foF2 Variations Research Articles

Critical Frequency foF2 Variations at Korhogo Station from 1992 to 2001 Prediction with IRI-2012

In this paper we report the foF2 data measured at Korhogo station (Lat. 9.3° N; Long. 354.6° E; dip. 0.6° S) compared to predictions with IRI-2012 subroutine URSI and CCIR for different solar cycle phases (minimum, ascending, maximum, descending) and different geomagnetic activity classes (quiet, fluctuating, recurrent, shock). According to our investigations, predictions with IRI are in agreement with the measured data during daytime and show significant differences between them at night-time and especially before sunrise. Except at solar minimum, the gap between predictions and measured data are more appreciable during recurrent and shock conditions compared to quiet and fluctuating conditions. Our results also show that only URSI model expresses the signature of EXB drift phenomenon at solar maximum phase during the recurrent days and at ascending phase for fluctuating activity.

FoF2 variability at a southern low-latitude station and the performance of IRI-2016 model during ascending phase of solar cycle-24

In this research, we have analyzed the data of the critical frequency of the F2 region (foF2) ionosphere over the southern low-latitude station COCO (Keeling) Island (Geog. Lat. 12.20°S; Geomag. Lat. 22.83°S; Geog. Long. 96.80°E) for the period 2009–2013 and thereof have compared the results with those obtained with the IRI-2016 model. Our analysis shows a good correlation between variations in foF2 and solar flux at an F10.7 cm wavelength. With regard to geomagnetic activity during 2009–2013, it has been found that the variability in the monthly mean Ap index remained below 20 nT throughout the period 2009–2013. Concerning quiet time F2 region variability, our results explicated solar cycle, semiannual, and seasonal/annual variations in foF2. Similar oscillations in foF2 have been explored using the Lomb-Scargle periodogram technique. We have also observed the fundamental mode (27 days) and its overtones (9- and 13-day) in the geomagnetic activity parameter (Ap). Analysis of foF2 showed the consistent presence of the semiannual anomaly and absence of the winter anomaly during the ascending phase of solar cycle-24. The presence of an annual component in normalized foF2 has been examined using regression analysis. A comparative study of ionosonde-observed foF2 with IRI-2016-modeled foF2 showed that the general ionospheric trends in IRI predictions are consistent with the observations for the diurnal, seasonal, and solar cycle variations. However, a deviation in the amplitude of foF2 up to the order of 5 MHz depending on local time, seasons, and phases of the solar cycle, respectively, has been observed.

Simplified Regional Prediction Model of Long-Term Trend for Critical Frequency of Ionospheric F2 Region over East Asia

To improve the accuracy of predictions and simplify the difficulty with the algorithm, a simplified empirical model is proposed in developing a long-term predictive approach in determining the ionosphere’s F2-layer critical frequency (foF2). The main distinctive features introduced in this model are: (1) Its vertical incidence sounding data, which were obtained from 18 ionosonde stations in east Asia between 1949 and 2017, used in reconstructing the model and verification; (2) the use of second-order polynomial and triangle harmonic functions, instead of linear ones, to obtain the relationship between the seasonal vs. solar-cycle variations of foF2 and solar activity parameters; (3) the flux of solar radio waves at 10.7 cm and sunspot number are together introduced in reconstructing the temporal characteristics of foF2; and (4) the use of the geomagnetic dip coordinates rather than geographic coordinates in reconstructing the spatial characteristics of foF2. The statistical results reveal that foF2 values calculated from the proposed model agree well with the trend in the monthly median statistical characteristics obtained from measurements. The results are better than those obtained from the International Reference Ionosphere model using both the CCIR and URSI coefficients. Furthermore, the proposed model has enabled some useful guidelines to be established for a more complete and accurate Asia regional or global model in the future.

