Critical Frequency Of F2 Layer Research Articles

On the long-term stability of the association between foF2 and EUV solar proxies

Solar extreme ultraviolet (EUV) radiation is the main source of heating and ionization of the Earth's upper atmosphere, forcing most of this system's time variability, which in annual scales corresponds to the solar activity ∼11-year cycle. Due to the difficulties in obtaining solar EUV time series covering extended periods of time or during periods without measurements available, the use of solar EUV proxies became a solution. In the case of the ionosphere, and in particular the F2-layer critical frequency (foF2), in addition to the solar activity cycle variation, it may also exhibit the effect of long-term trend forcings, like the monotonous increasing greenhouse gas concentration since the industrial revolution. To accurately detect and measure this weak trend against the solar activity variability, it is crucial to account for the solar forced variation. Traditionally, it is modeled as a linear association between foF2 and a given solar EUV proxy. However, the stability of this association has become a controversial issue. It would be reasonable to assume, in turn, that if the ionospheric environment is undergoing a trend forced by a non-solar diver, like the greenhouse gas concentration increase, the relationship between foF2 and solar proxies may be affected, ceasing to be stable if this additional driver is not introduced in the modeled association. Using rolling regressions over the period 1960–2023 to analyze this stability, our results suggest that the issue may not only lie in the steady trend expected in foF2 from a non-solar source or the need to include terms in the simple linear regression commonly used, but also in the possible deviation of the different proxies from the 'true' EUV solar flux, which is the ultimate main driver of F2 region ionization, a deviation that has been intensifying over the last two decades. We assert that it is a deviation from the actual EUV behavior because the indices diverge from one another, something that should not occur if they all reflect the same solar EUV.

Trends in the F2-layer parameters to 2023

Diurnal variations in the long-term trends in the F2-layer critical frequency, foF2, are studied in detail based on the observations to 2022 and use of several solar activity (SA) indices to remove the SA effects in the foF2 variations. The new results confirm the conclusions derived earlier based on the data only to 2014. In the winter months, two well pronounced periods are observed: the nighttime hours when the trends are small by their magnitude and the daytime hours when the negative trends are maximal. In March, the picture is close to that in winter months, however the daytime negative trends are weaker than in winter. No pronounced diurnal variation in the foF2 trends is found in the summer month June. A search for long-term trends in foF2 is performed based on the observations at three stations in the Northern Hemisphere (Juliusruh, Boulder, and Moscow) and three stations in the Southern Hemisphere (Townsville, Hobart, and Canberra). The data for two winter months in each hemisphere for five near-noon LT moments are analyzed. Three solar activity proxies (F30, Ly-α, and MgII) are used to get rid of the SA effects. Negative trends are obtained for all considered situations (station, month, LT moment, SA proxy). The trends for different stations agree well with each other. A tendency in intensification in the negative trends during the recent years is seen in all months.

Simultaneous monitoring of the limited area ionosphere with the use of GPS and ionosonde

The importance of this study lies in its contribution to the field of ionospheric science, particularly in the monitoring and prediction of ionospheric parameters. This study focuses on comparing two techniques for spatio-temporal kriging of critical ionospheric parameters, namely the F2 layer critical frequency (foF2) and Total Electron Content (TEC), under both quiet and disturbed ionospheric conditions. The first technique, Ordinary Kriging (OK), relies solely on foF2 data, while the second technique, Kriging with External Drift (KED), incorporates the dynamic relationship between foF2 and TEC data to improve the interpolation process. This study demonstrates how KED, which utilizes dynamic secondary information, offers advantages over OK. By simultaneously monitoring the ionosphere from a single ionosonde station and collecting data from visible GPS satellites, KED enhances the accuracy of spatial structural analysis, enabling more precise estimations of ionospheric characteristics across interpolation grid nodes. The research highlights that the non-uniform distribution of ionosonde stations, especially in regions with limited foF2 data, can benefit from KED's incorporation of TEC data. Additionally, the study reveals the potential for improving TEC maps by assimilating foF2 data located at the periphery of the observation area. Furthermore, the paper discusses the determination of semivariogram structures for both foF2 and TEC data, emphasizing the spatial correlations between data points. These findings contribute to a better understanding of ionospheric behavior and the potential for enhanced ionospheric modeling.

