foF2 Data Research Articles

Long-term trends in the ionospheric F2 region with different solar activity indices

Abstract. A new comprehensive data collection by Damboldt and Suessmann (2012a) with monthly foF2 and M(3000)F2 median values is an excellent basis for the derivation of long-term trends in the ionospheric F2 region. Ionospheric trends have been derived only for stations with data series of at least 22 years (124 stations with foF2 data and 113 stations with M(3000)F2 data) using a twofold regression analysis depending on solar and geomagnetic activity. Three main results have been derived: Firstly, it could be shown that the solar 10.7 cm radio flux F10.7 is a better index for the description of the solar activity than the relative solar sunspot number R as well as the solar EUV proxy E10.7. Secondly, the global mean foF2 and hmF2 trends derived for the interval between 1948 and 2006 are in surprisingly good agreement with model calculations of an increasing atmospheric greenhouse effect (Rishbeth and Roble, 1992). Thirdly, during the years 2007 until 2009, the hmF2 values and to a smaller amount the foF2 values strongly decrease. The reason for this effect is a reduction of the thermospheric density and ionization due to a markedly reduced solar EUV irradiation and extremely small geomagnetic activity during the solar cycle 23/24 minimum.

Temporal and spatial variation of equatorial ionization anomaly by using multistation ionosonde data for the 19th solar cycle over the Indian region

The paper deals with the study of temporal and spatial variation of equatorial ionization anomaly (EIA) phenomenon along with its dependence on solar activity and season during the 19th solar cycle by using seven Indian ionosonde stations. Present study is an attempt to carry out the comprehensive study of EIA by using the limited number of ground based instruments. This has been achieved by performing the Gaussian fitting over the latitudinal distribution of F2-region critical frequency (foF2) data. Results reveal that the phenomenon of EIA has a strong dependence on solar activity and seasons. The EIA crest exhibits the feature of latitudinal shifting and expansion with increasing solar activity. It is found out that the effect of solar cycle and seasons on EIA is local time dependent. The observations were also compared with the IRI-2001 model predictions and results reveal that the model values are in general agreement with the observed values with some discrepancies, particularly during the high solar activity period and morning sector. The results have been discussed in the light of relative contribution from transequatorial interhemispheric neutral wind and strength of equatorial fountain process during different local time, season and solar activity levels. Furthermore, an attempt is made to parameterize the location and foF2 of the EIA crest by using the regression analysis. These results can be used to predict the latitudinal position and foF2 of the EIA crest for any given 12-month running average sunspot number (R12).

Does the Equatorial Ionospheric Peak Electron Density Really Record the Lowest During the Recent Deep Solar Minimum?

AbstractThe solar activity in 2008–2009, located at the minimum phase of solar cycle 23/24, was unusually low, which attracts ionospheric physics scientists to explore the plasma behaviors in the ionosphere during the deep solar minimum. Some investigators reported a reduction in ionospheric electron density during this period, while others found a marginal and minor change. In this study we collected the critical frequency of the F2‐layer ionosphere (foF2) data retrieved from ionogram records observed by an ionosonde at Jicamarca (12°S, 76.9°W), an American station located near the dip equator, to examine the changes in electron density in equatorial regions and clarify what caused the inconsistence between the differences of solar minimum foF2 in published work. We determined the foF2 differences in moving yearly, seasonal and monthly medians and the Fourier series analysis. The picture of ionospheric changes is found to be related with the data analysis methods used. The seasonal median and moving yearly values of foF2 were smaller in 2008–2009 than in 1996–1997, both in the daytime and nighttime. In contrast, the monthly median foF2 is found to be of varying solar minimum‐to‐minimum differences. Although greater values in the last minimum are prevailing in the monthly median case at most local times, opposite changes are also found at some time intervals. Further analysis reveals that the reduction in foF2 during the current solar cycle minimum is certainly presented in the longer time scale variations of foF2. Therefore, the inconsistent changes in the published investigations reflect the impact of foF2 variability on the solar minimum‐to‐minimum difference over different time scales.

