wave. For a very large θ, (∼ 80°), the WW2 instability still has positive growth rates, and the UHW instability begins to have positive growth rates. Electromagnetic particle simulations were performed for parallel and three oblique cases, θ = 0°, 30°, 60° and 80°. When θ = 0°, whistler mode waves can hardly grow from the thermal fluctuation level because the electron beam which is supposed to provide free energy to the whistler mode waves is quickly diffused in the velocity space by much faster growing ESW. The ESW can lead to a secondary electrostatic instability. With θ = 30°, both electrostatic and whistler mode waves grow simultaneously. Also, the electrons diffused by the whistler mode instability to higher υ∥ velocity regions can lead to a secondary electrostatic instability. When θ is 60°, diffusion of the electron beam is controlled mainly by the WW2 instability. For θ = 80°, both WW2 and UHW grow despite their small growth rates. The simulations agree with linear analyses on the ESW growth rate for θ = 0°, but from our simulation data, growth rates of oblique whistler mode, electrostatic mode and quasi‐upper hybrid mode waves are usually smaller than those predicted by linear analyses. The electrostatic and whistler mode instabilities affect each other through their interactions with the electron beam. While the most intense ESW is generated for parallel propagation, the most intense whistler mode wave is observed at an oblique direction. Modulations between different electrostatic waves are found for θ = 0° after the electric field reaches its saturation level. Modulations between whistler waves (θ = 60°,80°) are also found while their magnetic fields increase nonlinearly and reach their saturation levels. A possible mechanism is proposed to explain the satellite observations of whistler mode chorus and accompanied electrostatic waves, whose amplitudes are sometimes modulated at the chorus frequency.

The generation of low frequency electrostatic mode by parametric decay of electromagnetic waves (EMWs) in the Earth’s inner magnetosphere with exponentially truncated kappa distributed hot electrons and cold electrons is studied. Nonlinear dispersion equation for the parametric process is derived from kinetic theory. The parametric instability of EMWs decay into low frequency electrostatic normal mode (ion acoustic like wave modes and electron acoustic wave modes) and electrostatic quasi–mode in the Earth’s inner magnetosphere are numerically analyzed. It is shown that parametric instability occurs only when the EMW is sufficiently strong if the collisions between ions and electrons are taken into account. The growth rate and the threshold conditions of the decay instability depend on the concentration and distribution of hot electrons. Because they change the dispersion and the damping rate of normal mode, the collisional damping of sideband EMW. In addition, the excitation of electrostatic normal mode by parametric decay of EMWs is more difficult than the excitation of electrostatic quasi–mode. The growth rate of EMWs decaying into electrostatic quasi–mode is much larger than the ones of decaying into electrostatic normal mode. But the frequency of electrostatic quasi–mode corresponding to the maximum growth rate can be as low as a few tens Hz. The mechanism may excite the electrostatic mode with frequency comparable to those of the ultra–low frequency electric fields observed in the Earth’s inner magnetosphere.

We present a statistical study of anomalous radar echoes observed in the auroral ionosphere thought to be signatures of Langmuir turbulence (LT). Data obtained with the European Incoherent Scatter Svalbard radar during the international polar year (IPY) were searched for these anomalous echoes in the auroral F region. In incoherent scatter radar experiments LT may in certain circumstances be observed as enhanced backscattered radar power at the ion line frequencies, plasma line frequencies, and at zero Doppler shift. The power enhancement at zero Doppler shift could arise due to Bragg scattering from nonpropagating density fluctuations caused by strong LT. In the IPY data set, around 0.02% of the data comply with our search criteria for altitudes above 190 km based on the ion line spectrum including enhancement at zero Doppler shift. The occurrence frequency of the identified events peaks in the premidnight sector and increases with local geomagnetic disturbance. Enhanced backscattered power is observed with limited altitude extent (below 20 km in 70% of the events), and the altitude distribution of identified radar signatures in the ion line channel has a peak at about 220 km. Enhancement of the plasma line is observed with the ion line enhancements in more than 60% of the events. Two classes of enhanced plasma lines occur. In the first class, plasma lines are limited in frequency and altitude and occur at altitudes of ion line enhancements. In the second class, the plasma lines are spread in frequency and range and are observed at lower altitudes than the first class (at about 170 km) with frequencies close to 3 MHz. Available optical data available indicate that the identified events to occur during auroral breakup with high‐energy electron precipitation.

