Plasma Density Perturbations Research Articles

Terahertz generation by a rotating relativistic electron beam in a magnetized plasma column

Terahertz (THz) field excitation by a rotating relativistic electron beam in a magnetized plasma column is described using numerical analysis and particle-in-cell simulation. A rotating electron beam propagating through a cylindrical plasma column excites plasma wakefields. The plasma wakefields couple with the electron beam to excite transverse currents at THz frequency. As a result, the energy of the wakefield directly converts into the form of electromagnetic radiation in the THz range. The magnetic field supports the transverse modes via electron cyclotron resonance. The strength of the THz field is enhanced due to scattering of the spiralling electron beam on the plasma density perturbation. The THz field amplitude is controllable by the electron beam velocity and beam density. On increasing the beam current, the THz field is enhanced significantly. The analytical results are compared with particle-in-cell simulations and are found to be in reasonable agreement.

Response of propagation of ELF electromagnetic waves through the morning ionosphere to small density variations caused by infrasonic wave

We consider the problem of the effect of small plasma density variations on the propagation and reflection of ELF (extremely-low-frequency) electromagnetic wave incident on the morning ionosphere from above. Modifications of the electromagnetic wave field near the Earth surface and coefficient of wave energy reflection from ionosphere caused by infrasonic wave are studied. The greatest modifications of the field on the Earth surface and reflection coefficient are related to the density perturbations at altitudes of strong electromagnetic wave attenuation (80–110 km). Reflection of electromagnetic waves is especially sensitive to the plasma density perturbations if the value of horizontal refractive index is close to a unit. In this case, modulation percentage of reflection coefficient can exceed the modulation percentage of density in the infrasonic wave by an order of magnitude. The parametric influence of plasma density perturbations in an infrasonic wave on the ELF electromagnetic field is analyzed. If the values of the horizontal wave numbers of the electromagnetic and infrasonic waves are close to each other, the field of the electromagnetic wave near the Earth surface can increase several times. The obtained results are important for understanding the interconnection of ionospheric wave processes of different nature. The study of the modulation of coefficient of ELF reflection from the ionosphere is relevant for explaining the operation modes of a plasma magnetospheric maser.

Influences of plasma density perturbations on ion cyclotron resonance heating

The scattering of waves in the ion cyclotron range by plasma density perturbations in the edge has been previously studied by Zhang et al with the help of antenna code RAPLICASOL (2020, Nucl. Fusion, 60, 096001). The further interesting question is whether the density perturbations have an effect on ion cyclotron resonance heating (ICRH) in the core. In this paper, finite element method based 2D full wave code integrating the core with the edge is used to study this issue. The analytical density perturbations are applied to study the influence of density perturbations on field distribution, power deposition, wave coupling, power partition among different species in the core, and the fraction of energy dissipation in the scrape-off layer. The influence of density perturbations becomes global and significant when the poloidal size of the density blob is comparable to the perpendicular wavelength. In addition, the strength of wave scattering is directly proportional to the amplitude and the radial size of the density blob. Finally, a typical experiment on the EAST is chosen and the influence of realistic density perturbations on ICRH is evaluated.

Coronal seismology by slow waves in non-adiabatic conditions

Slow magnetoacoustic waves represent an important tool for probing the solar coronal plasma. The majority of seismological methods with slow waves are based on a weakly non-adiabatic approach, which assumes the coronal energy transport has only weak effects on the wave dynamics. Despite it significantly simplifies the application of coronal seismology by slow waves, this assumption omits a number of important and confidently observed effects and thus puts strong limitations on the reliability of seismological estimations. We quantitatively assess the applicability of the weak thermal conduction theory to coronal seismology by slow waves. We numerically model the linear standing slow wave in a 1D coronal loop, with field-aligned thermal conduction κ‖ as a free parameter and no restrictions on its efficiency. The time variations of the perturbed plasma parameters, obtained numerically with full conductivity, are treated as potential observables and analysed with the standard data processing techniques. The slow wave oscillation period is found to increase with κ‖ by about 30%, indicating the corresponding modification in the effective wave speed, which is missing from the weak conduction theory. Phase shifts between plasma temperature and density perturbations are found to be well consistent with the approximate weakly conductive solution for all considered values of κ‖. In contrast, the comparison of the numerically obtained ratio of temperature and density perturbation amplitudes with the weak theory revealed relative errors up to 30–40%. We use these parameters to measure the effective adiabatic index of the coronal plasma directly as the ratio of the effective slow wave speed to the standard sound speed and in the polytropic assumption, which is found to be justified in a weakly conductive regime only, with relative errors up to 14% otherwise. The damping of the initial perturbation is found to be of a non-exponential form during the first cycle of oscillation, which could be considered as an indirect signature of entropy waves in the corona, also not described by weak conduction theory. The performed analysis and obtained results offer a more robust scheme of coronal seismology by slow waves, with reasonable simplifications and without the loss of accuracy.

