Irregularity Drift Research Articles

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.

Zonal drifts of ionospheric irregularities at temperate latitude in the Indian region

The systematic time differences observed in the onset of postsunset VHF scintillations recorded simultaneously at Ujjain (Geogr. lat. 23.2°N, Geogr. long. 75.6°E) and Bhopal (Geogr. lat. 23.2°N, Geogr. long. 77.6°E), situated at the peak of the anomaly crest in the Indian region, have been analysed to determine the zonal drifts of scintillation-producing irregularities. The method is based on the assumption that the horizontal movement of irregularities does not change while crossing the F-region cross-over points of these stations. The calculated velocities of irregularities indicate an eastward drift decreasing from about 180 m s–1 to 55 m s–1 during the course of night. In the premidnight period, the drifts are reduced under the magnetically disturbed conditions. The average east-west extension of irregularites is found to be in the range of 200–500 km.

Models for horizontal E- and F-region drifts

Plasma transport is very important for understanding the space-time variations of the ionosphere. Therefore, following a resolution of URSI Subcommission G4, an effort is made to create a computer code describing the main results of investigations the ionospheric drift which were not considered in IRI-1979. The experimental data from 23 stations in the Northern Hemisphere were obtained between 1957 and 1970. The worldwide coverage in geographic latitude is 7°N to 71°N (7.5° to 64.1° geomagnetic) and O° to 131°E geographic longitude. We have developed appropriate procedure which allow us to infer from these data the main parameters of the global ionospheric motions at E- and F-region levels. An algorithm for computing the zonal and meridional drift components VX, VY can be found in IRI-1990. The last version of the computer programm called DRIFT which does the test calculation of Ionospheric Drifts Global Model whith printing the tables at the Epson printer is written in Turbo ascal for the IBM PC AT 286/287 compatible computers. Program code (execute module) is about 25 Kbyte. Data files are about 10 Kbyte. E- and F-region horizontal ionospheric irregularities drift data, worldwide obtained from 1957 to 1970 by D1 and D3 methods, are statistically analysed and a computer code for the average velocity variations in latitude and local time for some solar activity levels is constructed. The PC program DRIFT allows to determine zonal and meridional drift velocities of ionospheric irregularities at the lower (90 < h < = 140 km) and upper (h > 140 km) ionosphere. The main block of the program DRIFT is the procedure DRIRR for calculating VX and VY for a period (P), geomagnetic (geographic) latitude (FI) and local time (LT) to be specified. The example of the program DRIFT calculation for F-region (REG=2) and for the whole period of observations (P=1) is in Table. VX > 0 to east, VY > 0 to north. FI is geomagnetic latitude.

Analysis of electromagnetic wave scattering by ionospheric irregularities

A quasi‐particle approach is used to analyze the effect of bottomside sinusoidal irregularities on the propagation of beacon satellite signals through the ionosphere. In this work, the radio wave is treated as a distribution of quasi‐particles described by a Wigner distribution function, and the irregularities are modeled as a two‐dimensional density modulation on an uniform background plasma. The collision of quasi‐particles with the irregularities modifies the quasi‐particle distribution and gives rise to the wave‐scattering phenomenon. The multiple scattering process is generally considered in this deterministic analysis of radio wave scattering off the ionospheric density irregularities. The analysis shows that this two‐dimensional density grating effectively modulates the intensity of the beacon satellite signals. This spatial modulation of the wave intensity is converted into time modulation due to the drift of the ionospheric irregularities, which then results in the scintillation of the beacon satellite signals. The modulation is due mainly to the transverse (to the wave propagation) grating. The results of the analysis agree well with the existing experimental data on the scintillation of Atmosphere Explorer satellites signal (Basu et al., 1986). The model is further based on to predict the scintillation of radio signals for their long distance propagation. For a fixed scintillation index (S4) the predicted propagation distance (d) versus the wave frequency (ƒ) follows a power law, namely, d∝ƒK, where k ≈ 2.

