Electron Density Perturbations Research Articles

Effect of ray and speed perturbations on ionospheric tomography by over-the-horizon radar: A new method

AbstractMost recent methods in ionospheric tomography are based on the inversion of the total electron content measured by ground‐based GPS receivers. As a consequence of the high frequency of the GPS signal and the absence of horizontal raypaths, the electron density structure is mainly reconstructed in the F2 region (300 km), where the ionosphere reaches the maximum of ionization, and is not sensitive to the lower ionospheric structure. We propose here a new tomographic method of the lower ionosphere, based on the full inversion of over‐the‐horizon (OTH) radar data. Previous studies using OTH radar for ionospheric tomography inverted only the leading edge echo curve of backscatter ionograms. The major advantage of our methodology is taking into account, numerically and jointly, the effect that the electron density perturbations induce not only in the speed of electromagnetic waves but also on the raypath geometry. This last point is extremely critical for OTH radar inversions as the emitted signal propagates through the ionosphere between a fixed starting point (the radar) and an unknown end point on the Earth surface where the signal is backscattered. We detail our ionospheric tomography method with the aid of benchmark tests. Having proved the necessity to take into account both effects simultaneously, we apply our method to real data. This is the first time that the effect of the raypath deflection has been quantified and that the ionospheric plasma density has been estimated over the entirety of Europe with an OTH radar.

First results on forecasting the spatial occurrence pattern of L-band scintillation and its temporal evolution

After a fairly successful attempt to forecast the temporal evolution of L-band scintillation over a given location, Trivandrum (8.5° N, 76.91° E, dip latitude 0.5° N) (Sridharan et al., 2012, J. Atmos. Sol. Terr. Phys. 80 230–238; Bagiya et al., 2014, J. Atmos. Sol. Terr. Phys. 110–111, 15–22), an attempt has been made here to generate the spatial–temporal maps of the occurrence pattern of L-band scintillation over the Indian region. To start with, the day time fluctuations in [f0F2]2 are used to forecast the temporal evolution of perturbations during the course of the night over Trivandrum. Similar to the earlier studies, here too it is taken that the electron density perturbations retain their characteristics throughout night and traverse with a uniform velocity. This implies that when the integrity of wave train of electron density perturbations is retained, any particular feature that passes over Trivandrum would have crossed over another location west of Trivandrum at an earlier time only dictated by the zonal velocity. With this assumption it becomes feasible to generate the probable spatial and temporal pattern of L-band scintillation. The consequences/limitations of the above assumptions are discussed in detail. The observed relation between the total duration of spread-F and the base height of the F-region (h'F) at 1930LT has been explained in terms of the favourable background neutral atmospheric conditions. Following Bagiya et al. (2013, J. Geophys. Res. 118, 1–8), the relation between h'F at 1930LT and the probable maximum latitudinal extent of the spread-F enables specification of the upper limit for the latitudes likely to be affected by the scintillation. It is believed that the presented results hold enough potential to generate the reliable L-band scintillation forecast maps and provide the necessary alerts to the satellite based air navigation users.

Finite orbital angular momentum states and Laguerre–Gaussian potential in two-temperature electron plasmas

Electron-acoustic waves are studied with orbital angular momentum (OAM) in an unmagnetized collisionless uniform plasma, whose constituents are the Boltzmann hot electrons, inertial cold electrons and stationary ions. For this purpose, we employ the fluid equations to obtain a paraxial equation in terms of cold electron density perturbations, which admits both the Gaussian and Laguerre–Gaussian (LG) beam solutions. Furthermore, an approximate solution for the electrostatic potential problem is found, which also allows us to express the components of the electric field in terms of LG potential perturbations. Calculating the energy flux of the electron-acoustic waves, an OAM density for these waves is obtained. Numerically, it is found that the parameters, such as, azimuthal angle, radial and angular mode numbers, and the beam waist strongly modify the LG potential profiles associated with electron-acoustic waves. The present results should be helpful to study the trapping and transportation of plasma particles and energy as well as to understand the electron-acoustic mode excitations produced by the Raman backscattering of laser beams in a uniform plasma.