Haiti Earthquake (Mw 7.2): Magnetospheric–Ionospheric–Lithospheric Coupling during and after the Main Shock on 14 August 2021

In the last few decades, the efforts of the scientific community to search earthquake signatures in the atmospheric, ionospheric and magnetospheric media have grown rapidly. The increasing amount of good quality data from both ground stations and satellites has allowed for the detections of anomalies with high statistical significance such as ionospheric plasma density perturbations and/or atmospheric temperature and pressure changes. However, the identification of a causal link between the observed anomalies and their possible seismic trigger has so far been prevented by difficulties in the identification of confounders (such as solar and atmospheric activity) and the lack of a global analytical lithospheric–atmospheric–magnetospheric model able to explain (and possibly forecast) any anomalous signal. In order to overcome these problems, we have performed a multi-instrument analysis of a low-latitude seismic event by using high-quality data from both ground bases and satellites and preserving their statistical significance. An earthquake (Mw = 7.2) occurred in the Caribbean region on 14 August 2021 under both solar quiet and fair weather conditions, thus proving an optimal case study to reconstruct the link between the lithosphere, atmosphere, ionosphere, and magnetosphere. The good match between the observations and novel magnetospheric–ionospheric–lithospheric coupling (M.I.L.C.) modeling of the event confirmed that the fault break generated an atmospheric gravity wave that was able to mechanically perturb the ionospheric plasma density, in turn triggering a variation in the magnetospheric field line resonance frequency.

Can Radioactive Emanations in a Seismically Active Region Affect Atmospheric Electricity and the Ionosphere?

—Anomalous variations in radon emissions from the soil are sometimes observed before and after strong earthquakes. In this paper, we theoretically analyze the known hypothesis that these variations in radioactive emanations in the surface layer of the atmosphere cause variations in the vertical background current in the atmosphere with subsequent changes in the electron density {{n}_{e}} in the lower ionosphere. For this purpose, we will first assess the relation between radon emission variations and the vertical atmospheric current flowing into the ionosphere. Then we will solve a model problem concerning the spatial distribution of plasma density and electric field perturbations occurring in the E-layer of the ionosphere caused by an inflowing vertical current. It was believed that the current flowing into this layer contains only an electron component and is attenuated only by the recombination of electrons. The estimate of the maximum variation {{n}_{e}} is at least 3–5 orders of magnitude smaller than the observed anomalous variations, which allows us to conclude that this hypothesis is not plausible.

Stimulation of Waves and Instabilities in Response to Irradiation of Electromagnetic Fields into Plasmas

Plasma is a unique phase of matter constituting positively or negatively charged atoms, excited atoms, neutral atoms, electrons, radicals, etcetera displaying a unique role in the nuclear fusion research besides studying electrical discharges in the domain of switching devices and biomedical applications lately. We discuss in this extensive proposition the fundamental plasma characteristics such as Debye length, plasma oscillations, plasma sheath and condition for sustainability and confinement of plasmas, besides examining the elementary waves in plasmas namely zero waves, electron plasma wave and ion plasma wave. The inherent electron plasma wave and ion plasma wave is associated with the driving of plasma currents which in turn depends upon the density perturbation and thermal velocities of the electrons and ions. The application of external electromagnetic radiation such as laser (pump wave) into the plasma modifies the dispersion relations of electron and ion plasma wave, respectively. The laser stimulates a plethora of waves in the plasma and undergoes remarkable physical phenomena such as self-focusing and filamentation of laser beams. The excitation of sideband waves of the laser beams into the plasma plays a key role in imparting ponderomotive force on the electron plasma waves leading to turbulence in the plasmas due to coupling of the waves. The oscillatory velocity of the electron due to pump wave, plasma density perturbation, ponderomotive force and current densities are associated with the excitation of instabilities in the plasma. Conclusively, such waves and instabilities in unmagnetized and magnetized plasma is comprehensively studied and concluded by proposing the investigation of unexplored twisted electromagnetic wave-plasma interaction.