Variability of foF2 over Rome and Gibilmanna during three solar cycles (1976‐2000)

Hourly validated values of the F2‐layer critical frequency (foF2) recorded at Rome, Italy (geographic coordinates 41.8°N, 12.5°E; geomagnetic coordinates 42.0°N, 93.8°E), and Gibilmanna, Italy (geographic coordinates 37.6°N, 14.0°E; geomagnetic coordinates 38.1°N, 93.6°E), along with the hourly quiet time reference values of foF2 (foF2QTRV) were considered around periods of minimum and maximum solar activity over the years 1976–2000. The foF2 data set was specifically organized in order to obtain an overall trend both for low and high solar activity, and different dispersion indices were used. The results obtained show that (1) at Rome, the foF2 variability is always greater during periods of high solar activity (HSA) in the hourly ranges 00:00–02:00 UT and 20:00–23:00 UT during winter months, and in the hourly ranges 00:00–10:00 UT and 04:00–16:00 UT during equinoctial and summer months respectively; (2) on the whole, around midday, for low solar activity (LSA), the foF2 variability is smaller at the equinoxes than at the solstices; for HSA, it is greater at equinoxes than at solstices; (3) for LSA, at Gibilmanna the foF2 variability is in general larger than at Rome, especially in summer, and it is characterized by a number of relative minimums and maximums greater than those observed at Rome; (4) at Rome, for both LSA and HSA, the passage of solar terminator at sunset significantly affects ionospheric variability in January, April, August, and November, at Gibilmanna in August, September, and November; (5) several variability peaks before sunrise and after sunset are observed in both stations; (6) on a monthly basis, for both LSA and HSA, a semiannual variation of foF2 variability is observed at both Rome and Gibilmanna; and (7) evidence of ionospheric variability at the typical heights of the F region, connected to upward propagating gravity waves triggered by solar terminator, is observed at Rome during some days characterized by HSA in the equinoctial months.

An empirical correction model for low-latitude storm-time ionospheric foF2 considering the equatorial [formula omitted] drift

The equatorial electric field plays an important role in low-latitude ionospheric plasma drift and its distribution at both quiet and disturbed periods, which should be involved in low-latitude ionospheric correction and prediction models. In this paper, an empirical model is established and utilized in real-time ionospheric correction, which contains the affiliation between the ionospheric disturbances and their electric field origins. This model is built using the nonlinear relations between ionospheric foF2 data from two low-latitude ionosondes and equatorial disturbance field data from an empirical model during fifty storms. The performance of the model during twelve intense storms is evaluated and compared with another empirical correction model, STORM. The latter relies on that long-lived negative storm effects are induced by neutral composition changes. The results of our model show that some sharp increase of foF2 could be captured, whereas STORM only gives the probable estimations. The averaged correction error is reduced by 17%, comparing with STORM. This indicates that the disturbance of the electric fields are also important in low-latitude storm-time ionospheric corrections.

Use of TEC global maps and the IRI model to study ionospheric response to geomagnetic disturbances

Period of October–November 2003 provides a wide range of geomagnetic conditions. This paper looks at three intervals: a moderate magnetic disturbance (13–16.10.2003) and two intense magnetic storms (29–31.10 and 20–21.11). One purpose of this paper is to define the behavior of the two parameters of the ionosphere, foF2 (or NmF2) and TEC, in this period. Data of foF2 of European ionosondes and TEC values of global maps for the corresponding coordinates are used. The emphasis is on the study of the synchrony of these parameters, which determine the behavior of the proportionality factor τ(obs) between the TEC and NmF2. The second objective is to compare two methods of determining foF2 during the disturbances, using: (i) the STORM-factor of the IRI model, (ii) the median equivalent slab thickness τ(med), together with the global TEC maps. It is shown that synchrony of variations of TEC and NmF2 does not always exist, which leads to unpredictable behavior of τ(obs). Acceptable option of the calculation of foF2 from TEC is to use τ(med), ensuring compliance with the experimental data better than 0.5MHz in the case of synchrony, and 1.5MHz when it does not exist. The relationship between τ(obs) and the plasmaspheric part of the TEC of the IRI-Plas model is considered. A comprehensive investigation of this relationship in cases where no synchrony may contribute to the development of model of τ(obs) during the disturbances.

