A. T. Karpachev

Morphology and causes of the Weddell Sea anomaly

The zone of anomalous diurnal variations in foF2, which is characterized by an excess of nighttime foF2 values over daytime ones, has been distinguished in the Southern Hemisphere based on the Intercosmos-19 satellite data. In English literature, this zone is usually defined as the Weddell Sea anomaly (WSA). The anomaly occupies the longitudes of 180°–360° E in the Western Hemisphere and the latitudes of 40°–80° S, and the effect is maximal (up to ∼5 MHz) at longitudes of 255°–315° E and latitudes of 60°–70° S (50°–55° ILAT). The anomaly is observed at all levels of solar activity. The anomaly formation causes have been considered based on calculations and qualitative analysis. For this purpose, the longitudinal variations in the ionospheric and thermospheric parameters in the Southern Hemisphere have been analyzed in detail for near-noon and near-midnight conditions. The analysis shows that the daytime foF2 values are much smaller in the Western Hemisphere than in the Eastern one, and, on the contrary, the nighttime values are much larger, as a result of which the foF2 diurnal variations are anomalous. Such a character of the longitudinal effect mainly depends on the vertical plasma drift under the action of the neutral wind and ionization by solar radiation. Other causes have also been considered: the composition and temperature of the atmosphere, plasma flows from the plasmasphere, electric fields, particle precipitation, and the relationship to the equatorial anomaly and the main ionospheric trough.

Causes of NmF2 longitudinal variations at mid- and subauroral latitudes under summer nighttime conditions

The main factors controlling NmF2 longitudinal variations at mid- and subauroral latitudes have been studied. The data of the Intercosmos-19 topside sounding, obtained at high solar activity for summer nighttime conditions, have been used in the analysis. The contributions of the solar ionization, neutral wind, and temperature and composition of the thermosphere to NmF2 longitudinal variations have been estimated based on ionospheric models. It has been indicated that NmF2 variations in the unsunlit midlatitude ionosphere mainly depends on the residual electron density and its decay under the action of recombination. At subauroral latitudes under summer nighttime conditions, the ionosphere is partially sunlit, and ionization by solar radiation mainly contributes to NmF2 longitudinal variations, whereas the effect of the neutral wind is slightly less significant. These results also indicate how the contribution of different factors to NmF2 longitudinal variations changes at different latitudes.

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.

Specific propagation of radiowaves from the Intercosmos-19 satellite in the region of the nighttime equatorial anomaly crest

It has been indicated how a complex ionogram of topside sounding near the outer slope of the winter southern crest of the equatorial anomaly, where a large NmF2, gradient and a deep hmF2, minimum are observed, is formed. The model latitudinal cross-sectio n of the ionosphere, used to perform trajectory calculations, has been constructed based on the corrected Intercosmos-19 data. The ray trajectories have been modeled using the method of characteristics. It has been indicated that a complex Intercosmos-19 ionogram is formed by an oblique reflection from the equatorial anomaly crest slope (the main trace) and by a strongly oblique reflection from the crest bottom as a result of the wave capture by a large-scale inhomogeneity (the additional trace).

Detection of a large-scale irregularity in the region of the main ionospheric trough according to the data of topside sounding on board the Intercosmos-19 satellite

Complicated ionograms of topside sounding on board the Intercosmos-19 satellite, which were registered on November 26, 1980, in the dusk sector (1800 LT) at the latitudes of the equatorward wall (55°–62° ILAT) of the main ionospheric trough (MIT), are analyzed. They are characterized by the presence of two extra traces at distances larger than the main traces. Approaching the MIT minimum, all traces become more scattered, converge, and join into one strongly diffusive trace. An attempt of interpretation of the complicated ionograms on the basis of trajectory calculations performed by the method of characteristics in the “complex” two-dimensional version (in two mutually intersecting planes) is undertaken. The modeling shows that the extra traces could be related to the presence of a large-scale irregularity stretched along the geomagnetic meridian at the equatorward wall of the MIT. The calculations make it possible to estimate the parameters of the irregularity: the intensity is δfoF2 ∼ 30%, the length is several hundred kilometers, the semi-thickness is 50–60 km, and the height is 350 km. The possible formation causes of the irregularity are discussed. The intensification of the diffuseness of all traces is related to the increase in the intensity of small-scale irregularities, which is usually observed when approaching the MIT minimum.

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.

