Vertical-incidence Sounding Research Articles

Investigation of Two Prediction Models of Maximum Usable Frequency for HF Communication Based on Oblique- and Vertical-Incidence Sounding Data

As one of the key technologies of HF communication, the maximum usable frequency (MUF) prediction method has been widely discussed. To experimentally confirm the reliability of commonly used MUFs prediction models for high-frequency communication, we have compared maximum observed frequencies (MOFs) and predicted MUFs to assess the accuracy of two typical prediction models. The root-mean-square error (RMSE) and relative RMSE (RRMSE) between oblique sounding MOFs and the predicted MUFs were used to assess the model’s accuracy. The oblique sounding path was from Changchun to Jinyang, and the vertical-sounding ionosonde was located in Beijing, which was approximately the midpoint of the oblique sounding circuit. The statistical analysis results show that: (a) the trend of prediction results from the Lockwood and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) model are in good agreement with the observations: the mean RMSE and RRMSE of the INGV model are less than those of the Lockwood model; (b) in the four different periods (sunrise, daytime, sunset, and nighttime) of the whole day, the maximum difference of RMSE between the Lockwood and INGV model is 0.14 MHz (the INGV performs better than the LWM), with the corresponding differences of RRMSE being 0.31% at sunrise and 0.68% at daytime; (c) in the four seasons of spring, summer, autumn, and winter, the minimum RMSE values of the Lockwood and INGV models are 1.51 MHz and 1.37 MHz, respectively, which are obtained in winter, and the corresponding RRMSEs are 11.47% and 11.79%, respectively; (d) in the high and low solar activity epochs, the mean RMSEs of the Lockwood and INGV models are 1.63 MHz, and 1.54 MHz, with corresponding mean RRMSE values of 11.47% and 11.55%. In conclusion, the INGV model is more suitable for MUF prediction over Beijing and its adjacent mid-latitude regions from the RMSE comparison of the two models.

Efficiency of Ionospheric Model Correction by Vertical-Incidence Sounding Data from an Ionosonde during Low Sunspot Activity

Numerical values of the criteria for the efficiency of the determination of the critical frequency of the F2 layer of ionosphere over European Russia are calculated with ionospheric models corrected based on vertical-incidence sounding data from an ionosonde. Two correction techniques are considered: the selection of an effective value of the input heliogeophysical index and linear-correlation correction of ionospheric models with respect to the critical frequency of the F2 layer of the ionosphere. Domains in which model correction by one point is effective are found. They determine the current coverage of the European Russia by the Russian ionospheric observation network. The regions in which the correction of median models is noticeable in comparison with the median models without correction are shown to be 7° in latitude and 15° in longitude at the midlatitudes. It is ascertained that it is mainly impossible to determine the ionospheric parameters at northern points and points in the region of the main ionospheric trough via correction based on data from midlatitude ionospheric observation sites. In these cases, the climate models without correction are more effective.

NVIS HF signal propagation in ionosphere using calculus of variations

Modeling Near Vertical Incidence Sounding (NVIS) High Frequency (HF) signal propagation in the ionosphere is important. Because, ionosondes which are special types of radars probing the ionosphere with certain HF frequencies (between 2 and 30 MHz), work mostly in NVIS mode (where elevation angle is between 89 and 90°). In this work, we are going to propose a new method for NVIS wave propagation in the ionosphere by discretizing the NVIS wave propagation path into mediums in which the refractive index changes linearly, where we solve the ray propagation in each medium analytically using calculus of variations and use Snell's Law at medium changes. The main advantage of the proposed solution is the reduced computational complexity and time. This algorithm can be used to simulate and compare the behavior of vertical ionosondes together with other ray tracing algorithms.

A new inversion algorithm for HF sky-wave backscatter ionograms

HF sky-wave backscatter sounding system is capable of measuring the large-scale, two-dimensional (2-D) distributions of ionospheric electron density. The leading edge (LE) of a backscatter ionogram (BSI) is widely used for ionospheric inversion since it is hardly affected by any factors other than ionospheric electron density. Traditional BSI inversion methods have failed to distinguish LEs associated with different ionospheric layers, and simply utilize the minimum group path of each operating frequency, which generally corresponds to the LE associated with the F2 layer. Consequently, while the inversion results can provide accurate profiles of the F region below the F2 peak, the diagnostics may not be so effective for other ionospheric layers. In order to resolve this issue, we present a new BSI inversion method using LEs associated with different layers, which can further improve the accuracy of electron density distribution, especially the profile of the ionospheric layers below the F2 region. The efficiency of the algorithm is evaluated by computing the mean and the standard deviation of the differences between inverted parameter values and true values obtained from both vertical and oblique incidence sounding. Test results clearly manifest that the method we have developed outputs more accurate electron density profiles due to improvements to acquire the profiles of the layers below the F2 region. Our study can further improve the current BSI inversion methods on the reconstruction of 2-D electron density distribution in a vertical plane aligned with the direction of sounding.

