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.     

Power spectra of VHF intensity scintillations from F2- and E-region ionospheric irregularities

An experiment is described for the routine study of scintillations and ionospheric irregularities at high-latitudes using NNSS satellites with additional coordinated observations by means of the EISCAT ionospheric radar facility. Early results, obtained during the development phase of the experiment, are presented of the power spectra of intensity fluctuations at 150 MHz observed at the equatorwards edge of the high-latitude irregularity zone. The spectra of 165 samples of night-time scintillation recorded during October 1982 to May 1983 show a spectral index with a mean value of −3.58 and a steepening of the spectral slope with increasing S₄. Some examples of scintillation arising from irregularities at E-layer height show spectral indices of magnitude generally smaller than for F-region cases. A few spectra have been found with a clear break in spectral slope at around 10 Hz, suggesting two regimes for irregularities of different scale sizes.     

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.     

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.     

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.     

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.     

The nature of ionospheric spread-F irregularities in mid-latitude regions

Initially, this paper considers earlier experimental results (some of them hitherto unpublished) obtained by making observations on signals returning from mid-latitude spread-F irregularities. These results suggest associations between spread-F irregularities and nighttime travelling ionospheric disturbances. Statistical analyses are then described which investigate the spread-F phenomenon at a number of mid-latitude stations with approximately the same latitudes but distributed over a range of longitudes. An east-west movement of spread-F irregularities is revealed when the occurrence at these stations is considered relative to days of enhanced occurrence at a particular station. All the experimental evidence presented in the paper supports the idea that the appearance of mid-latitude spread-F ionograms results primarily from specular reflections from relatively-large-scale structures which can be imagined as being in fact nighttime travelling ionospheric disturbances. These are, in turn, possibly related to internal gravity waves in the neutral atmosphere. It is suggested that the small-scale ionospheric structures (which are undoubtedly also present) are effective in inhibiting some of the specular reflections thus contributing to the diffuse nature of some records. This idea is quite contrary to the generally-accepted view that the spread-F traces are a direct consequence of scattering from these small-scale structures.     

Spatial and temporal distributions of midlatitude ionospheric scintillations observed by low-altitude satellites

Spatial and temporal distributions of ionospheric scintillations have been observed at Kashima (36.0°N, 140.7°E) using VHF and UHF signals from low-altitude satellites. From these observations, three different types of prevailing ionospheric scintillations seen from Japan are identified. Scintillations of type I are rather weak scintillations, occur most frequently during the daytime in summer and are primarily associated with the sporadic E-layer. However, considerable occurrences of type I scintillations are also observed during the night in summer and autumn, not necessarily due to the sporadic E-layer but occasionally due to F-layer irregularities which originate from localized midlatitude processes. Type II scintillations are much stronger than type I and occur near the equatorward horizon during spring, summer and autumn. Their occurrences start after sunset, reach a maximum before midnight and decrease subsequently, with a tendency for negative and positive correlations with the magnetic and solar activities, respectively. It is concluded that type II scintillations are the midlatitude aftermath of equatorial plume-associated irregularities and cause trans-equatorial propagation of VHF waves. From observations of type I and II scintillations, the boundary between midlatitude and equatorial scintillations is clearly identified. Type III scintillations are as strong as type II and appear only during magnetically active periods. They can be regarded as another aspect of the severe scintillation events observed on gigahertz waves from geostationary satellites as reported by Tanaka (1981).     

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.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.     

Generation of stationary irregularities due to heating of the high-latitude ionospheric E-region

The nonlinear equations describing the generation of artificial irregularities in the E-region of the high-latitude ionosphere due to Joule electron heating under the action of high-power radio waves are derived, including the low-frequency nonlinearities, and investigated in the three-wave approximation. Expressions for the stationary spectra of the short wavelength two-stream and long wavelength gradient-drift and current-convective irregularities are presented. The typical saturation amplitudes of the density fluctuations are obtained.     

Characteristics of ionospheric irregularities capable of producing quasi-horizontal traces on ionograms

This paper presents simulated ionograms calculated for a parabolic ionospheric layer containing irregularities in the form of small amplitude waves. With small amplitudes, perturbation techniques can be used enabling results for the irregular ionospheres to be calculated from the results for smooth ionospheres. This approach is relatively straightforward and avoids having to ray trace new paths each time the irregularity parameters are changed. It is, however, restricted to irregularities which do not cause multiple echoes. Irregularities with vertical wavelengths of up to a few kilometres can produce significant changes in the ionosphere over height intervals smaller than those involved in reflecting a single pulse. Consequently, in the simulation procedure, it is essential to consider not just the carrier frequency but the complete frequency spectrum of the pulse. Irregularities with vertical wavelengths of the order of 10 km or more can produce ripples in an ionogram trace. These will, of course, be more evident on ionograms with high frequency resolution. Irregularities with vertical wavelengths of up to several kilometres and amplitudes up to a few per cent can produce significant pulse spreading and splitting. The actual effects depend not just on the irregularity properties but also on the ionosonde pulse width, gain and frequency and height resolutions. Some simulations show trace splitting and quasi-horizontal traces similar in many respects to effects observed by Bowman (1987, J. atmos. terr. Phys. 49, 1007) and Bowmanet al. (1988, J. atmos. terr. Phys. 50, 797). Consequently it is suggested that, at least in some cases, small amplitude (≤3%) and small scale (≤4 km) irregularities produce the spread-ifF reported by these authors.     

