Horizontal Electron Density Gradients Research Articles

Systematic Ionospheric Residual Errors in GNSS Radio Occultation: Theory for Spherically Stratified Media

AbstractThe standard ionospheric correction of bending angles in Global Navigation Satellite System (GNSS) radio occultation (RO) measurements removes most of the influence from the ionosphere, but leaves small systematic residual errors in the corrected data. The main reasons for residual errors at stratospheric and mesospheric altitudes are the neglect of higher‐order terms in the expansion of the ionospheric refractive index, and the fact that the two GNSS signals follow slightly different paths through the ionosphere. Another reason is the neglect of the local electron density at the receiver in orbit. These residual errors depend on the geomagnetic field and the spatial distribution of the ionospheric electron density and its gradients. In this work we derive this dependency to high accuracy for a spherically stratified ionosphere, including the “bi‐local” situation where the ionosphere is different (though still spherically stratified) on the inbound and outbound side of the RO event tangent points. As part of the derivations, we find a small residual error term not previously noted, which can become appreciable for elliptical satellite orbits. The results are verified by ray tracing through simple ionospheric and geomagnetic field models. The accuracy of a higher‐order bi‐local ionospheric residual error correction based on these results would be limited by the uncertainty in knowledge of the electron density and by horizontal electron density gradients along the ray paths.

СИНТЕЗ ДОПЛЕРОГРАМ КВАЗІВЕРТИКАЛЬНОГО ЗОНДУВАННЯ ІОНОСФЕРИ ДОПЛЕРІВСЬКИМ ВЧ РАДАРОМ ІЗ РОЗНЕСЕНИМ ПРИЙМАННЯМ

Purpose: The ionospheric channel is widely used for the communication, radio navigation, radar, direction finding, radio astronomy, and remote radio probing systems. The radio channel parameters are characterized by nonstationarity due to the dynamic processes in the ionosphere, and therefore their study is one of the topical problems of space radio physics and earth-space radio physics of geospace. This work aims at presenting the results of synthesis of temporal variations in the Doppler spectra obtained by the Doppler probing of the ionosphere at vertical and quasi-vertical incidence. Design/methotology/approach: One of the most effective methods of ionosphere research is the Doppler sounding technique. It has a high time resolution (about 10 s), a Doppler shift resolution (0.01–0.1 Hz), and the accuracy of Doppler shift measurements (~0.01 Hz) that permits monitoring the variations in the ionospheric electron density (10–4–10–3) or the study of the ionospheric plasma motion with the speed of 0.1-1 m/s and greater. The solution of the inverse radio physical problem, consisting in determination of the ionosphere parameters, often means solving the direct radio physical problem. In the Doppler sounding technique, it belongs with the construction of variations in Doppler spectra and comparing them with the Doppler spectra measurements. Findings: For the radio wave ordinary component, three echoes being produced by three rays are observed. Influence of the geomagnetic fi eld and large horizontal gradients in the electron density of δ≥10 % give rise to complex ray structures with caustic surfaces. The ionospheric disturbances traveling along the magnetic meridian form the skip zones. The longitudinal and transverse displacement of the ray reflection point attains a few tens of kilometers along the vil. Haidary to vil. Hrakove quasi-vertical radiowave propagation path, for which the great circle range is 50 km. For the vertical incidence, the signal azimuth at the receiver coincides with the traveling ionospheric disturbance azimuth. The synthesis of temporal variations in the HF Doppler spectra has been made and compared with the temporal variations in the Doppler spectra recorded with the V. N. Karazin Kharkiv National University radar. The estimate of δ=15 % obtained confirms the existence of large horizontal gradients in electron density. Conclusions: Temporal variations in Doppler spectra and in azimuth have been calculated for the vertical and quasi-vertical incidence with allowance for large horizontal gradients of the electron density caused by traveling ionospheric disturbances. Key words: ionosphere, Doppler sounding at oblique incidence, synthesis of temporal variations in HF Doppler spectra, traveling ionospheric disturbances, electron density