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).

Ionospheric response over Europe during the solar eclipse of March 20, 2015

The solar eclipse on March 20, 2015 was a fascinating event for people in Northern Europe. From a scientific point of view, the solar eclipse can be considered as an in situ experiment on the Earth’s upper atmosphere with a well-defined switching off and on of solar irradiation. Due to the strong changes in solar radiation during the eclipse, dynamic processes were initiated in the atmosphere and ionosphere causing a measurable impact, for example, on temperature and ionization. We analyzed the behavior of total ionospheric ionization over Europe by reconstructing total electron content (TEC) maps and differential TEC maps. Investigating the large depletion zone around the shadow spot, we found a TEC reduction of up to 6 TEC units, i.e., the total plasma depletion reached up to about 50%. However, the March 20, 2015 eclipse occurred during the recovery phase of a strong geomagnetic storm and the ionosphere was still perturbed and depleted. Therefore, the unusual high depletion is due to the negative bias of up to 20% already observed over Northern Europe before the eclipse occurred. After removing the negative storm effect, the eclipse-induced depletion amounts to about 30%, which is in agreement with previous observations. During the solar eclipse, ionospheric plasma redistribution processes significantly affected the shape of the electron density profile, which is seen in the equivalent slab thickness derived by combining vertical incidence sounding (VS) and TEC measurements. We found enhanced slab thickness values revealing, on the one hand, an increased width of the ionosphere around the maximum phase and, on the other, evidence for delayed depletion of the topside ionosphere. Additionally, we investigated very low frequency (VLF) signal strength measurements and found immediate amplitude changes due to ionization loss at the lower ionosphere during the eclipse time. We found that the magnitude of TEC depletion is linearly dependent on the Sun’s obscuration function. By modelling TEC depletion and knowing the Sun’s obscuration function in advance, Global Navigation Satellite System (GNSS) operators may improve the broadcast ionospheric correction during a solar eclipse day.

Evaluation of ionospheric profilers using topside sounding data

AbstractAn operational system for deducing and imaging the vertical distribution of the electron density in the local ionosphere has been recently developed. The electron density profile is deduced from combined ground‐based measurements of the total electron content, ionospheric vertical incidence soundings, and empirically obtained values of the O+‐H+ ion transition height. The topside profile is permitted to take one of several forms: Exponential, Chapman, or Epstein. An evaluation of the above mentioned ionospheric profilers is needed in order to determine which one of them provides the best representation of the current ionospheric conditions. For this purpose, we use electron density profiles obtained from ionograms recorded by the topside sounders onboard the Alouette and ISIS satellites. Every profile has been fitted with each of the above mentioned theoretical ionospheric profilers and the corresponding approximation errors calculated. The results have been analyzed with respect to local time, geomagnetic latitude, season, magnetic and solar activity, ion transition height, and the ionospheric density peak characteristics. It has been found that, in the majority of cases, the best fit is provided by the exponential profiler, followed by the Chapman profiler. Also, while some influence of the underlying physical drivers on the topside electron density profile is detected, it is the use of ionospheric characteristics that offers more reliable selection criteria for the most appropriate profiler.

Semiempirical Model for Ionospheric Absorption based on the NRLMSISE‐00 atmospheric model

The strength of high‐frequency radio signals, when refracted by the ionosphere, can be strongly influenced by ionospheric absorption. Accurate modeling of the amount of this absorption is a vital part of many studies of radio waves propagating in the ionosphere. We have developed a new, flexible model of ionospheric absorption, the Semiempirical Model for Ionospheric Absorption based on the NRLMSISE‐00 atmospheric model (SiMIAN). This article describes the methods and formulae used by SiMIAN, a comparison of SiMIAN with an older absorption model, and validation work performed by simulating the power returned in vertical incidence soundings of the ionosphere and comparing the results with real soundings.