Dependence of annual asymmetry in <i>NmF2</i> on local time

Based on the global empirical model of the F2 layer critical frequency median (Satellite and Digisonde Data Model of the F2 layer, SDMF2), an analysis was made of the properties of diurnal variations in the annual asymmetry in the concentration of the F2 layer maximum NmF2 at different values of the solar activity index F. The AI index, which characterizes the relative difference in NmF2 averaged over all longitudes and latitudes between January and July at a given local time, was used as a parameter of this asymmetry. It was found that the diurnal variations of the AI index are dominated by a semidiurnal mode with maxima in the daytime and at night. The daytime maximum of the AI index is almost independent of the level of solar activity. The nighttime AI maximum decreases with increasing solar activity. For low solar activity, the daytime and nighttime AI maxima almost coincide in amplitude when AI = 16—17%. The difference in the solar radio flux between January and July due to the ellipticity of the Earth’s orbit relative to the Sun makes a significant contribution to the AI index at all hours of the day. On average, it is 3—4% and can reach 5% with low solar activity at night. The difference in the AI index for low and high activity according to the IRI model (with URSI and, especially, CCIR coefficients) is overestimated relative to the SDMF2 model at almost all hours of the day, apparently due to the limited number of experimental data when obtaining the CCIR and URSI coefficients especially over the oceans

HF radio channel modeling by a waveguide approach

We present a modified method of HF radio channel modeling based on a waveguide approach. The waveguide approach represents the electromagnetic field of radiation inside the Earth—ionosphere waveguide as an eigenfunction series of a radial boundary problem with impedance conditions on the Earth surface and radiation conditions at infinity. The transfer function of the radio channel is represented as a series of products of angular-operator Green functions, excitation coefficients, and coefficients for receiving individual normal modes. We have obtained a solution of the boundary value problem of determining the eigenfunctions and eigenvalues of the radial operator. The solution can be applied to the frequency range below the ionospheric F-layer critical frequency. We examine algorithms for calculating distance-frequency, frequency-angular, and amplitude characteristics of signals by analyzing and numerically summarizing the series in terms of strongly damped normal modes.

Моделирование КВ-радиоканала на основе волноводного подхода

We present a modified method of HF radio channel modeling based on a waveguide approach. The waveguide approach represents the electromagnetic field of radiation inside the Earth—ionosphere waveguide as an eigenfunction series of a radial boundary problem with impedance conditions on the Earth surface and radiation conditions at infinity. The transfer function of the radio channel is represented as a series of products of angular-operator Green functions, excitation coefficients, and coefficients for receiving individual normal modes. We have obtained a solution of the boundary value problem of determining the eigenfunctions and eigenvalues of the radial operator. The solution can be applied to the frequency range below the ionospheric F-layer critical frequency. We examine algorithms for calculating distance-frequency, frequency-angular, and amplitude characteristics of signals by analyzing and numerically summarizing the series in terms of strongly damped normal modes.

Large Scale Traveling Ionospheric Disturbances During Geomagnetic Storms of 17 March and 23 June 2015 in the Australian Region

AbstractLarge scale traveling ionospheric disturbances (LSTIDs) can be detected using the critical frequency of F2 layer (foF2) of the ionosphere. The HF interferometry (HF‐int) technique is applied to the ionosonde data from low and mid latitude stations over Australia. For the first time, we present a comparison of TIDs detected during the geomagnetic storms of 17 March and 23 June 2015. The TID response for these two comparable superstorms is significantly different owing to the differing storm evolution patterns. The storm associated TIDs are more intense and longer compared to the solar terminator. Results demonstrate that the morphology of parameters continually varies over time due to highly dynamic ionospheric response to geomagnetic storms. For the March storms, TIDs velocities were 400–700 m/s at the onset phase, ∼500 m/s during the main phase and ∼600 m/s during the recovery phase. Their propagation directions were equatorward with east/west deviations. Lower period (∼60 min) TIDs were determined during storm onset whereas higher periods (∼140 min) were detected during the recovery phase. The June storm produced a different response. The TID velocity at the onset phase was ∼500 m/s but during the main and recovery phases the velocities were greater than 1,000 m/s. There were large deviations in velocity vectors at the different stations suggesting multiple sources of TID generation and possible resonating effects at middle and low latitude stations. Our findings indicate that while TID detection and storm phases co‐occur, the propagation characteristics of TIDs change dramatically throughout the storm.

