Gradient Drift Waves Research Articles

Effect of Symmetry/Asymmetry of Shear Rotation of a Plasma Column in a Radial Electric Field on the Level of Turbulent Density Fluctuations

The influence of the properties of the profile of a radial static electric field E(r) on the evolution of an unstable ion temperature–gradient (ITG) drift wave in a nonuniformly rotating plasma column in a magnetic field is studied. The effect of symmetry on the decrease in the level of turbulent fluctuations, which are associated with the limiting state of the ITG wave during its destruction, is discussed. The level of turbulence is estimated in the framework of approximation of finite amplitudes depending on the electric field structure. It is shown that the maximum decrease in the level of fluctuations occurs with a symmetrical configuration of the radial electric field.

Study of high-frequency oscillations and waves in Hall thruster plasma using two-dimensional full kinetic axial-azimuthal model (2D 3V Full PIC)

The paper presents the results of Hall thruster plasma simulation using a 2D3V axial-azimuthal Full PIC model. Two types of solution are investigated in detail for two different magnitude of the magnetic field. In the calculations, different types of azimuthal waves were observed. By their characteristics these waves can be corresponded to the electron drift instabilities, gradient-drift waves and ion plasma resonance. The ion plasma resonance was observed in calculations with the magnetic field increase.

The mechanism of the onset of turbulent structures under the ion temperature gradient drift instability conditions

A possible mechanism of the appearance of turbulent structures in plasma is considered as a result of the development of unstable ion temperature gradient (ITG) drift waves, both with and without a sheared flow. A description of several stages of the drift wave evolution is proposed. The condition for limiting the growth of the wave amplitude is discussed. A qualitative agreement of the results with the theoretical and experimental results of other authors is shown. The motion of particles in the wave and vortex structures is studied on the basis of the proposed mechanism and ideas of other authors.

Interaction of counterstreaming rotating electron-positron beams with inhomogeneous electron-ion plasma

The interaction occurring between two counterstreaming rotating electron-positron beams and an inhomogeneous magnetized electron-ion plasma is studied with the focus of research on either positrons or electrons propagating in the direction of the magnetic field. Using the Vlasov theory along with geometrical optics, the linear eikonal equation corresponds to the gradient drift wave which is extracted in the background plasma, taking into account the beam contribution. The results reveal that the gradient drift instability is experienced where the gradients of density and temperature of electrons stand in the opposite directions, and in addition, the gradients act as destabilization effects. Regarding the beam contribution, when the electron beams propagate in the direction of the magnetic field, the parallel and perpendicular components of velocity and the Langmuir frequency of the rotating beams can induce stabilization effects on the unstable inhomogeneous configuration. However, as a considerable achievement, the mentioned stabilization effects vanish for the perpendicular velocity component lower than a certain threshold value. In addition, the destabilization effects of the characteristic parameters of the counterstreaming beams are observed as well, when the positron beams propagate in the direction of the magnetic field.

Gradient-drift instability applied to Hall thrusters

The stability of gradient-drift waves in a Hall-type plasma thruster is investigated within the framework of two-fluid ideal magnetohydrodynamics. The analysis is based on the dispersion relation, which includes the effects of equilibrium electron current, finite ion flow velocity, electron inertia, electron temperature, magnetic field and plasma density gradients, and also the Debye length effects. The features of unstable modes are calculated along the thruster channel. Three spatially separated areas of instability are revealed: (i) the near-anode region with long-wavelength azimuthal oscillations, (ii) the main part of the acceleration channel with short-wavelength axial modes destabilized by macroscopic ion flow, and (iii) the plume region characterized by short-wavelength oblique waves.