SpICE: A program to study small‐scale disturbances in the ionosphere

Ionospheric disturbances observed by bottom‐side soundings of the ionosphere appear at many temporal and spatial scales. Australia has many simultaneous observations from vertically orientated ionospheric sounders with spatial separations on the scale of 1000 km. However, with this spatial sampling only large scale ionospheric disturbances can be mapped and subsequently modeled. DSTO has an experimental program in progress to investigate the smaller spatial scale disturbances. These are often seen on vertical incidence soundings and are uncorrelated with soundings from greater than 500 km away. They can also be uncorrelated with soundings from the same site only 15 min later. The DSTO program to investigate these ionospheric disturbances is called SpICE, for Spatial Ionospheric Correlation Experiment. SpICE uses a small set of transmitters and receivers with varying separations to achieve a geographically spread set of near‐vertical incidence ionospheric “reflection” points separated by 50–150 km, allowing us to probe disturbances at this spatial scale. Using the latest digital receiver technology we can collect amplitude and phase information from the ionospheric returns of the continuous wave transmissions of a nearby transmitter that is rapidly sweeping through the HF band. The returned signal is processed at a very high resolution to achieve good signal‐to‐noise complex ionograms at better than one‐minute time updates. To date there have been three SpICE campaigns. This paper will discuss the SpICE program goals and highlight some of the unusual features observed in the first campaign. Following papers will look more closely at this data set and the subsequent campaigns.

Validation of a method for ionospheric electron density reconstruction by means of vertical incidence data during quiet and storm periods

A preliminary validation of the technique developed using the NeQuick ionospheric model and the «effective ionization parameter» Az, based on vertical total electron content data ingestion, was carried out in a previous study. The current study was performed to extend the analyzed conditions and confirm the results. The method to validate this technique is based on a comparison between hourly F2 peak values measured with Vertical Incidence (VI) soundings and those calculated with the new technique. Data corresponding to different hours and seasons (equinox, summer solstice, and winter solstice) during the period 2000-2003 (high and medium solar activity conditions) were compared for all available ionosonde stations. The results show a good agreement between foF2 and hmF2 values obtained with the new technique and measurements from vertical incidence soundings during quiet and storms conditions.

Analysis of a topside ionospheric model using GPS and ionosonde observables

Ground-based vertical incidence soundings are well suited to model the bottom-side ionosphere but are not so good for dependably modelling the topside ionosphere. This study aims to combine vertical incidence sounding and dual-frequency GPS measurements to reconstruct the topside profile. The reconstruction technique relays on the use of the so-called vary-Chap approach that use an α-Chapman function with a continuously varying scale height. We tested our technique with ionograms and GPS observations recorded at Ebro observatory, Spain (geomagnetic coordinates 43.6°N and 80.9°E). The contribution of the topside ionosphere to the vertical TEC was calculated as the difference between the vertical TEC estimated from GPS measurements and the contribution of the bottom-side ionosphere estimated by integration of the electron density obtained from ionograms. The obtained topside vertical TEC were used to estimate the parameters of the vary-Chap function that represents the topside profile, namely the transition height where the dominant ion species change from O + to H +, the scale height at the transition height, and the shape factor that controls the steepness of the O + to H + transition. The results are analyzed for solstices and equinoxes for a high solar activity year. The values obtained for the transition height are in agreement with those derived by other author. The topside profile seems to merge swiftly with an empirical plasmasphere model based on Radio Plasma Image measurements.

Ionospheric behavior over Europe during the solar eclipse of 3 October 2005

An annular eclipse occurred over Europe in the morning hours of 3 October 2005. The well-defined obscuration function of the solar radiation during the eclipse provided a good opportunity to study the ionospheric/thermospheric response to solar radiation changes. Since the peak electron density behavior of the ionospheric F2 layer follows the local balance of plasma production, loss and transport, the ionospheric plasma redistribution processes significantly affect the shape of the electron density profile. These processes are discussed here based on a comparison of vertical incidence sounding (VS) and vertical total electron content (TEC) data above-selected ionosonde stations in Europe. The equivalent slab thickness, derived with a time resolution of 10 min, provides relatively good information on the variation of the electron density profile during the eclipse. The computations reveal an increased width of the ionosphere around the maximum phase. As indicated by the available measurements over Spain, the photo production is significantly reduced during the event leading to a slower increase of the total ionization in comparison with the neighboring days. The supersonic motion of the Moon's cool shadow through the atmosphere may generate atmospheric gravity waves that propagate upward and are detectable as traveling ionospheric disturbances at ionospheric heights. High-frequency (HF) Doppler shift spectrograms were recorded during the eclipse showing a distinct disturbance along the eclipse path. Whereas the ionosonde measurements at the Ebro station/Spain in the vicinity of the eclipse path reveal the origin of the wave activity in the lower thermosphere below about 180 km altitude, the similar observations at Pruhonice/Czech Republic provide arguments to localize the origin of the abnormal waves in the middle atmosphere well below the ionospheric heights. Although ionosonde and HF Doppler measurements show enhanced wave activity, the TEC data analysis does not, which is an indication that the wave amplitudes are too small for detecting them via this interpolation method. The total ionization reduces up to about 30% during the event. A comparison with similar observations from the solar eclipse of 11 August 1999 revealed a quite different ionospheric behavior at different latitudes, a fact that needs further investigation.