Multiple Phase Screen Modeling of HF Wave Field Scintillations Caused by the Irregularities in Inhomogeneous Media

AbstractHigh frequency (HF) radio waves propagating through regions of ionospheric plasma irregularities exhibit random fluctuations in both amplitude and phase referred to as scintillations, which heavily influences the detection performance of HF sky‐wave radar and hybrid sky‐surface wave radar systems. In this paper, a numerical multiple phase screen model for HF sky‐wave propagation is developed, which can simulate the HF wave field scintillations caused by the irregularities in inhomogeneous media. In this model, the random phase changes due to irregularities and the diffraction effect due to phase mixing are considered separately and computed sequentially, where the phase change is numerically calculated using a geometric optical phase delay to allow for the consideration of horizontal electron density gradients. Utilizing the proposed model, we first simulate the observations of scintillation from the direct wave of the HF hybrid sky‐surface wave radar. The statistics of simulation and measured data, including power spectrum and scintillation indices on amplitude and phase, are considered to verify the effectiveness of the proposed model. Then, we analyze the effects of traveling ionospheric disturbances (TIDs) on the intensity of scintillation. The simulation indicates that the horizontal electron density gradients due to TIDs cause the fluctuation of the amplitude scintillation index .

Ionospheric Tilt Measurements: Application to Traveling Ionospheric Disturbances Climatology Study

AbstractIonospheric tilt is a concept used to characterize local horizontal electron density gradients in the ionosphere using a mirror reflection model. Experimental results are presented that illustrate the validity and accuracy of such approach. Analysis of the tilt measurements collected in 2012–2014 with a digisonde at Ebro observatory (40.8°N, 0.5°E) is presented. Digisonde systems allow measuring angles of arrival of ionospherically reflected radio signals, from which the ionospheric tilts are derived. The tilts are represented in terms of North‐South and East‐West components. Using several years of observations, a climatological distribution of wavelike variations in the tilt records presumed to be associated with traveling ionospheric disturbances (TIDs) is established. Most of the observed TIDs have their main periods of 30 min to 1.5 hr which is rather typical for medium‐scale TIDs. Summertime appears to have the most frequent occurrence of TIDs. There is a good agreement between the presence of TIDs and sporadic E layer occurrence, suggesting that some TIDs can be driven by instability in the electric field which is initiated via an interhemispheric link between the E and F regions of the ionosphere. Direction of disturbance propagation is also analyzed and compared to the modeled neutral wind. There are indications that during the daytime TIDs tend to propagate in the direction opposite to the background neutral wind. This suggests that daytime TIDs are produced by atmospheric gravity waves originating in the lower atmosphere and experiencing background wind filtering effect on their upward propagation.

Observations of ionospheric disturbances via U-shaped traces on ionograms

The ionospheric vertical sounding i s a basic technique for studying the Earth’s ionosphere. Taking into account the fact that conditions for propagation of radio waves depend on the operating frequency , d iagnostics of the ionosphere is performed using digital ionosondes that measure the delay s of decameter radio signal s of different frequencies. An ionogram is a display of the data produced by an ionosonde. It is a graph of the virtual reflection height of the ionosphere (actually , time between transmission and reception of a radio signal ) versus sounding frequency. Vertical and near-vertical ionograms provide the major share of information about the space-temporal structure of the ionospheric plasma above the ionosonde . Of particular interest is investigating dynamic processes from the series of ionograms taken once each minute. Studying seismic ionospheric effects by minute ionograms can be highly informative. It is known that the parameters of the ionospheric layers can significantly vary both before and after an earthquake , and such deviations are detectable from ionosonde data. Recent observations show that the effects from an earthquake can occur at large distances from the epicenter. For instance , s oundings in the Irkutsk region detected the anomalies that occurred several dozens of minutes after the main shock of the 11 .03. 201 1 earthquake in Japan. At the initial phase, the anomaly was recorded as a multicusp structure i n the ionogram, which was probably related to the multiple layers of the ionosphere. Th is structure was interpreted by iterative reconstruction of the electron density profile in a one-dimensional approximation (stratified ionosphere) , and the interperation was published . At the next p hase, the ionograms showed a U-shaped structure with a specific multiple reflection. Multipath propagation is usually associated with additional off-vertical ray paths, which are caused by traveling ionospheric disturbance s (TIDs) . In this regard, it is appropriate to use a 2D model of electron density , depending on both the vertical and horizontal coordinates. In our study , geometric optics approximation was used to simulat e vertical ionograms under the conditions of strong horizontal gradients of electron density. The study results show that d ifferent positions of the U-shaped structure with respect to the main tra ce of the ionogram can be obtained for a single TID . Specific ray paths forming the additional trac es of the ionogram are described . D epending on the TID location , the U-shaped structure can occur with or without a specific multipath reflection.

