Equatorial Irregularities Research Articles

Drift speeds of equatorial electrojet irregularities of kilometre and metre sizes

Drift speeds of electron density irregularities in the equatorial electrojet region of sizes in the kilometre range (from VHF scintillations) and a few metres range (from VHF backscatter radar) have been compared under type I and type II irregularity conditions. The drift speeds, over the irregularity size range studied, match only when type II conditions prevail. The implications of the results are discussed in the light of the electrojet irregularity characteristics.

Electron density irregularities observed on DE‐2

Observations of electron density irregularities have been made with the Langmuir probe (LANG) on DE‐2. The DE‐2 LANG data were examined for irregularities with scale sizes of 30 to 170 km. Such irregularities were found at all longitudes in the polar cap and auroral oval with stronger fluctuations in the oval. Night time equatorial passes having local times near 1900 or 2400 LT and occurring in an 80 day wide band about equinox were examined for irregularity occurrence. A definite longitude pattern was found in the data from several hundred orbits which showed an eastward shift at later local times. The equatorial irregularity occurrence pattern found in the LANG data is consistent with earlier in situ and remote observations of irregularities and spread F. In fact, the combined data set was found to closely follow the season‐longitude pattern determined by the condition of solar terminator alignment with magnetic field lines. Tsunoda (1985) first showed this correlation with scintillation data.

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 ( p 1 ~ 1.5) at long scalelengths (> 1km) and steep slope ( p 2 ~−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.

Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity

An enigma of equatorial research has been the observed seasonal and longitudinal occurrence patterns of equatorial scintillations (and range‐type spread F). We resolve this problem by showing that the seasonal maxima in scintillation activity coincide with the times of year when the solar terminator is most nearly aligned with the geomagnetic flux tubes. That is, occurrence of plasma density irregularities responsible for scintillations is most likely when the integrated E‐region Pedersen conductivity is changing most rapidly. Hence the hitherto puzzling seasonal pattern of scintillation activity, at a given longitude, becomes a simple deterministic function of the magnetic declination and geographic latitude of the magnetic dip equator. This demonstrated relationship is consistent with equatorial irregularity generation by the collisional Rayleigh‐Taylor instability and irregularity growth enhancement by the current convective and (wind‐driven) gradient drift instabilities. Some discrepancies in this relationship, however, have been found in scintillation data obtained at lower radio frequencies (below, say, 300 MHz) that suggest the presence of other irregularity‐influencing processes. The role of field‐aligned currents, associated with the longitudinal gradient in integrated E‐region Pedersen conductivity produced at the solar terminator, in equatorial irregularity generation via the current convective instability has not been discussed previously.

Conjugate studies of an isolated equatorial irregularity region

Coordinated ground and airborne measurements were conducted at three points along the Ascension Island magnetic meridian to investigate the distribution of scintillation‐producing (km scale) ionospheric irregularities within an isolated equatorial plasma depletion. Airglow measurements near the northern Appleton anomaly show that irregularities are confined to the region within a 6300‐Å airglow depletion. The airglow measurements are then used to map the depletion region along magnetic field lines for comparison with scintillation measurements near the magnetic equator and near the conjugate point at Ascension Island. Results show that the irregularity region is magnetic flux tube aligned and conjugate over north‐south distances of ∼3000 km.

Global morphology of ionospheric scintillations

Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

Comment on ‘The absolute scattering cross section at 50 MHz of equatorial electrojet irregularities’ by Farley et al.

Comment on ‘The absolute scattering cross section at 50 MHz of equatorial electrojet irregularities’ by Farley et al.

Occurrence of nighttime VHF scintillations near the equatorial anomaly crest in the Indian sector

The behavior of nighttime F region irregularities near the northern crest of the equatorial anomaly in the Indian sector has been investigated by using VHF amplitude scintillation measurements made at Calcutta (27°N dip subionospheric) during the period April 1977 through February 1980. With the increase in solar activity the occurrence of scintillations increases remarkably during the equinoxes and to a lesser extent during the December solstice, while the local summer occurrence shows little change. The observed patterns are assessed in terms of the variation of the F layer height with solar activity and upwelling motion of the depleted flux tubes associated with small scale irregularities. It is shown that the propagation path is more likely to intercept these equatorial irregularities during periods of high solar activity, while during periods of low solar activity the equatorial irregularities lie south of the propagation path.