High frequency electric field measurements from the AMPTE IRM plasma wave receiver are used to identify three simultaneously excited electrostatic wave modes in the Earth's foreshock region: the electron beam mode, the Langmuir mode, and the ion acoustic mode. A technique is developed which allows the rest frame frequency and wave number of the electron beam waves to be determined. Plasma wave and magnetometer data are used to determine the interplanetary magnetic field direction at which the spacecraft becomes magnetically connected to the Earth's bow shock. From the knowledge of this direction, the upstreaming electron cutoff velocity can be calculated. We take this calculated cutoff velocity to be the flow velocity of an electron beam in the plasma. Assuming that the wave phase speed is approximately equal to the beam speed and using the measured electric field frequency, we determine the plasma rest frame frequency and the wave number. We then show that the experimentally determined rest frame frequency and wave number agree well with the most unstable frequency and wave number predicted by linear homogeneous Vlasov theory for a plasma with Maxwellian background electrons and a Lorentzian electron beam. From a comparison of the experimentally determined and theoretical values, approximate limits are put on the electron foreshock beam temperatures. A possible generation mechanism for ion acoustic waves involving mode coupling between the electron beam and Langmuir modes is also discussed.

In the ionosphere, a sustained population of suprathermal electrons is generated due to photoionization or electron precipitation. At resonance, the phase velocity of the electrons matches the Langmuir phase velocity observed by the Incoherent Scatter Radar (ISR). As a result, the scattered power in the plasma line spectrum is enhanced, thus making it possible to detect them. Plasma lines can be used to improve the accuracy of the estimates of electron density and temperature, study features in the electron velocity distribution of the suprathermal electrons and provide an independent method to calculate ionospheric currents.   We analyzed the data collected with the EISCAT Tromsø UHF radar on 27 January 2022. We present a novel technique of data reduction to detect plasma lines and extract parameters. We use the method developed by Ivchenko (2017) to determine the times with enhanced plasma lines. For those times, we model the spectrum with a Gaussian function, where the plasma line intensity, frequency and bandwidth correspond to the amplitude, mean and variance, respectively. We observe photoelectron-enhanced plasma lines between 09:16:15 LT – 13:56:15 LT. All the detected plasma lines are field-aligned, except for 11:29:30 LT - 12:51:30 LT, when they are also detected in the vertical direction (i.e. at 11.67° to the magnetic field). The detection of the plasma lines is accompanied by an increase in the electron density estimates from the ion line. 

In the Incoherent Scatter Radar (ISR) spectrum, we observe a large amount of scattered power at frequencies corresponding to plasma electrostatic modes with wavenumber imposed by the radar. The commonly occurring resonant frequencies are ion and plasma lines. Ion lines are signatures of ion-acoustic waves travelling towards and away from the radar and are observable all the time. Plasma lines are signatures of high-frequency electrostatic waves and are observable when enhanced above the thermal level by a suprathermal electron population. In this work, we assume a Maxwellian thermal population together with a suprathermal population derived from the electron transport code Aeroplanets that calculates the angular electron flux. We provide the numerical electron velocity distribution function as an input to the Arbitrary Linear Plasma Solver (ALPS) to numerically solve the linear Vlasov-Maxwell dispersion relation and estimate the resonance frequency at angles to the magnetic field. The linear resonance frequency calculated from ALPS is then compared to the resonance frequency estimated from ISR plasma line measurements.  