Validation of the European Ionosonde Service nowcasting foF2 maps over the eastern Mediterranean

The Cyprus ionosonde (Digisonde) has been operating since 2009. Therefore it has been monitoring ionospheric conditions over the eastern Mediterranean region for more than 14 years. It is the only ionosonde operating in the area and it does not contribute to any existing European database and subsequently has not been utilised in the frames of any Space Situational Awareness (SSA) program. In the frames of the European Space Agency (ESA) activity “Potential Enhancements in Ionospheric Monitoring under SSA (PEIMSSA)” a quantitative validation of the the European Ionosonde Service (EIS) was undertaken. This validation is based on 8 years of manually scaled F-layer critical frequency (foF2) data that was compared with the corresponding interpolated values from EIS nowcasting foF2 maps over Europe. The aim of the study was to quantify EIS accuracy over the eastern Mediterranean with respect to manually scaled foF2 data from the Cyprus Digisonde. The findings of this study suggest that there are systematic underestimation-overestimation features that appear in all years considered. The most notable feature is an overestimation of EIS over Cyprus in the interval 00:00–03:00 UT, from October to March and an underestimation from April to August. An underestimation is also systematically observed in the interval, 14:00–16:00 UT from April to December and 22:00–23:00 UT in September to October. This overestimation-underestimation pattern is evident throughout all years under investigation with a higher mean % difference discrepancy in low than in high solar activity years. During disturbed days we observe, for all years, EIS slightly overestimating Cyprus foF2 values.

Statistical Analysis of the Critical Frequency foF2 Dependence on Various Solar Activity Indices

A description of the ionospheric F2-layer critical frequency foF2 dependence on solar activity byvarious indices (proxies) has been analyzed. The results of the vertical ionospheric sounding at the Juliusruhstation during 2 winter months (January and February), 2 equinox months (March and October), and a summermonth (June) are considered. Five solar proxies: Ly-α, MgII, Rz, F30, and F10.7 have been analyzed.The changes in foF2 are compared to the corresponding changes in 1957–1980. The determination coefficientR2 according to the Fisher F-test is used as a measure of the quality of the description of the foF2 dependenceon SA by each of the proxies. It has been found that a well-pronounced diurnal variation in the R2 valueis observed in winter months: this is higher at the near-noon hours than at night. In other words, all indicesdescribe the foF2 behavior better in the daytime than at night. A well-pronounced diurnal variation in R2 isalso observed for four proxies in the equinoctial months, whereas that variation is pronounced much less forRz: a strong scatter of the R2 values is seen. A diurnal variation in the R2 is absent completely in June: jumpsfrom one hour to another are observed. This analysis allows us to conclude that the most reliable SA proxiesfor description of the foF2 dependence on SA at all LT hours are MgII, F30, and Ly-α.

Spectral features of ionospheric plasma waves excited by powerful HF radio waves radiated at frequencies near electron gyroharmonics and F2-layer critical frequency

We present the result of the studies into characteristics of high-latitude ionospheric F-region longitudinal plasma waves (Langmuir and ion-acoustic), caused by the impact of powerful HF radio waves of ordinary (O-mode) or extraordinary (X-mode) polarization of the EISCAT/Heating facility, Tromsø, Norway. The powerful HF radio waves on October 20, 2012 and February 26, 2013 were emitted in the direction of the magnetic zenith with a step change in the effective radiation power (ERP). The radiation frequency fH of the EISCAT/Heating facility on February 26, 2013 was close to the F2-layer critical frequency foF2 (fH/foF2~1) and exceeded the electron gyroresonance frequency fH>5fce. On October 20, 2012, the conditions fH/foF2~0.85–0.95 and fH<6fce were fulfilled. Analysis of EISCAT measurements of an incoherent scatter radar (ISR) at a frequency of 930 MHz, spatially aligned with the heating facility for radiation conditions ERP<200 MW, has shown that parametric decay instabilities are excited at the ionospheric heights where the pump frequency is close to the plasma Langmuir frequency, fH≈fPL. We have studied peculiarities of excitation of the decay parametric instabilities as a function of height in the ionosphere and pump wave polarization, the ratios between fH and foF2, and also fH and nfce.

