Electron Content Gradients Research Articles

A semi-analytical and semi-numerical method for solving plasma instability of nonuniform two-dimensional electron gas

The plasma instability of two-dimensional electron gas (2DEG) is a crucial physical mechanism for generating terahertz radiation in field-effect transistors, especially in high electron mobility transistors (HEMTs). In this paper, we have proposed a new semi-analytical and semi-numerical method to deal with oscillation problems of any nonuniform 2DEG plasma, especially considering the steady-state distribution, which can be calculated and analyzed more quickly than only using numerical calculation. By constructing a wave equation, using the auxiliary function and Wentzel–Kramers–Brillouin approximation method, the wave vector of the plasma wave is obtained. On this basis, combined with the Dyakonov–Shur instability's boundary conditions, the oscillation frequency, the wave amplitude increment, and their correction caused by the nonuniformity can be obtained by numerical calculation. Furthermore, the analytical solution is obtained under reasonable approximate conditions for the linear distribution of electron concentration. It is proved that the electron concentration gradient in the channel will not only attenuate the wave increment but also decrease the plasmonic frequency in the case of linear distribution. Moreover, we get the reasons for the above conclusions through theoretical derivation. We also investigate the effects of various device parameters on attenuation, such as gate length, electron mobility, and voltage, which may explain the difference between the actual and theoretical values in HEMTs and provide new guidance for device design.

Wave Processes in Dusty Plasma near the Mercury’s Surface

Wave processes in dusty plasma near the surface of Mercury are discussed. The near-surface layers of Mercury’s exosphere have a number of common features with those of the exosphere of the Moon, e.g., there are dust particles above the illuminated side of both cosmic bodies that become positively charged due to the photoelectric effect. Mercury has its own magnetosphere that protects the surface from particles of the solar wind. However, the solar wind can reach the surface of the planet near the magnetic poles. Therefore, dust particles of the same size get different charges depending on their localization above the Mercury’s surface. A drift wave turbulence can appear in dusty plasma in the magnetic field near the Mercury’s surface in the presence of gradient of electron concentration. The solar wind that streams at speeds of about 400 km/s relative to plasma near the surface of the planet can induce longitudinal electrostatic oscillations with frequencies determined by the electron plasma frequency. We analyze wave processes taking into account the difference in parameters at aphelion and perihelion of the Mercury’s orbit, along with the fact whether the dust particles are located near the magnetic poles or far from them.

Variability of Vertical Total Electron Content Gradients in the Brazilian Sector

Ground Based Augmentation Systems are designed to meet demanding requirements during airport approach and landing sections of aircraft routes. It computes corrected and smoothed differential corrections and other information. These messages are transmitted to aircraft, which may use the corrections to improve their position estimation. However, residual correction errors remain, due to ionospheric spatial and temporal gradients between Ground Based Augmentation System reference receivers and approaching aircrafts. Large vertical ionospheric delay gradients detected during combinations of extreme geophysical environments indicate that there are problematic conditions for Ground Based Augmentation System operation over the Brazilian region. Motivated by the above conditions, data from Rede Brasileira de Monitoramento Contínuo, the Brazilian public network of dual-frequency Global Navigation Satellite System receivers operated by Instituto Brasileiro de Geografia e Estatística were analyzed to estimate ionospheric vertical Total Electron Content gradients. The effects from different locations (represented by the dip latitude) and geophysical conditions (represented by solar activity, season, and local time) on variations of ionospheric vertical Total Electron Content gradients have been statistically quantified. The results from the present work provide information to more detailed performance assessments of Ground Based Augmentation System procedures under equatorial and low-latitude ionospheric regions, aimed at flexibly delimiting the conditions for feasible operation in such regions.

Cathodoluminescence mapping of electron concentration in MBE-grown GaAs:Te nanowires

Cathodoluminescence mapping is used as a contactless method to probe the electron concentration gradient of Te-doped GaAs nanowires. The room temperature and low temperature (10 K) cathodoluminescence analysis method previously developed for GaAs:Si is first validated on five GaAs:Te thin film samples, before extending it to the two GaAs:Te NW samples. We evidence an electron concentration gradient ranging from below 1 × 1018 cm−3 to 3.3 ×1018 cm−3 along the axis of a GaAs:Te nanowire grown at 640 °C, and a homogeneous electron concentration of around 6–8 × 1017 cm−3 along the axis of a GaAs:Te nanowire grown at 620 °C. The differences in the electron concentration levels and gradients between the two nanowires is attributed to different Te incorporation efficiencies by vapor–solid and vapor–liquid–solid processes.