Diurnal variation of F2-layer critical frequency under solar activity recurrent conditions during solar cycles 21 and 22 at Ouagadougou Station: Prediction with IRI-2012

This paper presents foF2 data recorded at Ouagadougou Ionosonde Station and compares them with IRI-2012 model results through its two subroutines, Union Radio Scientifique Internationale (URSI) and Comite Consultatif International des Radio Communications (CCIR) during recurrent solar activity. Except for the solar cycle maximum period, measured data profiles corroborate with the signatures of E×B drift. IRI- 2012 subroutine URSI is not consistent with observed electrodynamics during any solar cycle phase, while CCIR predictions are consistent with the measured data during the solar maximum and declining phases. Investigation on the relative deviation module mean (RDMM) shows poor agreement between measurements and predictions with IRI most of the time. However, the deviation percentages indicate good correlation between URSI predictions and data from the ionosonde, except for solar ascending phase. Good correlation with CCIR is only obtained during the solar cycle minimum and maximum phases. From a quantitative point of view, this study shows that predictions with URSI are closer to experimental measurements. The investigations show good agreement between model and in situ measurement during the day. Significant differences are recorded at night, especially from midnight to sunrise. There is necessity to improve IRI model for equatorial regions to better predict foF2 variation. Key words: Ionosphere, foF2, Solar cycle, IRI model.

Model of the Ionospheric Trough for Daytime Winter Conditions Based on Data from Interkosmos-19 and Champ Satellites

The principles and algorithm for the construction of an empirical model of the ionospheric trough for quiet (Kр = 2) daytime (06−18 LT) winter conditions in the Northern and Southern Hemispheres are described. The model consists of models of the trough position and shape. The model of the trough position describes longitudinal variations of the trough position in terms of the geographic latitude for fixed moments of local time. The model of the trough shape describes the latitudinal–longitudinal variations of foF2 within the interval of geographic latitudes 40°−85° in both hemispheres. The model of the ionospheric trough is probabilistic, since the trough is not always observed under midday conditions. In other words, the problem consists of the construction of a model of foF2 in the daytime, high-latitude ionosphere that would describe the trough structure. Data from the Interkosmos-19 and CHAMP satellites were used to solve this problem. The IRI international model of the ionosphere was also widely used; however, according to the tests, the constructed model more adequately describes the diurnal, longitudinal, and latitudinal variations of foF2 than IRI-2016. This model, together with the previously constructed model of the nighttime trough, makes it possible to speak of the creation of a complete model of foF2 in the winter ionosphere. Soon, the complete foF2 model will be accessible at the IZMIRAN site http://www.izmiran.ru/ionosphere/sm-mit/ . The model make it possible to calculate the longitudinal variations of the trough position and the latitudinal–longitudinal variations of foF2 for any local time and any level of solar activity within the range F10.7 = 70−200 for all winter months.

Variability of the Critical Frequency foF2 for Equatorial Regions during Solar Cycle’s Minima and Maxima at Ouagadougou and Manila Stations

In this paper we report on the foF2 variabilities for two equatorial regions (Ouagadougou: Lat. 12.4°N; Long. 358.5°E, Dip. 1.43°S; and Manila: Lat. 14°36'15.12''N; Long. 120°58'55.92''E; Dip. 0.6°S) during solar cycles 20 and 21 minima and maxima phases. Many previous works have argued on the diurnal and seasonal variation of foF2 for different solar events conditions for latitudinal position. But there are few investigations for Africa equatorial region longitudinal variation. The present paper’s goal is to outline possible similarity in foF2 behavior between variations for better understanding of physical process lead to some observed phenomenon in Asia-Africa equatorial sector. The F-layer critical frequency (foF2) data observed from the two equatorial ionosonde stations have been used for the present comparative study. The results show significant similarity between the critical frequency (foF2) seasonal variations over the time intervals 1976-1996. During day-time measured data from Manila station are higher than those from Ouagadougou station. That may lie in that Manila is closer to equatorial ionization crest region. During solar minimum phase, the longitudinal variation of foF2 shows two crossing points (11:00 UT and 22:00 UT) between the foF2 profiles form the two stations for all seasons regardless of the solar cycle. However during intense solar activity condition, the number of crossing-point between measured data from Manila and Ouagadougou stations varies by seasons and solar cycle. This phenomenon may be due to the compilations of severe activities (storms, coronal mass ejection, heliosheet fluctuations) during the solar maximum phases.

