F-region ionospheric parameters observed in the equatorial and low latitude regions during medium solar activity in the Brazilian sector and comparison with the IRI-2007 model results

The ionospheric sounding observations using the Canadian Advanced Digital Ionosondes (CADIs) operational at Palmas (PAL; 10.2°S, 48.2°W; dip latitude 6.6°S; a near-equatorial station), and São José dos Campos (SJC, 23.2°S, 45.9°W; dip latitude 17.6°S; a low-latitude station located under the southern crest of the equatorial ionospheric anomaly), Brazil, are analyzed during the different seasons viz., winter (June and July 2003), spring (September and October 2003), summer (December 2003 and January 2004), and fall (March and April 2004). The period used has medium solar activity (sunspot number between 77.4 and 39.3). The seasonal mean variations (using only geomagnetically quiet days) of the ionospheric parameters foF2 (critical frequency of the F-region), hpF2 (virtual height at 0.834 foF2; considered to be close to hmF2 (peak height of the F-region)), and h’F (minimum virtual height of the F-region) are calculated and compared between PAL and SJC. The prominent differences between PAL and SJC are as follows: h’F variations show strong post-sunset enhancement at PAL during the seasons of spring, summer, and fall; hpF2 variations show pre-sunrise uplifting of the F-layer at both stations during all the seasons and the hpF2 values during the daytime are lower at SJC compared with PAL during all the seasons; the foF2 variations show mid-day bite-out at PAL during all the seasons and SJC shows strong equatorial ionospheric anomaly during summer and fall seasons. Also, the seasonal variations of the ionospheric parameters foF2 and hpF2 (with ±1 standard deviation) observed at PAL and SJC are compared with the IRI-2007 model results of foF2 and hmF2. In addition, variations of the foF2 and hpF2 observed at SJC are compared with the IRI-2001 model results of foF2 and hmF2. It should be pointed out that the ionospheric parameter hpF2 is much easier to obtain using computer program developed at UNIVAP compared with hmF2 (using POLAN program). During the daytime due to underlying ionization hpF2 estimated is higher (approximately 50 km) than the true peak height hmF2. During the nighttime hpF2 is fairly close to hmF2. The comparison between the foF2 variations observed at PAL and SJC with the IRI-2007 model results shows a fairly good agreement during all the seasons. However, the comparison between the hpF2 variations observed at PAL and SJC with the hmF2 variations with the IRI-2007 model results shows: (1) a fairly good agreement during the nighttime in all the seasons; (2) the model results do not show the pre-sunrise uplifting of the F-layer at PAL and SJC in any season; (3) the model results do not show the post-sunset uplifting of the F-layer at PAL; (4) considering that, in general, hpF2 is higher than hmF2 during the daytime by about 50 km, the model results are in good agreement at PAL and SJC during all the seasons except summer at SJC, when large discrepancies in the observed hpF2 and modeled hmF2 are observed. Also, it has been observed that, in general, hmF2 values for SJC calculated using IRI-2001 are higher than IRI-2007 during the daytime in winter, summer, and fall. However, hmF2 values for SJC calculated using IRI-2001, are lower than IRI-2007 during the nighttime in spring.     

Comparison of peak characteristics of F2 ionospheric layer over Tehran region at a low solar activity period with IRI-2001 and IRI-2007 models predictions

In the present work values of peak electron density (NmF2) and height of F2 ionospheric layer (hmF2) over Tehran region at a low solar activity period are compared with the predictions of the International Reference Ionosphere models (IRI-2001 and IRI-2007). Data measured by a digital ionosonde at the ionospheric station of the Institute of Geophysics, University of Tehran from July 2006 to June 2007 are used to perform the calculations. Formulations proposed by  and  are utilized to calculate the hmF2. The International Union of Radio Science (URSI) and International Radio Consultative Committee (CCIR) options are employed to run the IRI-2001 and IRI-2007 models. Results show that both IRI-2007 and IRI-2001 can successfully predict the NmF2 and hmF2 over Tehran region. In addition, the study shows that predictions of IRI-2007 model with CCIR coefficient has closer values to the observations. Furthermore, it is found that the monthly average of the percentage deviation between the IRI models predictions and the values of hmF2 and NmF2 parameters are less than 10% and 21%, respectively.     