Method of Detection Abnormal Features in Ionosphere Critical Frequency Data on the Basis of Wavelet Transformation and Neural Networks Combination

The research is focused on the development of automatic detection method of abnormal features, that occur in recorded time series of ionosphere critical frequency fOF2 during periods of high solar or seismic activity. The method is based on joint application of wavelet-transformation and neural networks. On the basis of wavelet transformation algorithms for the detection of features and estimation of their parameters were developed. Detection and analysis of characteristic components of time series are performed on the basis of joint application of wavelet transformation and neural networks. Method's approbation is performed on fOF2 data obtained at the observatory “Paratunka” (Paratunka settlement, Kamchatskiy Kray).

Seasonal, Diurnal, and Solar-Cycle Variations of Electron Density at Two West Africa Equatorial Ionization Anomaly Stations

We analyse the variability of foF2 at two West Africa equatorial ionization anomaly stations (Ouagadougou and Dakar) during three solar cycles (from cycle 20 to cycle 22), that is, from 1966 to 1998 for Ouagadougou and from 1971 to 1997 for Dakar. We examine the effect of the changing levels of solar extreme ultraviolet radiation with sunspot number. The study shows high correlation between foF2 and sunspot number (Rz). The correlation coefficient decreases from cycle 20 to cycle 21 at both stations. From cycle 21 to cycle 22 it decreases at Ouagadougou station and increases at Dakar station. The best correlation coefficient, 0.990, is obtained for Dakar station during solar cycle 22. The seasonal variation displays equinoctial peaks that are asymmetric between March and September. The percentage deviations of monthly average data from one solar cycle to another display variability with respect to solar cycle phase and show solar ultraviolet radiation variability with solar cycle phase. The diurnal variation shows a noon bite out with a predominant late-afternoon peak except during the maximum phase of the solar cycle. The diurnal Ouagadougou station foF2 data do not show a significant difference between the increasing and decreasing cycle phases, while Dakar station data do show it, particularly for cycle 21. The percentage deviations of diurnal variations from solar-minimum conditions show more ionosphere during solar cycle 21 at both stations for all three of the other phases of the solar cycle. There is no significant variability of ionosphere during increasing and decreasing solar cycle phases at Ouagadougou station, but at Dakar station there is a significant variability of ionosphere during these two solar-cycle phases.

GPS radio occultation measurements on ionospheric electron density from low Earth orbit

Since the proof-of-concept GPS/Meteorology (GPS/MET) experiment successfully demonstrated active limb sounding of the Earth’s neutral atmosphere and ionosphere via GPS radio occultation (RO) from low Earth orbit, the developments of electron density (n e) retrieval techniques and powerful processing systems have made a significant progress in recent years. In this study, the researches of n e profiling from space-based GPS RO observations are briefly reviewed. Applying to the Formosat-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) data, we also present a compensatory Abel inversion technique including the effects of large-scale horizontal gradients and/or inhomogeneous ionospheric n e obtained from an improved near real-time phenomenological model of the TaiWan Ionospheric Model. The results were evaluated by the ionosonde foF2 and foE data and showed improvements of rms foF2 difference from 29.2 to 16.5% in relative percentage and rms foE difference from 54.2 to 32.7% over the standard Abel inversion.