We have considered a Vlasov plasma with both the resonant and non-resonant mode waves. The non-resonant mode is considered as a perturbation to plasma where a turbulent field is present which is in resonant mode. The interaction of these waves is characterized by the Vlasov Maxwell set of equations. The evaluation process of the fluctuating parts of the distribution function owing to the presence of resonant mode wave, due to the modulation field, and the nonlinear fluctuating parts of distribution function due to the non-resonant wave is presented in this work.KeywordsNonlinear wave-particle interactionDensity and temperature gradientsDrift wave turbulence

We present the observations of the artificial ionospheric modification experiment of EISCAT on 18 October 2012 in Tromsø, Norway. When the pump of alternating O mode and X mode is switched on, the UHF radar observation shows some strong enhancements in electron density, ion lines and plasma lines. Based on some existing theories, we find the following: First, during the experiment, the frequency of plasma line (fL), ion line (fia) and pump (fh) matches fL = fh − 3fia and = fh − 5fia occasionally demonstrated that the cascade process occurred. Second, through quantitative calculation, we found that the O-mode component mixed in X-mode wave satisfies the thresholds of the parametric decay instability and the oscillation two-stream instability, from which we infer that the HF-induced plasma lines (HFPLs) and HF-enhanced ion lines (HFILs) observed in X-mode pulse could have been caused by the O-mode component mixed in X-mode wave. Third, the UHF radar observation shows some apparent enhancements over a wide altitude range (from approximately the reflection altitude to ~670 km) in electron density during X-mode pulse, which also does not, in fact, correspond to a true increase in electron density, but due to the enhancement in ion line or the enhancement in radar backscatter induced by some unknown mechanism.

Nonlinear interaction between two electrostatic normal modes of a warm pair-ion plasma, viz., ion plasma mode (Langmuir mode) and ion acoustic mode has been analyzed by employing a perturbation technique. It is shown that a gradual loss of phase coherence in the excited Langmuir wave dynamics (phase-mixing) occurs in such a plasma, leading to wave-breaking at arbitrarily low wave amplitudes. Nonlinear results provide an approximate expression for the phase-mixing time which is found to increase with the increase of the ratio of acoustic frequency to Langmuir frequency. The results of our investigation are expected to be relevant to the laboratory produced paired fullerene-ion plasmas.

The instability and electrostatic wave modes generation in a cold negative ion plasma which streams across a static magnetic field has been analysed by fluid dynamical model and computer simulation technique. In the theoretical approach, we observed different kinds of acoustic modes and cyclotron modes at an oblique angle to the static magnetic field. At some critical wave number, the positive and negative ion acoustic modes split up into two, fast and slow waves. The value of critical wave numbers and wave growth rates depends on the plasma streaming velocity, electron concentration in the system, and magnetic field intensity. In the presence of high intense external magnetic fields, we observed a wave coupling between different electrostatic modes. With the help of computer simulation performed by the Particle In Cell method, we confirmed the presence of theoretically proposed wave modes and studied the evolution of instability in detail.

During an experiment involving the alternating O / X mode pump, the Incoherent Scatter Radar (ISR) observation demonstrated that the high frequency enhanced ion line (HFIL) and plasma line (HFPL) did not immediately appear, but were delayed by a few seconds after the pump onset. By examining the initial behaviors of the ion line, plasma line and electron temperature as well as ionosphere condition, it is shown that (1) the HFIL and HFPL are delayed not only in the X mode pump but also in the O mode pump; (2) the HFIL can not be observed until the electron temperature is enhanced. The analysis suggests that (1) the leakage of the X mode to the O mode pump may not be ignored; (2) the spatiotemporal uncertainty, the spatiotemporal change in the profiles of ion mass and electron density, may play an important role in the lack of the Bragg condition; (3) nevertheless, the absence of parametric decay instability can not be ruled out due to the lack of matching condition caused by the spatiotemporal uncertainty.