IRI-90及其与中国电离层资料f_0F_2的比较

本文介绍了国际参考电离层IRI-90版中新增的内容,比较了它与IRI-86的主要差别。与我国电离层临频数据的统计分析表明,新版本对我国地区的适用程度有了改进。     

A comparison of ionosonde measured foF2 and IRI-2016 predictions over China

A comparison of the ionospheric F-region critical frequency (foF2) between ionosonde measurements and IRI-2016 predictions is studied over China during the period from January 2008 to October 2016. Four stations are selected, and the latitude coverage starts at 49.4°N and ends at 23.2°N with a sequential latitude interval of about 10°, the corresponding geomagnetic latitudes are from 39.5°N to 13.2°N. The results show that the variability of the observed foF2 versus latitudes, seasons, local time and levels of solar activity could be well reproduced by IRI-2016. However, the daily lowest value of foF2 from the IRI-2016 prediction occurs earlier than that from the ionosonde. Around the sunrise, the IRI-2016 prediction shows a very sharp rise and grows much faster than the observed foF2 in every month. The foF2 difference between the two options (URSI and CCIR) in IRI-2016 increases as the F10.7 index decreases. During 2008–2009, the annual average deviations of URSI and CCIR range from ?5% to ?10% and from 5% to ?5%, respectively. Generally, the CCIR performs better than URSI during postsunset under low solar activity or in Equatorial Ionization Anomaly (EIA) region over China, while it shows no large difference in performance with URSI in other locations or for other time.     

Diurnal and seasonal variations of F2 layer characteristics over Irkutsk during the decrease in solar activity in 2003–2006: Observations and IRI-2001 model predictions

The diurnal and seasonal variations of F2 layer characteristics (critical frequency, peak height and bottomside thickness) over Irkutsk, Russia (52.3 N and 104.3 E) are studied by the method of running medians. The comparison with the IRI-2001 model during the decrease in solar activity in 2003–2006 revealed cases of both close agreement and systematic differences between predictions and observations. The systematic difference is not the only reason for disagreement between IRI and observations; there are also intrayear variations which are not associated with seasonal behavior. The period of observation was too short to make conclusions about solar activity dependence of the noon bottomside thickness and the modification of its diurnal behavior with decreasing solar activity.     

On the time of minimum ionization in the F2 region

Using measurements of the critical frequency of F2 region (f_oF2) the validity of the International Reference Ionosphere model to predict the time of minimum ionization is checked. Data obtained at different ionospheric stations have been considered. The CCIR and URSI options are used to model calculations. For CCIR option the results show that good predictions were obtained for about 40% of the considered cases. For the rest of the considered data, the model predicts the minimum at times earlier than that observed in the measurements. The percentages of good predictions obtained with URSI option are lower than those corresponding to CCIR one.     

Comparison of GPS TEC measurements with IRI-2007 TEC prediction over the Kenyan region during the descending phase of solar cycle 23

This paper presents an analysis of the Total Electron Content (TEC) derived from the International GNSS Service receiver (formerly IGS) at Malindi (2.9°S, 40.1°E), Kenya for the periods 2004–2006 during the declining phase of solar cycle 23. The diurnal, monthly and seasonal variations of the TEC are compared with TEC from the latest International Reference Ionosphere model (IRI-2007). The GPS–TEC exhibits features such as an equatorial noon time dip, semi-annual variations, Equatorial Ionization Anomaly and day-to-day variability. The lowest GPS–TEC values are observed near the June solstice and September equinox whereas largest values are observed near the March equinox and December solstice. The mean GPS–TEC values show a minimum at 03:00 UT and a peak value at about 10:00 UT. These results are compared with the TEC derived from IRI-2007 using the NeQuick option for the topside electron density (IRI–TEC). Seasonal mean hourly averages show that IRI-2007 model TEC values are too high for all the seasons. The high prediction primarily occur during daytime hours till around midnight hours local time for all the seasons, with the highest percentage deviation in TEC of more 90% seen in September equinox and lowest percentage deviation in TEC of less than 20% seen in March equinox. Unlike the GPS–TEC, the IRI–TEC does not respond to geomagnetic storms and does overestimate TEC during the recovery phase of the storm. While the modeled and observed data do correlate so well, we note that IRI-2007 model is strongly overestimating the equatorial ion fountain effect during the descending phase of solar cycle, and this could be the reason for the very high TEC estimations.     

Estimation of foF2 variations using round-the-world sounding data

An estimation of the F2 ionospheric region critical frequency (foF2) variations using analysis of round-the-world radio sounding data has been made. Experimental data obtained by the Russian chirp-sounders network have been used. For the first time, using experimental data and numerical simulation, the quantitative dependency between the minimum foF2 magnitudes over round-the-world propagation paths and round-the-world maximum usable frequencies has been obtained.     