Characteristics of small‐scale ionospheric irregularities as deduced from scintillation observations of radio signals from satellites ETS‐2 and Polar Bear 4 at Irkutsk

This paper presents some new results on the small‐scale inhomogeneous ionospheric structure obtained at a facility for spaced‐antenna reception of transionospheric signals from ETS‐2 and Polar Bear 4 near Irkutsk (Eastern Siberia, 52°N, 104°E). A technique based on transferring time spectra of scintillations to spatial spectra using measured horizontal irregularity drift velocities is used to obtain an estimate of the mean spatial spectrum of midlatitude scintillations. Two different methods were used to determine the inclination index of the scintillation spectrum, which was found to be equal to −2, in agreement with the value recently predicted for small‐scale F region irregularities generated through mapping of small‐scale, turbulent electric fields from the E region to the F region. Drift velocities of the diffraction pattern, and also the altitudes at which ionospheric irregularities are located, agree well with results obtained by other authors for midlatitudes. Using simultaneous measurements for a geostationary satellite and an orbiting satellite, the supposition about the existence of the southern boundary of the scintillation region has been confirmed. Finally, analysis of quasi‐periodic (QP) scintillations and simultaneously determined diffraction pattern velocities is used to show that the height of isolated irregularities giving rise to QP scintillations corresponds to the maximum of the ionospheric F2 region.

Correction to “An assessment of the L shell fitting beam‐swinging technique for measuring ionospheric E region irregularity drift patterns”

Correction to “An assessment of the <i>L</i> shell fitting beam‐swinging technique for measuring ionospheric <i>E</i> region irregularity drift patterns”

Simultaneous observations of E and F region electric field fluctuations at the magnetic equator

Simultaneous daytime observations of E region horizontal irregularity drift velocities in the equatorial electrojet and F region vertical plasma drifts were made on a few magnetically quiet days at the magnetic equatorial station of Trivandrum (dip 0.5°N). Measurements of the electrojet irregularity velocities by VHF backscatter radar and the F region vertical plasma drifts by HF Doppier radar are used to deduce the daytime East-West electric fields in the E and F regions, respectively. The fluctuating components of the electric fields are separated and subjected to power spectral analysis. The E and F region electric field fluctuations are found to be well correlated; the estimated correlation coefficient is in the range of 0.52–0.8. The fluctuation amplitudes are of the order of 15% over the background for the E region and 25% for the F region. The spectral analysis reveals dominant components in the range of 30–90 min with F region components stronger than those of the E region by a factor of about 1.5 on the average. The F region electric fields during daytime being coupled from the low latitude E region, the good correlation observed between the E and F region perturbations suggests that the electric fields in the E region at low and equatorial latitudes are coherent for the temporal scales of the order of few tens of minutes. The spectral characteristics are such that the commonly occurring medium scale gravity waves could possibly be the source for the observed fluctuations in the E and F region electric fields.

An assessment of the L shell fitting beam‐swinging technique for measuring ionospheric E region irregularity drift patterns

The majority of current ground‐based radar experiments measuring ionospheric convection flows derive the ionospheric flow vectors from line‐of‐sight (l‐o‐s) velocity measurements made from a single radar site. In order to deduce vector information from such a set of scalar measurements, assumptions have to be made about the spatial and temporal structure of the flow pattern. These assumptions involve an actual or effective beam‐swinging technique. In this study the flow direction estimates from the application of sophisticated beam‐swinging software, developed by the Applied Physics Laboratory of the Johns Hopkins University, to over 20,000 estimates of the l‐o‐s ionospheric E region mean irregularity drift velocity from the Wick radar of the Sweden And Britain Radar‐auroral Experiment (SABRE) system, are compared with merged velocity vectors derived from data from both SABRE radar sites. The application of the beam‐swinging software to VHF E region backscatter data is found to lead to flow direction estimations which are in error a significant amount of the time, with beam‐swinging‐derived drift velocities lying outside a cone of half width 30° around the merged SABRE drift velocities in 30% or more of cases. Both curvatures and gradients in the ionospheric flow pattern measured by SABRE cause distortions of the beam‐swinging‐evaluated flow patterns. Moreover, from an examination of the Wick l‐o‐s velocity data and the data fitting calculations alone, it is often not possible to distinguish between accurate and inaccurate beam‐swinging flow direction estimates. Criteria are discussed which can be employed to increase confidence in the beam swinging results. Although the present comparison is confined to E region irregularity drift velocity measurements, these criteria probably have implications for all monostatic experiments employing a beam‐swinging technique (e.g., coherent and incoherent radars and optical instruments). It is concluded that it is highly desirable for the next generation of ionospheric radars to be capable of making common‐volume measurements from two well‐separated sites.