Ground based monitoring of the Earth’s environment at Nagycenk Observatory (NCK)

Ground based observations of the “Széchenyi István” Geophysical Observatory of the Hungarian Academy of Sciences (Nagycenk) offer a unique opportunity for investigation of the complex processes of Earth’s environment and of its effect on radio wave propagation. High temporal resolution recording of variations of the geomagnetic field, vertical incidence sounding of the ionosphere and observation of whistlers enable determination of plasma density profiles in the inner magnetosphere (plasmasphere) and their changes. Study of ionospheric processes and their effects in the upper atmosphere (thermosphere) is supported by a permanent GPS station. Both DC and AC fields of lightning discharges are also recorded in the Observatory. Measurement of the DC field is related to the atmospheric electric circuit, where the global thunderstorm activity is the generator and fine weather areas play the role of loading. Loading of the atmospheric electric field depends on rate of ionization, ionization being caused by galactic cosmic rays. However, galactic cosmic rays are modulated by solar activity. Similarly amplitude of Schumann resonances depend on thunderstorm activity and its distance from the recording site. Resonance frequencies occurring at wave-lengths approximating dimensions of the Earth-ionosphere waveguide are affected by conductivity variations of the upper boundary of the cavity resonator and thus, by changes of thickness of the cavity resonator.

What do we know of the long-term change of the Earth’s ionosphere?

In this paper, long-term change of the Earth’s ionosphere is discussed. It can most effectively be studied by means of ionospheric parameters recorded on ionograms obtained by vertical incidence sounding of the ionosphere. These measurements supply the longest ionospheric data series recorded by the greatest number of ionospheric stations. It has been found that in the F2 region long-term changes are due to geomagnetic activity and dynamical processes, from the 1970s effect of greenhouse gases is dominant [Mikhailov, A.V. Ionospheric long-term trends: can the geomagnetic control and the greenhouse hypothesis be reconciled? Ann. Geophys. 24, 2533–2541, 2006]. Spatial variation of hmF2 trends may be related to up- and downwelling at the neighbourhood of the boundary continent-sea. In the E and F1 regions, the sign of the foE, foF1 and h′E corrected trends, foF1 and foE increasing, h′E decreasing hint at greenhouse effect, which results in enhanced electron density and reduced height due to shrinking of the atmosphere as a consequence of cooling. Long-term changes in the lower ionosphere studied by LF phase height and ionospheric absorption measurements can be attributed to the effect of greenhouse gases [Bremer, J., Berger, U. Mesospheric temperature trends derived from ground-based LF phase-height observations at mid-latitudes: comparison with model simulations. J. Atmos. Solar-Terr. Phys. 64, 805–816, 2002].

Topside plasma scale height retrieved from radio occultation measurements

The plasma scale height is one of the important ionospheric characteristics due to its intrinsic connection to the ionospheric plasma temperature and composition, and thus to the shape of the electron density profile. Therefore, the knowledge of the plasma scale height values and variation is of crucial importance when addressing several open scientific and technological questions such as the upper ionosphere temperature balance, ion composition, storm dynamics, etc. While the plasma scale height value in the bottomside ionosphere can be deduced directly and reliably enough by vertical incidence sounding, the plasma scale height in the topside ionosphere is difficult to obtain. The ionospheric radio occultation (IRO) technique, based on low-earth-orbiting (LEO) satellites, is capable of delivering valuable information on the topside plasma scale height behaviour and of providing a rich database for consequent development of new empirical models of the plasma scale height and density. The purpose of this paper is to present a new procedure for retrieval of the topside plasma scale height value from IRO measurements by the LEO satellite CHAMP. First results from the analysis of the topside plasma scale height’s temporal and spatial variations are also provided.