A New Way of Estimating the Spatial and Temporal Components of the Vertical Total Electron Content Gradient Based on UPC‐IonSAT Global Ionosphere Maps

AbstractThe determination of the ionospheric perturbation degree is essential to describe the ionosphere state for space weather monitoring. A new method for estimating the spatial and temporal components of the Vertical Total Electron Content (VTEC) gradient is introduced. The new method is based on VTEC estimated at each grid point of Global Ionosphere Map (GIM) by the UPC‐IonSAT research group of the Universitat Politècnica de Catalunya. Depending on the requirement, the VTEC spatial gradients can be derived at selected regions or grid points of the GIM on a global scale. According to the comparison with previous studies over the Europe region during quiet ionosphere state and two severe geomagnetic storms, the new method has proven to be reliable and has a great potential for the monitoring of ionospheric perturbation degree on a global scale. In addition, the associated warning of disturbed ionosphere might be available in the context of the on‐going development of real‐time GIMs.

The Deployable Low‐Band Ionosphere and Transient Experiment

AbstractThe Deployable Low‐Band Ionosphere and Transient Experiment (DLITE) is a four‐element interferometric radio telescope made from mostly commercial off‐the‐shelf parts to minimize costs and maximize ease of deployment. It operates in the high frequency and very high frequency (VHF) regimes, nominally in a 30–40 MHz band, but with good sensitivity (sky‐noise dominated) in the 20–80 MHz range. Its configuration is optimized to probe ionospheric structure using the so‐called “A‐Team,” exceptionally bright sources of cosmic radio emission. Methods have been developed to track the apparent positions and intensities of A‐Team sources without the need for beam forming to enable measurements of VHF scintillations as well as total electron content gradients. Time difference of arrival and frequency difference of arrival methods have been adapted for all‐sky imaging to facilitate both statistical measurements of scintillation levels and time domain astronomy. This study provides a detailed description of the system design, the analysis algorithms, and the science that can be conducted using results from two prototype DLITE systems in Maryland and New Mexico.

Highly stable Na metal anode enabled by a multifunctional hard carbon skeleton

Sodium (Na) metal and hard carbon are regarded as promising and competitive alternatives as anode materials for sodium-ion batteries (SIBs). Constructing a Na metal/hard carbon composite anode to exert a synergistic effect on the electrochemical performance of SIBs is highly desirable but rarely reported. In this work, we report a Na metal/N-functionalized hard carbon (NHC) composite as a high-performance anode for SIBs. The introduction of the N-functionalized groups transforms the surface chemistry of the hard carbon from sodiophobic to sodiophilic, thus significantly promoting the dispersion of the hard carbon in Na metal and facilitating the formation of an intimate interfacial contact between them. In addition, during Na stripping/plating cycling, the sodiated NHC particles homogenize the local electron distribution, while the desodiated NHC particles function as an artificial SEI film, which facilitate the bottom-up deposition of Na due to electron concentration gradient and mechanically inhibit the growth of Na dendrites. Consequently, the Na–NHC anode in the symmetric cell exhibits excellent cycling stability up to 1700 h without a short circuit, which significantly exceeds that of the bare Na anode. In the full cell with the Na–NHC anode, NaTi2(PO4)3 exhibits a significantly extended cycling life.

Hydrodynamic Terahertz Plasmons and Electron Sound in Graphene with Spatial Dispersion

The intrinsic transverse-magnetic modes of graphene with spatial dispersion are investigated theoretically in the hydrodynamic regime in a terahertz frequency range. It is revealed that spatial dispersion is caused by the diffusion electron-transport mechanism due to the spreading of electron-concentration gradients under the effect of pressure in an electron liquid. The cases of a screened and unscreened plasmon as well as electron sound are considered.