A new global version of M(3000)F2 prediction model based on artificial neural networks

A new version of global empirical model for the ionospheric propagation factor, M(3000)F2 prediction is presented. Artificial neural network (ANN) technique was employed by considering the relevant geophysical input parameters which are known to influence the M(3000)F2 parameter. This new version is an update to the previous neural network based M(3000)F2 global model developed by Oyeyemi et al. (2007), and aims to address the inadequacy of the International Reference Ionosphere (IRI) M(3000)F2 model (the International Radio Consultative Committee (CCIR) M(3000)F2 model). The M(3000)F2 has been found to be relatively inaccurate in representing the diurnal structure of the low latitude region and the equatorial ionosphere. In particular, the existing hmF2 IRI model is unable to reproduce the sharp post-sunset drop in M(3000)F2 values, which correspond to a sharp post-sunset peak in the peak height of the F2 layer, hmF2. Data from 80 ionospheric stations globally, including a good number of stations in the low latitude region were considered for this work. M(3000)F2 hourly values from 1987 to 2008, spanning all periods of low and high solar activity were used for model development and verification process. The ability of the new model to predict the M(3000)F2 parameter especially in the low latitude and equatorial regions, which is known to be problematic for the existing IRI model is demonstrated.     

Dependence of the F-region peak electron density (foF2) on solar activity observed in the equatorial ionospheric anomaly region in the Brazilian sector

The long-term (solar cycle) changes in the Sun and how it affects the ionospheric F-region observed at São José dos Campos (23.2° S, 45.9° W), Brazil, a location under the southern crest of the equatorial ionospheric anomaly, have been investigated in this paper. The dependence of the F-region peak electron density (foF2) on solar activity during the descending phase of the 23rd solar cycle for the periods of high, medium, and low solar activity has been studied. The ionospheric F-region peak electron densities observed during high and medium solar activity show seasonal variations with maxima close to the equinox periods, whereas during the low solar activity the maxima during the equinox periods is absent. However, during the low solar activity only change observed is a large decrease from summer to winter months. We have further investigated changes in the different ionospheric F-region parameters (minimum virtual height of the F-region (h′F), virtual height at 0.834foF2 (hpF2), and foF2) during summer to winter months in low solar activity periods, 2006–2007 and 2007–2008. Large changes in the two ionospheric parameters (hpF2 and foF2) are observed during summer to winter months in the two low solar activity periods investigated.     

The response of African equatorial foF2 to geomagnetic storms: Comparison between observations and IRI-2007 predictions

This study examines the response of the African equatorial ionospheric foF2 to different levels of geomagnetic storms, using the foF2 hourly data for the year 1989 from Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N). The study also compares the observed data for the selected storm periods with the latest IRI model (IRI-2007). The foF2 values (both observed and predicted) show typical features of daytime peak and post-midnight minimum peak. The response of the ionospheric foF2 over Ouagadougou to storms events, during the night-time and post-midnight hours indicates negative responses of the ionospheric foF2, while that of the daytime hours indicates positive responses. For the investigation on the variability of the observed foF2 with respect to IRI-2007 model, with the exception of the analysis of the 20–22, October, 1989 data, where a midday peak was also observed on the first day, this study reveals two characteristic daily foF2 variability peaks: post-midnight and evening peaks. In addition, for all the geomagnetic storms considered, the URSI and CCIR coefficients of the IRI-2007 model show reasonable correspondence with each other, except for some few discrepancies. For instance, the event of 28–30 August, 1989 shows comparatively higher variability for the URSI coefficient, and at the foF2 peak values, the event of 20–22 October, 1989 shows that the CCIR coefficient is more susceptible to foF2 variability than the URSI coefficient. This study is aimed at providing African inputs for the future improvement of the IRI model.     

Comparison of GPS-TEC measurements with NeQuick2 and IRI model predictions in the low latitude East African region during varying solar activity period (1998 and 2008–2015)

This paper examines the performances of NeQuick2, the latest available IRI-2016, IRI-2012 and IRI-2007 models in describing the monthly and seasonal mean total electron content (TEC) over the East African region. This is to gain insight into the success of the various model types and versions at characterizing the ionosphere within the equatorial ionization anomaly. TEC derived from five Global Positioning System (GPS) receivers installed at Addis Ababa (ADD, 5.33°N, 111.99°E Geog.), Asab (ASAB, 8.67°N, 116.44°E Geog.), Ambo (ABOO, 5.43°N, 111.05°E Geog.), Nairobi (RCMN, ?4.48°N, 108.46°E Geog.) and Nazret (NAZR, 4.78°N, 112.43°E Geog.), are compared with the corresponding values computed using those models during varying solar activity period (1998 and 2008–2015). We found that different models describe the equatorial and anomaly region ionosphere best depending on solar cycle, season and geomagnetic activity levels. Our results show that IRI-2016 is the best model (compared to others in terms of discrepancy range) in estimating the monthly mean GPS-TEC at NAZR, ADD and RCMN stations except at ADD during 2008 and 2012. It is also found that IRI-2012 is the best model in estimating the monthly mean TEC at ABOO station in 2014. IRI show better agreement with observations during June solstice for all the years studied at ADD except in 2012 where NeQuick2 better performs. At NAZR, NeQuick2 better performs in estimating seasonal mean GPS-TEC during 2011, while IRI models are best during 2008–2009. Both NeQuick2 and IRI models underestimate measured TEC for all the seasons at ADD in 2010 but overestimate at NAZR in 2009 and RCMN in 2008. The periodic variations of experimental and modeled TEC have been compared with solar and geomagnetic indices at ABOO and ASAB in 2014 and results indicate that the F10.7 and sunspot number as indices of solar activity seriously affects the TEC variations with periods of 16–32?days followed by the geomagnetic activity on shorter timescales (roughly periods of less than 16?days). In this case, NeQuick2 derived TEC shows better agreement with a long term period variations of GPS-TEC, while IRI-2016 and IRI-2007 show better agreement with observations during short term periodic variations. This indicates that the dependence of NeQuick2 derived TEC on F10.7 is seasonal. Hence, we suggest that representation of geomagnetic activity indices is required for better performance over the low latitude region.     

