G. A. Zhbankov

RADIATIVE TRANSFER IN A LAYER OF MAGNETIZED PLASMA WITH RANDOM IRREGULARITIES

Abstract The problem of radio wave reflection from an optically thick plane uniform layer of magnetized plasma is considered in the present work. The plasma electron density irregularities are described by a spatial spectrum of arbitrary form. The small-angle scattering approximation in invariant ray coordinates is proposed as a technique for the analytical investigation of the radiation transfer equation. The approximate solution describing the spatial and angular distribution of radiation reflected from a plasma layer is obtained. The solution obtained is investigated numerically for the case of ionospheric radio wave propagation. Two effects occur as a consequence of multiple scattering: a change in the reflected signal intensity and an anomalous refraction.The problem of radio wave reflection from an optically thick plane monotonous layer of magnetized plasma is considered at present work. The plasma electron density irregularities are described by spatial spectrum of an arbitrary form. The small-angle scattering approximation in the invariant ray coordinates is suggested for analytical investigation of the radiation transfer equation. The approximated solution describing spatial-and-angular distribution of radiation reflected from a plasma layer has been obtained. The obtained solution has been investigated numerically for the case of the ionospheric radio wave propagation. Two effects are the consequence of multiple scattering: change of the reflected signal intensity and anomalous refraction.

Anomalous attenuation of extraordinary waves in ionosphere heating experiments

Multiple scattering from artificial random irregularities HF-induced in the ionosphere F region causes significant attenuation of both ordinary and extraordinary radio waves together with the conventional anomalous absorption of ordinary waves due to their conversion into the plasma waves. To study in detail features of this effect, purposeful measurements of the attenuation of weak probing waves of the extraordinary polarization have been performed at the Sura heating facility. Characteristic scale lengths of the involved irregularities are ~0.1-1 km across the geomagnetic field lines. To determine the spectral characteristics of these irregularities from the extraordinary probing wave attenuation measurements, a simple procedure of the inverse problem solving has been implemented and some conclusions about the artificial irregularity features have been drawn. Theory and details of experiments have been stated earlier. This paper reports results of two experimental campaigns carried out in August 2000 and June 2001 under support of Russian Foundation for Basic Research (grants No. 99-02-17525 and No. 01-02-31008). Particularity of these experiments consisted in using of lower heating power (20-80 MW ERP). Regular character of the multiple scattering effects has been confirmed.Multiple scattering of radio waves by artificial random irregularities HF-induced in the ionosphere F region may cause significant attenuation of both ordinary and extraordinary waves together with common anomalous absorption of ordinary waves due to their non-linear conversion into plasma waves. To demonstrate existence and strength of this effect, direct measurements of attenuation of both powerful pump wave and weak probing waves of extraordinary polarization have been carried out during an experimental campaign on September 6, 7 and 9, 1999 at the Sura heating facility. The attenuation magnitude of extraordinary waves reaches of 1-10 dB over a background attenuation caused by natural irregularities. It is interpreted in the paper on the base of the theory of multiple scattering from the artificial random irregularities with characteristic scale lengths of 0.1-1 km. Simple procedure for determining of irregularity spectrum parameters from the measured attenuation of extraordinary waves has been implemented and some conclusions about the artificial irregularity formation have been obtained.

Distant earthly reflections on ionograms of the intercosmos-19 satellite

Complex ionograms from the Intercosmos-19 satellite with strongly delayed and sometimes multiple reflections from the Earth are considered. An analysis shows that these reflections are usually associated with sharp horizontal gradients of the ionospheric plasma. Such gradients are formed on the walls of the main ionospheric trough, at peaks of electron density, and on the inner and, especially frequently, on the outer slope of the crest of the equatorial anomaly. In one case, distant reflections from the Earth (DREs) formed near the equator, when the satellite in perigee was lower than the F2-layer maximum height. A quantitative interpretation of the most typical cases of DREs is given based on ray tracing. For this purpose, the model of the ionosphere under the satellite is developed, ray paths are calculated, and model ionograms are formed. The good agreement between experimental and model ionograms allows us to conclude that the task of interpreting complicated ionograms obtained by Intercosmos-19 with DRE has been solved successfully.

On the theory of anomalous absorption of an ordinary wave

We obtain and analyze numerically exact formulas for anomalous absorption of a probing ordinary wave due to its transformation into plasma waves under vertical sounding of an ionospheric plasma layer with given spectrum of irregularities. The nature of possible approximations and their validity limits are analyzed.