Emergence of plasma turbulent structures in the presence of ITG instability

A possible mechanism of turbulent structure formation is considered. Specifically, a transition from unstable ion temperature gradient (ITG) drift wave to a turbulent state of plasma. A theoretical model is developed. Increase and decay of the drift wave are taken into account. A condition of wave decay is proposed. Direct relationship between dimension of turbulent structures, fluctuations, plasma and drift wave parameters are established for the cases of existence and absence of velocity shear. Approximation of plasma density fluctuations is obtained. The approximation satisfactorily corresponds with the experimental data and theoretical estimates of other authors. Experimental results of beam emission spectroscopy (BES) diagnostic are discussed. It is proposed that velocity shear prevents but does not break down large turbulent structures at the H-mode regime.

A comparison of the relative effect of the Earth's quasi‐DC and AC electric field on gradient drift waves in large‐scale plasma structures in the polar regions

AbstractRadio signals traversing polar cap plasma patches and other large‐scale plasma structures in polar regions are prone to scintillation. This implies that irregularities in electron concentration often form within such structures. The current standard theory of the formation of such irregularities is that the primary gradient drift instability drives a cascade from larger to smaller wavelengths that manifest as variations in electron concentration. The electric field can be described as the sum of a quasi‐DC and an AC component. While the effect of the quasi‐DC component has been extensively investigated in theory and by modeling, the contribution of the AC component has been largely neglected. This paper investigates the relative contributions of both components, using data from the Dynamics Explorer 2 satellite. It concludes that the contribution of the AC electric field to irregularity growth cannot be neglected. This has consequences for our understanding of large‐scale plasma structures in polar regions (and any associated radio scintillation) as the AC electric field component varies in all directions. Hence, its effect is not limited to the trailing edge of such structures, as it is for the quasi‐DC component. This raises the need for new experimental and modeling investigations of these phenomena.

Direct Observation of Energy Transfer between Electron Temperature Gradient Mode and Drift Wave Mode in Transient Experiment

Direct Observation of Energy Transfer between Electron Temperature Gradient Mode and Drift Wave Mode in Transient Experiment

On the altitude of initiation of the gradient drift waves at different longitude sectors in the vicinity of the dip equator

In order to understand the variation in the altitudes of initiation (haoi) of E region gradient drift (GD) waves at different longitude sectors in the vicinity of the dip equator, the linear growth rate expression was examined. This revealed that the growth rate of the primary GD waves in an altitude region of 85–93km depends on the square of the geomagnetic field strength (B). This is shown to explain the lower haoi of GD waves over Indian longitude vis-a-vis American longitudes. The available observations on the GD waves from different longitude sectors reveal that the haoi is inversely proportional to B2.

Coordinated radar observations of plasma wave characteristics in the auroral F region

Abstract. Properties of decameter-scale plasma waves in the auroral F region are investigated using coordinated observations of plasma wave characteristics with the Kodiak HF coherent radar (KOD) and Poker Flat Incoherent Scatter Radar (PFISR) systems in the Alaskan sector. We analyze one event on 14 November 2012 that occurred during the first PFISR Ion-Neutral Observations in the Thermosphere (PINOT) campaign when exceptionally good F region backscatter data at 1 s resolution were collected by KOD over the wide range of locations also monitored by PFISR. In particular, both radar systems were observing continuously along the same magnetic meridian, which allowed for a detailed comparison between the line-of-sight (l-o-s) velocity data sets. It is shown that l-o-s velocity correlation for data points strictly matched in time (within 1 s) depends strongly on the number of ionospheric echoes detected by KOD in a given post-integration interval or, equivalently, on the KOD echo occurrence in that interval. The l-o-s velocity correlations reach 0.7–0.9 for echo occurrences exceeding 70%, while also showing considerable correlations of 0.5–0.6 for occurrences as low as 10%. Using the same approach of strictly matching the KOD and PFISR data points, factors controlling coherent echo power are investigated, focusing on the electric field and electron density dependencies. It is demonstrated that the signal-to-noise ratio (SNR) of F region echoes increases nearly monotonically with an increasing electric field strength as well as with an increasing electron density, except at large density values, where SNR drops significantly. The electric field control can be understood in terms of the growth rate of the gradient-drift waves being proportional to the convection drift speed under conditions of fast-changing convection flows, while the density effect may involve over-refraction at large density values and radar backscatter power proportionality to the perturbation density.

