A method of estimating the excess electron density in random irregularities embedded in the ionosphere

Summary Wave propagation through the ionospheric plasma containing irregularities has been studied. The excess electron number density due to the presence of irregularities has been calculated in a way which may be suitably used for practical purposes.     

Scattering of radio waves from the ionosphere in presence of time varying random irregularities

Summary In this paper the modified formula for scattering cross-section of radio waves from the ionosphere has been deduced in presence of time-varying random irregularities. The anisotropic behaviour of the medium which arises due to the presence of the geomagnetic field along with the time-varying irregularities of the medium has been considered. The derived expression for the dielectric tensor for the above medium has been utilised to obtain the required cross-section. The latitude, altitude, azimuth and polarisation angle dependence of scattering of radio waves from the ionosphere has been shown. The evaluated formula may be verified experimentally and the importance of the geomagnetic field and irregularities may then be analysed.     

On effective conductivity of the ionosphere with random irregularities

The influence of stochastic irregularities of the ionosphere on its effective conductivity has been estimated. The study was carried out for large scale inhomogeneities and quasistationary electromagnetic fields. It is found, that Pedersen conductivity sharply increases in a strong geomagnetic field even for small stochastic ionospheric irregularities of the electron density. This peculiarity has to be taken into account during analysis of ionospheric and magnetospheric measurements.     

Determining dynamic parameters of different-scale ionospheric irregularities over northern Siberia

In 1995–1996, observations were carried out at Norilsk (geomagnetic latitude and longitude 64.2°N and 160.4°E) to determine dynamic parameters of irregularities in the high-latitude ionosphere. The short-baseline spaced-receiver method that has been implemented at the ionospheric facility of the Norilsk Integrated Magnetic–Ionospheric Station, provides a means of simultaneously measuring parameters of small-scale irregularities (spatial scale of 3–5 km) by the Similar-Fading Method (SFM), as well as of medium-scale irregularities (time scale of 10–30 min, spatial scale of hundreds of kilometres) by the Statistical Angle-of-arrival and Doppler Method (SADM). About 20 h of the observational data for the F2-layer under quiet geomagnetic conditions (K_p < 3), 20 h under disturbed conditions (K_p ≥ 3) and about 15 h for the sporadic E-layer (K_p ≈ 3) were processed. It has been found that the propagation directions and velocities of different-scale irregularities do not coincide. Small-scale irregularities of the F2-layer travel predominantly eastward or westward. The velocity of the F2-layer irregularities is about 100 m/s, and under disturbed conditions it is up to 200–250 m/s. Small-scale irregularities of the sporadic E-layer travel mostly in the northward direction. It is confirmed that the E_s-layer is characterised by high velocities of the irregularities (as high as 1000 m/s). Medium-scale irregularities with periods in the range of 10–30 min travel mostly in a southward direction with velocities of 20–40 m/s.     

Local enhancements in the horizontal distribution of the electron density at heights of the topside ionosphere near the dayside polar cap boundary

The structure of the electron density horizontal distribution at heights of the topside ionosphere in the Northern Hemisphere under quiet magnetic conditions in the polar peak region has been analyzed based on the measurements conducted on board the STSAT-1, DMSP F13, and DMSP F15 satellites during the period of moderate solar activity. The sector near 1000 MLT (magnetic local time) for the spring equinox season has been studied most thoroughly. It has been obtained that the polar peak consists of localized irregularities in the electron density of a specific form, inside which the electron density exceeds its background values by several tens of percent. The characteristic dimensions of the irregularities vary from a few degrees to several tens of degrees in the meridional and zonal directions, respectively. The irregularities are centered along the dayside boundary of the polar cap and coincide with the region of “soft” electron precipitation (with the average energy lower than 500 eV and with the flux density above 10⁷ electrons cm^?2 s^?1 sr^?1).     

Specification and forecasting of scintillations in communication/navigation links: current status and future plans

The ionosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves to cause amplitude and phase scintillation and affect satellite communication and GPS navigation systems. The effects are most intense in the equatorial region, moderate at high latitudes and minimum at middle latitudes. The thermosphere and the ionosphere seem to internally control the generation of irregularities in the equatorial region and its forcing by solar transients is an additional modulating factor. On the other hand, the irregularity generation mechanisms in the high-latitude ionosphere seem to be driven by magnetospheric processes and, therefore, high-latitude scintillations can be tracked by following the trail of energy from the sun in the form of solar flares and coronal mass ejections. The development of a global specification and forecast system for scintillation is needed in view of our increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillation. Such scintillation specification systems are being developed for the equatorial region. An equatorial satellite equipped with an appropriate suite of sensors, capable of detecting ionospheric irregularities and tracking the drivers that control the formation of ionospheric irregularities, has also been planned for the purpose of specifying and forecasting equatorial scintillations. In the polar region, scintillation specification and forecast systems are yet to emerge although modeling and observations of polar cap plasma structures, their convection and associated irregularities have advanced greatly in recent years. Global scintillation observations made during the S-RAMP Space Weather Month in September 1999 are currently being analyzed to study the effects of magnetic storms on communication and navigation systems.     