Substorm‐associated radar auroral surges

We report a recurrent convection signature observed in the E region ionosphere within ∼2 hours of the dusk meridian by the SABRE radar facility. In a typical event, the irregularity drift speed in the SABRE field of view is seen to increase from about 300 m s−1 to of the order of 1 km s−1 in the space of about 10 min. The speed subsequently remains at the enhanced level for 10 min or longer before declining as rapidly as its onset. The total event duration ranges between 30 min and 1 hour. As the irregularity drift speed increases the direction of the drift velocity changes, rotating poleward. At the same time, the radar backscatter power decreases. The onset of the drift speed enhancement crosses the SABRE field of view as a front moving from east to west. Detailed study of individual events indicates that the events are associated with increases in the |AL| index and with the injection of energetic particles into geosynchronous orbit. We thus suggest that the events are part of the magnetospheric response to the onset of a geomagnetic substorm. However, while each event appears to be associated with a substorm onset, not every substorm onset is associated with an event, at least not at SABRE. We estimate the speed at which the substorm‐initiated ionospheric flow enhancement moves from the nightside to be 1–4 km s−1, a figure that is consistent with the rate at which the drift velocity front crosses the SABRE field of view. Although the front is associated with a rotation in the drift velocity, we see little evidence of strong vertical vorticity as the front passes. However, shears in the flow do develop subsequently which seem likely to correspond to field‐aligned current. Although associated with substorm onset, we argue that these events are distinct from westward traveling surges and appear to differ from the midlatitude phenomenon known as subauroral ion drifts. We thus call this new‐found signature a substorm‐associated radar auroral surge or SARAS event.

Irregularity drift velocity estimates in radar auroral backscatter

It is known that, in VHF radio aurora, the observed irregularity drift velocity V ir is often much lower than the ambient electron drift velocity V d particularly when the latter exceeds ~400 m s −1. The main reason for this discrepancy is believed to be the phase velocity saturation of the two stream instability plasma waves to values near the ion acoustic speed. On the other hand, experimental evidence suggests that additional factors are involved which make the relationship between V ir and V d more complex. In this paper a simultaneous coherent and incoherent scatter data set is used to investigate the irregularity drift vs electron drift relationship in the auroral electrojet plasma. To explain the irregularity drift velocity measurements, an empirical model is introduced which considers, in addition to azimuthal phase velocity anisotropies, altitude integration effects. The latter are due to altitudinal changes of the various plasma parameters and the influences of both electron density and magnetic aspect angle height variations on the mean scattering cross-section which affect indirectly the measured mean irregularity drift. The results suggest that the proposed method is somewhat more effective, compared to previous approaches, but still it cannot account for many discrepancies observed between V ir and V d .

Time variability of pulsar dispersion measures

Energetic events like supernova explosions, stellar winds, and expanding H II regions create turbulent irregularities in the interstellar electron density. The dispersion measure (DM) of a pulsar fluctuates as these irregularities drift past the line of sight. To monitor changes in the interstellar electron density we conducted 47 and 430 MHz timing observations of seven pulsars, once a month for 2 yr with the Arecibo radio telescope. DM variations were detected toward six of the seven objects. The root mean square variations were largest for the most distant pulsars, confirming that the fluctuations were indeed interstellar

Zonal irregularity drifts and neutral winds measured near the magnetic equator in Peru