Characteristics of E-region background ionosphere and plasma waves measured over the dip equator during total solar eclipse campaign

A unique set of rocket flight measurements of plasma parameters from the dip equator (Thumba; 8.5°N, 76.5°E, dip 0.5°S) was carried out at two obscuration levels (40% and 72%) on a total solar eclipse day (16 February 1980) which was also under a moderately disturbed geomagnetic condition. The path of totality was 400km north of Thumba. Another rocket flight was conducted on 17 February 1980 to obtain the control day measurement at the same local time. As expected, the electron densities are found to be less throughout the measured altitude region on a solar eclipse day with the 72% obscuration level compared to the control day at the same local time (same solar zenith angle). In the presence of average electron density scale length of 10 and 9km in the altitude region of 88–100km, the initiation of the gradient drift waves is observed at altitudes of 91 and 93km during 40% and 72% obscuration levels respectively on a solar eclipse day. However, on a control day, in the presence of average electron density scale length of 9km, these waves are found at altitude as low as 88km. In addition, the amplitude of the gradient drift waves is found to be the largest during the 72% obscuration level compared to those during the 40% obscuration level and control day. In the absence of electric field measurements, the magnetometer observations are used to infer an increase in the polarization electric field when the obscuration level is around 72%. This along with steeper gradient can account for the increase in the amplitude of gradient drift waves during 72% obscuration compared to 40% obscuration. The relative role of the growth and decay of the gradient drift waves is discussed in the context of these observations.

Sheared-flow induced confinement transition in a linear magnetized plasma

A magnetized plasma cylinder (12 cm in diameter) is induced by an annular shape obstacle at the Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)]. Sheared azimuthal flow is driven at the edge of the plasma cylinder through edge biasing. Strong fluctuations of density and potential ({delta}n/n{approx}e{delta}{phi}/kT{sub e}{approx}0.5) are observed at the plasma edge, accompanied by a large density gradient (L{sub n}={nabla}lnn{sup -1}{approx}2cm) and shearing rate ({gamma}{approx}300kHz). Edge turbulence and cross-field transport are modified by changing the bias voltage (V{sub bias}) on the obstacle and the axial magnetic field (B{sub z}) strength. In cases with low V{sub bias} and large B{sub z}, improved plasma confinement is observed, along with steeper edge density gradients. The radially sheared flow induced by ExB drift dramatically changes the cross-phase between density and potential fluctuations, which causes the wave-induced particle flux to reverse its direction across the shear layer. In cases with higher bias voltage or smaller B{sub z}, large radial transport and rapid depletion of the central plasma density are observed. Two-dimensional cross-correlation measurement shows that a mode with azimuthal mode number m=1 and large radial correlation length dominates themore » outward transport in these cases. Linear analysis based on a two-fluid Braginskii model suggests that the fluctuations are driven by both density gradient (drift wave like) and flow shear (Kelvin-Helmholtz like) at the plasma edge.« less

Gradient drift instability as a source of spread F in the region of large-scale irregularities in the low-latitude topside ionosphere

The expression for the increment of instability and decrement of diffusion damping of gradient drift waves for ionospheric altitudes above the F 2 layer maximum is obtained. The gradient drift instability is used to interpret the observations of spread F in the region of large-scale horizontal irregularities of the electron density. Two types of such irregularities observed on board the Intercosmos-19 (IC-19) satellite in the region of low latitudes (a peak of the density in the dusk ionosphere and a trough of the density in the dawn ionosphere) are considered. It is shown that the observed gradients of the density and electric field values in the dawn and dusk ionospheric sectors are quite sufficient for the instability development criterion to be satisfied in both considered cases.