Atmospheric Meteorological Parameters and Ionospheric F2 Layer Critical Frequency (foF2) Observation for 6th December, 2016 Indonesia Earthquake (M 6.5): A Case Study

On 6th December, 2016, an earthquake with M 6.5 occurred at the tectonic plate boundary, southwest of Sumatra, Indonesia (Latitude: 0.5897° S, Longitude: 101.3431° E). In this case, ionosphereic critical frequency of F2 layer (foF2) variations and meteorological parameters, viz., air temperature, relative humidity, atmospheric pressure and wind speed variations were investigated so as to detect any anomalies. Data are obtained from different websites freely available for researchers. In the absence of real ionosonde foF2 data, IRI 2016 model data were used. For each parameter, anomaly window were defined when values fell beyond ± 6 °C, < 70 %, ± 4 mb and ± 3.5 km h-1 from the event day value and one third of total foF2 values broke the limits of the upper and lower bounds. Certain random anomalies in temperature, relative humidity, pressure, wind speed and foF2 frequencies were observed different days prior to occurrence of the quake but each parameter showed anomalies 12 days before the occurrence. Also, geomagnetic tranquility was justified through Kp and Dst indices. This study reveals that continuous monitoring of atmospheric meteorological parameters and regular ionospheric foF2 observations might help us to predict an earthquake about a week prior to the occurrence.

Irregular Phenomena in Magnetically Conjugate Regions of the F2 Layer of the Ionosphere

The paper focuses on the behavior of relative variations in the critical frequency of the F2 layer of the ionosphere δfoF2 in magnetically conjugate regions under quiet and disturbed conditions. Comparison of changes in δfoF2 over time shows that the electron density level at stations in both hemispheres is about the same. The coupling of conjugate regions and the correlation of δfoF2 variations increase noticeably during ionospheric storms. In addition, the predawn effect in foF2 variations is investigated, which is caused by sunrise at the magnetically conjugate point. Statistics of the effect are presented for a month in winter. Examples of δfoF2 variations at the magnetically conjugate point during solar eclipses are given.

Dynamics of sunspot cycle ascending and descending phases asymmetry in equatorial ionospheric variability

Ionospheric variation from the climatological mean has been extensively studied due to its effect on ionospheric high frequency radio propagation. This work reports foF2 variation at Ouagadougou (geogr. 12oN, 1.8oW, dip ∼3°), complementing works that have been done at this station. Nighttime variability is higher than daytime and varies with season but with no consistent pattern. There is an earlier post-midnight peak variability in June solstice (around 0100LT) and occurred between 0400 and 0500LT in other seasons for all sunspot classifications except solar maxima. In this study, we have paid attention to features in the ascending/decending phase of the sunspot cycle and we observed that nighttime ionospheric variability may not always decrease with increasing sunspot number, particularly in the ascending phase. The mechanisms for the difference between observations in the ascending and descending phases could not be identified.

FoF2 variations measured by the Rome observatory during solar minimum in the last three solar cycles

The last three solar cycles have been considered for a study on the variation of the ionospheric parameter foF2 during the solar minimum. Hourly observations recorded by the ionospheric station of Rome have been considered for the years 1985, 1986, 1987, 1995, 1996, 1997, 2007, 2008 and 2009. A background value of foF2 have been computed considering a 27-day running median. A variation of foF2 of more than ± 15% over the background level and with a minimum duration of 3 hours, is here considered as an anomaly. For each anomaly, a study of the geomagnetic condition has been carried, considering the value of the Ap index at the time when the anomaly is detected and for the previous 24 hours. The auroral electrojet index AE has also been investigated in relationship with each observed foF2 anomaly, considering 6 hours before the occurrence of the anomaly since TADs, related to upsurges of auroral activity, can reach middle latitudes and consequently perturb the F2 layer. A characterization of the observed foF2 anomalies with respect to the related geomagnetic conditions is presented in this work. Two foF2 storms are here studied in the details, analysing the solar sources.

