Equatorial electrojet and radio scintillations

It is shown that the day-time scintillations of VHF radio waves at the equatorial station, Huancayo, are very small, of the order of 1–2 dB peak, during the equatorial electrojet condition. If the event of complete or partial counter-electrojet occurring either on quiet or during disturbed conditions is followed by the occurrence of blanketing type of Es, then only strong day-time scintillations are observed. Counter-electrojet events followed by only the absence of Es are not found to produce any additional scintillations. Thus the day-time VHF scintillations near the magnetic equatorial regions are due to the sharp plasma gradient associated with blanketing type of sporadic E region.     

VHF radio scintillations at Bombay

The simultaneous recordings of the amplitude scintillations of VHF radio signals from nearby geostationary satellites FLEETSAT (at 73°E long.) and SIRIO (at 65°E) received at Bombay (geog. lat. 19°N, geog. long. 73°E, mag. lat. 15°N) have revealed systematic time shifts in the starting and the ending of the individual scintillation events. The ionosphere crossover points of the two transmission paths were separated by only 80 km in the east-west direction, which was smaller than the average size of the irregularity patches. Scintillations normally started after 1930 h, reached a maximum at 2200 h and slowly decreased till 1000 h, after which no scintillations were observed. The speed of the irregularity patches computed from the time shifts of these events was about 150 m s⁻¹ in the early hours of the night, decreasing to about 100 m s⁻¹ by midnight and showing much lower velocities in the post-midnight hours.     

Mid-latitude quasi-periodic scintillations of satellite beacon signals

Simultaneous observations of amplitude fluctuations at two separated stations and of the relative phase of the arriving signals are used to study quasi-periodic scintillations of the kind that are sometimes called ‘ringing irregularities’. Since one set of measurements is sufficient to locate the irregularity it is possible to assess the utility of the simple model proposed by Elkins and Slack by using it in a computer simulation to predict the temporal behaviour of the other observed quantities. The resulting agreement leaves no doubt that scintillations arise in the ionosphere by a mechanism having the gross features of the one suggested, although the additional redundant observations permits the derivation of information about the ionospheric irregularities not encompassed by the Elkins and Slack model. This simple geometric model which assumes specular reflection is extremely useful and the more complete scattering analysis is needed only where detailed information about the irregularity is required. Evidence is also presented for the occasional occurrence of large angle scattering from ionospheric irregularities.     

Night-time ionospheric scintillations at the magnetic equator

The nocturnal and seasonal variations of equatorial ionospheric scintillations are presented. Scintillations are classified into two classes, namely, class I and class II depending on their fading rates and association with bottomside spread-F. Power spectra and frequency indices for class I and class II scintillations are presented and their theoretical implications are discussed.     

Electrojet irregularity parameters from daytime equatorial scintillations

The second moment of the complex amplitude or the mutual coherence function (MCF) for transionospheric VHF radio waves transmitted from the geostationary satellite ATS-6 is computed from daytime amplitude and phase scintillations recorded at an equatorial station, in order to study the structure of electrojet irregularities. The shape of the correlation function for fluctuations in the integrated electron content along the signal path is deduced by using a theoretical relationship between this correlation function and the MCF which is based on the assumption that the irregularities are “frozen”. Further, using a power-law spectrum to describe the electrojet irregularities, the outerscale l_o associated with the spectrum as well as the r.m.s. density fluctuation are estimated from theoretical fits to the computed values. The irregularity drift speeds V_o transverse to the signal path, for the scintillation events studied here, are derived from power spectra of weak scintillations. On the basis of a relationship between l_o and V_o suggested by a linear theory of the gradient-drift instability, the effective Hall conductivity is estimated to be about five times the effective Pedersen conductivity in the electrojet region.     

Equatorial spread-F: recent results and outstanding problems

Recent results and outstanding problems in the field of equatorial spread-F are reviewed. The discussion is organized about four distinct wavelength regimes: long (≳ 20 km). intermediate (20 km–100 m). transitional (100 m–10 m) and short (≲ 10 m). The intermediate waves are best understood, although the enhanced turbulent power near 1 km wavelength needs explanation, as does the saturation mechanism itself. The role of shear flow and irregularity seeding by ‘geophysical noise’ in the neutral atmosphere form important future research topics at the longest scales. Identification of drift waves in the transitional wavelength range has been a major new step in explaining the full spread-F phenomenon. This drift wave turbulence in the transition regime will be actively studied in the near future, particularly with regard to its role in determining the saturated amplitude of the intermediate waves, as well as in the anomalous diffusion of plasma at high altitudes. Although waves as short as 11 cm have been unambiguously detected and linear theories exist, the origin and amplitude spectra of these short wavelength waves inside topside plumes remain in some doubt and should also be examined in more detail.     