Correlation between scintillation indices and gradient drift wave amplitudes in the northern polar ionosphere

A model is developed of the gradient drift instability growth rate in the north polar cap ionosphere, utilizing a novel approach employing an ionospheric imaging algorithm. The growth rate values calculated by this model are in turn used to estimate how the amplitudes of actual gradient drift waves vary over time as the plasma drifts and the growth rates change with time. Ionospheric imaging is again used in order to determine plasma drift velocities. The final output from the model is in turn used to assess the linear correlation between the scintillation indices S4 and σ recorded by several GPS L1 band scintillation receivers stationed in the north polar cap and mean gradient drift wave amplitudes. Four separate magnetic storm periods, totaling 13 days, are analyzed in this way. The results show weak but significant linear correlations between the mean wave amplitudes calculated and the observed scintillation indices at F layer altitudes.

East-west and vertical spectral asymmetry associated with equatorial type I waves during strong electrojet conditions: 2. Theory

[1] In order to explain vertical and east-west type I asymmetries uncovered in a companion paper and in references therein, we introduce the notion that the generating large-scale gradient drift waves break down first in elongated isolated enhancement and depletion regions (blobs and holes) that follow an asymmetric nonlinear evolution. The nonlinear evolution is accompanied by a rotation of the electric field inside the structures, be they blobs or holes. It is shown here that holes have a tendency to rotate more than blobs but that as a result the electric field inside holes is also weaker. These two features (contrasting rotations and contrasting field strengths) explain the predominant sign of the up-down and east-west asymmetries during weakly to moderately driven conditions. They also explain how the asymmetry is observed to disappear and even to reverse its sign under more strongly driven conditions.

Modelling high-power large-aperture radar meteor trails

Despite decades of research, many questions remain about the global flux of meteoroids at Earth, their influence on the atmosphere, and their use as upper atmospheric diagnostics. We see high-power large-aperture (HPLA) radar observations of meteor phenomena called head echoes and non-specular trails as a valuable tool for answering these questions. In the past we conducted plasma simulations demonstrating that meteor trails are unstable to growth of Farley–Buneman gradient-drift (FBGD) waves that become turbulent and generate large B-field aligned irregularities (FAI). These FAI result in reflections called non-specular meteor trails. Using these and other results, we have developed a model that follows meteor evolution from ablation and ionization through the creation of radar head echoes and non-specular trail reflections. This paper presents results from this model, showing that we can reproduce many aspects of these large radar observations, such as the general altitude profile of head echoes and non-specular trails. Additionally we show that trail polarization due to E-fields or neutral winds causes a noticeable trail feature as well as may be responsible for trails lasting longer than about 1 s. We also demonstrate how such a model is a valuable tool for deriving meteoroid properties such as flux, mass, and velocity. Finally, such a model could also provide some composition information, and diagnose the atmosphere and ionosphere where meteors produce their trails.

E-region echo characteristics governed by auroral arc electrodynamics

Abstract. Observations of a pair of auroral arc features by two imagers, one ground- and one space-based, allows the associated field-aligned current (FAC) and electric field structure to be inferred. Simultaneous observations of HF radar echoes provide an insight into the irregularity-generating mechanisms. This is especially interesting for the E-region echoes observed, which form the focus of our analysis, and from which several conclusions can be drawn, summarized as follows. Latitudinal variations in echo characteristics are governed by the FAC and electric field background. Particularly sharp boundaries are found at the edges of auroral arcs. Within regions of auroral luminosity, echoes have Doppler shifts below the ion-acoustic speed and are proportional to the electric field, suggesting scatter from gradient drift waves. Regions of downward FAC are associated with mixed high and low Doppler shift echoes. The high Doppler shift component is greatly in excess of the ion-acoustic speed, but seems to be commensurate with the driving electric field. The low Doppler shift component appears to be much depressed below expectations.Key words. Ionosphere (ionospheric irregularities; electric fields and currents)