Study the behavior of the ionospheric frequencies at Karachi during three consecutive solar minima of cycle 21, 22 & 23 and their comparison with IRI-2016

The behavior of critical frequencies of ionospheric E and F2 layers (foE & foF2) along with minimum ionospheric frequency (fmin) is studied for solar minima of cycle 21 (1986), 22 (1996) and 23 (2008) over Karachi (24.95°N, 67.13°E), Pakistan. The station is located at the crest of equatorial ionization anomaly region. Beside seasonal differences, pronounced change in the values of frequencies is noted from one solar minimum to another solar minimum. A strong and direct correlation of foF2 with Smoothed Sunspot Number (SSN) and F10.7 cm solar flux is observed. In the minimum of cycle 23, reduction in foF2 is noted due to reduction of solar EUV as compared to other minima. Also disappearance of semi-annual variations in foF2 is noted in cycle 23 minimum. Unexpectedly higher values of foE and fmin are observed in minimum of cycle 23 as compared to other minima. It is difficult to explain this unusual behavior of fmin and foE along with disappearance of semi-annual variation in foF2. It is possible that during very low solar activity, thermospheric conditions are changed which in turn altered the ionosphere. Further investigation of atmosphere-ionosphere coupling is required to understand this complex behavior. On comparison of observed values with IRI-2016, higher deviations are observed in foE before noon hours while in case of foF2, large deviations are noted during daytime. The absence of foF2 semi-annual variation in cycle 23 is not reproduced by IRI-2016. It is suggested that IRI-2016 need some modification for extremely low solar activity condition.

The possible effect of stratospheric quasi-biennial oscillation on the critical frequency of the ionospheric F2-layer

In this study, the coupling between the stratospheric quasi-biennial oscillation (QBO) and the critical frequency of the ionospheric F2-layer (foF2) was analysed statistically. The multiple regression model was used as a statistical tool. The model was developed by adding the sunspot number (SSN), which affects the foF2 (measured for Madras, Kodaikanal, Bogota, Manila and Tahiti) in the ionosphere at a significant level. Four different ‘Dummy’ sets of data were used in the model in order to observe the effect of the direction (east–west) and the magnitude (for both directions, between 0 and 15 m / s and between 0 and 16 m / s and the largest value) of QBO. It was observed that the variations of foF2 in the range of 60–78% in the model could be explained by SSN and $$\hbox {SSN}^{2}$$ . The change of 2–13% that occurred in foF2 could be explained by the whole set of QBO. It was also observed that the effect of the direction and magnitude of QBO on foF2 differed between the stations.

Diurnal and Longitudinal Variations of the Structure of an Equatorial Anomaly During Equinoxes According to Intercosmos-19 Satellite Data

Longitudinal and local time variations in the structure of the equatorial anomaly under high solar activity in the equinox are considered according to the Intercosmos-19 topside sounding data. It is shown that the anomaly begins to form at 0800 LT, when the southern crest is formed. The development of the equatorial anomaly is associated with well-known variations in the equatorial ionosphere: a change in the direction of the electric field from the west to the east, which causes vertical plasma drift W (directed upward) and the fountain effect. At 1000 LT, both anomaly crests appear, but they become completely symmetrical only by 1400 LT. The average position of the crests increases from I = 20° at 1000 LT to I = 28° at 1400 LT. The position of the crests is quite strong, sometimes up to 15°, varies with longitude. The foF2 value above the equator and the equatorial anomaly intensity (EAI) at 1200–1400 LT vary with the longitude according to changes in the vertical plasma drift velocity W. At this time, four harmonics are observed in the longitudinal variations of W, foF2, and EAI. The equatorial anomaly intensity increases to the maximum 1.5–2 h after the evening burst in the vertical plasma drift velocity. Longitudinal variations of foF2 for 2000–2200 LT are also associated with corresponding variations in the vertical plasma drift velocity. The equatorial anomaly intensity decreases after the maximum at 2000 LT and the crests decrease in size and shift towards the equator, but the anomaly is well developed at midnight. On the contrary, after midnight, foF2 maxima in the region of the anomaly crests are farther from the equator, but this is obviously associated with the action of the neutral wind. At 0200 LT, in contrast to the morning hours, only the northern crest of the anomaly is clearly pronounced. Thus, in the case of high solar activity during the equinoxes, a well-defined equatorial anomaly is observed from 1000 to 2400 LT. It reaches the maximum at 2000 LT.