A description of the Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE)

The Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE) was an extensive experiment undertaken by the Defence Science and Technology (DST) Group that focused on collecting ionospheric sensor data from multiple overlapping ionospheric paths in the Australian region in order to improve understanding of the characteristics of ionospheric variability and its effect on HF radio propagation. The experiment ran from July to October 2015 and included a period of three weeks of increased sample density and three days of dedicated over-the-horizon (OTH) radar operations. It was anticipated that ELOISE would sample a wide range of environmental conditions and present an opportunity to characterise periods of “normal variability” and periods of “exceptional variability” in the ionosphere.This report is a general description of the aims of this experiment and the types of data collected. Particular interest focused on observing and measuring variability in ionospheric electron density gradients and their effect on oblique HF propagation. To this end, ELOISE established a pair of two dimensional HF receiver arrays to directly measure the oblique angle of arrival (AoA) on many overlapping oblique paths. ELOISE also established a dense sub-network of spatially separated quasi-vertical incidence soundings in the vicinity of Alice Springs in central Australia. This enabled a comparison of gradients observed in a dense network of vertical sounders with gradient effects observed in oblique propagation passing overhead. Several additional ionospheric observing systems were also used to give complementary pictures of the fine scale characteristics of ionospheric variability in the region. Plain Language SummaryThis paper is an overview of the 2015 Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE), an experiment designed to observe and characterise mid-latitude ionospheric disturbances in the Australian region and understand their impact on high frequency (HF) signal propagation.ELOISE involved the simultaneous operation of a large collection of ionospheric sounders enabling ionospheric variability to be characterised on a finely sampled large scale.Particular efforts were made to provide direct high fidelity measurements of the angle of arrival (AoA) on many oblique HF propagation paths. These direct AoA measurements imply horizontal electron density gradients that can be compared to ionospheric gradients estimated from conventional models of the ionosphere derived from the spatial network of sounder sites.The size and scope of the experiment are detailed in this paper and some preliminary results are presented.

The influence of the spatial and temporal collocation windows on the comparisons of the ionospheric characteristic parameters derived from COSMIC radio occultation and digisondes

GPS radio occultation (RO) ionospheric products obtained by Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission during the year of 2014 and the observations from 3 digisonde stations which are located at different latitudes are used to study the influence of different time and space collocation windows on the comparisons of the ionospheric characteristic parameters (ICPs), including the peak density and peak height, derived from the two techniques. The results show that the correlation coefficients (CC) and the standard deviation of the absolute biases (SDAB) between the ICPs derived from the two techniques vary distinctly under different spatial and time collocation windows. Generally, the CC (SDAB) of the ICPs decrease (increase) as the size of the collocation window increases in time dimension or in space dimension. The rate of change of the statistic parameters with the increase in the size of the collocation window in time dimension and space dimension is analyzed for each digisonde station. It is found that within the collocation window of 60min,20°,20°, the influence of the increase of 1° in the space window on the statistical comparison is much more significant than that of the increase of 1 min in the time window, and it is suggested that there can be appropriate relaxation on the time window within the threshold of 60 min to get a balance between the quality of the comparison results and the number of the matched pairs. In addition, it is found that the same variations in the longitude window and in the latitude window may have different influences on the comparison results when the horizontal gradients in electron density are distinctly different along different directions at the digisonde station, and strict space collocation window is preferred when comparing the observations from COSMIC RO with those from the digisonde station in such cases.