Influence of Szigeti effective charge on coherent Raman scattering of laser radiation in magnetized direct gap semiconductors

By considering the hydrodynamic model of semiconductor-plasmas, an analytical investigation is performed to study the influence of Szigeti effective charge (qs) on coherent Raman scattering (CRS) of laser radiation in a transversely magnetized direct gap semiconductor arising from electron density perturbations and molecular vibrations of the medium both produced at the transverse optical phonon frequency. Assuming that the origin of CRS lies in the third-order susceptibility of the medium, an expression is obtained for Raman gain coefficient (gR). Numerical estimates are made for a representative n-type InSb crystal duly shined by pulsed 10.6 μm CO 2 laser. The threshold field (E0 th ) for inciting CRS is identified. The dependence of gR on pump field strength E0, externally applied magnetic field (in terms of electron cyclotron frequency ωc) and doping concentration (in terms of plasma frequency ωp) is reported for both cases viz., qs = 0 and qs ≠ 0. In absence of qs, the results are found to be well in agreement with available literature.

Horizontal parameters of daytime thermospheric gravity waves and E region neutral winds over Puerto Rico

We report on the electron density perturbation amplitudes, periods (up to 60 min), horizontal and vertical wavelengths, phase speeds, and propagation directions of daytime traveling ionospheric disturbances (TIDs) from 115 to 300 km altitude using dual‐beam experiments at the Arecibo Observatory (AO), Puerto Rico. As in previous studies, we find a near continuum of waves above the AO. While the TIDs propagate in nearly all directions except purely westward, we find that most propagate southward southeastward. We find that TID amplitudes increase nearly exponentially with increasing period, although with a much smaller slope for periods >30 min. TID amplitudes peak on the bottomside of the F region. Typical vertical wavelengths increase from less than 50 km at low altitudes to ∼100–300 km. Horizontal wavelengths increase from ∼70–100 km to ∼150–500 km over the same altitude range. Median vertical wavelengths, horizontal wavelengths, and periods increase with altitude up to z∼<225 km and are approximately constant at higher altitudes. Horizontal phase speeds are >100–150 m/s. We also measure the E region horizontal neutral winds and find that they peak at ∼150 m/s near z∼105 km in the middle of the day. Wave phase speeds are in general greater than these ambient winds. In addition, by tracing individual wave packets vertically in altitude, we find that a packet's vertical wavelength generally peaks near the altitude where its inferred ion velocity amplitude is maximum. The vertical wavelength generally decreases above this altitude, an observation that is consistent with gravity wave packet theory.

Semianalytical description of the modulator section of the coherent electron cooling

In the coherent electron cooling, the modern hadron beam cooling technique, each hadron receives an individual kick from the electric field of the amplified electron density perturbation created in the modulator by this hadron in a co-propagating electron beam. We developed a method for computing the dynamics of these density perturbations in an infinite electron plasma with any equilibrium velocity distribution -- a possible model for the modulator. We derived analytical expressions for the dynamics of the density perturbations in the Fourier-Laplace domain for a variety of 1D, 2D, and 3D equilibrium distributions of the electron beam. To obtain the space-time dynamics, we employed the fast Fourier transform (FFT) algorithm. We also found an analytical solution in the space-time domain for the 1D Cauchy equilibrium distribution, which serves as a benchmark for our general approach based on numerical evaluation of the integral transforms and as a fast alternative to the numerical computations. We tested the method for various distributions and initial conditions.

Electrostatic waves in carbon nanotubes with an axial magnetic field

Based on a linearized hydrodynamic model and within the quasi-static approximation, the dispersion relation of electrostatic waves propagating through single-walled carbon nanotubes subject to an axial magnetic field is theoretically explored. In the classical limit, we obtain two main possible waves which in turn are divided into two branches, a low-frequency acoustical and a high-frequency optical plasmon branch. In the quantum case, we have found that the dispersion relation is substantially modified when the electron wavelength becomes large enough compared to the propagation wavelength of the electrostatic waves in the quantum plasma. We also show that the axial magnetic field manifest itself on the perturbed electron density through the quantum term and gives rise to the propagation of the electrostatic waves within the quantum plasma. As a result, the effect of the magnetic field is pronounced in the plasma dispersion relations in such a way that their curves approach to zero when the magnetic field is weak; and for the strong magnetic field, they asymptotically meet the constant lines.