Modeling ionospheric <I>fo</I>F2 by using empirical orthogonal function analysis

Abstract. A similar-parameters interpolation method and an empirical orthogonal function analysis are used to construct empirical models for the ionospheric foF2 by using the observational data from three ground-based ionosonde stations in Japan which are Wakkanai (Geographic 45.4° N, 141.7° E), Kokubunji (Geographic 35.7° N, 140.1° E) and Yamagawa (Geographic 31.2° N, 130.6° E) during the years of 1971–1987. The impact of different drivers towards ionospheric foF2 can be well indicated by choosing appropriate proxies. It is shown that the missing data of original foF2 can be optimal refilled using similar-parameters method. The characteristics of base functions and associated coefficients of EOF model are analyzed. The diurnal variation of base functions can reflect the essential nature of ionospheric foF2 while the coefficients represent the long-term alteration tendency. The 1st order EOF coefficient A1 can reflect the feature of the components with solar cycle variation. A1 also contains an evident semi-annual variation component as well as a relatively weak annual fluctuation component. Both of which are not so obvious as the solar cycle variation. The 2nd order coefficient A2 contains mainly annual variation components. The 3rd order coefficient A3 and 4th order coefficient A4 contain both annual and semi-annual variation components. The seasonal variation, solar rotation oscillation and the small-scale irregularities are also included in the 4th order coefficient A4. The amplitude range and developing tendency of all these coefficients depend on the level of solar activity and geomagnetic activity. The reliability and validity of EOF model are verified by comparison with observational data and with International Reference Ionosphere (IRI). The agreement between observations and EOF model is quite well, indicating that the EOF model can reflect the major changes and the temporal distribution characteristics of the mid-latitude ionosphere of the Sea of Japan region. The error analysis processes imply that there are seasonal anomaly and semi-annual asymmetry phenomena which are consistent with pre-existing ionosphere theory.

Performance of IRI‐based ionospheric critical frequency calculations with reference to forecasting

Ionospheric critical frequency (foF2) is an important ionospheric parameter in telecommunication. Ionospheric processes are highly nonlinear and time varying. Thus, mathematical modeling based on physical principles is extremely difficult if not impossible. The authors forecast foF2 values by using neural networks and, in parallel, they calculate foF2 values based on the IRI model. The foF2 values were forecast 1 h in advance by using the Middle East Technical University Neural Network model (METU‐NN) and the work was reported previously. Since then, the METU‐NN has been improved. In this paper, 1 h in advance forecast foF2 values and the calculated foF2 values have been compared with the observed values considering the Slough (51.5°N, 0.6°W), Uppsala (59.8°N, 17.6°E), and Rome (41.8°N, 12.5°E) station foF2 data. The authors have considered the models alternative to each other. The performance results of the models are promising. The METU‐NN foF2 forecast errors are smaller than the calculated foF2 errors. The models may be used in parallel employing the METU‐NN as the primary source for the foF2 forecasting.

Variation of F-region critical frequency (foF2) over equatorial and low-latitude region of the Indian zone during 19th and 20th solar cycle

The effect of solar cycle and seasons on the daytime and nighttime F-layer ionization has been investigated over the equatorial and low-latitude region during 19th (1954–1964) and 20th (1965–1976) solar cycle. The F-layer critical frequency (foF2) data observed from the three Indian Ionosonde stations has been used for the present study. The dependence of foF2 on solar cycle has been examined by performing regression analysis between the foF2 values and R 12 (twelve month running average sunspot number). The result shows that the magnitude of the cycle, seasons and the location of station has considerable effects on foF2. There is a significant nonlinear relationship between the foF2 values and R 12 during 19th solar cycle as compared to 20th solar cycle. Further, the nighttime saturation effect is prominently seen during the 19th solar cycle and summer season. It is also observed that the most profound saturation effect appears at the equatorial ionization anomaly crest region. Seasonally, it is seen that all the stations exhibits semiannual anomaly. The phenomenon of winter anomaly decays as we move higher along the latitude and is prominently seen during the intense solar activity.