Neutral Air Turbulence in the Mesosphere and Associated Polar Mesospheric Summer Echoes (PMSEs)

The first true common volume observations of the PMSE source region with 4 radars are presented in this paper. Radar frequencies of 8, 56, 224, and 930 MHz are used in this study. Three days of experimental observations at EISCAT are presented. Numerical simulations of mesospheric dusty/ice plasma associated with the observed radar frequencies are presented. The effect of neutral air turbulence on the generation and strength of plasma density perturbations associated with PMSE using four radar frequencies and in the presence of various dust parameters is investigated. Using the model it is shown that the well-known neutral air turbulence in the presence of heavy dust particles and neutral air turbulence combined with dust density (dusty turbulence) can largely explain the observed radar cross-section at four radar frequencies. The effect of neutral air turbulence amplitude along with dust charging and diffusion in the presence of various dust parameters is investigated using the computational model. Specifically, the response of diffusion to charging time scales, plasma density fluctuation amplitude and background dusty plasma parameters are discussed. Several key parameters in the dusty plasma responsible for the PMSE observations are determined. Qualitative comparison of radar echo strength at 4 frequencies with numerical results is provided. Unlike the previous studies that required large dust particles of 20 nm for PMSE formation, the present work demonstrates the possibility of small dust particles to explain the experimental observations.

Modeling of Kilometer-Scale Ionospheric Irregularities at Mars

Recently, kilometer-scale Martian ionospheric irregularities have been measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission (Fowler et al., 2020). In this study, we carried out a simulation of these irregularities, assuming a uniform Martian zonal neutral wind and a cosinusoidal perturbation of the plasma density as the seeding source. Results show that a vertical electric field shear could be induced by such a plasma density perturbation. We find that the vertical electric field shear causes a velocity shear of the plasma between the topside and bottomside ionosphere, which in turn is able to produce kilometer-scale ionospheric irregularities — irregularities of a smaller scale than were seen in previous simulations (Jiang CH et al., 2021). These kilometer-scale variations with altitude, in plasma density and magnetic field profiles, are comparable to the MAVEN observations.

Q2DW-Tide and -Ionosphere Interactions as Observed From ICON and Ground-Based Radars.

A quasi-2-day wave (Q2DW) event during January-February, 2020, is investigated in terms of its propagation from 96 to 250 km as a function of latitude (10°S to 30°N), its nonlinear interactions with migrating tides to produce 16 and 9.6-h secondary waves (SWs), and the plasma drift and density perturbations that it produces in the topside F-region (590-607 km) between magnetic latitudes 18°S and 18°N. This is accomplished through analysis of coincident Ionospheric Connections Explorer (ICON) measurements of neutral winds, plasma drifts and ion densities, and wind measurements from four low-latitude (±15°) specular meteor radars (SMRs). The Q2DW westward-propagating components that existed during this period consist of zonal wavenumbers s = 2 and s = 3, that is, Q2DW+2 and Q2DW+3 (e.g., He, Chau et al., 2021, https://doi.org/10.1029/93jd00380). SWs in the ICON measurements are inferred from Q2DW+2 and Q2DW+3 characteristics derived from traditional longitude-UT fits that potentially contain aliasing contributions from SWs ("apparent" Q2DWs), from fits to space-based zonal wavenumbers that each reflect the aggregate signature of either Q2DW+2 or Q2DW+3 and its SWs combined ("effective" Q2DWs), and based on information contained in published numerical simulations. The total Q2DW ionospheric responses consists of F-region field-aligned and meridional drifts of order ±25 ms-1 and ±5-7 ms-1, respectively, and total ion density perturbations of order (±10%-25%). It is shown that the SWs can sometimes make substantial contributions to the Q2DW winds, drifts, and plasma densities.

