Vertical Electron Density Distribution Research Articles

Iterative Gradient Correction (IGC) Method for True Height Analysis of Ionograms

AbstractInversion of precise true height electron density profile from the measured virtual heights by the Ionosonde is a quite challenging and ill‐posed problem. In this paper, we present a new method to compute the true height profiles from ionograms that relies on computing the propagation path of radio waves with time. This method does not use predefined polynomial functions to fit the vertical electron density distribution; hence, it is free from fitting errors. Instead, this method implements iterative corrections in the electron density gradient between the successive points and progressively reconstructs the true height profile. This Iterative Gradient Correction (IGC) method assures minimizing the error to below a tolerance limit at all sampled points on the ionogram. The true height profiles derived from this method exhibit better accuracy than those derived from the widely used POLynomial ANalysis, particularly, at cusp and F2‐peak regions. Further, the IGC method gives the best results at higher sampling resolutions of ionograms and is less sensitive to scaling errors.

Ionospheric Vertical Correlation Distance Calculation Based on COSMIC Electron Density Profile Data

AbstractBackground error covariance is an important part of ionospheric assimilation and contains correlation information of the ionospheric background field and error information of the model background field, determines the influence degree of observations on models, and controls the vertical electron density distribution of assimilation results. For more accurate background error covariance determination along the ionosphere vertical direction, we used Constellation Observing System for Meteorology Ionosphere and Climate occultation electron density profile data for vertical correlation distance calculation considering IRI 2016 model errors. The vertical direction correlations are asymmetrical above and below the reference altitude, and with increasing altitude, the vertical correlation distance growth rate may decline or remain unchanged. There are correlation differences in different solar activity years, varying with local time, geomagnetic latitude, and altitude. The vertical correlation distances in high‐solar activity years are greater than that in low‐solar activity years. The correlation distances difference at different times decreases with latitude, with the largest correlation distances at low latitudes. The nighttime distances vary slightly with latitude, while the sunrise correlation distances vary the most relative to other periods. At the same altitude, the correlation distances in the daytime low‐latitude region are the smallest overall, while the correlation distances between the daytime mid‐latitude region and sunrise time are the largest. The point where vertical correlation distances growth stabilize may be correlated with ionospheric hmF2 and upper transition. The IRI model is the most widely used background model in ionospheric assimilation, and the study results could facilitate more accurate vertical background error covariance for ionospheric assimilation.

A New Method for converting Slant Ionospheric Delays to Vertical for SBAS Ionospheric corrections

SBAS gives ionospheric correction data to GNSS users for more accurate positioning. SBAS uses obliquity factor of thin shell model to convert slant ionospheric delays to vertical delays which is not precise especially at lower elevation angle. This paper suggests a new method for estimating the vertical delays which are needed for generating the ionospheric corrections. The new method uses Chapman profile assumption by which the vertical electron density distribution can be modeled. It is also assumed that Chapman profile’s parameters are given and the vertical ionospheric delay can be linearly modeled. We divided ionosphere into multi-layer and mapped ionospheric vertical delays with a linear function. Converting vertical delays of each layer to slant delays, we can set a linear equation for the vertical ionospheric delay at IPP. We used IRI model to get information about ionosphere for simulation to verify our new method. Estimation error of vertical ionospheric delay at IPP was decreased about 40% in average when using our new method.

Reconstruction of Plasmaspheric Density Distributions by Applying a Tomography Technique to Jason‐1 Plasmaspheric TEC Measurements

AbstractWe have developed a tomography algorithm and applied it to a plasmaspheric total electron content data set from Jason‐1 satellite (~1,336 km), which has a Global Positioning System receiver onboard. To invert the measured plasmaspheric total electron content into vertical distribution of electron density, we adopted a multiplicative algebraic reconstruction technique with an assumed initial density distribution. The reconstruction of the plasmaspheric density distribution was performed on Indian (70°E − 90°E), Pacific (200°E − 220°E), and Atlantic (320°E − 340°E) longitudinal planes during the periods of high solar activity (F10.7 > 100) and low geomagnetic activity (Ap < 12) from 2002 to 2005. The reconstructed density distribution displays general climatological characteristics of the plasmasphere. For all three longitudinal sectors, the reconstructed density distributions show weak diurnal variations being greater during daytime (09–15 LT) than nighttime (21–03 LT). In Atlantic sector, the reconstructed plasmaspheric density exhibits an annual anomaly (higher density in December than in June), which was not apparent in other longitude sectors. By fitting the reconstructed density profiles, we derived empirical functions of equatorial plasmaspheric density profiles for 4 months (March, June, September, and December) in the three longitude sectors. The parameters associated with these functions were found to exhibit diurnal variation and annual anomaly characteristics.