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.

Time evolution and dynamics of equatorial backscatter plumes 1. Growth phase

Much of our understanding of equatorial spread‐F phenomena has resulted from VHF radar studies of backscatter ‘plumes’ i.e., patterns of backscatter that appear to extend from the bottomside into the topside of the F layer. By using ALTAIR, a fully steerable backscatter radar, to map selected plumes repeatedly, it was found that the dynamic behavior of plumes can be characterized by a growth and a decay phase. The growth‐phase characteristics described in this paper are found to be generally consistent with equatorial irregularity generation as predicted by the Rayleigh‐Taylor and gradient‐drift instabilities. In particular, (1) plumes are generated primarily in the local time around F layer sunset, (2) plumes develop from local perturbations in plasma density in the bottomside of the F layer, (3) plumes develop upward with growth velocities ranging from 125 to 350 m/s, and (4) backscatter strength increases during upward plume development and decreases when upward plume growth slows or ceases. However, certain characteristics, namely, an east‐west asymmetry in backscatter strength found in the bottomside F layer, have been found that suggest key roles played by the eastward neutral wind and by altitude modulation of the bottomside F layer in establishing the initial conditions for plume growth.

Influence of the equatorial irregularities and precipitations in the South Atlantic Magnetic Anomaly on the generation of auroral‐type plasma instabilities

Observational evidence of upward field‐aligned beams in the keV range has been obtained in the sub‐equatorial ionosphere above South America. These events can be related to coupled magnetic and ionospheric activity (magnetic storm, ionospheric irregularities). This result is in opposition with the current theory of the low‐latitude ionosphere. Its interpretation must assume that conditions exist for the growth of plasma instabilities. This implies a low plasma density, a close coupling between the ionosphere and the magnetosphere, and field‐aligned currents. Such suitable conditions have independently been observed in ionospheric irregularities (density, currents) or during magnetic storms (energetic particle precipitation) or they are deduced from the structure of the Anomaly (field‐aligned currents). This allows us to suggest that the South Atlantic Anomaly sometimes compares to the auroral oval and may develop some current‐driven plasma instabilities.

Long‐term 1.5 GHz amplitude scintillation measurements at the magnetic equator

The diurnal and seasonal behavior of long‐term (approximately 20 months) 1.54 GHz amplitude scintillation measurements under low sunspot conditions observed at Huancayo, Peru are presented. The measurements refer to a low elevation geometry in the E‐W plane at the magnetic equator. The data show clear equinoctial maxima of occurrence and confinement to the pre‐midnight hours. There is a total absence of scintillations during the May‐July period.The present results are critically compared to previously reported GHz measurements in the context of geometry of observations, equatorial irregularity strength variation and spatial structure information obtained from in‐situ data.

The dynamics of equatorial irregularity patch formation, motion, and decay

Using scintillation observations from a series of equatorial propagation paths as well as backscatter and airglow data, the development, motion, and decay of equatorial irregularity patches have been studied. Assembling the results of earlier studies in the field with our observations, we find the following: the patch has limited east‐west dimensions with a minimum of 100 km. Several patches may be melded together to reach an extent of 1500 km. Its magnetic north‐south dimensions are often greater than 2000 km; the most intense irregularities (as evidenced by the Jicamarca radar at the dip equator) are from 225 to 450 km in altitude, although irregularities are found as high as 1000 km. The patch initially has a westward expansion following the solar terminator, then, maintaining its integrity, moves eastward. Evidence over a limited series of experiments suggests that premidnight patches are formed within 1½ hours after ionospheric sunset in the absence of special magnetic conditions. From Ascension Island (∼16°S dip latitude) the individual patches can be clearly distinguished. The decay of patches in the midnight time period was studied, pointing to a rapid decrease in scintillation intensity in this time period.

True height calculation from ionograms

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.