Interaction between plasma structures in an unstable ionospheric plasma

A vortex structure renders additional stability to plasma irregularities stretched along magnetic field lines. Plasma irregularities extended over several tens of kilometers are registered with rocket and satellite equipment in the topside ionosphere. The registered scale of irregularities depends on the spatial and time resolution of the equipment used. Irregular structures were registered in the ionosphere during experiments with barium clouds and jets, when a plasma irregularity separated into strata extended over several meters and several kilometers across the geomagnetic field. It has been indicated that plasma vortices can be generated in an unstable plasma in a situation when its quasi-neutrality is disturbed. Local geomagnetic field disturbances will be caused by the appearance of a proper vortex magnetic field. Plasma vortices can interact in an inhomogeneous plasma with an unstable electron component. Such interactions are related to the transformation of the phase volume of free electrostatic oscillations in the frequency-wave vector space.     

The role of natural E-region plasma turbulence in the enhanced absorption of HF radio waves in the auroral ionosphere:Implications for RF heating of the auroral electrojet

Physical processes which affect the absorption of radio waves passing through the auroral E-region when Farley-Buneman irregularities are present are examined. In particular, the question of whether or not it is legitimate to include the anomalous wave-enhanced collision frequency, which has been used successfully to account for the heating effects of Farley-Buneman waves in the auroral E-region, in the usual expression for the radio-wave absorption coefficient is addressed. Effects also considered are those due to wave coupling between electromagnetic waves and high-frequency electrostatic waves in the presence of Farley-Buneman irregularities. The implications for radio-wave heating of the auroral electrojet of these processes are also discussed. In particular, a new theoretical model for calculating the effects of high-power radio-wave heating on the electron temperature in an electrojet containing Farley-Buneman turbulence is presented.     

Modeling of vertical sounding ionograms for nonmonotonous ionospheric layers

The problem of radiosignal reflection from a plane layered isotropic ionosphere has been considered. The vertical sounding ionograms for ionospheric layers with a complicated structure have been modeled. It has been demonstrated that the structure of reflected signals depends on the degree of ionospheric irregularities. Diffuse reflection of radiosignals, caused by the vertical irregularity of the ionosphere, has been detected.     

Modification of electron concentration in the ionosphere in the pump-wave plasma resonance region

We discuss the propagation of sounding radio waves in the inhomogeneous ionosphere, in the reflection area of which there are small-scale artificial magnetically-positioned irregularities. The propagation of radio waves in such an area, where the lateral dimensions of strongly elongated artificial irregularities are smaller than the wavelength, has a diffraction nature. It is shown that the calculation of diffraction parameters makes it possible to derive the amplitude of density irregularities and their relative area perpendicular to the magnetic field direction. Comparison of theoretical calculations with experimental studies on modification of the electron density altitude profile by heating of the ionosphere with midlatitude stand Sura showed that the relative area of the negative density perturbations can reach several percent.     

Determining the parameters of the layer of scattering irregularities,forming coherent echo,according to the Irkutsk IS Radar data

The method for determining the parameters of the layer of scattering irregularities, form coherent echo, based on the Irkutsk Incoherent Scatter (IS) Radar data is presented. It has been indicted that the method has the necessary accuracy. The processing of experimental data for the periods of September 25–26, 1998, and July 15–16, 2000, indicated that our results are in good agreement with the data obtained by other researchers. The analysis of the experiments with a high time resolution made it possible to reveal the time variation in the determined parameters. The time variations in the layer thickness and height do not contradict the data obtained by other researchers, and the time variations in the angular sensitivity of irregularities has been observed for the first time and requires further studies.     

Variation of the ionospheric scintillation index with elevation angle of the transmitter

We have analyzed GPS data from 2007–2011 to determine the nature of variation of scintillation index with elevation of the direction of propagation at an observing point Warsaw, Poland, and Hornsund, Svalbard. To compare with the theory, the intensity scintillation index is simulated as a function of elevation angle, azimuth, magnetic field inclination, and shape of irregularities, using the phase screen model of scintillation as formulated by Rino (1979). Data analysis has been done for the seasonal as well as geomagnetic activity dependence of ionospheric scintillation. Scintillation index is a power-law function of the cosecant of the elevation angle. Results show that the power law strongly depends on the form of irregularities, being larger than in isotropic case for irregularities with dimension along the magnetic field direction smaller than those across the magnetic field. The present work also shows the need to use experimentally derived dependence on elevation.     

Diagnosing the effective parameters of the ionospheric fine structure from statistical characteristics of radio waves in the vicinity of a regular caustic

This paper is concerned with the oblique propagation of decametric radio waves in the ionosphere with random electron density irregularities. Effective parameters are introduced for calculating the influence of irregularities on the wave field structure. A technique is proposed for determining these parameters from measurements of statistical characteristics of the signal in the vicinity of a regular caustic. The technique uses asymptotic expressions obtained using the interference integral method and perturbation theory, as well as matching them to the numerical solution on the basis of the method of characteristics. A global semi-empirical model that is updated for current ionospheric conditions is used to specify the background medium. The proposed technique has been tested using data from a number of mid-latitude paths. Results obtained in this study testify that the technique deserves a practical implementation.     