Characteristics of the Growth Increment of Drift-Dissipative Instability at the Fronts of Equatorial Plasma Bubbles

Data from ground-based and satellite measurements, as well as the results of numerical modeling of the spatial structure of equatorial ionospheric bubbles, show that the longitude gradients of the logarithm of the electron concentration at the vertical boundaries of the bubbles can reach 10–3 m–1. At such electron concentration gradients, the development of drift-dissipative instability is possible. This can generate ionospheric plasma inhomogeneities with spatiotemporal scales characteristic of the equatorial F scatter. This article presents the results of calculations of the increment of the increase in gradient-drift instability on the side walls of ionospheric bubbles obtained on the basis of numerical modeling of the structure of equatorial plasma bubbles and the dispersion equation of drift-dissipative instability. The equatorial plasma bubbles were simulated on the basis of a two-dimensional numerical model of the Rayleigh-Taylor instability in the Earth’s equatorial ionosphere, which is suitable for modeling Rayleigh-Taylor and gradient type inhomogeneities that are strongly elongated along the magnetic field lines. The results of numerical experiments confirm that significant longitudinal plasma gradients at the fronts of the developed equatorial plasma bubble can generate drift-dissipative instability of the ionospheric plasma.

The Properties and Origins of Corotating Plasmaspheric Irregularities as Revealed Through a New Tomographic Technique

AbstractThis paper describes insights into the nature of corotating plasmaspheric irregularities (CPIs) enabled by a newly developed method for range‐ and time‐resolved tomographic images of these structures. The method is based on high‐precision measurements of total electron content gradients measure toward cosmic radio sources using an interferometric radio telescope, the Very Large Array (VLA). Exploiting hundreds of hours of data from the VLA Low‐band Ionosphere and Transient Experiment, we confirm that CPIs are predominantly detected at night and at shells 2–4. Combining this large data set with other ionospheric, atmospheric, and space weather databases, we have established that CPI detection rates are much higher when levels of solar and geomagnetic activity are low. We also demonstrate that higher detection rates coincide with drivers of perturbations in the ionospheric electric field, including sporadic E and gravity waves. The gravity wave sources most commonly associated with CPIs are jet stream bend/shear and substorm‐triggered atmospheric disturbances. We also show that simulations of the plasmaspheric impact of both gravity wave‐driven electric field perturbations and polarization electric fields associated with electrobuoyancy waves/sporadic E produce results that are very similar to what is observed with the VLA.

Edge Doping in Graphene Devices on SiO2 Substrates

The conductivity of single layer and bilayer graphene ribbons 0.5–4 μm in width fabricated by oxygen plasma etching is studied experimentally. The dependence of the electron concentration in graphene on the ribbon width is revealed. This effect is explained by the edge doping of graphene due to defects arranged in SiO2 near the ribbon edges. The method of the formation of abrupt p–n junctions in graphene and structures with a constant electron concentration gradient in the graphene plane with the help of edge doping is proposed.

Краевое легирование в графеновых приборах на подложках SiO-=SUB=-2-=/SUB=-

AbstractThe conductivity of single layer and bilayer graphene ribbons 0.5–4 μm in width fabricated by oxygen plasma etching is studied experimentally. The dependence of the electron concentration in graphene on the ribbon width is revealed. This effect is explained by the edge doping of graphene due to defects arranged in SiO_2 near the ribbon edges. The method of the formation of abrupt p – n junctions in graphene and structures with a constant electron concentration gradient in the graphene plane with the help of edge doping is proposed.

Assessment of Ionospheric Activity Tolerances for Epoch of Reionization Science with the Murchison Widefield Array

Abstract Structure imprinted in foreground extragalactic point sources by ionospheric refraction has the potential to contaminate Epoch of Reionization (EoR) power spectra of the 21 cm emission line of neutral hydrogen. The alteration of the spatial and spectral structure of foreground measurements due to total electron content gradients in the ionosphere creates a departure from the expected sky signal. We present a general framework for understanding the signatures of ionospheric behavior in the 2D neutral hydrogen power spectrum measured by a low-frequency radio interferometer. Two primary classes of ionospheric behavior are considered, corresponding to dominant modes observed in Murchison Widefield Array (MWA) EoR data, namely, anisotropic structured wave behavior and isotropic turbulence. Analytic predictions for power spectrum bias due to this contamination are computed and compared with simulations. We then apply the ionospheric metric described in Jordan et al. to study the impact of ionospheric structure on MWA data, by dividing MWA EoR data sets into classes with good and poor ionospheric conditions, using sets of matched 30-minute observations from 2014 September. The results are compared with the analytic and simulated predictions, demonstrating the observed bias in the power spectrum when the ionosphere is active (displays coherent structures or isotropic turbulence). The analysis demonstrates that unless ionospheric activity can be quantified and corrected, active data should not be included in EoR analysis in order to avoid systematic biases in cosmological power spectra. When data are corrected with a model formed from the calibration information, bias reduces below the expected 21 cm signal level. Data are considered “quiet” when the median measured source position offsets are less than 10.″–15.″.