Measurements of zonal irregularity drifts were made by the spaced receiver scintillation and radar interferometer techniques from Huancayo and Jicamarca, respectively. The Fabry-Perot Interferometer operated at Arequipa provided the zonal neutral winds. These simultaneous measurements were performed during evening hours in the presence of equatorial spread- F on three nights in October 1988. The zonal drift of 3-m irregularities obtained with the 50-MHz radar showed considerable variation as a function of altitude. The drift of hundreds of m-scale irregularities obtained by the scintillation technique agreed with the drift of 3-m irregularities when the latter were measured near the F-peak. The neutral winds, on the other hand, sometimes exceeded the irregularity drifts by a factor of two. This is a possible result of the partial reduction of the vertical polarization electric field in the F-region caused by the effects of integrated Pedersen conductivity of the off-equatorial night-time E-region coupled to the F-region at high altitudes above the magnetic equator.

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.

Smoothness of the HF virtual reflector in the quiet, daytime mid-latitude F-region

Traveling ionospheric disturbances (TIDs) modulate the amplitude and phase of received F-region HF skywave signals. This TID-generated complex modulation usually masks the weaker effect of fine-scale ionospheric irregularities, complicating application of phase-screen analyses to HF array data. We employ knowledge of the instantaneous TID trace velocity, measured by compact-array velocimetry, in separating the effects of TIDs from those of finer-scale irregularities in the same data set. A classical phase-screen analysis is then applied to the suitably transformed data. We find that during normal conditions the irregularities consist simply of weak random tilts (a few milliradians r.m.s.), on scales exceeding 1 km. On the other hand, application of phase-screen analysis directly to the untransformed data, containing TID effects, would give erroneous conclusions that the random tilts are significantly stronger and on shorter scales. These results confirm a long-standing critique of full-reflection irregularities drift measurements: the effect of TIDs largely dominates that of drifting irregularities, making use of the latter (as a marker of mean drift) difficult or impossible.

Ionospheric boundary conditions of hydromagnetic waves: The dependence on azimuthal wavenumber and a case study

A statistical study investigating Pc5 pulsation ionospheric boundary conditions is reported. The study utilizes data from the Sweden And Britain Radar-auroral Experiment (SABRE) VHF coherent auroral radar. The ratio of the pulsation radar backscatter intensity modulations to the ionospheric irregularity drift velocity modulations is calculated. This enables the pulsation ionospheric electric field boundary condition to be established, allowing the wave mode in the radar field of view to be determined. Increasingly compressional Pc5 waves are observed as the azimuthal wavenumber increases in the range m = 1–2. One possible interpretation of this result is that the dayside, continuous pulsations observed with SABRE are caused by an impulsive excitation of the magnetosphere. Three compressional pulsations with similar characteristics are examined in more detail and a full case study, which includes data from an extensive ground magnetometer array, as well as the Scandinavian Twin Auroral Radar Experiment (STARE), is conducted on one of these pulsations. The case study event has a constant period and a small phase variation between 51° and 67° N geomagnetic latitude and 78° and 110° E geomagnetic longitude, with a clear amplitude maximum close to 63° N latitude. The ionospheric electric field boundary condition reveals the pulsation to be a fast mode wave in the SABRE and STARE fields of view. These Pc5 pulsation observations are discussed in terms of recent global mode modelling.

Flow and aspect angle dependence of 3‐m irregularities in the auroral E region

Measurements from the BARS radars have been used to derive the dependence of the relative backscatter cross section on flow angle, aspect angle and irregularity drift velocity. Aspect angles between 87° and 81° and drift velocities up to 700 m/s could be covered. The variation of the backscatter cross section shows general agreement with the growth rate for two‐stream instability. The flow angle dependence is approximately a cos2 variation, whose amplitude increases with velocity and decreases with aspect angle. The aspect angle dependence shows almost no variation with velocity; the cross section decreases steeply between 87° and 86° and stays constant between 86° and 81°.