Спектральные характеристики плазменных волн ионосферы при возбуждении мощными КВ-радиоволнами на частотах излучения вблизи гирогармоник электронов и критической частоты слоя F2

We present the result of the studies into characteristics of high-latitude ionospheric F-region longitudinal plasma waves (Langmuir and ion-acoustic), caused by the impact of powerful HF radio waves of ordinary (O-mode) or extraordinary (X-mode) polarization of the EISCAT/Heating facility, Tromsø, Norway. The powerful HF radio waves on October 20, 2012 and February 26, 2013 were emitted in the direction of the magnetic zenith with a step change in the effective radiation power (ERP). The radiation frequency fH of the EISCAT/Heating facility on February 26, 2013 was close to the F2-layer critical frequency foF2 (fH/foF2~1) and exceeded the electron gyroresonance frequency fH>5fce. On October 20, 2012, the conditions fH/foF2~0.85–0.95 and fH<6fce were fulfilled. Analysis of EISCAT measurements of an incoherent scatter radar (ISR) at a frequency of 930 MHz, spatially aligned with the heating facility for radiation conditions ERP<200 MW, has shown that parametric decay instabilities are excited at the ionospheric heights where the pump frequency is close to the plasma Langmuir frequency, fH≈fPL. We have studied peculiarities of excitation of the decay parametric instabilities as a function of height in the ionosphere and pump wave polarization, the ratios between fH and foF2, and also fH and nfce.

Seasonal Variation of the F2-Layer Critical Frequency and Comparison with the IRI-2016 Model during Two Extremes of Solar Activity Phase of Solar Cycle 22

Abstract: Introduction: The variation of the ionosphere is mostly studied using the critical frequency of the F2-layer (foF2) whose values can also be predicted by an ionospheric model. The widely used model for predicting ionospheric parameters is the International Reference Ionosphere (IRI). Aim: This paper aims to demonstrate how well the current International Reference Ionosphere (IRI-2016) model performs in predicting the critical frequency of the F2-layer (foF2) over two equatorial stations during two extremes of solar activity phase of solar cycle 22.. Methods: The hourly foF2 experimental data collected during the Maximum Phase of Solar Activity MPSA year (1989) and Minimum Phase of Solar Activity MnPSA year (1986) at Ougadougou (Geomagnetic Latitude 0.59 oN, Geomagnetic Longitude 71.46 oE) in the African longitudinal sector and Manila (Geomagnetic Latitude 3.4 oN, Geomagnetic Longitude 191.1 oE) in the Asia longitudinal sector as well as the predicted foF2 data by the IRI-2016 model were used in this study. Sunspot data from Zurich was utilized as a measure of solar activity phase. The foF2 data were grouped into four seasons before analysis began. Comparing the seasonal means of the experimental foF2 data and the IRI-2016 modeled foF2, it was possible to determine how closely the model matched the experimental data at the different seasons and longitudinal sectors Results: The results showed that the IRI-2016 model overestimate and underestimate the observed foF2 at different periods of the day during the equinox and solstice seasons. Observation showed that the highest positive and negative percentage deviations were observed mostly during the post-midnight hours. Observation also showed that Seasonal mean values of the IRI-2016 model of both options showed remarkable improvement at this two stations since their values have little difference from the observed foF2 values. Conclusion: The discrepancy (underestimation and overestimation) in the IRI-2016 model is found larger during MPSA year than during MnPSA year. The URSI option performs better than the CCIR option since its predicted values are much closer to the observed values. Both options of the model perform better in the Asian longitudinal sector than the African longitudinal sector.

Wavelet coherence analysis foF2 over Boulder station during different geomagnetic activity


 
 
 
 
 
 
 
 A geomagnetic storm is a major disturbance of Earth’s magnetosphere that occurs when there is a very efficient exchange of energy from the solar wind into the space environment surrounding Earth. During solar and geomagnetic activities, critical frequency of F2 layer (foF2) varies in a great extent. The focus of the current study is on variations in foF2, the critical frequency of the F2 layer, during the intense geomagnetic storm of solar cycle 24 and quiet storm of solar cycle 25. The foF2 data of mid latitude station BC840 (Boulder) in the American region have been used to study geomagnetic and solar wind properties linked with foF2. Time series analysis and Wavelet Coherence analysis have been used to establish the relationship between those parameters. We have found considerable effect of solar and geomagnetic storm disturbances on foF2 during the geomagnetic storm time. The foF2 is enhanced prior to the storm and depletes during recovery phase with no significant changes observed during weak storm. The enhancement or depletion in foF2 may be used as precursor of geomagnetic storms. With long trend analysis and larger database we can make a catalog which can be used to predict the geomagnetic storm and deepen our understanding in impacts of storms on communication signals due to energy injected in ionosphere.
 
