Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—I. A method of estimating electric fields

Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V_p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E_y in the EEJ can be deduced from the experimental height profiles of V_p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E_y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated.It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E-region.     

F-layer irregularities' formation at auroral latitudes: radio wave scintillation and EISCAT observations

A coordinated experiment involving scintillation observations using orbital satellite beacons and CP-3-F program measurements by means of the EISCAT ionospheric radar facility is described. The results reveal the location of patches, containing kilometre-scale irregularities, in the vicinity of a region of an electron density minimum and an electron temperature increase. In the daytime under quiet geomagnetic conditions, the region of scintillations coincided closely with the southwards gradient in electron density, while a plasma drift velocity was mainly westwards V_E-W ≲ 0.3 km/s. In the evening, the region of the most intense irregularities was transformed to the northwards sense of the electron density gradient simultaneously with the plasma drift velocity reverse and the arrival of a significant southwards component V_N-S ≲ (1.5−1.0) km/s. EISCAT data demonstrated the patches' location in regions of an electron temperature increase. Processes operating to create kilometer-scale irregularities were analysed and estimated according to the data obtained. The assessments suggest that irregularities with a cross-field scale, equal to or greater than 1 km, and a field-aligned scale, equal to or greater than 30 km, were the result of growth of the thermomagnetic instability.     

Equatorial scintillations: advances since ISEA-6

Since the last equatorial aeronomy meeting in 1980, our understanding of the morphology of equatorial scintillations has advanced greatly due to more intensive observations at the equatorial anomaly locations in the different longitude zones. The unmistakable effect of the sunspot cycle in controlling irregularity belt width and electron concentration responsible for strong scintillation in the GHz range has been demonstrated. The fact that night-time F-region dynamics is an important factor in controlling the magnitude of scintillations has been recognized by interpreting scintillation observations in the light of realistic models of total electron content at various longitudes. A hypothesis based on the alignment of the solar terminator with the geomagnetic flux tubes as an indicator of enhanced scintillation occurrence and another based on the influence of a transequatorial thermospheric neutral wind have been postulated to describe the observed longitudinal variation.A distinct class of equatorial irregularities known as the bottomside sinusoidal (BSS) type has been identified. Unlike equatorial bubbles, these irregularities occur in very large patches, sometimes in excess of several thousand kilometers in the E-W direction and are associated with frequency spread on ionograms. Scintillations caused by such irregularities exist only in the VHF band, exhibit Fresnel oscillations in intensity spectra and are found to give rise to extremely long durations (~ several hours) of uninterrupted scintillations. These irregularities maximize during solstices, so that in the VHF range, scintillation morphology at an equatorial station is determined by considering occurrence characteristics of both bubble type and BSS type irregularities.The temporal structure of scintillations in relation to the in situ measurements of irregularity spatial structure within equatorial bubbles has been critically examined. A two-component irregularity spectrum with a shallow slope (p₁ ~ 1.5) at long scalelengths (> 1km) and steep slope (p₂ ~−3) at shorter scalelengths has been found in both vertical and horizontal spectra. Phase and intensity scintillation modelling was found to be consistent with this two-component irregularity spectrum.Finally, the information provided by the major experimental undertaking represented by Project Condor in the fields of night-time scintillations and zonal irregularity drifts with be briefly outlined.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

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.     

Small-scale ionospheric structures associated with mid-latitude spread-F

Slant-F traces on ionograms recorded by a modern ionosonde in a sunspot-minimum period have revealed the existence of field-aligned irregularities at times of spread-F occurrence. This appears to be the first investigation in a mid-latitude region around 36° (geomagnetic) to detect these irregularities at F₂-region heights using an ionosonde. Although such traces were observed frequently near sunspot minimum they were seldom recorded for periods close to sunspot maximum. Also, for a specific spread-F event in August 1989, both the ionograms from the modern ionosonde and scintillations of 150 MHz transmissions from a Transit satellite indicate the existence in the ionosphere of periodic structures (period around 11 min). The scintillation recording also included rapidly fading signals indicative of small-scale structures. The satellite had a path close to the magnetic meridian which passed through the recording station (Brisbane, Australia). Because of the enhanced signal fluctuations in the scintillation recording on this occasion it seems likely (with the support of other evidence on the ionograms) that the small-scale structures present were field-aligned.     

Ionospheric scintillation observations with radio interferometry

Radio astronomical interferometric observations are affected by atmospheric refraction, being particularly sensitive to inhomogeneities in the atmosphere. At frequencies below 2 GHz the influences of the ionosphere are significant in radio astronomy, especially for single dish observations and for connected element interferometry.Analytical expressions for the manifestations of weak ionospheric scintillation in radio interferometric observations, are derived. We indicate which ionospheric scintillation parameters can be derived from radio interferometric measurements. It is shown that the baseline dependence of the observed amplitude scintillation index implies a direct determination of the height of the region of random irregular electron distribution. Furthermore, the linear scale of the irregularities causing scintillation can be determined directly from the baseline dependence of the scintillation index S₄. From the mean square phase fluctuations as a function of interferometer baseline, the spatial scale of the irregularities responsible for this effect can also be determined. From a comparison with observational mid-latitude data we find indications that scintillation irregularities occur in the lower parts of the F2-layer. The spatial scale of irregularities causing amplitude scintillation is of the order of about 25 to about 500 metres. Phase scintillations are caused by irregularities with dimensions which are an order of magnitude larger.     