New insights from a nonlocal generalization of the Farley-Buneman instability problem at high latitudes

Abstract. When their growth rate becomes too small, the E-region Farley-Buneman and gradient-drift instabilities switch from absolute to convective. The neutral density gradient is what gives the instabilities their convective character. At high latitudes, the orientation of the neutral density gradient is close to the geomagnetic field direction. We show that this causes the wave-vector component along the geomagnetic field to increase with time. This in turn leads to wave stabilization, since the increase goes hand-in-hand with an increase in parallel electric fields that ultimately short-circuits the irregularities. We show that from an equivalent point of view, the increase in the parallel wave vector is accompanied by a large upward group velocity that limits the time during which the perturbations are allowed to grow before escaping the unstable region. The goal of the present work is to develop a systematic formalism to account for the propagation and the growth/decay of high-latitude Farley-Buneman and gradient-drift waves through vertical convective effects. We note that our new formalism shies away from a plane wave decomposition along the magnetic field direction. A study of the solution to the resulting nonlinear aspect angle equation shows that, for a host of initial conditions, jump conditions are often triggered in the parallel wave-vector (defined here as the vertical derivative of the phase). When these jump conditions occur, the waves turn into strongly damped ion-acoustic modes, and their evolution is quickly terminated. We have limited this first study to Farley-Buneman modes and to a flow direction parallel to the electron E × B drift. Our initial findings indicate that, irrespective of whether or not a jump in aspect angle is triggered by initial conditions, the largest amplitude modes are usually near the ion-acoustic speed of the medium (although Doppler shifted by the ion motion), unless the growth rates are small, in which case the waves tend to move at the same drift as the ambient electrons.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; plasma waves and instabilities)

Stability properties of high‐pressure geotail flux tubes

Kinetic theory is used to investigate the stability of ballooning interchange modes in the high‐pressure geotail plasma. A variational form of the stability problem is used to compare new kinetic stability results with MHD, fast MHD, and Kruskal and Oberman [1958] stability results. Two types of drift modes are analyzed: A kinetic ion pressure gradient drift wave with a frequency given by the ion diamagnetic drift frequency ω*pi and a very low frequency mode |ω| ≪ ω*pi, ωDi that is often called a convective cell or the trapped particle mode. In the high‐pressure geotail plasma a general procedure for solving the stability problem in a 1/β expansion for the minimizing δB‖ is carried out to derive an integral differential equation for the kinetically valid displacement field ξψ for a flux tube. The plasma energy released by these modes is estimated in the nonlinear state. The role of these instabilities in substorm dynamics is assessed in the substorm scenarios described by Maynard et al. [1996].

Ionospheric electric field estimates from radar observations of the equatorial electrojet

Estimates of the zonal ionospheric electric field have been made on the basis of JULIA radar observations of the equatorial electrojet at Jicamarca. Two observing techniques were used and their merits compared. One technique (here referred to as the “oblique” technique) made use of a new, small antenna array with a broad beam in the equatorial plane. Electric fields were estimated from the Doppler shifts of type II radar echoes at low elevation angles as a function of range in the manner of Balsley [1969a]. For the other technique the main Jicamarca antenna array was used, and interferometry was employed to measure altitude profiles of the phase speeds of intermediate‐scale primary gradient drift waves in the electrojet directly overhead. Electric fields were inferred from these profiles following a modeling approach similar to the one described by Murthy and Ravindran [1994], Zonal ionospheric electric field measurements comparable to seasonal average incoherent scatter radar measurements were obtained using both methods. However, the interferometric observations reveal the presence of intense waves and jets in the zonal neutral wind similar to what has been seen in recent chemical release sounding rocket experiments. Zonal winds mainly control the zonal phase speeds of the primary gradient drift waves in the late afternoon and evening. Although the prereversal enhancement of the zonal electric field is evident in data from the oblique technique, there is no evidence of it in measurements derived from the interferometric technique.