 
 
 
 
 
 

Statistical analysis of the critical frequency foF2 dependence on various solar activity indices

A detailed analysis of the description of the F2-layer critical frequency foF2 dependence on solar activity indices (proxies) is analyzed. To do that, the results of the vertical ionospheric sounding at Juliusruh station during two winter months (January and February), two equinox months (March and October), and a summer month (June) are considered. Five solar proxies: Ly-α, MgII, Rz, F30, and F10.7 are analyzed. The determination coefficient R2 according to Fisher F-test is used as a measure of the quality of the description of the foF2 dependence on SA by each of the proxies It is found that a well pronounced diurnal variation in the R2 is observed: this value is higher at the near-noon hours than at night. In other words, all indices describe better the foF2 behavior in the daytime than at night. A well pronounced diurnal variation in R2 is observed also for four proxies in the equinoctial months, whereas that variation is pronounced much worse for Rz: a strong scatter of the R2 values is seen. A diurnal variation in the R2 is absent completely in June: jumps of that value from one hour to another are observed. A few examples of the same analysis are presented also based on the Boulder stations data. The aforementioned analysis allows us to conclude that the most reliable SA proxies for description of the foF2 dependence on SA are (in the declining order) F30, MgII, and Ly- α.

Validation of F2-layer critical frequency variations in the ionosphere with radio observations of solar bursts

The high-frequency (HF) ionospheric cutoff restricts the possibilities of radio astronomical observations from the Earth, but on the other hand, it allows one to estimate the F2-layer critical frequency of the ionosphere. This effect has been measured using a new active antenna meant for receiving cosmic radio emission in the frequency range 1–40 MHz. The instrument was implemented near the Ukrainian T-shape Radiotelescope, second modification (UTR-2) and serves as a prototype of the HF antenna for a future radio array on the farside of the Moon. We detected directly a storm of solar radio bursts on May 22, 2021 and observed clearly their cutoff due to the ionosphere. For comparison our analysis of the experimental data was supplemented by simultaneous measurements from ionosondes located close to the UTR-2. The distance between the closest ionosonde near Zmiiv (Kharkiv region, Ukraine) and the radio astronomical antenna used in the experiment is 46.5 km. Results obtained from the solar radio records are consistent with the ionosonde data. We show that such an antenna implementation provides us with new opportunities to study the F2-layer critical frequency of the ionosphere.

Особенности возбуждения искусственной ионосферной турбулентности при О- и Х-нагреве вблизи критической частоты слоя F2

We present experimental results from the studies of large-scale inhomogeneities along the external magnetic field with increased electron density, electron temperature, and excitation of elongated plasma waves (Langmuir and ion-acoustic), induced by the ordinary (O-mode) and extraordinary (X-mode) HF heating near the F2-layer critical frequency, in the high-latitude ionospheric F-region. The experiments have been carried out at the EISCAT/Heating facility (Tromsø, Norway). Powerful HF radio waves radiated towards the magnetic zenith through a step change in the effective radiated power at frequencies fH near and below the F2-layer critical frequency fₒF2. The EISCAT incoherent scatter radar (930 MHz), co-located with the EISCAT/Heating facility, was utilized for diagnostics of ionospheric modification effects. We calculated the electric field of a powerful HF radio wave near the reflection altitude, taking into account the non-deflective absorption along the propagation path. We determined the conditions for electric field generation and its threshold (minimum) values required for electron density enhancements in a wide altitude range, excitation of Langmuir and ion-acoustic plasma waves under fH~fₒF2 and fH<fₒF2. The possible generation mechanisms for the electron density enhancements above the reflection altitude of the powerful HF radio wave of O- and X-polarization are discussed.