Ionospheric variations over Chinese EIA region using foF2 and comparison with IRI-2016 model

In the present work, we have analyzed data of critical frequency of the F2 region (foF2) for the period, 2008–2013 over low latitude Chinese station Guangzhou (Geog. Lat. 23.10°N, Geog. Long. 113.40°E, dip, Lat. 13.49°N) and results thereof have been compared with IRI-2016 model. foF2 data set of the present study encompasses period of unusual and extended solar minimum, i.e., the years 2008–2009 and rising phase of solar cycle 24. IRI data have been obtained by choosing topside electron density profile IRI-NeQuick for two F peak models, CCIR and URSI. It is found that the general trend of variation in foF2 closely follows the trend of the solar flux during the period of study. A linear regression analysis gave a correlation coefficient of 0.98 which shows strong dependence of foF2 variation over solar flux variation. Semi-annual and annual oscillations are clearly brought out in the foF2 data using the Lomb-Scargle periodogram. A presence of semiannual and winter anomaly in observed as well as modeled foF2 at Guangzhou have found to be consistent throughout the period 2008–2013 irrespective of the phases of the solar activity. Our results also show the stronger presence of winter anomaly during the years of higher solar flux and it has been confirmed by normalizing the difference of winter to summer foF2 values for each year. Comparative results of ionosonde observation and IRI-2016 model show a significant discrepancy with regard to values of foF2 in different seasons and local time variations. IRI 2016 model underestimates the foF2 values in winter and equinoxes and overestimates foF2 values in summer. IRI modeled foF2 values using CCIR and URSI F peak models were found greater during forenoon hours and smaller during afternoon hours than the observed foF2 values throughout the period 2008–2013.

Ionosonde observations of the effects of the major magnetic storm of September 22–26, 1999 at equatorial station in west Africa

A new approach to study the mechanisms of storm-time variations in the F-layer height and critical frequency at dip-equator is proposed. The latitudinal variations in the magnetic disturbance index DP were combined with h’F and foF2 data from an IPS 42-type ionosonde at Korkogo (9.2° N, 5° W; 2.4° S dip lat), Ivory Coast, to investigate the nighttime ionospheric effects of the geomagnetic storm of September 22–26, 1999 in the West-African sector. A clear equatorward penetration of magnetic disturbances from high latitudes regions was observed. At dip-equator, the DP magnetic disturbance pattern showed up to four distinct regimes of disturbance electric fields, each associated with a specific phase of the storm. A regime of westward transient electric fields followed by a regime of eastward transient electric fields occurred during the main phase of the storm. This was preceded by a period of quasi-absence of disturbance during the compression phase, the whole followed by a regime of westward persistent disturbance electric fields during the recovery phase. From the latitudinal variations and the shapes of these perturbations, we could associate the regime of westward (resp. eastward) disturbance electric fields with prompt penetration (resp. overshielding) occasioned by magnetospheric convections and the persistent one with a cumulative effect of storm-time winds and magnetospheric convections from high latitudes regions. The h’F variations were found to be strongly correlated with the DP ones, clearly providing evidence for the prevalence of these electric fields on the observed F-layer motions. Additionally, the foF2 variations showed two periods of depleted electron density, one in the evening during the compression phase of the storm and the other near midnight. We discussed the mechanisms of these ionospheric negative storms in the light of earlier investigations of storm-time ionospheric disturbances and validated our method by comparison of the above results with those based on criteria by other authors.

Investigation of the relationship between ionospheric foF2 and earthquakes

Variations of the ionospheric F2 region critical frequency (foF2) have been investigated statistically before earthquakes during 1980–2008 periods in Japan area. Ionosonde data was taken from Kokubunji station which is in the earthquake preparation zone for all earthquakes. Standard Deviations and Inter-Quartile Range methods are applied to the foF2 data. It is observed that there are anomalous variations in foF2 before earthquakes. These variations can be regarded as ionospheric precursors and may be used for earthquake prediction.