Ionospheric perturbation\xa0during\xa0the South American total solar eclipse on 14th December 2020 revealed with the Chilean GPS eyeball

The influence of the South American total solar eclipse of 14th December 2020 on the ionosphere is studied by using the continuous Chilean Global Positioning System (GPS) sites across the totality path. The totality path with eclipse magnitude 1.012 passed through the Villarrica (Lon. 72.2308°W and Lat. 39.2820°S) in south Chile during 14:41:02.0 UTC to 17:30:58.1 UTC and maximum occurred ~ 16:03:49.5 UTC around the local noon. The vertical total electron content (VTEC) derived by GPS sites across the totality path for two PRN’s 29 and 31 show almost 20–40% of reduction with reference to ambient values. The percentage reduction was maximum close to totality site and decreases smoothly on both sides of totality sites. Interestingly, the atmospheric gravity waves (AGWs) with a period ~ 30–60 min obtained using wavelet analysis of VTEC timeseries show the presence of strong AGWs at the GPS sites located north of the totality line. But the AGWs do not show any significant effect on the VTEC values to these sites. Our analysis suggests, possibly an interplay between variability in the background plasma density and eclipse-generated AGWs induced plasma density perturbation could explain the observations.

Measuring the self-generated magnetic field and the velocity of plasma flow in a pulsed plasma accelerator

In phase of radial compression the unsteady plasma flow at the exit of a pulsed plasma accelerators is related to the generation of fast high-energy charged particle beams, to plasma density perturbations, to the internal plasma pressure is opposite to the external magnetic pressure (B2/2μ0) associated with plasma column’s self-generated magnetic field. The last one listed above is of particular interest. The investigation of the magnetic field of plasma flow relatively gives information about the above mutually close factors cause to plasma flow instability.In this paper a miniature high-frequency magnetic probe are made for measuring and further investigating the pulsed self-generated magnetic field of plasma flow, in particular, at the exit of the accelerator in an experimental setup of PPA (located in KazNU, IETP). The magnetic probe dimensions are following: coil diameter is 2,12 mm, number of turns is 7, copper wire diameter is 150 μm, and winding length is 1, 1 mm. The coil inductance is about 0,113 μH and the temporal resolution of the magnetic probe is 2,3 ns. The magnetic probe was calibrated using a Hall sensor on the magnetic field of a multi-layered solenoid connected to an alternating power source of 15 A. Thus, the evaluated measurement error of the probe was no more than 10%.

Comparison of seasonal and longitudinal variation of daytime MSTID activity using GPS observation and GAIA simulations

The total electron content (TEC) data derived from the GAIA (Ground-to-topside model of Atmosphere Ionosphere for Aeronomy) is used to study the seasonal and longitudinal variation of occurrence of medium-scale traveling ionospheric disturbances (MSTIDs) during daytime (09:00–15:00 LT) for the year 2011 at eight locations in northern and southern hemispheres, and the results are compared with ground-based Global Positioning System (GPS)-TEC. To derive TEC variations caused by MSTIDs from the GAIA (GPS) data, we obtained detrended TEC by subtracting 2-h (1-h) running average from the TEC, and calculated standard deviation of the detrended TEC in 2 h (1 h). MSTID activity was defined as a ratio of the standard deviation to the averaged TEC. Both GAIA simulation and GPS observations data show that daytime MSTID activities in the northern and southern hemisphere (NH and SH) are higher in winter than in other seasons. From the GAIA simulation, the amplitude of the meridional wind variations, which could be representative of gravity waves (GWs), shows two peaks in winter and summer. The winter peak in the amplitude of the meridional wind variations coincides with the winter peak of the daytime MSTIDs, indicating that the high GW activity is responsible for the high MSTID activity. On the other hand, the MSTID activity does not increase in summer. This is because the GWs in the thermosphere propagate poleward in summer, and equatorward in winter, and the equatorward-propagating GWs cause large plasma density perturbations compared to the poleward-propagating GWs. Longitudinal variation of daytime MSTID activity in winter is seen in both hemispheres. The MSTID activity during winter in the NH is higher over Japan than USA, and the MSTID activity during winter in the SH is the highest in South America. In a nutshell, GAIA can successfully reproduce the seasonal and longitudinal variation of the daytime MSTIDs. This study confirms that GWs cause the daytime MSTIDs in GAIA and amplitude and propagation direction of the GWs control the noted seasonal variation. GW activities in the middle and lower atmosphere cause the longitudinal variation.