Features of the Wave Disturbances in the Ionosphere During Periodic Heating of the Plasma by the “Sura” Radiation

We present the results of observations of the Doppler-spectrum and Doppler-frequency shift variations of the ionosphere-reflected radio signals from a high-frequency vertical-sounding radar when the ionospheric plasma is affected by the high-power periodic radiation of the “Sura” heating facility. The distance from the heater to the Doppler radar is about 960 km. It has been confirmed that high-power radiation leads to the generation (amplification) of wave disturbances in the parameter range of internal gravity waves (periods 10–30 min and velocities 360–460 m/s). Soliton-like wave packets with a period of 10–15 min, a duration of 20–30 min, and a relative amplitude of electron-density perturbations from 3 to 8 % have been detected. The same parameter for other wave disturbances was 1–3 %. It has been confirmed that the generation efficiency of periodic disturbances in the parameter range of internal gravity waves depends significantly on the heater operation regime.

Chemical and electrical impact of lightning on the Earth mesosphere: The case of sprite halos

A one‒dimensional electrochemical model is developed to describe, in a self‒consistent way, the response of the Earth mesosphere to different types of lightning discharges between 50 and 87 km of altitude. This model is applied to the case of sprite halos, one of the most common types of transient luminous events (TLE). We have studied the time‒altitude evolution of more than 20 chemical species. Our model predicts an increase of up to 70 cm−3 in the electron density from ambient electron density values between 55 and 81 km of altitude in the +CG lightning cases and a negligible mesospheric electron density perturbation in the −CG lightning case. For all the +CG and some −CG (200 kAkm) cases considered, the model also shows an enhancement of several orders of magnitude in the concentration of ground‒state negative and positive ions and electronically excited positive ions such as responsible for the Meinel emissions and . On the other hand, for the first positive group of N2the calculated emission brightness exceeds the threshold of visibility (1 MR) for a halo of 100 km of diameter at an altitude of 77 km for all the CG lightning discharges studied (except for the −CG case with 100 kAkm current peak) and for relatively lower altitudes when +CG lightnings are considered. Moreover, the calculated concentration of the metastables and O(1D) exhibit an enormous enhancement (of more than 10 orders of magnitude) over their ambient values that, for +CG, remains high (5–7 orders of magnitude above ambient values) for long times (up to 500 s), below 55 km.

Perturbed recombination from dark matter annihilation

We show that dark matter annihilation around the time of recombination can lead to growing ionization fraction perturbations, that track the linear collapse of matter over-densities. This amplifies small scale cosmological perturbations to the free electron density by a significant amount compared to the usual acoustic oscillations. Electron density perturbations distort the CMB, inducing secondary non-gaussianity. We calculate the CMB bispectrum from recombination, that is marginally observable by Planck. Even though electron perturbations can be markedly boosted compared with the Standard Model prediction, the dark matter effect in the CMB bispectrum turns out to be small and will be difficult to disentangle from the standard model in the foreseeable future.

Interaction between high- and low-frequency waves

We outline models for the interaction between high- and low-frequency waves that arise in plasma physics and atmosphere/ocean dynamics. The original ideas are embodied in the Zakharov equations, which describe the interaction between high-frequency (Langmuir) electrostatic waves and low-frequency (ion-acoustic) electron density perturbations. Analogous equations have been developed in atmospheric physics (describing the interaction between acoustic waves and internal gravity waves) and atmosphere/ocean dynamics in which the coupling between the higher-frequency inertial waves and lower-frequency planetary Rossby waves is analyzed. The last two cases are more complicated than the original Zakharov equations because of the dispersive and anisotropic nature of the natural modes of the system.

The Propagation of Circularly Polarized Waves in Quantum Plasma

The quantum effects on the propagation circularly polarized waves have been investigated in electron magnetized quantum plasmas. We obtain the dispersion equations of the propagation of circularly polarized laser beam through cold plasma. The results show that the laser can be propagated due to the quantum effects which enhance the propagation phase velocity. For this purpose, the quantum hydrodynamic (QHD) equations with magnetic field and Maxwell’s equations system is used to derive these dispersion relations. The perturbed electron density and current due to the interaction of laser beam with quantum plasma have been investigated. It is shown that the external magnetic field which is parallel to the propagation waves has strong effect on the dispersion relation for the laser propagation in quantum model than the classical regime.

