Ionogram Inversion Research Articles

A profile inversion method for vertical ionograms

Inversion of ionograms from vertical measurements is significant for studying the ionospheric structure and ionospheric wave propagation, and it has attracted widespread attention. The model method is a relatively common inversion method for vertical ionograms. In this paper, a model inversion algorithm based on the multivariate QP model (single quasi-parabolic ionospheric profile model) is proposed, which is different from the traditional QPS model (where the multiple quasi-parabolic segment ionospheric profile model uses quasi-parabolic or anti-parabolic models to represent E, valley, and F1 and F2 layers). In other words, the electron density height profile of each layer in the ionosphere is no longer described by a single QP model. Still, it is based on QP as the basic unit and characterized by a combination of multiple QP units, and the entire vertical ionospheric profile consists of a series of QP unit models. Moreover, in the case of the multivariate QP model, determining the parameters of each layer becomes more complex. This study provides a more straightforward method, which can determine the entire QP unit profile by selecting one of the three parameters of each unit QP. Based on this model, the inversion of vertical ionograms was achieved, and the effectiveness of the inversion algorithm was verified based on typical vertical ionogram data.

Variable-step 3D numerical ray-tracing method for simulating oblique ionogram via inversion analysis

Oblique ionogram inversion is an important tool for understanding the ionosphere. Vertical plasma density profiles are the most common inversion results for the majority of the inversion algorithms. However, the inversion of the dynasonde ionogram provides not only the vertical plasma density profile, but also two tilts profiles directly related to the horizontal gradients of the plasma density. The main reason is that the dynasonde ionogram inversion uses a 3D numerical ray-tracing technique that considers the geomagnetic field and the horizontal inhomogeneity of the ionosphere. The theory and the ray-tracing performance of the 3D numerical ray-tracing method are presented. The ray-tracing method uses a variable-step strategy to speed up the simulation. And three plasma density models with traveling ionospheric disturbances and small-scale irregularities and without them are considered when using the presented ray-tracing method to simulate oblique ionograms. The simulation results show that the ionosphere disturbances can eventually be embodied in the simulated oblique ionograms.

An Improved Method for the Inversion of Backscatter Ionograms by Using Neighborhood-Aided and Multistep Fitting

To solve the problem that a parameter search easily falls into a local optimum and the two-dimensional electron density profile construction error is large in the process of backscatter ionogram inversion, an improved method using neighborhood-aided and multistep fitting is proposed. The ionospheric parameter inversion results in the adjacent space are combined and reconstructed by using the neighborhood-aided correction method. The introduction of auxiliary information sources addresses the defects of the conventional genetic algorithm. The local region multistep fitting method is used to describe the local uniformity and global inhomogeneity of the two-dimensional electron density profile by dividing the fitting region. The experimental results show that the proposed method can improve the accuracy of backscatter ionogram inversion and provide reliable support for tracking radio ray trajectories.

Inferring thermospheric composition from ionogram profiles: a calibration with the TIMED spacecraft

Abstract. We present a method for augmenting spacecraft measurements of thermospheric composition with quantitative estimates of daytime thermospheric composition below 200 km, inferred from ionospheric data, for which there is a global network of ground-based stations. Measurements of thermospheric composition via ground-based instrumentation are challenging to make, and so details about this important region of the upper atmosphere are currently sparse. The visibility of the F1 peak in ionospheric soundings from ground-based instrumentation is a sensitive function of thermospheric composition. The ionospheric profile in the transition region between F1 and F2 peaks can be expressed by the “G” factor, a function of ion production rate and loss rates via ion–atom interchange reactions and dissociative recombination of molecular ions. This in turn can be expressed as the square of the ratio of ions lost via these processes. We compare estimates of the G factor obtained from ionograms recorded at Kwajalein (9∘ N, 167.2∘ E) for 25 times during which the Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) spacecraft recorded approximately co-located measurements of the neutral thermosphere. We find a linear relationship between G and the molecular-to-atomic composition ratio, with a gradient of 2.55±0.40. Alternatively, using hmF1 values obtained by ionogram inversion, this gradient was found to be 4.75±0.4. Further, accounting for equal ionisation in molecular and atomic species yielded a gradient of 4.20±0.8. This relationship has potential for using ground-based ionospheric measurements to infer quantitative variations in the composition of the neutral thermosphere via a relatively simple model. This has applications in understanding long-term change and the efficacy of the upper atmosphere on satellite drag.