Variations of the ionospheric parameters and vertical electron density distribution at the northern edge of the EIA from 2010 to 2015 along 95°E and comparison with the IRI-2012

The vertical electron density profiles over Dibrugarh (27.5°N, 95°E, 43° dip) a low mid latitude station normally located at the northern edge of the EIA for the period of July 2010 till October 2015 are constructed from the measured bottom side profiles and ionosonde-GPS TEC assisted Topside Sounder Model (TSM) topside profiles. The bottom side density profiles are obtained by using POLAN on the manually scaled ionograms. The topside is constructed by the modified ionosonde assisted TSM model (TaP-TSM assisted by POLAN) which is integrated with POLAN for the first time. The reconstructed vertical profile is compared with the IRI predicted density profile and the electron density profile obtained from the COSMIC/FORMOSAT radio occultation measurements over Dibrugarh. The bottom side density profiles are fitted to the IRI bottom side function to obtain best-fit bottom side thickness parameter B0 and shape parameter B1. The temporal and solar activity variation of the B-parameters over Dibrugarh are investigated and compared to those predicted by IRI-2012 model with ABT-2009 option. The bottom side thickness parameter B0 predicted by the IRI model is found to be similar to the B0 measured over Dibrugarh in the night time and the forenoon hours. Differences are observed in the early morning and the afternoon period. The IRI doesn’t reproduce the morning collapse of B0 and overestimates the B0 over Dibrugarh in the afternoon period, particularly in summer and equinox. The IRI model predictions are closest to the measured B0 in the winter of low solar activity. The B0 over Dibrugarh is found to increase by about 15% with solar activity during the period of study encompassing almost the first half of solar cycle 24 but solar activity effect was not observed in the B1 parameter. The topside profile obtained from TaP profiler is thicker than the IRI topside in equinox from afternoon to sunrise period but is similar to the IRI in summer daytime. The differences in the bottom side may be attributed to the non-inclusion of ground measurements from 90°E to 100°E longitude in the ABT-2009 model while differences in the topside could be due the non-uniform longitudinal distribution of topside sounder profiles data and the stronger fountain effect in this longitude.

High-resolution ionospheric observations and modeling over Belgium during the solar eclipse of 20 March 2015 including first results of ionospheric tilt and plasma drift measurements

The ionospheric behavior over Belgium during the partial solar eclipse of 20 March 2015 is analyzed based on high-resolution solar radio flux, vertical incidence sounding, and GPS TEC measurements. First results of ionosonde-based ionospheric plasma drift and tilt observations are presented and analyzed, including some traveling ionospheric disturbances caused by the eclipse. Also, collocated ionosonde and GPS measurements are used to reconstruct the time evolution of the vertical electron density distribution using the Royal Meteorological Institute (RMI) ionospheric specification system, called Local Ionospheric Electron Density profile Reconstruction (Liedr).

Ionosonde tracking of infrasound wavefronts in the thermosphere launched by seismic waves after the 2010 M8.8 Chile earthquake

AbstractIonospheric disturbances associated with the M8.8 Chile earthquake (35.91°S, 72.73°W) on 27 February 2010 were observed at Kazan, Russia (55.85°N, 48.81°E). Rapid‐run ionograms at 1 min intervals exhibited multiple‐cusp signatures (MCSs) for more than 30 min, which have been observed several times after large earthquakes. The ionospheric disturbances were caused by infrasound propagating upward in the atmosphere, which modified the electron density distribution through ion‐neutral collisions. The anomaly of the vertical electron density distribution responsible for the MCSs was analyzed by converting the ionogram traces into real height profiles. The density profiles at 1 min intervals allowed the tracking of the vertical propagation of infrasound and provided information on parameters of acoustic waves, which was not possible from the previous measurements such as standard ionograms at 5–15 min intervals, HF Doppler soundings, and total electron content using satellite beacon signals. The speed of acoustic waves in the thermosphere was evaluated from the consecutive ionograms with MCSs, and it was found that the thermospheric temperature was slightly higher than that calculated using the Mass Spectrometer and Incoherent Scatter Radar empirical model (NRLMSISE‐00).