Terahertz radiation generation from short-pulse laser interaction with electron-hole plasmas

In semiconductors, electrons and their corresponding holes exist as free carriers that are created in pairs in inter-band transitions. At very high carrier densities, the carrier-carrier collisions dominate over carrier-lattice collisions and these carriers begin to behave collectively to form a plasma. In this study, we present phase-matched terahertz (THz) radiation generation from laser interaction with electron-hole plasmas. Two collinear laser pulses of finite spot size propagate in an electron-hole plasma to generate THz radiation. The lasers beat and exert a ponderomotive force on electrons and holes. The carriers oscillate with oscillatory velocity at the laser beat frequency. The laser beat wave interacts with the plasma density perturbation and produces a nonlinear transverse current with nonzero curl. This nonlinear current drives coherent THz radiation. Spacial density modulation of hole density is considered to satisfy the phase-matching condition for resonant interaction. The systematic analysis of this mechanism shows an efficient and tunable source of THz radiation, which varies with various parameters such as the density modulation, laser intensity and other laser parameters.

Statistical Study of Chorus Modulations by Background Magnetic Field and Plasma Density

AbstractIn this study, we use observations of THEMIS and Van Allen Probes to statistically study the modulations of chorus emissions by variations of background magnetic field and plasma density in the ultra low frequency range. The modulation events are identified automatically and divided into three types according to whether the chorus intensity correlates to the variations of the magnetic field only (Type B), plasma density only (Type N), or both (Type NB). For the THEMIS observations, the occurrences of the Types B and N are larger than Type NB, while for the Van Allen Probes observations, most events are of Type N. The chorus intensity is mostly correlated to the magnetic field strength negatively and plasma density positively. The chorus intensity tends to increase when the magnitude of the magnetic field perturbation increases, but little dependence on plasma density perturbation amplitude is found.

Ionospheric monitoring with the Chilean GPS eyeball during the South American total solar eclipse on 2nd July 2019

The impact of total solar eclipse of July 2, 2019 on the Ionosphere is studied using 24 Chilean GPS stations north–south of the totality path. The total solar eclipse passed through Coquimbo region from ~ 16:38 CLT (~ 20:38 UTC) to ~ 16:40 CLT (~ 20:40 UTC) and maximum eclipse was observed ~ 16:39 CLT (~ 20:39 UTC). The total electron content (TEC) derived from GPS signals shows peculiar features. At the totality stations TEC variations are small (~ 0.39 TECu), but it shows significant decrease (maximum ~ 2.24 TECu) for stations located south and increase (maximum ~ 3.89 TECu) for the stations located north of totality of the surface. The wavelet analysis of VTEC timeseries shows the presence of strong atmospheric gravity waves (AGWs) of duration ~ 30 to 60 min at the stations located north of totality. Thus, the results suggest an interplay between eclipse effect on the ionosphere plasma density and eclipse generated AGWs induced plasma density perturbation provided the peculiar features.