Equatorial predictions from a new neural network based global foF2 model

A new neural network (NN) based global empirical model for the foF2 parameter, which represents the peak ionospheric electron density, has been developed using extended temporal and spatial geophysical relevant inputs. It has been proposed that this new model be considered as a suitable replacement for the International Union of Radio Science (URSI) and International Radio Consultative Committee (CCIR) model options currently used within the International Reference Ionosphere (IRI) model for the purpose of F2 peak electron density predictions. The most recent version of the model has incorporated data from 135 global ionospheric stations including a number of equatorial stations. This paper concentrates on the ability of this new model to predict foF2 for the equatorial sector, an area that has been identified as problematic within the current IRI peak prediction setup. The improvement in the predictions of the foF2 parameter by the new model as compared to the URSI and CCIR model options of the IRI is demonstrated and the requirement for additional foF2 data from the equatorial zone for the purpose of global modeling of foF2 is highlighted in this paper.

Study of variation in slab thickness of ionospheric F-region during earthquake by wavelet analysis

The present work deals with the anomalies in ionospheric slab thickness of F-region associated with strong seismic activity. In this regard we have considered ionosonde data of Critical frequency of F2 layer (foF2) and Ionospheric Total Electron Content (ITEC) observed for Athens [38°N, 24°E] and Rome [42°N, 13°E] stations. We have considered three cases of earthquake occurring on December 20, 2007, June 08, 2008 and April 06, 2009 and analyzed the ionosonde data of foF2 and ITEC for slab thickness of F-region of the ionosphere in one-hour interval time series record by filtering the geomagnetic disturbances. The result of the study shows that some unusual perturbations were observed in slab thickness of F-region for some days before the main seismic event. It may be due to the generation of seismogenic electric field above the surface of the earth well before the seismic event. This anomalous behavior of perturbations may be used as earthquake precursor.

Association of Total Electron Content (TEC) and foF2 variations with earthquake events at the anomaly crest region

The paper attempts to identify ionospheric parameters in association with earthquake at anomaly crest station through VHF Radio Beacon data and ground based ionosonde measurements while the Total Electron Content(TEC) parameters from RB observations are based mainly on data taken over Guwahati (26.2°N, 91.75° E), foF2 data used in the analysis were collected at Ahmadabad (23.01°N, 72.36°E). The paper describes methods and techniques adopted to examine modifications on these parameters if any, due to earthquake preparatory processes at equatorial anomaly crest stations. The mechanism of inducement of density changes in the ionosphere is sought through the generation of strong fountain effect possibly by the development of electric field during the earthquake preparatory process.

Can the polarization tagging of the ionogram trace deceive autoscaling methods? The Learmonth case

This paper focuses on the problem of invalid O/X polarization tagging of an ionogram and how this can affect ionogram autoscaling methods. To illustrate this problem, 623 ionograms recorded in March and April 2004 (days 80-105) by the digisonde 256 installed at Learmonth (22.3° S, 114.1° E) were considered. These ionograms, often characterized by very unreliable O/X polarization tagging of the echoes because of unresolved antenna issues, have been autoscaled by both ARTIST 4.2 and Autoscala. Results of comparisons between automatically and manually scaled foF2 data are shown for both programs, considering as acceptable an autoscaled value that lies within 0.5 MHz of the manual value. Autoscala values of foF2 agree with the manually-scaled values for ~99% of ionograms, while ARTIST values of foF2 agree with the manually-scaled values for ~75% of ionograms. While ARTIST was coded on the assumption of valid polarization tagging, the fact remains that it produces invalid results when equipment issues cause invalid tagging. Autoscaling procedures that do not use the polarization tagging will generally work better than ARTIST in such cases. However, these other procedures are susceptible to failure in other situations.

Within-the-hour variability: levels and their probabilities

The study of foF2 data measured every 5-min and of TEC measurements made every 10-min shows that the within-the-hour variability is different in the two parameters. Deciles of this variability for foF2 and for TEC are determined together with the probabilities of exceeding a given level of variability. Furthermore, considering hourly values, it is found that the variability in TEC is like an «intrinsic noise» throughout the day of the order of less than 5% of the hourly value; but at sunrise and often at sunset large values take place. A seasonal dependence is evident. Besides, a within-the-hour variability in foF2 is always present with large values at sunrise or sunset depending on the season, and also during disturbed ionospheric conditions.