Особенности возбуждения искусственной ионосферной турбулентности при О- и Х-нагреве вблизи критической частоты слоя F2

We present experimental results from the studies of large-scale inhomogeneities along the external magnetic field with increased electron density, electron temperature, and excitation of elongated plasma waves (Langmuir and ion-acoustic), induced by the ordinary (O-mode) and extraordinary (X-mode) HF heating near the F2-layer critical frequency, in the high-latitude ionospheric F-region. The experiments have been carried out at the EISCAT/Heating facility (Tromsø, Norway). Powerful HF radio waves radiated towards the magnetic zenith through a step change in the effective radiated power at frequencies fH near and below the F2-layer critical frequency fₒF2. The EISCAT incoherent scatter radar (930 MHz), co-located with the EISCAT/Heating facility, was utilized for diagnostics of ionospheric modification effects. We calculated the electric field of a powerful HF radio wave near the reflection altitude, taking into account the non-deflective absorption along the propagation path. We determined the conditions for electric field generation and its threshold (minimum) values required for electron density enhancements in a wide altitude range, excitation of Langmuir and ion-acoustic plasma waves under fH~fₒF2 and fH<fₒF2. The possible generation mechanisms for the electron density enhancements above the reflection altitude of the powerful HF radio wave of O- and X-polarization are discussed.

Trends in the Critical Frequency of the F2 Layer during the Recent Decade

Trends in the F2-layer critical frequency foF2 are calculated based on the vertical sounding data atJuliusruh station over a period of 1996–2022. Four solar activity indices: Ly-α, MgII, F10.7mon, and Rz wereused. The conclusion drawn previously is confirmed: seasonal (stronger in winter than in summer) and diurnal(stronger in the daytime than at night) variations exist in the foF2 negative trends. It is shown that theresults for the first three indices agree very well, whereas the results using Rz provide slightly different results.It has been found that during the recent 5–7 years, the maximum (winter, daytime) negative trends in foF2became stronger than for the entire considered interval by a factor of 2–3

Trends in foF2 to 2022 and various solar activity indices

The trends in the F2-layer critical frequency foF2 are analyzed based on the observations at Juliusruh station to 2022. Five solar activity indexes (proxies): Ly-a, MgII, Rz, F10.7mon, and F30 are used to remove the SA effects. It is found that the conclusions obtained earlier on the diurnal (in the daytime stronger than at night) and seasonal (in winter stronger than in summer) variations in the amplitude of negative trends in foF2, k(foF2) are confirmed using all five proxies. The maximal negative values of k(foF2) are 0.03–0.04 MHz per year. Use of the “Delta” method confirms these conclusions. Indications are found that the negative trends in foF2 are stronger during the recent decade than during the entire considered period 1996–2022.

Ionospheric Variability over the Brazilian Equatorial Region during the Minima Solar Cycles 1996 and 2009: Comparison between Observational Data and the IRI Model

The behavior of the Brazilian equatorial ionosphere during the solar minimum periods, 1996 and 2009, which cover the solar cycles 22/23 and 23/24, respectively, is investigated. For this, the F2 layer critical frequency (foF2) and peak height (hmF2) registered by a Digisonde operated at São Luis (2.33° S; 44° W) are carefully analyzed. The results show that the seasonal mean values of the foF2 and the hmF2 in the equinoxes and winter during 2009 were lower than in 1996. In the summer, an anomalous response to solar variability was observed. In this case, the hmF2 in 2009 is higher than in 1996 during a specific daytime interval. Besides that, it was verified that the prereversal enhancement of the zonal electric field (PRE) during the equinoxes in 2009 occurred a few minutes earlier than in 1996. Additionally, a Fast Fourier Transform (FFT) analysis was used to investigate the impacts of solar atmospheric tides (amplitude, diurnal, semidiurnal, and terdiurnal modes) on foF2 and hmF2 parameters with respect to its seasonality. Significant differences were observed between their values during the two minima, mainly in the amplitude of hmF2, which was higher in 1996 than in 2009 for all days analyzed. Moreover, the seasonality in the diurnal and semidiurnal modes for both periods presented an annual variability, while the terdiurnal mode exhibited annual and semiannual components. The results are compared with the International Reference Ionosphere (IRI) model, and the main differences between the observation and the model results are discussed in this work.