Phase and amplitude scintillations due to electrojet irregularities

Features of the power spectra of weak amplitude and phase scintillations on a VHF signal, transmitted from a geostationary satellite and recorded at an equatorial station, are found to be in good agreement with theory. Irregularity drift speeds transverse to the signal path were extracted from the first few pronounced Fresnel minima in the power spectra. For some data intervals, a low frequency peak, associated with an irregularity wavelength greater than the Fresnel dimension, could be identified in the phase spectrum. The dominant wavelength of the large scale waves is found to be ≳ 1.5km.     

Distributions of the irregularities which produce ionospheric scintillations

The distribution of nighttime irregularities which produce satellite scintillation has been examined for a midlatitude location using a large array of receivers. The irregularities are aligned along the earth's magnetic field and appear to extend from top to bottom of the F-region, being preferentially observed near the F-region ionization peak where they produce the strongest scintillations. A new method of mapping the horizontal distribution shows patches of various shapes and sizes but with no systematic structure.     

Equatorial spread-F: a review of recent experimental results

A number of striking experimental verifications of the Rayleigh-Taylor instability, as applied to equatorial spread-F, have been made in the past few years. We review these observations and attempt to synthesize them with parallel theoretical developments. The results also pose some new questions concerning the turbulent cascade of energy and the evolution of an inner scale for the phenomenon. We review recent observations of bottomside altitude modulation and discuss the possible role of gravity waves in initiating the equatorial spread-F process.     

Faraday polarization fluctuations associated with amplitude scintillations at Lunping

The diurnal, seasonal and solar cycle variations of Faraday polarization fluctuations (FPF) associated with amplitude scintillations observed at Lunping, Taiwan (25.0°N, 121.2°E : geographic) during the period 1978–1981 are presented. The occurrence of polarization fluctuations is maximum in the premidnight hours. FPFs occur either simultaneously or with a time lag after the onset of amplitude scintillations. There is an increase in FPF activity with an increase in sunspot activity. Occurrence of FPF peaks in the equinoxes. There had been a moderate activity in summer while the winter occurrence is a minimum. The seasonal occurrence pattern compared with reports from other locations indicates a longitudinal control on FPF activity. The maximum probable duration of FPF ranges from 15 to 30 min. It was found that the association of FPF with range spread-F is much better than that with frequency spread-F. Large ambient ionization densities corresponding to plasma frequencies greater than 15 MHz appear to create a favourable environment for the occurrence of FPF.     

Spectral characteristics of magnetic storm-induced F-region scintillations extending into daytime

Magnetic storm-induced F-region scintillations extending into daytime were recorded over Bombay, situated near the anomaly crest region in India, on 12 November 1991. The scintillations at 244 MHz using the radio beacon onboard FLEETSAT (73°E), lasted till 1312 h IST (77.5°E). Observations at Trivandrum, situated close to the magnetic equator also show strong daytime scintillations lasting till 1030 h. The scintillation event followed a sudden commencement at 1748 h UT (2318 h IST) on 11 November 1991 and the ionosonde observations, both over Ahmedabad in the anomaly crest region and Kodaikanal near the magnetic equator, show large upward drift of about 50 m s⁻¹ around 0300 h IST. The scintillation index S₄, autocorrelation function and power spectra have been computed from the digital data recorded at Bombay. The time variation of S₄ shows large fluctuations with a periodicity of about half an hour. The 50% decorrelation time of the signal fluctuations is of the order of seconds. The spectral index n, of the temporal power spectra, where P(F)αF⁻ⁿ, varies between 1.5 and 5.0, with a mean value of 3.0, and shows a dependence on the S₄ index. These features are similar to those reported for night time scintillations recorded over Ahmedabad.     

Drift analysis of random and quasiperiodic scintillations in the ionosphere

Radio signals in the VHF/UHF range from the geostationary satellite ATS-6 were recorded using a system of three spaced antennas at Slough. Simultaneously, the integrated electron content (TEC) was monitored between the satellite and ground. Full correlation analysis and similar fade techniques were used to deduce the drift velocities of irregularities responsible for random and quasiperiodic (QP) ‘ringing’ scintillations. Similar drift velocities were found for the disturbances responsible for both types of scintillations at the times when QP and random scintillations occurred in a sequential pattern. A southward-drifting disturbance was responsible for rare, multiple QP scintillations which were followed by large scale fluctuations in electron density. In general, QP-scintillation-producing irregularities drifted southward, with velocities whose median magnitude and azimuth were 64 m s⁻¹ and 178°E of N respectively.The sequential occurrence pattern of QP-random scintillations as well as the time delay between occurrences of large fluctuations in TEC and QP scintillations, appear to be consistent with a reflection model of generation of the ringing fading of the signal.     

Experimental results on satellite scintillations due to field-aligned irregularities at mid-latitudes

Experiments using multi-station networks receiving signals from the VHF beacon of a geostationary satellite have been carried out in order to clarify the geometrical factor involved in ionospheric intensity scintillations due to field-aligned irregularities. The characteristics of scintillation observed in the daytime agree with the theoretical value expected for weak diffractive scattering by ionospheric irregularities with an elongation of 10 along the geomagnetic field. However, those in the night-time show much marked enhancement along the field-line due to strong refractive scattering by irregularities having the same elongation. Finally, it is shown that the geometrical factor in scintillation at mid-latitudes can be expressed as a function of the propagation angle between the radio path and the geomagnetic field in the ionosphere. The maximum values of the geometrical factor are respectively about 5 in the daytime and 14 at night.     