Numerical simulation of the mutual conversion of ordinary and extraordinary waves in a magnetised plasma by artificial ionospheric turbulence

Some effects of normal mode coupling in weakly anisotropic inhomogeneous plasma are analysed on the basis of the numerical solution of transfer equations for the Stokes parameters. The numerical analysis of normal mode conversion on a set of isolated irregularities demonstrates the possibility of an effective polarisation transformation on such structures. It is shown that by the appropriate selection of discrete irregularities of the external magnetic field direction or plasma electron concentration resulting in radio-wave refraction, one can control electromagnetic radiation polarisation characteristics. Analytical expressions for mean Stokes parameters have been obtained for the rare isolated irregularities. The opportunity for the simulation of mutual wave conversion processes in the ionospheric heating experiments is discussed.     

波在含有各向异性不均匀体介质中的散射理论

关于波在无序起伏的介质中传播问题曾在文献[1,2]中给予统计讨论,当满足一定条件λ<相似文献   

Equatorial field-aligned irregularities deduced from the echo-train and hybrid whistlers

Summary Equatorial field-aligned irregularities have been studied by using low-latitude echotrain and hybrid whistlers observed at Sakushima Station (geomag. lat. 24°). The structure is discussed in relation to the propagation mode trapped by field-aligned irregularities. We then find that the field-aligned irregularities responsible for the trapping of low latitude whistlers are of such small dimension as the HF ducts.     

Spatial observations by the CUTLASS coherent scatter radar of ionospheric modification by high power radio waves

Results are presented from an experimental campaign in April 1996, in which the new CUTLASS (Co-operative UK twin-located Auroral Sounding System) coherent scatter radar was employed to observe artificial field aligned irregularities (FAI) generated by the EISCAT (European Incoherent SCATter) heating facility at Tromso, Norway. The distribution of back-scatter intensity from within the heated region has been investigated both in azimuth and range with the Finland component of CUTLASS, and the first observations of artificial irregularities by the Iceland radar are also presented. The heated region has been measured to extend over a horizontal distance of 170 ± 50 km, which by comparison with a model of the heater beam pattern corresponds to a threshold electric field for FAI of between 0.1 and O.OlV/m. Differences between field-aligned and vertical propagation heating are also presented.     

电离层不规则结构漂移的GPS测量及其初步结果

本文阐述了利用GPS接收机台阵测量到的闪烁和TEC变化率ROT快速起伏图样估计F层不规则结构漂移的原理和方法,并利用实测数据估计了静日和暴时电离层不规则结构的水平漂移速度.短间距台网和超短间距台链观测实例的计算结果表明,暴时武汉地区引起TEC快速起伏的电离层不规则结构沿纬圈向西漂移,21∶30至03∶00 LT,西向漂移速度在约40 m/s至130 m/s的范围内变化;在桂林地区,磁静日午夜前后引起L波段电波闪烁的电离层不规则结构沿纬圈向东漂移,漂移速度从约70 m/s下降到约55 m/s,磁扰日午夜前不规则结构向西漂移,速度从约150 m/s下降到约50 m/s,午夜后转为向东漂移,速度从约25 m/s上升到约65 m/s.文中还提出了由单站多卫星观测估计F层不规则结构漂移的设想.实例分析与计算结果表明,利用单站多卫星观测估计电离层不规则结构漂移是一种合理可行的方法.     

Radar observations of ionospheric irregularities at Syowa Station, Antarctica: a brief overview

We briefly overview the radar observations that have been made for 30 years at Syowa Station, Antarctica for studying small-scale electron-density irregularities in the southern high-latitude E- and F-region ionosphere. Some observational results (i.e., long-term variations of radio aurora, Doppler spectra with narrow spectral widths and low Doppler velocities, and simultaneous observations of radar and optical auroras) from VHP radars capable of detecting 1.3- to 3-m scale irregularities are presented. A new 50-MHz radar system equipped with phased-antenna arrays began operation in February 1995 to observe two-dimensional behaviors of E-region irregularities. An HF radar experiment also began in February 1995 to explore decameter-scale E- and F-region irregularities in the auroral zone and polar cap. These two radars will contribute to a better understanding of the ionospheric irregularities and ionospheric physics at southern high latitudes.     

Seasonal variations of equatorial spread-F

The occurrence of spread-F at Trivandrum (8.5^N, 77^E, dip 0.5^N) has been investigated on a seasonal basis in sunspot maximum and minimum years in terms of the growth rate of irregularities by the generalized collisional Rayleigh-Taylor (GRT) instability mechanism which includes the gravitational and cross-field instability terms. The occurrence statistics of spread-F at Trivandrum have been obtained using quarter hourly ionograms. The nocturnal variations of the growth rate of irregularities by the GRT mechanism have been estimated for different seasons in sunspot maximum and minimum years at Trivandrum using h’F values and vertical drift velocities obtained from ionograms. It is found that the seasonal variation of spread-F occurrence at Trivandrum can, in general, be accounted for on the basis of the GRT mechanism.