On the region of mid-latitude spread-F

Mid latitude spread-F has extensively studied recently as compared to the earlier periods. In the course of these investigations two concepts have appeared in the literature concerning the origin of the mid-latitude spread-F. The first concept based on radar measurements attributes the development of spread-F to plasma instabilities (Fukao et al. 1988), while according to the other concept based on the vertical incidence sounding of the ionosphere the occurrence of mid-latitude spread-F is related to travelling ionospheric disturbances (Bowman 1981). Therefore, it seemed worthy to study mid-latitude spread-F from the point of view of its origin. This study has been carried out by analysing the connection between the occurrence of spread-F and the occurrence of sporadic E layers, keeping in mind that the development of sporadic E layers at mid-latitude is due to atmospheric gravity waves, which initiate plasma instabilities. On the other hand, the relation of the occurrence of spread-F to enhancements of the auroral electrojet index (AE) has been studied indicating the occurrence of large scale travelling ionospheric disturbances.

Some results referring to the long-term change of ionospheric parameters

Mid latitude spread-F has extensively studied recently as compared to the earlier periods. In the course of these investigations two concepts have appeared in the literature concerning the origin of the mid-latitude spread-F. The first concept based on radar measurements attributes the development of spread-F to plasma instabilities (Fukao et al.\ 1988), while according to the other concept based on the vertical incidence sounding of the ionosphere the occurrence of mid-latitude spread-F is related to travelling ionospheric disturbances (Bowman 1981). Therefore, it seemed worthy to study mid-latitude spread-F from the point of view of its origin. This study has been carried out by analysing the connection between the occurrence of spread-F and the occurrence of sporadic E layers, keeping in mind that the development of sporadic E layers at mid-latitude is due to atmospheric gravity waves, which initiate plasma instabilities. On the other hand, the relation of the occurrence of spread-F to enhancements of the auroral electrojet index (AE) has been studied indicating the occurrence of large scale travelling ionospheric disturbances.

Further improvements to the inversion of elevation-scan backscatter sounding data

Based upon a multi-frequency sounding scheme, an algorithm to improve the inversion of HF elevation-scan backscatter sounding data is described. The efficiency of the algorithm is evaluated by computing the mean and the standard deviation of the differences between inverted parameter values and ‘true’ values obtained from vertical incidence sounding. The influence of the use of an estimator weighted over frequencies by the a posteriori probability density and the influence of a model of the lower ionosphere on the quality of the results are both considered. From both daytime and night-time data, it is found that vertical incidence parameter values are recovered from the inversion of mid-latitude oblique sounding backscatter data with a mean error of less than 4% for foF2 and 11 % for hmF2 and with a standard deviation less than 11 % for both parameters. The usefulness of the algorithm to aid coordinate registration for over-the-horizon (OTH) radar is discussed and demonstrated.

Developing countries service with ground-based vertical-incidence sounding observations for enhanced utilization of space data

Regular ground-based vertical-incidence sounding observations are available to specify the ionospheric quiet and disturbed conditions however their accuracy is affected by missed or abserrant hourly data which comprise a few percents of occurrence at mid-latitudes but can reach 30 to 70% of times at equatorial, arctic and antarctic zones. Positive and negative deviations of ground-based and space borne F2 layer peak parameters from local or regional median are taken as the ionosphere disturbance measure. It is shown that the long-term ionospheric variability is correlated with geomagnetic indices. Decreasing number of geomagnetic quiet days with a rate of 1.5% days per year suggest growing the ionospheric and geomagnetic disturbances in 20th century. World-wide services could be arranged in developing countries to produce hourly, 3-hours and other time scales ionospheric disturbance indices in support of space observation.

Enhanced variability in the topside ionosphere

Variability of total electron content (TEC) observed by the Faraday rotation method at Florence has been stud- ied with the same technique applied independently to the ionospheric parameters foF2 and M(3000)F2 of the ground-based vertical-incidence sounding database (VID). Results of daily and monthly TEC disturbance indices at sub-ionospheric point are compared with variability of the ionosphere at Rome and Gibilmanna (de- duced from VID) for a period of 1976 to 1991. During moderate and high solar activity the variability of TEC is greater than the variability of VID, whereas during solar minimum the situation is opposite. In this context joint TEC and VID observations distinguish either the F region peak or the topside ionosphere heights where the dynamic processes dominate at different times.