On the coexistence of kilometer‐ and meter‐scale irregularities in the nighttime equatorial F region

Nighttime multifrequency scintillation and 50‐MHz radar backscatter observations simultaneously performed over a nearly common ionospheric volume at the dip equator in Peru during March 1977 were used to study the relationship between the large‐scale irregularities (∼0.1–1 km) giving rise to scintillations and small‐scale irregularities (3 m) causing 50‐MHz backscatter. It is shown that during the generation phase of equatorial irregularities in the evening hours, the kilometer‐ and meter‐scale irregularities coexist, whereas in the later phase, approximately an hour after the onset, the meter‐scale irregularities decay but the large‐scale ones continue to retain their high spectral intensities. Further, multistation scintillation observations from a host of geostationary satellites as well as from the Wideband satellite indicate that eastward‐drifting irregularity structures detected around midnight cause significant scintillations at UHF and L band but generally fail to give rise to appreciable backscatter. Thus, contrary to expectations, it is possible to have even L band scintillations without any plume structure on backscatter maps. This indicates that at later local time a cutoff of the spectral intensity probably occurs at some scale length between 100 and 3 m. These observational results are discussed in the context of current theories of plasma instability in the equatorial ionosphere.

Ionospheric holes and equatorial spread F: Chemistry and transport

Existing experimental and theoretical results are synthesized with new ion mass spectrometer data from Atmosphere Explorer C to provide a more comprehensive perspective on the understanding of nighttime equatorial irregularities. Particular attention is given to possible relationships between ionospheric holes and smaller‐scale (3 m) irregularities observed by ground‐based radar. Ion chemistry and transport considerations are found to support the concept that equatorial holes and equatorial spread F can be one and the same phenomenon, with the smaller‐scale irregularities imbedded within the much larger scale ionospheric depletions.

Equatorial irregularity belt and its movement during a magnetic storm

GEOSTATIONARY satellites are able to monitor amplitude, phase, plane of polarisation (Faraday rotation) and group retardation of radio waves passing through the ionosphere with improved temporal resolution, although some of these parameters were earlier measured using orbiting satellites. An extensive experiment has been conducted on Faraday rotation measurements at a chain of stations covering a latitude region from the magnetic equator to 45°N dip (30°N geog.) in the Indian sub-continent using 140 MHz radio beacons received from ATS-6 satellite1. We report here the first evidence of an irregularity belt and its movement during a magnetic storm.

Auroral and equatorial two-stream irregularities: Difference in nonlinear state

It is shown that the electrojet velocity is reduced to the ion acoustic velocity by the two-stream irregularities if the width of the unstable layer is larger than, say, several hundred meters. However, if it is of the order of 100 m or smaller, the electrojet velocity is not appreciably changed, but the electron density is modified, the instability thereby being stabilized. On the basis of these results it is postulated that electrojets responsible for diffuse radar auroras are occasionally highly irregular and consist of many narrow streams with widths of the order of 100 m. Then the observational fact that irregularities with velocities larger than the ion acoustic velocity exist can be explained.

Equatorial scintillations: A review

With the advent of satellite communications systems at frequencies varying from \sim140 MHz to 1600 MHz as well as navigation and ranging systems in the 1200-1600 MHz portions of the spectrum, the effect of equatorial irregularities on fading signals has become of importance. Recent observations of the signal statistics of scintillations at frequencies ranging from 136 MHz to 6 GHz reveals a power law fall off of irregularity sizes. Power spectra are now available for a variety of conditions and for frequencies from VHF to microwaves. During periods of intense equatorial activity, at frequencies to 360 MHz, Rayleigh scattering is frequently experienced. The latitudinal extent of the scintillation irregularity region has been established with a half occurrence width during years of moderate solar flux of plus and minus 12\deg . A correlation of in-situ measurements of irregularities from satellites had revealed the great variations in longitudinal patterns during any season. It has also allowed the \Delta N obtained from in-situ measurements to be utilized to predict scintillation excursions. New facets of scintillation activity in the equatorial region recently reported include weak daytime scintillation and the patchy nature of irregularities (small irregularities embedded in large structures) particularly noted during periods of low solar flux. Future studies will assist in the delineation of the extent of the equatorial region at frequencies from 1.5 to 6 GHz, and in the UHF range. From the viewpoint of the communicator the morphology of scintillations at microwaves has still not been reported from any long term program of measurements.

Review of equatorial scintillation phenomena in light of recent developments in the theory and measurement of equatorial irregularities

Recent developments in the understanding of equatorial irregularities have come from a unique combination of rocket, satellite, radar, theoretical, and computer simulation investigations. These will be reviewed with emphasis on the effect of the irregularities upon the propagation of radio waves. Recent scintillation calculations based upon in situ rocket and satellite irregularity measurements are also reviewed and a new composite model presented here which explains many properties of the scintillation phenomena associated with equatorial spread- F.