The lower ionosphere disturbances observed during the chain of the meteotsunamis in the Mediterranean Sea in June 2014

In June 2014, a number of meteotsunamis were detected in the Mediterranean and Black Sea area. These meteotsunamis were initiated by a unique high-altitude dynamical system which was initially originated above Spain and traveled across the Mediterranean Sea towards Black Sea and Turkey. Meteotsunamis unlike tsunamis driven by strong earthquakes are local events, and their formation has different mechanism. Atmospheric internal gravity waves (IGWs) are one of the main known sources of meteotsunamis (e.g. Vilibic et al. in Pure Appl Geophys 165:2169–2195, 2008). The synoptic system produced short-lived and small-scale atmospheric pressure perturbations which drifted with the jet stream-like bubbles and generated tsunami-like waves in the open waters. The bubbles with typical dimensions 15–60 km continuously form and collapse in the atmosphere at altitudes of 3–6 km. Such a “boiled” atmosphere generated IGWs propagating both downward, where they produced meteotsunamis (presumably under Proudman resonance condition) and upward into the ionosphere, with following dissipation and excitation of plasma density perturbations. One of the few experimental techniques, which can monitor perturbations of the ionization within the lower ionosphere, uses long-wave probing by very low and low frequency (VLF/LF) radio signals. To study the ionospheric disturbances observed during the chain of meteotsunamis affecting the Mediterranean Sea, we used VLF/LF data collected in South Europe by “The International Network for Frontier Research on Earthquake Precursors”. By applying the spectral analysis method to the anomalous VLF/LF signals, it was found that revealed periods of the signal variations were from 10 to 40–70 min in different stations, which are in the range of the atmospheric pressure oscillations and the meteotsunami events. These periods also correspond to the periods of IGWs.

Estimation of plasma density perturbation from dusty plasma injection by laser irradiation on tungsten target in DiPS

To investigate the interaction of dusty plasma with magnetized plasmas at divertor plasma simulator, radial profiles of plasma density (ne) and electron temperature were measured in terms of plasma discharge currents and magnetic flux intensity by using a fast scanning probes system with triple tips. Dusty plasma with dusts (a generation rate of 3 μg s−1 and a size of 1–10 μm) was produced via interactions between a high-power laser beam and a full tungsten target. As ne increases, the scale of the effects of dusty plasma injection on magnetized plasmas was decreased. Also, the duration of transient fluctuation was reduced. For numerical estimation of plasma density perturbation due to dusty plasma injection, the result was ∼10% at a core region of the magnetized plasma with ne of (2–5) × 1011 cm−3 at steady state condition.

Spatial evolution of small wavelength fluctuations in a Hall Thruster

The spatial evolution of small wavelength (<1 cm) fluctuations in the Hall direction of a 9-kW class magnetically shielded Hall effect thruster is experimentally characterized. High-speed electrostatic probes are used to measure plasma density perturbations in the acceleration region and near field plume of this cross field device. Two types of waves are observed: broadband turbulence in the low hundreds of kilohertz and megahertz oscillations characterized by distinct peaks in the measured power spectrum. The lower frequency content is shown to be commensurate with linear ion acousticlike waves, while the higher frequency oscillations are indicative of cyclotron resonances. It is found that the higher frequency content dominates the power spectrum in the upstream acceleration zone and saturates at a fraction (<0.002) of the thermal energy. Downstream of this region, the lower frequency content becomes dominant, growing until the total wave energy again saturates. It is proposed that both the upstream and downstream modes are the same oscillation, the electron drift instability, but in different limits. The transition from upstream to downstream dispersion may be explained by the electron drift instability undergoing an inverse energy cascade as it forms and propagates with the ion drift in the discharge.

Generation of runaway electrons during the initial stage of the T-10 tokamak plasma discharge

The formation of runaway electron beams in the initial stage of discharge is one of the characteristic features of fusion facilities with magnetic plasma confinement, such as tokamaks. They are generated due to the presence of the high electric fields and strong plasma-wall interaction in plasma. The runaway electrons produced during the breakdown stage also remain in the plasma in the quasistationary stage of the discharge. Studies carried out at the T-10 tokamak show that the electron energy and the heat flux density can reach 5–10 MeV and 1–3 GW/m2, respectively. Thus, the interaction with electrons causes erosion and damage to the limiter surface. In this article, the energy distribution of high-energy electrons formed in the initial stage of plasma discharge in the T-10 tokamak during the formation of runaway electron beams in a strong longitudinal magnetic field was studied. The effect of the plasma density and MHD perturbations on the generation and acceleration of runaway electrons was revealed. The energy distributions of runaway electrons were estimated for different stages of the plasma discharge development.