Study of nonlinear ohmic heating and ponderomotive force effects on the self-focusing and defocusing of Gaussian laser beams in collisional underdense plasmas

In the present paper, the propagation characteristics of a linearly polarized gaussian laser beam in a non-isothermal underdense collisional plasma is studied. By considering the effects of the ponderomotive force and ohmic heating of plasma electrons as the nonlinear mechanisms, the second order differential equation of the dimensionless beam width parameter has been obtained and solved at several initial ion temperatures. Furthermore, by using the nonlinear dielectric permittivity of the mentioned plasma medium in the paraxial approximation and its dependence on the propagation characteristics of the gaussian laser pulse, the perturbed electron density ne/n0e is obtained and its variation in terms of the dimensionless plasma length is analyzed at different initial ion temperatures. Our results show that the dimensionless beam width parameter is strongly influenced by the initial plasma ion temperature. It is found that, for the self-focusing regime, the plasma electron density perturbation continuously oscillates between the initial density distribution and a minimum, and for the defocusing regime, the plasma electron density perturbation continuously oscillates between the initial density distribution and a maximum.

Is it possible to derive quantitative information on polarization of electron density from the multipolar model?

The accuracy of electrostatic properties estimated from the Hansen-Coppens multipolar model was verified. Tests were carried out to determine whether the multipolar model is accurate enough to study changes of electrostatic properties under the influence of a crystal field. Perturbed and unperturbed electron densities of individual molecules of amino acids and dipeptides were obtained from cluster and perturbation theory calculations. This enabled the changes in electrostatic properties values caused by polarization of the electron density to be characterized. Multipolar models were then fitted to the subsequent theoretical electron densities. The study revealed that electrostatic properties obtained from the multipolar models are significantly different from those obtained directly from the theoretical densities. The electrostatic properties of isolated molecules are reproduced better by multipolar models than the electrostatic properties of molecules in a crystal. Changes of electrostatic properties caused by perturbation of electron density due to the crystal environment are barely described by the multipolar model. As a consequence, the electrostatic properties obtained from multipolar models fitted to the perturbed theoretical densities derived either from cluster or periodic calculations do not differ much from those estimated from multipolar models fitted to densities of isolated molecules. The main reason for this seems to be related to an inadequate description of electron-density polarization in the vicinity of the nuclei by the multipolar model.

HF waves heating ionosphere F-layer

To predict and evaluate the effects of ionospheric modification quantitatively, the electron temperature and density perturbations are analyzed at different heating conditions, based on a developed model including momentum equation, continuity equation and energy equation. The results show that (1) powerful HF radio waves can cause the electron temperature to increase markedly, and the largest perturbation is near the reflection layer, but the electron density decreases at this area; (2) the lower neutral gas density in the ionosphere, the larger disturbance of the electron temperature and density; the higher radio frequency, the lower increment of electron temperature; and the higher radio power, the larger disturbance of electron temperature and density are caused, but there are no linear relationships between the radio power and disturbance amplitude; (3) the perturbations at nighttime are larger than at daytime when heated by HF radio waves with the same powers; the electron temperature perturbations in the solar minimum are larger than that in solar maximum; and the electron temperature perturbations in spring and autumn are larger than in winter and summer; (4) compared with the low-latitude, the mid-latitude has the smaller perturbations, the maximum electron temperature perturbation is ∼30% (at daytime of winter), and the density is ∼5%.

Long‐lastingD‐region ionospheric modifications, caused by intense lightning in association with elve and sprite pairs

Observations show that intense +CG lightning discharges which trigger both an elve and a sprite are associated with long‐lasting conductivity modifications in the upperD‐region ionosphere. They are observed as strong perturbations in VLF signals propagating through the disturbed region, manifested asLOng Recovery Early VLF events (LORE), which can last up to 30 minutes. These same ionospheric modifications are also responsible for step‐like changes, seen mostly in off‐storm VLF transmissions, which offset signal levels even for longer times. The evidence suggests that when a very intense positive cloud to ground lightning stroke leads to an elve and a high altitude sprite, and possibly a sprite halo as well, there is production of long lasting elevations in electron density at VLF reflection heights that cause LOREs and severe effects on VLF propagation. The present results confirm past predictions and postulations that elves may be accompanied by long‐lasting electron density perturbations in the lower ionosphere.