Diurnal and Longitudinal Variations of the Structure of an Equatorial Anomaly During Equinoxes According to Intercosmos-19 Satellite Data

Longitudinal and local time variations in the structure of the equatorial anomaly under high solar activity in the equinox are considered according to the Intercosmos-19 topside sounding data. It is shown that the anomaly begins to form at 0800 LT, when the southern crest is formed. The development of the equatorial anomaly is associated with well-known variations in the equatorial ionosphere: a change in the direction of the electric field from the west to the east, which causes vertical plasma drift W (directed upward) and the fountain effect. At 1000 LT, both anomaly crests appear, but they become completely symmetrical only by 1400 LT. The average position of the crests increases from I = 20° at 1000 LT to I = 28° at 1400 LT. The position of the crests is quite strong, sometimes up to 15°, varies with longitude. The foF2 value above the equator and the equatorial anomaly intensity (EAI) at 1200–1400 LT vary with the longitude according to changes in the vertical plasma drift velocity W. At this time, four harmonics are observed in the longitudinal variations of W, foF2, and EAI. The equatorial anomaly intensity increases to the maximum 1.5–2 h after the evening burst in the vertical plasma drift velocity. Longitudinal variations of foF2 for 2000–2200 LT are also associated with corresponding variations in the vertical plasma drift velocity. The equatorial anomaly intensity decreases after the maximum at 2000 LT and the crests decrease in size and shift towards the equator, but the anomaly is well developed at midnight. On the contrary, after midnight, foF2 maxima in the region of the anomaly crests are farther from the equator, but this is obviously associated with the action of the neutral wind. At 0200 LT, in contrast to the morning hours, only the northern crest of the anomaly is clearly pronounced. Thus, in the case of high solar activity during the equinoxes, a well-defined equatorial anomaly is observed from 1000 to 2400 LT. It reaches the maximum at 2000 LT.

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.

Global model SMF2 of the F2-layer maximum height

A global model SMF2 (Satellite Model of F2 layer) of the F2-layer height was created. For its creation, data from the topside sounding on board the Interkosmos-19 satellite, as well as the data of radio occultation measurements in the CHAMP, GRACE, and COSMIC experiments, were used. Data from a network of ground-based sounding stations were also additionally used. The model covers all solar activity levels, months, hours of local and universal time, longitudes, and latitudes. The model is a median one within the range of magnetic activity values Kp< 3+. The spatial–temporal distribution of hmF2 in the new model is described by mutually orthogonal functions for which the attached Legendre polynomials are used. The temporal distribution is described by an expansion into a Fourier series in UT. The input parameters of the model are geographic coordinates, month, and time (UT or LT). The new model agrees well with the international model of the ionosphere IRI in places where there are many ground-based stations, and it more precisely describes the F2-layer height in places where they are absent: over the oceans and at the equator. Under low solar activity, the standard deviation in the SMF2 model does not exceed 14 km for all hours of the day, as compared to 26.6 km in the IRI-2012 model. The mean relative deviation is by approximately a factor of 4 less than that in the IRI model. Under high solar activity, the maximum standard deviations in the SMF2 model reach 25 km; however, in the IRI they are higher by a factor of ~2. The mean relative deviation is by a factor of ~2 less than in the IRI model. Thus, a hmF2 model that is more precise than IRI-2012 was created.

Waveguide radio-wave propagation in the equatorial ionosphere according to the Interkosmos-19 data

Additional strongly remote (up to 2000 km) radio-signal reflection traces on Intercosmos-19 ionograms obtained in the equatorial ionosphere have been considered. These traces, as a rule, begin at frequencies slightly lower than the main trace cutoff frequencies, which indicates that an irregularity with a decreased plasma density exists here. The waveguide stretched along the magnetic-field line is such an inhomogeneity in the equatorial ionosphere. The ray tracing confirm that radio waves propagate in a waveguide and make it possible to determine the typical waveguide parameters: −δNe ≥ 10%, with a diameter of 15–20 km. Since the waveguide walls are smooth, an additional trace is always recorded distinctly even in the case in which main traces were completely eroded by strong diffusivity. Only one additional trace (of the radio signal X mode) is usually observed one more multiple trace is rarely recorded. Waveguides can be observed at all altitudes of the equatorial ionosphere at geomagnetic latitudes of ±40°. The formation of waveguides is usually related to the formation of different-scale irregularities in the nighttime equatorial ionosphere, which result in the appearance of other additional traces and spread F.