Regional representation of F2 Chapman parameters based on electron density profiles

Abstract. Understanding the physical processes within the ionosphere is a key requirement to improve and extend ionospheric modeling approaches. The determination of meaningful parameters to describe the vertical electron density distribution and how they are influenced by the solar activity is an important topic in ionospheric research. In this regard, the F2 layer of the ionosphere plays a key role as it contains the highest concentration of electrons and ions. In this contribution, the maximum electron density NmF2, peak height hmF2 and scale height HF2 of the F2 layer are determined by employing a model approach for regional applications realized by the combination of endpoint-interpolating polynomial B splines with an adapted physics-motivated Chapman layer. For this purpose, electron density profiles derived from ionospheric GPS radio occultation measurements of the satellite missions FORMOSAT-3/COSMIC, GRACE and CHAMP have been successfully exploited. Profiles contain electron density observations at discrete spots, in contrast to the commonly used integrated total electron content from GNSS, and therefore are highly sensitive to obtaining the required information of the vertical electron density structure. The spatio-temporal availability of profiles is indeed rather sparse, but the model approach meets all requirements to combine observation techniques implicating the mutual support of the measurements concerning accuracy, sensitivity and data resolution. For the model initialization and to bridge observation gaps, the International Reference Ionosphere 2007 is applied. Validations by means of simulations and selected real data scenarios show that this model approach has significant potential and the ability to yield reliable results.

Local ionospheric electron density profile reconstruction in real time from simultaneous ground-based GNSS and ionosonde measurements

The purpose of the LIEDR (local ionospheric electron density profile reconstruction) system is to acquire and process data from simultaneous ground-based total electron content (TEC) and digital ionosonde measurements, and subsequently to deduce the vertical electron density distribution above the ionosonde’s location. LIEDR is primarily designed to operate in real time for service applications and, for research applications and further development of the system, in a post-processing mode. The system is suitable for use at sites where collocated TEC and digital ionosonde measurements are available. Developments, implementations, and some preliminary results are presented and discussed in view of possible applications.

Recent advances in real-time analysis of ionograms and ionospheric drift measurements with digisondes

Reliable long distance RF communication and transionospheric radio links depend critically on space weather, and specifically ionospheric conditions. Modern ground-based ionosondes provide space weather parameters in real-time including the vertical electron density distribution up to ∼1000 km and the velocity components of the ionospheric F region drift. A global network of digisondes distributes this information in real-time via internet connections. The quality of the automatic scaling of the echo traces in ionograms was a continuous concern ever since first attempts have been reported. The modern low-power ionosonde with ∼100 W transmitters (compared to several kilowatt for the older ionosondes) relies on more sophisticated signal processing to enhance the signal-to-noise ratio and to retrieve the essential ionospheric characteristics. Recent advances in the automatic scaling algorithm ARTIST have significantly increased the reliability of the autoscaled data, making the data, in combination with models, more useful for ionospheric now-casting. Vertical and horizontal F region drift velocities are a new real-time output of the digisondes. The “ionosonde drift” is derived from the measured Doppler frequency shift and angle of arrival of ionospherically reflected HF echoes, a method similar to that used by coherent VHF and incoherent scatter radars.

Ionospheric GPS radio occultation measurements on board CHAMP

The GPS radio occultation technique is a rather simple and inexpensive tool for getting information about the global characteristics of the vertical electron density distribution. No other ionospheric sounding technique (bottomside/topside vertical sounding, incoherent scatter) unifies vertical profiling through the entire ionosphere with global coverage. The paper addresses retrieval methods and algorithms applied for the generation of operational products including their limitations in accuracy and spatial resolution. Preliminary results of ionospheric radio occultation (IRO) measurements carried out onboard the German CHAMP satellite are reported. The achieved accuracy of the retrieved electron density profiles (EDPs) is estimated in particular by comparing the IRO results with independent vertical sounding data from European stations. It is concluded that CHAMP-IRO measurements have the potential to establish global data sets of EDPs, contribute to ionospheric research, develop and improve global ionospheric models and to provide operational space weather information.