Estimating the Characteristics of Traveling Ionospheric Disturbances from Vertical Incidence Ionograms within a Compound Parabolic Layer Model

The characteristic U-shaped traces (cusps) in ionograms are associated with the passage of traveling ionospheric disturbances (TIDs), which lead to horizontal electron density gradients in the ionosphere and, therefore, to off-angle reflections in radio sounding. A new form of representation is considered for daily ionospheric sounding data. A compound parabolic layer model is proposed, which allows analytical calculation of ray paths to speed up the “homing-in” of the rays. Changes in the shape of the trace in the ionogram due to varying the TID characteristics are examined. A discussion is given of the possibilities for estimating TID characteristics from digitized vertical sounding ionograms.

Ionospheric electron density inversion for Global Navigation Satellite Systems radio occultation using aided Abel inversions

AbstractThe Abel inversion of ionospheric electron density profiles with the assumption of spherical symmetry applied for radio occultation soundings could introduce a greater systematic error or sometimes artifacts if the occultation rays trespass regions with larger horizontal gradients in electron density. The aided Abel inversions have been proposed by considering the asymmetry ratio derived from ionospheric total electron content (TEC) or peak density (NmF2) of reconstructed observation maps since knowledge of the horizontal asymmetry in ambient ionospheric density could mitigate the inversion error. Here we propose a new aided Abel inversion using three‐dimensional time‐dependent electron density (Ne) based on the climatological maps constructed from previous observations, as it has an advantage of providing altitudinal information on the horizontal asymmetry. Improvement of proposed Ne‐aided Abel inversion and comparisons with electron density profiles inverted from the NmF2‐ and TEC‐aided inversions are studied using observation system simulation experiments. Comparison results show that all three aided Abel inversions improve the ionospheric profiling by mitigating the artificial plasma caves and negative electron density in the daytime E region. The equatorial ionization anomaly crests in the F region become more distinct. The statistical results show that the Ne‐aided Abel inversion has less mean and RMS error of error percentage above 250 km altitudes, and the performances for all aided Abel inversions are similar below 250 km altitudes.

On the determination of the effect of horizontal ionospheric gradients on ranging errors in GNSS positioning

An alternative approach to the traditionally employed method is proposed for treating the ionospheric range errors in transionospheric propagation such as for GNSS positioning or satellite-borne SAR. It enables the effects due to horizontal gradients of electron density (as well as vertical gradients) in the ionosphere to be explicitly accounted for. By contrast with many previous treatments, where the expansion of the solution for the phase advance is represented as the series in the inverse frequency powers and the main term of the expansion corresponds to the true line-of-sight distance from the transmitter to the receiver, in the alternative technique the zero-order term is the rigorous solution for a spherically layered ionosphere with any given vertical electron density profile. The first-order term represents the effects due to the horizontal gradients of the electron density of the ionosphere, and the second-order correction appears to be negligibly small for any reasonable parameters of the path of propagation and its geometry for VHF/UHF frequencies. Additionally, an “effective” spherically symmetric model of the ionosphere has been introduced, which accounts for the major contribution of the horizontal gradients of the ionosphere and provides very high accuracy in calculations of the phase advance.

A model-assisted radio occultation data inversion method based on data ingestion into NeQuick