Using the STARE double pulse system to obtain the mean velocity and velocity of long-lived irregularities

The STARE and SABRE auroral radars use double pulses to measure one value of the autocorrelation function (ACF) to obtain the Doppler velocity of the irregularities in the E-region auroral zone. If the pulse separation of each double pulse is τ, then the measured velocity tends towards the mean velocity V_DP as τ → 0, and towards the velocity of the long-lived irregularities V_LL as τ → ∞. A practical implementation using these results is presented, which allows measurement of these two physically important velocities from just four pulse spacings, rather than the 11 or 15 currently used to obtain the full velocity spectrum in the STARE and SABRE radars, and thus gains greater precision or better time resolution. The two line-of-sight velocities, V_DP and V_LL, are in general not equal, with V_LL greater in magnitude than V_DP (sometimes much greater). Comparison of V_DP and V_LL for the two STARE stations and one of the SABRE stations shows interesting differences, and a number of statistically significant features. V_LL probably measures the velocity of primary irregularities whenever these exist with the appropriate k vector. Both velocities ought to be measured routinely, and also for comparison with other techniques.     

Equatorial scintillations—a review

The scintillation technique, as is well known, provides an integrated measure of phase and amplitude fluctuations imposed on radio signals over a wide range of frequencies during their propagation through the ionosphere. The large amplitude of equatorial irregularities necessitates the use of frequencies in the GHz band to obtain unambiguously the temporal variation of irregularity intensity and the effect of irregularity anisotropy. Recent observations of equatorial scintillations will be reviewed with an emphasis on GHz measurements. The steep spatial gradients observed in in-situ data and their relationship to intense GHz scintillations will be explored. Co-ordinated measurements of equatorial irregularities by such techniques as radar backscatter, in-situ rocket and satellite, total electron content and 6300 Å airglow will be discussed, insofar as they provide a better understanding of the scintillation phenomena. While it is difficult to critically assess results that are so recent and constantly evolving, we have attempted to focus attention on the outstanding problems that still remain in the field.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—III. Comparison of observed ΔH values with those estimated from measured electric fields

From VHF backscatter radar measurements at Thumba (dip: 56′S) of the phase velocities of type II irregularities in the equatorial electrojet (EEJ), electric field (E_y) values are estimated for different times of the day. Using the electric field values thus deduced and the Pedersen and Hall conductivities calculated using model values of electron densities and the collision frequencies of ions and electrons, the height integrated current intensity in the EEJ is estimated. The surface level geomagnetic field perturbation ΔH produced by this ionospheric current is then calculated. The calculated values of ΔH are compared with observed values of ΔH (after subtracting the magnetospheric contribution of D_st) for a number of days. The comparisons show good agreement between observed ΔH values and those calculated from measured electric fields. The agreement is found to be good even when type I irregularities are present at higher altitudes in the EEJ. This comparative study demonstrates the validity of estimating electric field values from VHF radar measurements and it indicates the possibility of deducing electric field values from ground level ΔH values, at least for statistical studies.     

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.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—II. Characteristics of electric fields on quiet and disturbed days

Using the Doppler spectra of VHF radar signals, the height profiles of the phase velocity (V_p) of 2.7 m irregularities in the equatorial electrojet (EEJ) over Thumba (dip: 56′S) are obtained. The day-time east-west electric fields (E_y) are deduced by matching experimentally observed V_p profiles with theoretically deduced ones for a number of quiet and disturbed days. The experimental E_y values show: (i) a large day-to-day variability; (ii) a large decrease in the afternoon hours on some days (quiet and disturbed); (iii) the frequent presence of short period fluctuations with amplitudes of 30–50% of the background value and with typical time scales of 30–60 min, on moderately disturbed days (9 ⩽ A_p ⩽ 30); (iv) a significant decrease of the average E_y on disturbed days compared to that on quiet days during 0900–1200 h L.T.     

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).     

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.     

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.     

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.     

Ion-neutral collisions and the equatorial electrojet

There are various theoretical models of ion-neutral collision and the resultant collision frequencies (v_in) differ significantly in their values. Also there is a range of uncertainty associated with each of the theoretical values of v_in. The effects of the differing theoretical values and the uncertainty in the v_in on the estimates of the equatorial electrojet (EEJ) current strength are examined numerically. It is found that the differences in various v_in-models affect the amplitude of the EEJ, but leave the peak altitude of the EEJ nearly unchanged. However, modification to v_in by the order of its uncertainty does change the peak altitude of the EEJ by at least 2km.     

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.     

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.