Ionospheric assimilation techniques for ARGOS Low‐Resolution Airglow and Aurora Spectrograph (LORAAS) tomographically reconstructed equatorial electron density profiles

The LORAAS instrument aboard the ARGOS satellite observes line‐of‐sight ultraviolet limb intensities from ionosphere and thermosphere airglow. This study uses tomographically reconstructed electron density profiles (EDPs) from the nightside emissions. The ionospheric reconstruction is performed using a two‐dimensional O+ 1356Å radiative recombination forward model and discrete inverse theory. The forward model assumes a Chapman layer for the vertical electron density distribution from which hmF2, NmF2, and topside scale height are derived for every 90 s limb scan, which is equivalent to 5° resolution in latitude. Since ARGOS is in a near Sun‐synchronous orbit, these EDPs form a latitude slice through the equatorial anomaly structures at approximately 0230 LT. These data reflect ongoing ionospheric processes, and it is necessary to assimilate or compare with a model that contains appropriate ionospheric evolution such as the ionospheric forecast model (IFM). This study addresses the reasonableness of both the reconstructed EDPs and the IFM in describing the equatorial anomalies' diurnal and weather variability. The comparison of the LORASS EDPs with those of IFM for October 2000 show that the EDP reconstruction results compare favorably to the IFM EDPs in peak height and topside scale height. Additionally, the sector‐to‐sector climatology of the observed and modeled equatorial anomalies is similar to within the resolution of the instrument and model. The variability observed in each pass of the satellite is much larger than the IFM variability. The LORASS observation variability indicates that careful assessment of the representation error of the observations should be addressed through supplemental observations.

Real-time reconstruction of the vertical electron density distribution from GPS TEC measurements

Presented is a new operational model for real-time reconstruction of the vertical electron density distribution from concurrent GPS-based total electron content and ionosonde measurements. The model is developed on the basis of a novel approach for deducing the topside ion scale heights assuming Exponential, Epstein, or Chapman type of vertical density distribution. The required input data are submitted on-line to an operational centre where processing is carried out immediately and the electron density profile is derived. The method is suitable for use at middle and high latitude locations where ionosonde measurements are available. Several tests have been carried out and preliminary results have been presented and discussed.

A new method for reconstruction of the vertical electron density distribution in the upper ionosphere and plasmasphere

Ground‐based ionosphere sounding measurements alone are incapable of reliably modeling the topside electron density distribution above the F layer peak density height. Such information can be derived from Global Positioning System (GPS)‐based total electron content (TEC) measurements. A novel technique is presented for retrieving the electron density height profile from three types of measurements: ionosonde (foF2, foE, M3000F2, hmf2), TEC (GPS‐based), and O+‐H+ ion transition level. The method employs new formulae based on Chapman, sech‐squared, and exponential ionosphere profilers to construct a system of equations, the solution of which system provides the unknown ion scale heights, sufficient to construct a unique electron density profile at the site of measurements. All formulae are based on the assumption of diffusive equilibrium with constant scale height for each ion species. The presented technique is most suitable for middle‐ and high‐geomagnetic latitudes and possible applications include: development, evaluation, and improvement of theoretical and empirical ionospheric models, development of similar reconstruction methods utilizing low‐earth‐orbiting satellite measurements of TEC, operational reconstruction of the electron density on a real‐time basis, etc.

First application of the radioholographic method to wave observations in the upper atmosphere

Wave phenomena in the upper atmosphere can be studied using the high‐precision Global Positioning System (GPS) radio navigational field. In this paper, basic principles, accuracy, and vertical resolution of the radioholographic technique for studies of ionospheric wave phenomena are presented for the general case when the orbits of the satellites are arbitrary. Results of testing of the radioholographic method are discussed using orbital station MIR and geostationary satellites (MIR/GEO) and GPS/Meteorology (GPS/MET) radio occultation data. The radioholographic method has high vertical (12–30 m) and angular (4–8 μrad) resolution, which has been validated by directly observing multibeam propagation in the atmosphere and revealing signals, reflected from the sea, in GPS/MET and MIR/GEO radio occultation data. We show that this method allows one to determine the vertical profile of the electron density and monitoring wave structures in the upper atmosphere. As an example of this approach, observations of the summer Antarctic mesosphere on 7 February 1997 are presented. We show, by combining phase and amplitude analysis, that a vertical resolution of 0.3–0.5 km reveals wavelike structures with spatial periods from 1–2 km to 8–10 km in the vertical electron density distribution in the D and E regions. Variations in the gradient of the electron density from ±5 × 103 to ±8 × 103 el/(cm3 km) at altitudes of 72–95 km were observed. The obtained results demonstrate the high‐technology level of the radioholography approach and open new perspectives for radio occultation experiments: measurements of the characteristics of the natural processes in the atmosphere, mesosphere, and ionosphere and observations of the state of the sea surface by measuring parameters of reflected signal simultaneously with radio occultation experiments.