Inverse Abel transform is the most common method to invert radio occultation (RO) data in the ionosphere and it is based on the assumption of the spherical symmetry for the electron density distribution in the vicinity of an occultation event. It is understood that this ‘spherical symmetry hypothesis’ could fail, above all, in the presence of strong horizontal electron density gradients. As a consequence, in some cases wrong electron density profiles could be obtained. In this work, in order to incorporate the knowledge of horizontal gradients, we have suggested an inversion technique based on the adaption of the empirical ionospheric model, NeQuick2, to RO-derived TEC. The method relies on the minimization of a cost function involving experimental and model-derived TEC data to determine NeQuick2 input parameters (effective local ionization parameters) at specific locations and times. These parameters are then used to obtain the electron density profile along the tangent point (TP) positions associated with the relevant RO event using NeQuick2. The main focus of our research has been laid on the mitigation of spherical symmetry effects from RO data inversion without using external data such as data from global ionospheric maps (GIM). By using RO data from Constellation Observing System for Meteorology Ionosphere and Climate (FORMOSAT-3/COSMIC) mission and manually scaled peak density data from a network of ionosondes along Asian and American longitudinal sectors, we have obtained a global improvement of 5% with 7% in Asian longitudinal sector (considering the data used in this work), in the retrieval of peak electron density (NmF2) with model-assisted inversion as compared to the Abel inversion. Mean errors of NmF2 in Asian longitudinal sector are calculated to be much higher compared to American sector.

The field of a point source in a horizontally inhomogeneous short-wave radio channel with inaccurate parameters. Part I. General relationships

The reference problem method is used to construct an expression for the field of a point source in a 2D inhomogeneous medium as a function of the distance from the source. The short-wave fields in an ionispheric reflective channel with allowance for horizontal electron density gradients of the ionosphere are described. The results make it possible to solve the problem of a short-wave radio channel with inaccurate parameters.

A new technique for absolute total electron content determination using the CITRIS instrument on STPSat1 and the CERTO beacons on COSMIC

[1] Highly accurate measurements of total electron content (TEC) of the ionosphere are made using beacon transmitters and a beacon receiver on low Earth orbit satellites. The Naval Research Laboratory Scintillation and Tomography Receiver in Space (CITRIS) instrument, in a nearly circular orbit at 560 km altitude and 35° inclination, uses signals from more than 12 VHF/UHF radio beacons also in low Earth orbits. CITRIS recordings of differential phase observations estimate TEC between two satellites using a differential phase technique and are limited by the 2π ambiguities of the measurements. During space-to-space conjunctions, nearby passes allow determination of absolute TEC by assuming that the TEC is zero for extrapolations to zero separation. Data from CITRIS conjunctions with the Coherent Electromagnetic Radio Tomography (CERTO) beacons on the Taiwanese COSMIC satellites provide absolute TEC and horizontal electron density gradients. Accurate absolute TEC measurements, though difficult to obtain, are important for successful space weather modeling. These types of high-resolution ionospheric observations have been routinely collected for midlatitude and low-latitude regions of the ionosphere with CITRIS for the past 2 years.

L dependence of energetic electron precipitation driven by magnetospherically reflecting whistler waves

Ray tracing analysis and a first‐order treatment of the wave frequency and electron energy dependence of gyroresonant pitch angle scattering reveals the L shell dependence of the time‐integrated energetic electron (>150 keV) precipitation flux in the drift loss cone due to a single cloud‐to‐ground lightning stroke. Primary features of the L dependence are determined by the ray paths of magnetospherically reflecting nonducted whistler waves, as well as the refraction of the wave normal vectors due to ionospheric horizontal electron density gradients at low latitudes. Calculations for lightning source latitudes of 25°, 35°, and 45°, with an assumed “average” ionospheric electron density profile, as well as an International Reference Ionosphere model profile, indicate that horizontal ionospheric electron density gradients can cause the whistler wave energy to focus near the geomagnetic equator on the first magnetospheric traverse, resulting in a spatially narrow precipitation signature at low L shells (1.4 < L < 1.65). The magnitude of this low‐L peak is found to be sensitively dependent on the local ionospheric density gradients. Subsequent magnetospheric reflections result in a broad precipitation peak at higher L shells (1.8 < L < 4.2), which is robustly present, with magnitude being relatively insensitive to lightning source‐latitude and ionospheric horizontal gradients. The calculated L dependences were found to be in excellent agreement with drift‐loss cone fluxes measured aboard the SAMPEX satellite.