Quantitative Characteristics of Equatorial Ionization Gradient Above 150 km at Low Solar Activity

AbstractThe electron ionization gradient (dN/dh) characteristics were quantitatively presented using data from Ilorin (8.50°N, 4.68°E) Digisonde during low solar activity. Ionogram inversion using calculated average representative profile program was employed in obtaining the profile data. Results were presented for both the minimum and maximum gradient positions above the 150‐km altitude. The daily hours were classified into four segments. Our result revealed that at sunset/postsunset and nighttime hours, the ionization magnitude at the point of maximum gradient during the equinoxes is twice the value in solstices. This feature of the higher ratio in the equinox is less distinct at sunrise/presunrise and noon/postnoon hours and not pronounced at all hours of the minimum gradient position. The time of occurrence of the maximum gradient for the sunrise/prenoon, noon/postnoon, and nighttime hours, respectively, are 07 LT, 17 LT, and 23 LT for all seasons. For this same diurnal segments at the point of minimum gradient, it is 05 LT, 12 LT, and 04 LT, respectively. The height (hdN) at which the highest gradient was reached is ≈310 km, occurring during the sunset/postsunset hours for both the maximum and minimum gradient positions. The lowest (≈235 km) was reached during the sunrise/prenoon hours. hdN at the position of maximum ionization is lower than the real height (hmF2) across all seasons. The percentage difference in dN/dh magnitude at the position of maximum and minimum gradients is lowest at noon/postnoon period and highest at sunrise/prenoon hours. The various physical processes involved were discussed. We established an inverse relationship between the pattern of dN/dh and vertical drift velocity.

A METHOD OF RECONSTRUCTING HORIZONTALLY‐INHOMOGENEOUS IONOSPHERIC STRUCTURE USING HF SKY‐WAVE BACKSCATTER IONOGRAMS

AbstractHigh‐frequency (HF) sky‐wave backscatter sounding, as a powerful tool for detecting the ionosphere and studying the characteristics of radio channels, can be used to monitor the ionosphere continuously at a remote distance and to acquire the ionospheric parameters in a large area. Backscatter ionograms show the relationship between operating frequency, group path, and echo amplitude. Since an ionogram carries the information of the ionospheric profile along the detection path, the ionospheric parameters can be evaluated through the inversion technique. An algorithm for backscatter sounding ionogram inversion is developed based on the restriction of solution space to reconstruct the horizontally inhomogeneous structures of the ionosphere. Furthermore, the Newton‐Kontorovich method that generally treats nonlinear operator equations and the Tikhonov regularization method that generally deals with ill‐posed problems are effectively combined to resolve the equations. The algorithm can get a stable and unique solution under the solution space limitation. The algorithm we have developed was tested against both model data and observation data, and compared with the method of Fridman and Fridman (1994). Test results prove that the method in this paper has reliable convergence, is insensitive to measurement errors, and has higher accuracy for the inversion of ionospheric structures than the method of Fridman and Fridman (1994). Our algorithm not only can inverse horizontal changes in electron density under quiet conditions (at night or during the daytime at mid‐latitudes), but also can diagnose the horizontally inhomogeneous structures of the ionosphere during sunrise or sunset periods with high accuracy. This consequently demonstrates the application value of the proposed algorithm in the treatment of the complex and volatile measured backscatter ionograms.

Analysis of the ionospheric time-varying effects on the radar echoes based on ionogram inversion

Ionogram is one of the most important sources to analyze the ionospheric real-time state. For doing this, the ionograms must be inverted into several parameters firstly. The algorithm of partial derivative iteration is often used to finish this work. But the accuracy of this inversion results are dependent on the selection of initial values. A criterion for the selection of the initial values is proposed to solve this problem. After inversion, a polynomial fitting method is used to establish the dynamic ionospheric model. Then, based on the model and the path-distance transformation, the radar echoes can be obtained. At last, changing the working bandwidth and the ionospheric parameters, we can find which parameters are the key factors to affect the radar spectrum.