Effect of unresolved electrojet microstructure on measurements of irregularity drift velocity in auroral radar backscatter

The existence of microstructure in the distribution of electron concentration within a scattering volume can cause non-linear effects in the relationship between the irregularity drift velocity measured by a coherent radar such as STARE and the electron drift velocity measured by an incoherent scatter radar such as EISCAT. If there is an anticorrelation between electron concentration and electric field, and the average electric field is just below the threshold value for generating irregularities, scattering will only occur in regions where the electric field is above average and so the drift velocity measured by coherent radar will be overestimated. For larger electric fields, where the threshold is exceeded throughout the plasma volume, the strongest scattered signals will come from regions where the electron concentration is above average, and the electric field below average, leading to an underestimate in the drift velocity measured by coherent radar.

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10 4 cm −3. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.

Directional Changes in the Irregularity Drift during Artificial Generation of Striations

Resonance instabilities generated by high power HF radio waves have been observed to produce 1 meter field-aligned electron density depletions in the high latitude E-region. The striations have in a smaller portion of the experiments given a clockwise turning in the artificial irregularity drift velocity with respect to the natural drift of irregularity waves in the electrojet.Two sets of conditions in the experiments offer explanations of the clockwise turning. One theory takes into account the steady state conductivity changes by assuming that the striations can be presented as homogeneous plasma columns elongated in the magnetic field direction. The electrostatic polarization electric field due to changes in temperature and density modifies the artificial irregularity wave direction. The idealized model of the striations explains the observations for parameters expected by the theory. The simple model gives maximum changes in the altitude of 95 km for |ΔNe/Ne0| > 10-2. The driving term in the production of the electric polarization field is ΔNe/Ne0, while enhanced temperatures reduce the effect of ΔNe/Ne0.Another consideration takes into account the possibility that the formation of striations takes place in a different altitude than the electrojet height. This condition gives a twisting in the drift direction in comparison to the phase velocity direction of natural two-stream and gradient drift irregularities when the striations follow the maximum growth direction of natural irregularities. Height differences between striations and electrojet show the largest effect for altitudes above 110 km. Calculations show that this effect is minor with respect to the above proposed for altitudes less than 108 km.

Gigahertz scintillations and spaced receiver drift measurements during Project Condor Equatorial F Region Rocket Campaign in Peru

Radar backscatter at 50 MHz, rocket, and VHF/GHz scintillation measurements of spread F irregularities at the magnetic equator in Peru were made during the Project Condor campaign in March 1983. The paper discusses the coordinated set of observations on two evenings, March 1 and March 14, 1983, when the altitude of the F region peak differed by more than 150 km. The full complement of equatorial spread F phenomena, namely, the occurrence of 3‐m plume structures and VHF/GHz scintillations, were recorded on both these evenings. It was found that the radar backscatter with extended plumes occurs in association with maximum 1.7‐GHz scintillations. This established that the height‐integrated rms electron density deviation of ∼200‐m scale irregularities causing 1.7‐GHz scintillations maximizes in extended 3‐m plume structures. The magnitude of 1.7‐GHz scintillations recorded at high elevation angles (∼76°) near the magnetic equator did not exceed a value of S4 = 0.2 (4 dB) in contrast to the near saturated 1.5‐GHz scintillations routinely observed at Ascension Island near the crests of the equatorial anomaly of F2 ionization. The observed scintillation magnitudes at 1.7 GHz have, however, been found to be compatible with the ambient F region and the irregularity parameters measured by the rocket. The spectral index n of scintillations has been found to be relatively less steep (n ∼ −3) on March 1, 1983, when the F region was high, and the index is steep (n ∼ −5) on March 14, 1983, when the F region was at a lower altitude. The F region rocket, however, measured nearly identical one‐dimensional spectral indices of intermediate‐scale irregularities on both evenings, compatible with the less steep spectral index of scintillations. The irregularity drift velocities measured by the spaced receiver scintillation measurements were in general agreement with the radar interferometer results except that the spaced receiver drifts were 20% higher. The zonal drift was observed to be ∼100 m/s on the night of March 1 and ∼200 m/s on the night of March 14. This result may be a consequence of the day‐to‐day variability of the measured zonal neutral winds and field line integrated Pedersen conductivities.