An automated system for the study of ionospheric spatial structures

The system is designed for the study of the vertical distribution of electron density and the parameters of medium-scale ionospheric irregularities over the sounding site as well as the reconstruction of the spatial distribution of electron density within the range of up to 300 km from the sounding location. The system comprises an active central station as well as passive companion stations. The central station is equipped with the digital ionosonde “Basis”, the measuring-and-computing complex IVK-2, and the receiver-recorder PRK-3M. The companion stations are equipped with receivers-recorders PRK-3. The automated comlex software system includes 14 subsystems. Data transfer between them is effected using magnetic disk data sets. The system is operated in both ionogram mode and Doppler shift and angle-of-arrival mode. Using data obtained in these two modes, the reconstruction of the spatial distribution of electron density in the region is carried out. Reconstruction is checked for accuracy using data from companion stations.

The Mariner 10 Radio Occultation Measurements of the Ionosphere of Venus

Data from the Mariner 10 radio occultation experiment have been utilized to determine the vertical electron density distribution in the ionosphere of Venus. The ingress measurements, which were made at latitude 1.3 deg N on the nightside of the planet, show two distinct layers. The main layer was located at 142 km altitude and had a peak density of 9000 electrons per cu cm. A secondary layer with a peak density of 7000/cu cm was detected at 124 km altitude. During egress, the ionosphere was probed at latitude 56.0 deg S on the dayside of Venus. The solar zenith angle in this region was 67.0 deg. The dayside ionosphere consisted of a main layer with a peak density of 290,000 per cu cm at 142 km altitude and several minor layers. At the top of the dayside ionosphere, the measurements showed an abrupt drop in the density from 2000 per cu cm at 335 km altitude to below the level of detectability, i.e., less than 200 per cu cm, at 360 km altitude. This abrupt density change may be the ionopause where the solar wind plasma interacts with the ionized components of the atmosphere.

Horizontal Diffusion and the Geomagnetic Anomaly in the Equatorial F-Region

DR. LYON concludes that, at the magnetic equator, the vertical distribution of electron density N takes the form N ∝ e−2z above the F2 peak, the height z being measured in units of the scale height of the ionizable gas. This is in contrast to the situation at high latitudes where vertical diffusion is not hindered by Earth's magnetic field, and the ionization takes up a distribution resembling that of a gas, the molecular weight of which is half that of the positive ions (for example, ref. 1); that is, N ∝ e−½z.

The Faraday fading of radio waves from an artificial satellite

Faraday fading of signals from an artificial satellite is analyzed in terms of the difference between the Doppler shifts of the ordinary and extraordinary components in the ionosphere. A procedure is outlined for determining the vertical distribution of electron density in the upper ionosphere. Explanations are given for the apparently excessive values of electron content yielded by measurements of Faraday fading and for the observation that the rate of Faraday fading is not exactly inversely proportional to the square of the wave frequency.

A quick method for analysing ionospheric records

A method is described by which routine (h′ - f) records can be analysed quickly to give information about the vertical distribution of electron density in the ionosphere. The method is approximate but is simple and quick to use, and is therefore convenient for making analyses of the type required for testing theories of the ionosphere. It consists in assuming, after Appleton, that the electron distribution is parabolic and then in constructing a series of curves, similar to those of Booker and Seaton, on a transparent scale, in such a way that they can be matched directly to the photographic records. The important parameters can then be read directly from the scale. Retardation in the F1 layer can be allowed for when the F2 layer is being analysed. Scales based on other electron distributions are also described and are useful in the analysis of unusual records of the type sometimes encountered at Huancayo. An account is given of the calculation of the total number of electrons in a unit column of the F2 layer below the level of the maximum. The calculations are made on the assumption that the earth's magnetic field is zero, and the effect of removing this limitation is discussed.