Dynamics of vertical ionospheric inhomogeneities over Irkutsk during 06:00–06:20UT 11/03/2011 caused by Tohoku earthquake

We study dynamics of vertical ionospheric irregularities caused by Tohoku earthquake 11/03/2011 (38.3N, 142.4E) at a distance of 3400km from the epicenter. Equivalent horizontal velocities of propagation in the ionosphere and vertical quasi-wavelength of the wave-like ionospheric irregularities, generated by the earthquake, are calculated.Based on the data of quasi-vertical sounding at Usolie–Tory path (midpoint – 52.3N, 103E, 120km distance), dynamics of vertical mid-scale inhomogeneities of the plasma frequency profile was reconstructed with a fine temporal resolution (60s).As a result of numerical simulation involving the data from TALAYA seismic station (TLY, 51.7N, 103.7E) and comparison with the experiment it is shown that vertical ionospheric irregularities of 5–40km quasi-wavelength observed from 06:00 to 06:20 UT are qualitatively explained by traveling of the acoustic shock wave cone (Mach cone ) from a supersonic ground source – the seismic wave.It is demonstrated that the most likely sources of the shock wave were Z and E components of the seismic oscillations observed at TLY station. Irregularities observed after 06:20UT were apparently linked with other mechanisms. It is found that current temporal resolution of CHIRP ionosonde (60s) and accuracy of the ionogram inversion technique used are not enough for detailed diagnosis of dynamics of the fine spatial vertical structures generated by the earthquake.

The effect of collisions in ionogram inversion

The results of this paper demonstrate that the effect of collisions on the group refraction index is small, when the ordinary ray is considered. If, however, in order to improve the performance of a system for automatic interpretation of ionograms, the information contained in ordinary and extraordinary traces is combined, the effect of collisions between the electrons and neutral molecules should be taken into account for the extraordinary ray. The magnitude of these differences is generally very small and must be compared with the resolution in the virtual vertical height of the ionosonde, resolution which is typically of the order of few kilometers.

Ionogram inversion for MARSIS topside sounding

In the present paper, we propose a method of ionogram inversion to retrieve the electron density profile, Ne(h), of the Martian ionosphere from the topside ionogram, which is measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board the Mars Express spacecraft. The new inversion technique is developed from Titheridge’s method by replacing the prior polynomials with empirical orthogonal functions (EOFs), which are estimated from the archived Ne(h) observation by the radio occultation of Mars Global Surveyor (MGS). The EOF-based technique has achieved quick convergence and good stability. It is concluded that the newly developed method is an alternative tool for the analysis of MARSIS ionograms.

Retrieval of thermospheric parameters from routine ionospheric observations: assessment of method’s performance at mid-latitudes daytime hours

A new method has been developed to retrieve neutral temperature T n and composition [O], [N 2 ], [O 2 ] from electron density profiles in the daytime mid-latitude F 2 -region under both quiet and disturbed conditions. A comparison with CHAMP neutral gas density observations in the vicinity of Millstone Hill Incoherent Scatter Radar (ISR) has shown that the retrieved neutral gas densities coincide with the observed ones within the announced accuracy of CHAMP observations, provided that accurate N e ( h ) ISR profiles are used for the retrieval. The performance of the method has also been tested ingesting Digisonde N e ( h ) profiles. In this case the agreement with CHAMP neutral gas density observations is less successful. Possible factors that can influence the performance accuracy are investigated. These are mostly related to limitations due to the ionogram scaling and inversion methods, including performance limitations of the sounding technique itself, like for instance during G-conditions. Several tests presented here demonstrate that discrepancies in the hmF2 values provided by the Digisondes could have an important impact on the performance of the method. It should be noted that in all tests performed here using Digisonde N e ( h ) profiles, the topside part is approximated with the NeQuick model and any assessment concerning the impact of the topside profiler on the accuracy of the method is beyond the scope of this investigation. Despite the limitations related to the use of Digisonde profiles, the proposed method has the potential to monitor the thermosphere at least with ISR N e ( h ) profiles. Digisonde electron density profiles can also be used if quality improvements are made concerning the ionogram inversion methods.