Equatorial F-layer irregularity patches at anomaly latitudes

Recent studies of the physics of F-layer irregularities in the equatorial ionosphere have been concerned with the development of plumes or patches. A series of observations in the equatorial anomaly region in a year of high solar flux has been analyzed for the radio propagation effect of scintillations. The observations were made on patches in the developing, mature and decay phases. Although irregularities develop on the west wall of the patches, the intensity of scintillation does not appear to diminish within the patch; the patches contain bursts of high level activity.Patch characteristics at microwave wavelengths match airglow depletion images when two considerations are introduced, i.e. the westward tilt of the patch as shown by optical and radar observations and the effective path length of the irregularities affecting the radio propagation path. Using optical images of depletions the effective thickness of the layer of irregularities above the peak of the F2-layer can be estimated; it is relatively short, i.e. of the order of 70 km for the gigaHertz frequencies and 150 km for the 257 MHz transmissions. The total path length is 110 km for the microwave frequencies and 220 km for the lower levels of scintillation at 257 MHz. The decrease in microwave scintillations compared to meter wavelength observations in the midnight and post-midnight time period in these anomaly observations is due to the combination of decay of electron density as well as the relatively rapid decay of smaller scale irregularities, as has previously been noted in observations at the magnetic equator.     

Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum

A 5-yr study (1987–1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20–55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found: so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorward edge of auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988–1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.     

HF produced ionospheric electron density irregularities diagnosed by UHF radio star scintillations

High frequency waves incident on an overdense ionosphere (i.e. HF < penetration frequency) are known to produce large-scale irregularities with scale sizes of several hundred meters in the F-region of the ionosphere. Three observations of radio star intensity fluctuations at UHF are reported for HF ionospheric modification experiments performed at the Arecibo Observatory. Two observations at 430 MHz and one observation at 1400 MHz indicate that the thin phase screen theory is a good approximation to the observed power spectra. However, the theory has to be extended to include antenna filtering. Such filtering is important for UHF radio star scintillations since the antenna usually has a narrow beam width. HF power densities of less than 37 μW m⁻² incident on the ionosphere produce electron density irregularities larger than 13% of the ambient density (at 260 km) having scale sizes of ~510 m perpendicular to the geomagnetic field. The irregularities form within 20–25 s after the HF power is turned on. From the observed power spectra driftvelocities of the irregularities can be estimated.     

Regimes of ionospheric turbulence from fractal analysis of satellite radio signal scintillations

Samples of amplitude scintillations of the radio signal from a geostationary satellite obtained at a midlatitude station near Irkutsk were processed. For calculating the fractal dimensionalities the Grassberger and Procaccia [(1983) Physica D9, 189] algorithm was used. Results of the data processing tend to divide into two groups. One group includes those realizations for which it was possible to obtain reliable estimates of dimensionality. Three of the seven realizations considered were in this group, and the fractal dimensionalities were found to be low (3.12 4.5). The other data fall within the second group; a reliable estimate of dimensionality for them is unobtainable in terms of the method used. We suppose that this is attributable to the high dimensionality of the process. Power spectra of the signals of this group are close to those with an exponent of −2. The spectra of the signals of the first group are markedly steeper. On the basis of the data analyzed it is supposed that there exist two modes of ionospheric turbulence in midlatitudes, namely the mode with low dimensionality typical of localized turbulent processes, and the mode with high dimensionality typical of homogeneous turbulence that covers an extensive region of the ionosphere.     

Equatorial spread-F and neutral atmospheric turbulence : a review and a comparative anatomy

A number of satellite and rocket plasma density spectra obtained during equatorial spread-F conditions are presented and discussed in the light of similar measurements in the neutral atmosphere. We discuss this comparison in some detail and find both distinct similarities and subtle differences. The horizontal spectral measurements show a peak at an outer scale quite similar to the scale of the undulations caused by gravity wave interactions with the ionosphere. This feature is similar to a buoyancy subrange but it is easy to show that the amplitudes of the plasma fluctuations are too large to be directly driven by the neutral atmosphere. At intermediate scales the plasma fluctuations have a one-dimensional horizontal spectrum with a power law well described by a ($\text{−5}\text{3}$) slope. Once again it can be shown that the neutral fluid cannot be similarly structured at 400 km altitude due to the high viscosity coefficient. A plasma cascade process seems to be operating but it is not at all clear how the spectrum is formed. Furthermore, vertical power spectra seem to run the gamut in spectral form with slopes (n) varying in the range from −1 to −3. So the horizontal spectra are near universal in form, while the vertical spectra are quite variable.