Evaluation of extreme ionospheric total electron content gradient associated with plasma bubbles for GNSS Ground‐Based Augmentation System

AbstractAssociated with plasma bubbles, extreme spatial gradients in ionospheric total electron content (TEC) were observed on 8 April 2008 at Ishigaki (24.3°N, 124.2°E, +19.6° magnetic latitude), Japan. The largest gradient was 3.38 TECU km−1 (total electron content unit, 1 TECU = 1016 el m−2), which is equivalent to an ionospheric delay gradient of 540 mm km−1 at the GPS L1 frequency (1.57542 GHz). This value is confirmed by using multiple estimating methods. The observed value exceeds the maximum ionospheric gradient that has ever been observed (412 mm km−1 or 2.59 TECU km−1) to be associated with a severe magnetic storm. It also exceeds the assumed maximum value (500 mm km−1 or 3.08 TECU km−1) which was used to validate the draft international standard for Global Navigation Satellite System (GNSS) Ground‐Based Augmentation Systems (GBAS) to support Category II/III approaches and landings. The steepest part of this extreme gradient had a scale size of 5.3 km, and the front‐normal velocities were estimated to be 71 m s−1 with a wavefront‐normal direction of east‐northeastward. The total width of the transition region from outside to inside the plasma bubble was estimated to be 35.3 km. The gradient of relatively small spatial scale size may fall between an aircraft and a GBAS ground subsystem and may be undetectable by both aircraft and ground.

Solar effect on the Rayleigh-Taylor instability growth rate as simulated by the NCAR TIEGCM

The TIEGCM (Thermosphere Ionosphere Electrodynamics General Circulation Model) is used to investigate the solar effect on the equatorial ionospheric Rayleigh-Taylor (R-T) instability growth rate, which is responsible for the occurrence of the plasma bubbles. The R-T growth rate is calculated for the solar maximum year 2003 and minimum 2009. The growth rate is strongly dependent on the solar activity. During solar maximum, the pre-reversal enhancement is much stronger leading to higher R-T growth rate. The R-T growth rates from the TIEGCM follow the same solar dependence as the observed occurrence of equatorial plasma bubbles by DMSP satellites. The R-T growth rate also enhances when the day/night terminator is parallel to the magnetic field line near the equator. The R-T growth rate does not correlate well with the solar F10.7 index on a short time scale (~10 days) because the field-line integrated electron content gradient cancels out the positive correlation between the vertical ion drift with the F10.7 index. The TIEGCM result shows the importance of the electron content gradient to the R-T growth rate and the plasma bubble occurrence. The bubble occurrence rates were estimated based on the vertical ion drift simulation results.

Tuning a Kalman filter carrier tracking algorithm in the presence of ionospheric scintillation

Strong ionospheric electron content gradients may lead to fast and unpredictable fluctuations in the phase and amplitude of the signals from Global Navigation Satellite Systems (GNSS). This phenomenon, known as ionospheric scintillation, is capable of deteriorating the tracking performance of a GNSS receiver, leading to increased phase and Doppler errors, cycle slips and also to complete losses of signal lock. In order to mitigate scintillation effects at receiver level, the robustness of the carrier tracking loop, the receiver weakest link under scintillation, must be enhanced. Kalman filter (KF)-based tracking algorithms are particularly suitable to cope with the variable working conditions imposed by scintillation. However, the effectiveness of this tracking approach strongly depends on the accuracy of the assumed dynamic model, which can quickly become inaccurate under randomly variable situations. This study first shows how inaccurate dynamic models can lead to a KF suboptimum solution or divergence, when both strong phase and amplitude scintillation are present. Then, to overcome this issue, it proposes two self-tuning KF-based carrier tracking algorithms, which self-tune their dynamic models by exploiting the knowledge about scintillation that can be achieved through scintillation monitoring. The algorithms have been assessed with live equatorial data affected by strong scintillation. Results show that the algorithms are able to maintain the signal lock and provide reliable scintillation indices when classical architectures and commercial ionospheric scintillation monitoring receivers fail.