Ionogram inversion F1-layer treatment effect in raytracing

This paper shows the importance of the F1-layer shape in the electron density profiles obtained from ionograms with different inversion techniques when the profiles are used in ray tracing. This layer often controls the propagation on the path with ranges less than about 2000 km, particularly for spring and summer periods. Ionograms from two different stations, Hainan (19.4N, 109E) and El Arenosillo (37.1N, -6.7E), obtained during the month of July 2002 (average sunspot number: 99.6) during geomagnetic quiet conditions (Ap-index between 9 and 15) are analyzed. The profiles obtained with two different inversion techniques with different options are used together with the ray tracing program of the Proplab-Pro software. This program calculates the features of the received signal as angle of arrival, path length, height of reflection and range for each given profile assumed to define a spherically symmetric ionosphere in the region along the path. For each ionospheric condition (location, day, hour) the difference between range values obtained with Proplab-Pro program using profiles from the two techniques and the different options (POLAN no valley, POLAN valley, POLAN1-layer and NHPC) are considered.

Comparisons of GPS/MET retrieved ionospheric electron density and ground based ionosonde data

The Global Positioning System/Meteorology (GPS/MET) mission has been the first experiment to use a low Earth orbiting (LEO) satellite (the MicroLab-1) to receive multi-channel Global Positioning System (GPS) carrier phase signals and demonstrate active limb sounding of the Earth’s atmosphere and ionosphere by radio occultation technique. Under the assumption of spherical symmetry at the locality of the occultation, the dual-band phase data have been processed to yield ray-path bending angle profiles, which have then been used to yield profiles of refractive index via the Abel integral transform. The refractivity profiles can then, in turn, yield profiles of ionospheric electron density and other atmospheric variables such as neutral atmospheric density, pressure, and temperature in the stratosphere and upper troposphere, and water vapor in the lower troposphere with the aid of independent temperature data. To approach a near real-time process, electron density profiles can also be derived by the Abel transform through the computation of total electron content (TEC) assuming straight-line propagation (neglecting bending). In order to assess the accuracy of the GPS/MET ionospheric electron density retrievals, coincidences of ionosonde data with GPS/MET occultations have been examined. The retrieved electron density profiles from GPS/MET TEC observations have been compared with ionogram inversion results derived from digital ionospheric sounders operated by the National Central University (the Chung-Li digisonde; 24.6°N, 121.0°E) and by Utah State University (the Bear-Lake dynasonde; 41.9°N, 111.4°W). A fuzzy classification method for the automatic identification and scaling of ionogram traces has been applied to recorded ionograms, and then bottomside ionospheric electron density profiles are determined from true-height analysis. The comparison results show better agreement for both of the derived electron density profiles and the F2-layer critical frequency ( foF2) at mid-latitude observations than at low-latitude observations. The rms foF2 differences from the GPS/MET retrievals are 0.61 MHz to the Bear-Lake dynasonde measurements and 1.62 MHz to the Chung-Li digisonde measurements.

Reconstruction of a three‐dimensional ionosphere from backscatter and vertical ionograms measured by over‐the‐horizon radar

A method for inversion of backscatter ionograms is described. Source data for the method consist of leading edge of backscatter ionograms from all azimuthal sectors of the backscatter sounder and the quasi vertical incidence (QVI) ionogram. Such data are routinely collected at over‐the‐horizon radar (OTHR) installations operated by the U.S. Navy. The output is a three‐dimensional model of the ionosphere for the coverage area of the sounder. The model produced is consistent with all the input data. The method is based on the Newton‐Kontorovich method for nonlinear operator equations and Tikhonov's regularization technique. Theory of the method and principles of its numerical realization are described. The method is applied to data acquired by the backscatter sounder operating at the Chesapeake, Virginia, OTHR facility. Maps of plasma frequency at a constant altitude over a geographic region that covers a 64°–80° azimuthal sector out to distances of more than 2000 km are presented. The backscatter ionogram inversion technique suggested makes it possible for OTHR backscatter sounders to provide real‐time monitoring of the ionosphere over a large geographical area.