The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs

Abstract. Simultaneous all-sky camera and HF radar observations of the visual and E-region radar aurora in the west-ward electrojet suggest a close relationship between a pair of parallel east-west-aligned auroral arcs, separated by ~ 30 km, and a region of strong radar backscatter. Poleward of this a broader region of radar backscatter is observed, though the spectral characteristics of the echoes in these two regions differ considerably. We suggest that the visual aurorae and their radar counterparts are produced in a region of upward field-aligned current (FAC), whereas the backscatter poleward of this is associated with downward FAC. Relatively low electric fields ( ~ 10 mV m-1) are observed in the vicinity of the arc system, suggesting that in this case, two-stream waves are not directly generated through the electrodynamics of the arc. Rather, the generation of irregularities is most probably associated with the gradient drift instability operating within horizontal electron density gradients produced by the filamentary nature of the arc FAC system. The observation of high Doppler shift echoes superimposed on slow background flow within the region of backscatter poleward of the visual aurora is argued to be consistent with previous suggestions that the ion-acoustic instability threshold is reduced in the presence of upwelling thermal electrons carrying downward FAC.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; particle precipitation)

An evaluation of a new two‐dimensional analytic ionospheric ray‐tracing technique: Segmented method for analytic ray tracing (SMART)

Analytic ray tracing is considerably less time consuming than numerical ray tracing. However, despite the advantage of speed, analytic ray tracing has a major limitation: the difficulty of including the effects on the ray path of horizontal gradients in electron density. In the past this difficulty has been addressed using ionospheric models whereby an effective tilt is introduced by displacing the center of the ionosphere from the center of the Earth. In this paper, ray equations for the ground range, phase path, group path, and divergent power loss are developed for a new two‐dimensional analytic ray tracing technique, known as the segmented method for analytic ray tracing (SMART), which is able to deal with horizontal gradients in electron density. Ray tracing using two tilting methods are compared with ray tracing using SMART. Further, these results are compared with a numerical ray tracing program known as homing‐in ray tracing (HIRT).

Imaging of electron density troughs by tomographic techniques

Troughs in the latitudinal distribution of electron density are a well‐known feature of the ionosphere from subauroral to polar latitudes. The location and depth of the trough minimum, the width of the feature, and the horizontal gradients in electron density associated with the trough walls are all quantities of interest or concern to practical applications of radio systems involving the ionosphere. In practice, the precise characteristics of trough‐like structures have been difficult to monitor using ground‐based methods. Ionospheric tomography represents a new development that is maturing into a technique ideally suited to the study of electron density troughs. Results are presented from a variety of observations made during tomographic campaigns in northern Europe. A long‐term investigation has been made of the main trough from a network of stations in the United Kingdom. The position of the trough minimum and the wall gradients have been studied on a diurnal basis using tomographic images reconstructed from measurements for a succession of passes of Navy Navigation Satellite System satellites. With stations deployed for more than 6 months, the average behavior has also been studied. Examples are shown of extreme behavior of the trough under very disturbed geomagnetic conditions, during which tomography continues to yield images while the limitations of ionosondes are exposed. Studies of narrow troughs with very steep gradients seen at auroral latitudes have been used to investigate some of the successes and limitations of the tomographic method. Measurements made in the polar cap show the depleted densities of the polar hole in the center of the dawn convection cell and illustrate the power of the tomographic method at high latitudes. Finally, the dayside trough at the high‐latitude boundary between corotating and counterstreaming flux tubes in the afternoon sector has been revealed in a tomographic image extending over some 30° latitude, made using a chain of six stations in Scandinavia.