A wavelet analysis of F2 layer critical frequency over solar cycle 23

AbstractTemporal oscillations of F2 layer critical frequency are direct outcome of solar EUV variability. The hourly data of F2 layer critical frequency (foF2) during solar cycle 23 over eight ionosonde stations which falls within same longitudinal span are evaluated using Continuous Wavelet Transform (CWT) to estimate the ionospheric variations. The quasi triennial, annual, semiannual, 27 day and diurnal variations of foF2 are clearly evident in the wavelet power spectra of all the stations. Quasi triennial oscillations which show a clear latitudinal dependence is more evident in southern stations. A strong quasi biennial oscillation (QBO) is also noticed in higher latitudes which was not observable in equatorial latitude. The present study reveals that the semiannual variations are more obvious over the annual variation in the equatorial and low latitude stations while the annual variations are prominent in higher latitudes.

A new short-term forecasting model for the total electron content storm time disturbances

This paper aims to introduce a new model for the short-term forecast of the vertical Total Electron Content (vTEC). The basic idea of the proposed model lies on the concept of the Solar Wind driven autoregressive model for Ionospheric short-term Forecast (SWIF). In its original version, the model is operationally implemented in the DIAS system (http://dias.space.noa.gr) and provides alerts and warnings for upcoming ionospheric disturbances, as well as single site and regional forecasts of the foF2 critical frequency over Europe up to 24 h in advance. The forecasts are driven by the real time assessment of the solar wind conditions at ACE location. The comparative analysis of the variations in foF2 and vTEC during eleven geomagnetic storm events that occurred in the present solar cycle 24 reveals similarities but also differences in the storm-time response of the two characteristics with respect to the local time and the latitude of the observation point. Since the aforementioned dependences drive the storm-time forecasts of the SWIF model, the results obtained here support the upgrade of the SWIF’s modeling technique in forecasting the storm-time vTEC variation from its onset to full development and recovery. According to the proposed approach, the vTEC storm-time response can be forecasted from 1 to 12–13 h before its onset, depending on the local time of the observation point at storm onset at L1. Preliminary results on the assessment of the performance of the proposed model and further considerations on its potential implementation in operational mode are also discussed.

Electron Bulk Surface Density Effect on Critical Frequency in the F2-Layer

Ionosphere layer is the atmosphere region which reflects radio waves for telecommunication. The density in particles in this layer influences the quality of communication. This study deals with the effects of Total Electron Contents (TEC) on the critical frequency of radio waves in the F2-layer. Total Electron Contents parameter symbolizes electron bulk surface density in ionosphere layer. Above critical frequency value in F2 layer (foF2), radio waves pass through ionosphere. The knowledge of this value enables to calibrate transmission frequencies. In this study, we consider TEC effects on foF2 under quiet time conditions during the maximum and the minimum of solar cycle 22, at Ouagadougou station, in West Africa. The study also considers the effects of seasons and the hourly variability of TEC and foF2. This work shows winter anomaly on foF2 and TEC on minimum and maximum of solar cycle phase respectively. Running International Reference Ionosphere (IRI) model enables to carry out the effects of TEC on foF2 by use of their monthly average values. This leads to a new approach to calibrate radio transmitters.

Annual and Diurnal Variabilities in the Critical Frequency (foF2) during Geomagnetic Fluctuating Activity over Solar Cycles 21 and 22 at Ouagadougou

Geomagnetic activity is characterized by four solar wind conditions. Each condition has specific impact on ionosphere. This paper review on fluctuating activity effects on ionosphere F2 layer through its critical frequency foF2 variations. Under fluctuating wind conditions, we have investigated on annual, diurnal and seasonal variations of foF2 during solar cycles 21 and 22 phases covered by Ouagadougou ionosonde station data (Lat: 12.5&#176N; Long: 358.5&#176E; dip: 1.43&#176). Our investigations show that foF2 annual’ variability is in phase with solar cycle. The diurnal variation is “noon bite out” most of the time except for the solar maximum where we have a morning peak testifying to the fact that the vertical drift E × B is disturbed. The seasonal variations show that the fluctuating activity has no particular effect on certain characteristics of the equatorial ionosphere such as electrojet and vertical drift E × B. However, the increase of the electric field pre-reversal phenomenon in autumn is a characteristic effect observed during the fluctuating activity.