Improving the inversion of ionograms by combining neural network and data fusion techniques

Data fusion (integration) techniques are combined with MultiLayer Feedforward (backpropagation) neural networks in order to improve the inversion —extraction of the key describing parameters — of oblique-incidence ionograms (plots of apparent height of reflection versus transmission frequency). Two separate investigations were undertaken: first, the incorporation of vertical ionogram data to improve inversion; and secondly, the fusion of ionogram data gathered from a 2D array of ionosondes (the ground-based radio frequency transmitters). With the former, the average percentage errors obtained by incorporating data fusion dropped by a factor of five when compared with single ionogram inversion. Moreover, gradients of ionospheric parameters (critical frequency, layer height and thickness) were also obtained. In the case of the latter, the error rate dropped by a similar factor, and by even more when vertical ionograms were incorporated. Better results were forthcoming when a hierarchical network was used to invert the ionograms prior to fusion, compared with directly fusing the ionogram array data.

Study of ionospheric variability at fixed heights using data from South America

A series of about 800 electron density profiles calculated by inversion of ionograms obtained at four stations in Argentina (low to middle geomagnetic latitudes) have been used to study the variability of electron density at fixed heights. The variability index used is the percentage ratio of the standard deviation over the mean. A set of groups of days has been selected representing different seasons and two years with different levels of solar activity. Also used were “composite” noon profiles of San Juan (in three years 219 profiles each representative for the given hour in a group of five consecutive days). Around noon minimum variability is consistently found at 170 – 190 km. Variability increases with height in the F2 region both by day and night, reaching a maximum somewhere below 300 km. At greater heights the variability show a tendency to decrease. Night-time variability is apparently larger. By day the variability decreases with increasing solar activity. This behaviour is reversed by night above 220 km. A certain increase of the variability with latitude appears to exist.

Accuracy comparison of ionogram inversion methods

Results obtained with a new inversion method /1/ have been compared with the inversions as given by the ARTIST approach /2/. Future generalizations are suggested.

A method of determining horizontal structure of the ionosphere from backscatter ionograms

We have developed a technique lor oblique backscatter sounding (OBS) ionogram inversion as a diagnostic tool for the horizontally inhomogeneous structure of the ionosphere. Input data for the method include the leading edge of a backscalter ionogram that is measured through soundings in a given direction, and the vertical electron density profile measured over the sounding station or over some other site lying in the sounding direction. The method may be useful for reconstructing the two-dimensional electron density distribution in a vertical plane aligned with the direction of sounding. The inverse problem has been solved using the Newton Konlorovich method and the Tikhonov regularization method. The algorithm we have developed was tested against model data, that is, OBS ionograms synthesized using geometrical optics calculations for different models of the inhomogeneous ionosphere. Test results demonstrate that our method converges reliably, is stable to measurement errors and provides a good accuracy of reconstruction of inhomogeneous structures with scales of 100 2000 km. This indicates that this method shows promise as an operative remote diagnostic tool for ionospheric irregularities of natural and artificial origin.

Realistic requirements for inverting ionograms to profiles

The requirements for an accurate ionogram inversion are discussed briefly and quantitative estimates of possible accuracy are made using Dynasonde data. The presence of tilts in the ionosphere seriously limits the accuracy with which the effects of missing data (valleys, D-region) can be estimated. The very accurate range estimates from stationary phase [ca 60m] and evaluations of prevailing tilts are essential for reliable valley and underlying ionization corrections.

Ionogram inversion for a tilted ionosphere

Digital ionosondes such as the Dynasonde disclose that the ionosphere is seldom horizontal even when it is plane stratified to a good approximation. The local magnetic dip does not then determine correctly the radiowave propagation angle for inversion of the ionogram to a plasma density profile. The measured echo direction of arrival can be used together with the known dip for an improved propagation angle. The effects are small for simple one‐parameter laminae but become important when differential (ordinary, extraordinary) retardations are used to aid correction for “valley” and “starting” ambiguities. The resulting profile describes the plasma distribution along the direction of observation, rather than the vertical; it thus conveys information about horizontal gradients. Observations suggest that advances in inversion methods may be practicable for application to modern ionosonde recordings, by which local lateral structure can be described in greater detail.