A linear scale height Chapman model supported by GNSS occultation measurements

AbstractGlobal Navigation Satellite Systems (GNSS) radio occultations allow the vertical sounding of the Earth's atmosphere, in particular, the ionosphere. The physical observables estimated with this technique permit to test theoretical models of the electron density such as, for example, the Chapman and the Vary‐Chap models. The former is characterized by a constant scale height, whereas the latter considers a more general function of the scale height with respect to height. We propose to investigate the feasibility of the Vary‐Chap model where the scale height varies linearly with respect to height. In order to test this hypothesis, the scale height data provided by radio occultations from a receiver on board a low Earth orbit (LEO) satellite, obtained by iterating with a local Chapman model at every point of the topside F2 layer provided by the GNSS satellite occultation, are fitted to height data by means of a linear least squares fit (LLS). Results, based on FORMOSAT‐3/COSMIC GPS occultation data inverted by means of the Improved Abel transform inversion technique (which takes into account the horizontal electron content gradients) show that the scale height presents a more clear linear trend above the F2 layer peak height, hm, which is in good agreement with the expected linear temperature dependence. Moreover, the parameters of the linear fit obtained during four representative days for all seasons, depend significantly on local time and latitude, strongly suggesting that this approach can significantly contribute to build realistic models of the electron density directly derived from GNSS occultation data.

A new angle for probing field‐aligned irregularities with the Murchison Widefield Array

AbstractElectron density irregularities in the ionosphere are known to be magnetically anisotropic, preferentially elongated along the lines of force. While many studies of their morphology have been undertaken by topside sounding and whistler measurements, it is only recently that detailed regional‐scale reconstructions have become possible, enabled by the advent of widefield radio telescopes. Here we present a new approach for visualizing and studying field‐aligned irregularities (FAIs), which involves transforming interferometric measurements of total electron content gradients onto a magnetic shell tangent plane. This removes the perspective distortion associated with the oblique viewing angle of the irregularities from the ground, facilitating the decomposition of dynamics along and across magnetic field lines. We apply this transformation to the data set of Loi et al. (2015a), obtained on 15 October 2013 by the Murchison Widefield Array (MWA) radio telescope and displaying prominent FAIs. We study these FAIs in the new reference frame, quantifying field‐aligned and field‐transverse behavior, examining time and altitude dependencies, and extending the analysis to FAIs on subarray scales. We show that the inclination of the plane can be derived solely from the data and verify that the best fit value is consistent with the known magnetic inclination. The ability of the model to concentrate the fluctuations along a single spatial direction may find practical application to future calibration strategies for widefield interferometry, by providing a compact representation of FAI‐induced distortions.

Application of ionospheric tomography for high frequency oblique incidence ray tracing

<p>Recognizing the applicability of tomographic imaging in the ionosphere, the reconstruction method has been applied to generate more realistic low latitude ionospheric background based on the Ionex TEC data and also direct measurement TEC data over West Bengal, India. Using these backgrounds, ray tracing of high frequency waves has been performed. Influence of Earth’s magnetic field and also the existence of horizontal gradient of electron concentration in the ionosphere are considered separately for analyzing the ray tracing algorithm. Evaluation of ray tracing parameters and maximum usable frequency for different ground range ensures the possibility of application of ionospheric tomography technique for accurate ray tracing. The analysis shows accurate ray tracing for HF waves, thus defeating the restriction of choice of ionospheric model for analytical ray tracing methods. Performance analysis of the ray tracing technique presented in this paper ensures the consistency of speed with the other methods.</p>

L-band scintillations and calibrated total electron content gradients over Brazil during the last solar maximum

This work presents a contribution to the understanding of the ionospheric triggering of L-band scintillation in the region over São Paulo state in Brazil, under high solar activity. In particular, a climatological analysis of Global Navigation Satellite Systems (GNSS) data acquired in 2012 is presented to highlight the relationship between intensity and variability of the total electron content (TEC) gradients and the occurrence of ionospheric scintillation. The analysis is based on the GNSS data acquired by a dense distribution of receivers and exploits the integration of a dedicated TEC calibration technique into the Ground Based Scintillation Climatology (GBSC), previously developed at the Istituto Nazionale di Geofisica e Vulcanologia. Such integration enables representing the local ionospheric features through climatological maps of calibrated TEC and TEC gradients and of amplitude scintillation occurrence. The disentanglement of the contribution to the TEC variations due to zonal and meridional gradients conveys insight into the relation between the scintillation occurrence and the morphology of the TEC variability. The importance of the information provided by the TEC gradients variability and the role of the meridional TEC gradients in driving scintillation are critically described.