Region of permanent generation of large-scale irregularities in the daytime winter ionosphere of the Southern Hemisphere

With the use of data from topside sounding on board the Interkosmos-19 (IK-19) satellite, the region of permanent generation of large-scale irregularities in the daytime winter ionosphere of the Southern Hemisphere is differentiated. This region is characterized by low values of foF2 and hmF2 and occupies a rather large latitudinal band, from the equatorial anomaly ridge to ~70° S within the longitudinal range from 180° to 360°. Irregularities with a dimension of hundreds kilometers are regularly observed in the period from 0700–0800 to 1800–1900 LT, i.e., mainly in the daytime. In the IK-19 ionograms, they normally appear in the form of an extra trace with a critical frequency higher than that of the main trace reflected from the ionosphere with lower density. The electron density in the irregularity maximum sometimes exceeds the density of the background ionosphere by nearly a factor of 3. A model of the ionosphere with allowance for its irregular structure was created, and it was shown on the basis of trajectory calculations how the IK-19 ionograms related to these irregularities are formed. A possible mechanism of the generation of large-scale irregularities of the ionospheric plasma is discussed.

Pearl diffuse structures on ionograms of Intercosmos-19 associated with irregularities of the low-latitude ionosphere

Unusual complex ionograms obtained by the Intercosmos-19 satellite are considered, in which four diffuse clouds with a characteristic shape are strung like pearls on the main path of the reflected signal. Ray tracing has been used to show that they are associated with 26 layers of irregularities located at altitudes from hmFs2 up to ~900 km. The sizes of the irregularities range from a few kilometers to 100 kilometers, and the intensity of δNe reaches 100%. The heights of irregular layers increase towards the equator, together with a rise of the F2 layer, and are not associated with magnetic field lines. Complex ionograms have been observed on the outer slope and at the top of the crest of the equatorial anomaly. They are probably caused by the processes occurring in the equatorial ionosphere.

Transionospheric radiosounding with allowance for the reflection of radiowaves from the earth

Further development of the method of transionospheric radiosounding of the ionosphere in the range of its plasma frequencies is presented. The trajectories of radiowaves with a reflection from the Earth are newly included in the consideration. A modeling of the transionospheric radiosounding was performed in conditions of an irregular ionosphere including moderate-dimensional irregularities with drawing possible trajectories, as well as the obtainment and analysis of the corresponding transionograms.

Spread F in the Midlatitude Ionosphere According to DPS-4 Ionosonde Data

The results of studying spread F obtained from the DPS-4 ionosonde data at the observatory of the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (Moscow) are presented. The methodical questions that arise during the study of a spread F phenomenon in the ionosphere are considered; the current results of terrestrial observations are compared with previously published data and the results of sounding onboard an Earth-satellite vehicle. The automated algorithm for estimation of the intensity of frequency spread F, which was developed by the authors and was successfully verified via comparison of the data of the digisonde DPS-4 and the results of manual processing, is described. The algorithm makes it possible to quantify the intensity of spread F in megahertz (the dFs parameter) and in the number of points (0, 1, 2, 3). The strongest spread (3 points) is shown to be most likely around midnight, while the weakest spread (0 points) is highly likely to occur during the daytime. The diurnal distribution of a 1–2 point spread F in the winter indicates the presence of additional maxima at 0300–0600 UT and 1400–1700 UT, which may appear due to the terminator. Despite the large volume of processed data, we can not definitively state that the appearance of spread F depends on the magnetic activity indices Kp, Dst, and AL, although the values of the dFs frequency spread interval strongly increased both at day and night during the magnetic storm of March 17–22, 2015, especially in the phase of storm recovery on March 20–22.

Scattered reflections and multiple traces in the range of 7–10 MHz on ionograms of the interkosmos-19 satellite

The scattered reflections and multiple traces regularly recorded on the topside sounding ionograms of the Interkosmos-19 satellite in the frequency range of 7–10 MHz are considered. The reflected radio signals in this frequency range appear both above and below the critical frequency of the regular layer F2. They are observed at all altitudes of the topside ionosphere from hmF2 to a satellite altitude of 1000 km. It is shown that these phenomena regularly appear at high latitudes (≥60° ILAT) and, less often, in the equatorial region. The scattered reflections indicate the presence of small-scale irregularities, and continuous traces are a consequence of total internal reflection from large-scale irregularities. Small-scale irregularities evidently form within a large-scale irregularity. Ray tracing shows that the size of large-scale irregularities is hundreds of kilometers in height and tens of kilometers in latitude. The appearance of scattered reflections and multiple traces at high latitudes is nearly independent of local time; in the equatorial region, they appear only in the interval of 20–08 LT. All of this agrees well with other observations of irregularities in the ionospheric plasma of different scales.

Investigation of optical path functional for high and low ionospheric radio rays

Optimization method for point-to-point ionospheric ray tracing based on the direct variational principle for the optical path is proposed and applied to calculations of both high and low ionospheric rays. The main problems of low ray determination and possible ways for solving them are outlined.