Twisted dust acoustic waves in dusty plasmas

We examine linear dust acoustic waves (DAWs) in a dusty plasma with strongly correlated dust grains, and discuss possibility of a twisted DA vortex beam carrying orbital angular momentum (OAM). For our purposes, we use the Boltzmann distributed electron and ion density perturbations, the dust continuity and generalized viscoelastic dust momentum equations, and Poisson’s equation to obtain a dispersion relation for the modified DAWs. The effects of the polarization force, strong dust couplings, and dust charge fluctuations on the DAW spectrum are examined. Furthermore, we demonstrate that the DAW can propagate as a twisted vortex beam carrying OAM. A twisted DA vortex structure can trap and transport dust particles in dusty plasmas.

Traveling ionospheric disturbances generated due to periodic plasma heating by high-power high-frequency radiation

The results of observations of variations in the Doppler-frequency shift of electromagnetic radiation of decameter waves reflected from the ionosphere during the plasma modification by highpower radio-frequency radiation from the “Sura” facility and in reference days are presented. The distance from the facility to the diagnostic tools (Doppler radar) was about 1000 km. The traveling ionospheric disturbances corresponding to a delay time of about 50 min with respect to the time of switching high-power periodic radiation and a propagation velocity of 300–400 m/s were observed. The relative amplitude of the electron-density perturbations was 1–4 %, while their oscillation period amounted to tens of minutes. The parameters of the traveling ionospheric disturbances significantly depended on the “Sura” facility radiation mode.

Dissipative soliton-like plasmon-polariton pulses in extended medium

The propagation of the coupled state of the electron density perturbation in an extended metallic medium and the excitation of a two-level resonant medium are analyzed. The one- and two-photon transitions in the resonant medium are considered. The electron density perturbation is described using the hydrodynamic approximation. The formation of plasmon-polariton pulses is analyzed in the case when losses in the extended medium are compensated for by the pumping of the two-level dielectric medium. Numerical analysis carried out for the two models revealed that the losses in a soliton-like pulse in a thin metallic medium can be compensated for due to the energy transfer from the amplifying medium to electron density waves. It is shown that the dispersion of a medium containing a two-level component may considerably affect the characteristics of the pulses. The possibility of effectively controlling the evolution of soliton-like pulses by varying an external electromagnetic field and the characteristics of the matrix is demonstrated.

Direct Evidence for CH···π Interaction Mediated Stabilization of Pro-cisPro Bond in Peptides with Pro-Pro-Aromatic motifs

Although weak interactions play subtle but important roles in dictating protein structures, their experimental detection is nontrivial. From NOE experiments we provide direct evidence for the presence of CH···π interaction, operational between the C(α)-H of the first Pro and the aromatic (Aro) side chain of Xaa, in a peptide series with the general sequence Ac-Pro-Pro-Xaa-NH(2). Indirect evidence of CH···π interaction is provided from ring current-induced upfield displacement of Pro(1) C(α)-H chemical shifts and restriction of side-chain (χ1) rotation of Xaa. A consequence of this interaction is the enhanced stability of the Pro-cisPro conformer in Ac-Pro-Pro-Xaa-NH(2) when Xaa is aromatic. The free energies associated with trans to cis transformation of the Pro-Pro moiety are 0.35, 0.59, 0.64, and 0.82 kcal/mol when Xaa is Tyr, Trp, Phe, and His (pH of 8.4), respectively. In comparison, the corresponding free energy is ∼1.55 kcal/mol when Xaa is nonaromatic. The observed population of Pro-cisPro-His and the pH-induced perturbation of electron density of the His side chain were correlated, providing further evidence for a direct role of CH···π interaction in modulating the stability of Pro-cisPro population in Ac-Pro-Pro-Aro-NH(2). Our study establishes Pro-Pro-Aro to be a new sequence motif that can stabilize Pro-cisPro peptide bonds. This study not only identifies a new structurally biased sequence motif but also directly demonstrates the role played by CH···π interactions in subtly altering conformational preferences of three-residue peptide sequences with implications on the role played by cis-peptide bonds in unfolded proteins.