Electrodynamical control of the ambient ionization near the equatorial anomaly crest in the Indian zone during counter electrojet days

The variability of ambient ionization near the crest of the equatorial anomaly on the days of counter electrojet (CEJ) is studied using a long‐term (1980–1990) database of total electron content (TEC) obtained from Calcutta (geographic: longitude, 88.38°E; latitude, 22.58°N; dip: 32°N), situated virtually below the northern crest of the equatorial anomaly, in conjunction with the equatorial electrojet (EEJ) data of the Indian subcontinent and foF2 data from the equatorial stations. On the days of CEJ events the diurnal variations of TEC exhibit deviations (decreases) from monthly mean values, but the feature is not regular throughout the observing period. A larger percentage of CEJ days show decreases in the descending phase (1980–1985) of solar cycle compared to the ascending one (1986–1990). During low to moderate solar activity years the ionosphere near the anomaly crest is found to be more susceptible to CEJ related equatorial electrodynamics than in the high solar activity periods. On the seasonal basis, though the solstitial months of ascending phase exhibit higher percentage occurrence of decreases, the feature is not so prominent in the descending epoch. An attempt is made to relate the decreases in TEC with the prevailing electrodynamics at the magnetic equator for which EEJ, under quiet geomagnetic conditions, may be treated as a proxy index. A statistical analysis reveals positive correlations of TEC decreases with strength and duration of CEJ but anticorrelations with rising slope, strength, and time‐integrated value of EEJ as well as initiation time of CEJ. To represent better correspondence of the TEC variability with the equatorial electrodynamics a function combining various EEJ parameters has been generated, which seems to exhibit significant association with TEC decreases on the days of CEJ events.

Evaluation of the IRI model for the European region

The aim of this paper is to investigate various aspects of the International Reference Ionosphere (IRI) performance in European area and to evaluate its accuracy and efficiency for: long term prediction of the critical frequencies foF2 and the maximum usable frequencies (MUF); using storm-time correction option (ST); the total electron content (TEC) and the maximum observable frequency (MOF) updating. Data of foF2, TEC, MOF are related to 2005. It is obtained that median values of foF2 can be predicted with the mean error σ(med)∼ 0.49 MHz. For median values of MUF absolute σ was 1.39 MHz and relative σ r was 8.8%. For instanteneous values estimates are increased to 1.58 σ(med) MHz for foF2 and could reach 3.84 MHz for MUF. Using correction ST-option and TEC values provided ∼30% improvement but TEC seems to be more preferable. However, from considered parameters of the IRI updating (ST-factor, TEC, MOF) the best results were demonstrated by MOF. Using the IRI2007 to calculate TEC gives 20–50% improvement of TEC correspondence to experimental values but this improvement is not enough to treat TEC without the IRI model adaptation.

The variability of foF2 in different phases of solar cycle 23

In this paper we examined the variations of the foF2 with solar activity for different local time and different seasons. Beside this we evaluated International Reference Ionosphere (IRI) models at different phases of solar cycle 23, different latitudes and different local time. We studied F2 layer critical frequency (foF2) of the ionosphere by using the flare index calculated by the Kandilli Observatory. For this purpose, we identified the months similar with high flare activity during the solar cycle 23. We chose 6 months which represented the different phases (ascending branch, maximum and descending branch) of the solar cycle. We also took into account the fact that these months were in different seasons. The hourly monthly means of observed foF2 data from four ionosonde stations for 6 months were calculated. On the other hand, the identical foF2 values of the same months were calculated for the year 1996, which is the minimum year of the previous solar activity cycle. We subtracted the foF2 values of 1996 from the values of the selected months of the last solar cycle to obtain the residuals, Δ(foF2). Then the magnitude of the residuals is compared through the cycle. We used IRI-2007 as well as IRI-2001 models to see the degree of deviation of the observed results from the predicted ones. We found that the predicted values of the ΔfoF2, which are calculated by the IRI-2007, fitted well with the observed Δ(foF2) and showed that the Δ(foF2) are dependent on the solar cycle variations in general.