The radar constant calibration in incoherent scatter radar ion line processing is essential for the data quality and was not paid enough attention in previous studies. In this investigation, based on several experiments made by the newly built Sanya incoherent scatter radar (SYISR), we made and evaluated the ion line calibration by plasma line both in case study and statistically. The calibration factor had local time and altitude variations, due to the corresponding variations of the transmitted power, the radar gain, and the noise temperature. We obtained a mean calibration factor of 1.35 by the simultaneous measured plasma line and ion line electron density and applied it to a one-month ion line observation calibration. Through a co-located ionosonde measured foF2 evaluation, the calibration decreased the mean deviation from −1.92 MHz (−18%) to −0.33 MHz (−3%), which resulted in much better agreement between the ion line foF2 after calibration and the ionosonde results. The existed deviations between after calibration and ionosonde results were due to the uncertainties either in the used calibration factor or the ionosonde measurements. An empirical Te/Ti usage in raw electron density estimation and ignoring the seasonal and short-term variations of the effecting factors might influence the calibration performance. Using the to-be-completed SYISR Tristatic System, the performance of plasma line calibration technique is expected to be improved in the future.

In this work, we investigated the discharge characteristics and heating mechanisms of argon helicon plasma in different wave coupled modes with and without blue core. Spatially resolved spectroscopy and emission intensity of argon atom and ion lines were measured via local optical emission spectroscopy, and electron density was measured experimentally by an RF-compensated Langmuir probe. The relation between the emission intensity and the electron density was obtained and the wavenumbers of helicon and ‘Trivelpiece-Gould’ (TG) waves were calculated by solving the dispersion relation in wave modes. The results show that at least two distinct wave coupled modes appear in argon helicon plasma at increasing RF power, i.e. blue core (or BC) mode with a significant bright core of blue lights and a normal wave (NW) mode without blue core. The emission intensity of atom line 750.5 nm (I ArI750.5nm) is related to the electron density and tends to be saturated in wave coupled modes due to the neutral depletion, while the intensity of ion line 480.6 nm (I ArII480.6nm) is a function of the electron density and temperature, and increases dramatically as the RF power is increased. Theoretical analysis shows that TG waves are strongly damped at the plasma edge in NW and/or BC modes, while helicon waves are the dominant mechanism of power deposition or central heating of electrons in both modes. The formation of BC column mainly depends on the enhanced central electron heating by helicon waves rather than TG waves since the excitation of TG waves would be suppressed in this special anti-resonance region.

In this paper, we present the first results of a new technique for measuring the electron temperature in the daytime ionosphere using the Arecibo incoherent scatter radar (ISR). The technique utilizes the plasma line component of the incoherent scatter spectrum. The difference in the up‐ and down‐shifted plasma line frequencies is related to the density and temperature of the ionosphere, as well as more minor effects resulting from photoelectrons, currents, and other sources. The shift is very small (the order of 1 kHz in a plasma line frequency of several MHz) but can be measured quite accurately with the coded long pulse plasma line technique. We compare the results to ion line measurements of the electron temperature, and the two independent techniques show good agreement. In addition to providing a measure of the electron temperature that is independent of the ion line, the approach allows for a sensitive test of kinetic plasma theory including a magnetic field, gives us the ability to study photoelectron populations and electron currents, and will allow us to constrain ion line fits in the bottomside (and possibly topside) regions to more accurately fit for composition.

Emisson spectra and time-resolved two-dimensional (2D) emission images of the electron-ion dielectronic recombination (i.e. a reversal process of auto-ionization) line of neutral Cu atoms, the selectively excited Cu ionic line, and normal Cu atomic line were observed for understanding the excitation mechanisms of Cu neutral and ionic lines in a low-pressure laser-induced plasma (LP-LIP) of Ar. From the observations, the number of charged particles around the emitting species seems to increase with increasing Ar pressure. Different time-resolved 2D emission images were observed among the selectively excited Cu ionic line and other Cu emission lines resulting from the different excitation mechanisms of the respective emission lines. Collisions of the second kind and electron-ion recombinations were found to be one of the major excitation mechanisms of Cu in Ar LP-LIP.