Periodic amplitude variations as precursors of plumes of equatorial ionospheric irregularities

Periodic amplitude fluctuations of VHF signals from a geostationary satellite monitored from near the magnetic equator have been observed in the evening hours as precursors of strong Rayleigh fading associated with plumes of irregularities. These periodic fluctuations called “amplitude waves” exhibit amplitude changes of only 1 to 2 dB and have been observed for up to 30 minutes before the onset of strong scintillations. Individual fades are correlated over distances of at least 120 km in the magnetic eastwest direction. The velocity of these wavelike disturbances has been found to be approximately 140 ms⁻¹ eastward with a corresponding wavelength of 25 km. No wavelike behavior of Faraday rotation, a measure of the background changes in TEC, was observed during these times. Several mechanisms are examined as the cause of these amplitude waves; however, none was found to be completely satisfactory in explaining the observations.     

The global distribution of ionospheric small-scale irregularities from topside sounding data

This paper examines the global distribution of electron density irregularities with scales of the order of several tens to hundreds of meters in the ionosphere by using topside sounder data from the COSMOS-1809 satellite obtained in May–June and December 1987. The diffuse traces of Z-waves on topside ionograms in a frequency band just below the upper hybrid resonance are used for diagnostics. These traces are attributed to the scattering of sounder-generated ordinary and slow extraordinary mode waves.     

Mid-latitude ionospheric scintillations of VHF radio signals associated with peculiar fluctuations of Faraday rotation

Simultaneous measurements of the scintillation and Faraday rotation of 136 MHz radio waves radiated by the Japanese geostationary satellite ETS-2 have been studied at Kokubunji Japan during April to May, 1977.These preliminary results show that there is a close association between scintillations and the peculiar fluctuations of the Faraday rotation due to ionospheric spread F, and an empirical formula relating them is presented.Furthermore, the successive daily values of the two parameters show a close correspondence with a cross correlation coefficient of about 0.8. An unexpected result is that cross-correlation between the incidence of Es and the Faraday fluctuations on a daily bases is found to be about 0.65.     

spread-F ionospheric irregularities and their relationship to stable auroral red arcs at magnetic mid-latitudes

The signature of the stable auroral red arc (SAR arc) as it appears on ionograms is described. The key features are a very significant increase in the amount of spread-F and a reduction in the maximum plasma density compared with regions just equatorward and poleward of the SAR arc Identification of the SAR arc signature is made by using complementary data from the global auroral imaging instrument on board the Dynamics Explorer-1 satellite.At sunspot minimum there is a positive correlation between the occurrence of spread-F on ionograms from Argentine Islands, Antarctica (65°S, 64°W; L = 2.3) and magnetic activity. In contrast, at sunspot maximum there is a weak negative correlation when the K magnetic index is less than 6. but a significant increase in spread-F occurrence at K ⩾ 6. Detailed study of ionograms shows that there are two distinct regions where considerable spread-F is observed. These are the region where SAR arcs occur and the poleward edge of the mid-latitude ionospheric trough. They are separated by a region associated with the trough minimum, where comparatively little spread-F is seen. It is suggested that the movement of these features to lower latitudes with increasing magnetic and solar activity can explain the lack of correspondence between variations of spread-F occurrence as a function of magnetic activity at sunspot maximum compared with that at sunspot minimum at Argentine Islands.     

Small scale ionospheric irregularities near regions of soft particle precipitation: scintillation and EISCAT observations

Results are presented from a coordinated experiment involving scintillation observations using transmissions from NNSS satellites and simultaneous measurements with the EISCAT ionospheric radar facility. The scintillation was used to indicate the presence of sub-kilometre scale irregularities while the radar yielded information on the larger structures in the background ionosphere. Two examples are discussed in which localised patches of scintillation were observed at L-shells near ‘blob’ like enhancements in F-region ionisation density. Elevated electron temperatures indicated that the enhancements may have had their origins in soft particle precipitation. While structuring of the precipitation on the 100 m scale cannot be completely ruled out as a source of the irregularities, in one case the blob gradient can be shown to be stable to the E λ B mechanism. The most likely cause of the irregularities appears to be shearing of the high velocity plasma flow in a region adjacent to the density enhancement. This region is characterised by a high ion temperature while the resulting scintillation has a shallow spectral slope.     

Stationary large-scale irregularities of the ionosphere

Ionospheric absorption measurements (Al method) made in the course of eight voyages by Soviet and other research vessels indicate that the global distribution of absorption shows a distinctive regional structure. Areas of abnormally high absorption in the neighbourhood of the equator have been located in the Pacific near the west coast of South America and in the Indian Ocean. The west Mediterranean area also shows abnormally high absorption. In some cases these areas of high absorption appear to coincide with areas of low nocturnal F-region electron density.     

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.