High‐frequency Doppler radar observations of magnetic aspect sensitive irregularities in the midlatitude E region ionosphere

In this paper, an experiment designed for dual‐frequency azimuthal Doppler spectrum studies of decameter‐scale field‐aligned irregularities in the midlatitude E region ionosphere is introduced, and the first results are presented. The observations were made in July 1993 with a large HF radar facility near Valensole (43.8°N, 6.1°E), in the south of France. The radar system is an oblique ionospheric sounder that employes long antenna arrays of vertical monopole pairs and covers the HF frequency band from 4 to 30 MHz in 1‐kHz steps. A scheme of broad beam width transmission and narrow‐beam, phased‐array, multireceiver coverage was used to scan with 2° step an azimuthal sector from about 24° east to 60° west of geomagnetic north. The 15‐gate viewing region was confined in range between 100 and 370 km in order to cover an area of E region magnetic aspect sensitive backscatter near 37° invariant magnetic latitude (L ≃ 1.7, magnetic dip, ∼60°). In this configuration, each azimuthal scan was completed in 80 s over a 15×42 spatial grid with the full Doppler spectrum at each grid point recorded in real time. The experiment provided observations simultaneously at two frequencies, 9.0 MHz and 14.8 MHz, that correspond to backscatter from plasma waves with wavelengths of 16.7 and 10.1 m, respectively. Here, we present an overview of the observations that include azimuthal and range‐time echo characteristics as well as mean Doppler shift and spectrum width properties. The first results show aspect sensitive decameter‐wavelength irregularities having mean phase velocities at least as large as 120 m/s to act as tracers of wavelike dynamic structures that drift westward with speeds in the 40‐ to 80‐m/s range and have characteristic times between 10 and 30 min and typical scale lenghts between 40 and 90 km. In our interpretation, we consider these structures to be sporadic Es ionization patches, possibly affected by the passage of atmospheric gravity waves, which are accompanied by intense horizontal electron density gradients and enhanced electric fields. Although secondary generation cannot be excluded entirely, the evidence supports the possibility that the gradient drift instability is the basic physical mechanism for direct generation of decameter aspect sensitive plasma waves in the midlatitude E region ionosphere.

Delay, Doppler, and amplitude characteristics of HF signals received over a 1300‐km transauroral sky wave channel

Channel probe observations of propagation conditions along a 1294‐km transauroral path between Sondrestrom, Greenland, and Keflavik, Iceland, were made during the period from March 13 to April 2, 1992. The midpoint of this path was located at a corrected geomagnetic latitude of 72°. The objective of these measurements was to supplement the existing data base describing propagation conditions on the HF transauroral channel with data pertaining to a period around the time of solar maximum. Received signals for this path fell into three distinct groups depending on their amplitude and delay and Doppler spread characteristics. These are (1) strong, specularly reflected ionospheric returns characteristic of a quiescent daytime ionospheric channel during magnetically quiet conditions; (2) strong specular multipath signals reflected from horizontal gradients of electron density and regularly encountered at night; and (3) weak scatter returns that are also a persistent nighttime phenomenon. The scatter returns are usually observed at delays exceeding those anticipated for the one‐hop return and, very often, at frequencies that are well above the MUF for the great circle propagation path. The multipath and scatter returns exhibit large delay and Doppler spreads indicative of spatially extensive distributions of drifting and randomly moving irregularities. Two measurement events are discussed which illustrate these conclusions: a noontime measurement with Kp = 3, and a midnight measurement with Kp = 2. The noontime measurement exhibited a scatter return from an isolated irregularity region in addition to the usual ionospheric reflected signals. A simple irregularity drift model produced delay and Doppler shift curves that were consistent with those observed for the scatter component of the received signal and supported a hypothesis of an irregularity region drift speed of 1200 m s−1 parallel to the great circle propagation path.

Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere (MU) radar, we have been able to track the passage of gravity waves and measure the horizontal electron density gradients that they produce. Our previous work on this topic, on one day of observations, verified that the gradient structure and fluctuations were consistent with a gravity wave source. The present work (1) looks at several days of observation to confirm the routine existence of these gradient structures; (2) identifies individual wave packets; (3) finds evidence that long trains of gravity wave fluctuations are composed of several juxtaposed wave packets of one or two cycles each rather than a single wave of many cycles; (4) finds that during quiet periods strong waves are present only during the daytime, but during disturbed periods they are present day and night; and (5) notes several other characterizations.