Comparison between observed ionospheric foF2 and IRI-2001 predictions over periods of severe geomagnetic activities at Grahamstown,South Africa

The observed ionospheric F2 critical frequency (foF2) values over a South Africa mid-latitude station, Grahamstown, (geographic coordinates: 33.3°S, 26.5°E), were analysed and compared with International Reference Ionosphere (IRI) model, using the CCIR (Comite´ Consultatif International des Radio communications) and URSI (Union Radio-Scientifique Internationale) coefficients, during four geomagnetically disturbed days in the year 2000. These days are April 5, May 23, August 10 and September 15. The data were analysed for five days around the storm day. Comparisons between the IRI-2001 predicted foF2 values, using both CCIR and URSI coefficients and the observed values are shown with their root-mean-square error (RMSE) and the relative deviation module mean (rdmm) for the various storm periods. The CCIR option performed more accurately than the URSI option.     

Ionospheric behavior of the F2 peak parameters foF2 and hmF2 at Hainan and comparisons with IRI model predictions

Monthly median values of foF2, hmF2 and M(3000)F2 parameters, with hourly time interval resolution for the diurnal variation, obtained with DPS-4 digisonde observations at Hainan (19.4°N, 109.0°E) are used to study the low latitude ionospheric variation behavior. The observational results are compared with the International Reference Ionospheric Model (IRI) predictions. The time period coverage of the data used for the present study is from March 2002 to February 2005. Our present study showed that: (1) In general, IRI predictions using CCIR and URSI coefficients follow well the diurnal and seasonal variation patterns of the experimental values of foF2. However, CCIR foF2 and URSI foF2 IRI predictions systematically underestimate the observed results during most time period of the day, with the percentage difference ΔfoF2 (%) values changing between about −5% and −25%, whereas for a few hours around pre-sunrise, the IRI predictions generally overestimate the observational ones with ΔfoF2 (%) sometimes reaching as large as ∼30%. The agreement between the IRI results and the observational ones is better for the year 2002 than for the other years. The best agreement between the IRI results and the observational ones is obtained in summer when using URSI coefficients, with the seasonal average values of ΔfoF2 (%) being within the limits of ±10%. (2) In general, the IRI predicted hmF2 values using CCIR M(3000)F2 option shows a poor agreement with the observational results. However, when using the measured M(3000)F2 as input, the diurnal variation pattern of hmF2 given by IRI2001 has a much better agreement with the observational one with the detailed fine structures including the pre-sunrise and post-sunset peaks reproduced reasonably well. The agreement between the IRI predicted hmF2 values using CCIR M(30,000)F2 option and the observational ones is worst for the afternoon to post-midnight hours for the high solar activity year 2002. During daytime hours the agreement between the hmF2 values obtained with CCIR M(30,000)F2 option and the observational ones is best for summer season. The discrepancy between the observational hmF2 and that obtained with CCIR M(30,000)F2 option stem from the CCIR M(3000)F2 model, which does not produce the small scale structures observed in the measured M(3000)F2.     

Evaluation of the improvement of IRI-2016 over IRI-2012 at the India low-latitude region during the ascending phase of cycle 24

This paper investigated the performance of the latest International Reference Ionosphere model (IRI-2016) over that of IRI-2012 in predicting total electron content (TEC) at three different stations in the Indian region. The data used were Global Positioning System (GPS) data collected during the ascending phase of solar cycle 24 over three low-latitude stations in India namely; Bangalore (13.02°N Geographic latitude, 77.57°E Geographic longitude), Hyderabad (17.25°N Geographic latitude, 78.30°E Geographic longitude) and Surat (21.16°N Geographic latitude, 72.78°E Geographic longitude). Monthly, the seasonal and annual variability of GPS-TEC have been compared with those derived from International Reference Ionosphere IRI-2016 and IRI-2012 with two different options of topside electron density: NeQuick and IRI01-corr. It is observed that both versions of IRI (i.e., IRI-2012 and IRI-2016) predict the GPS-TEC with some deviations, the latest version of IRI (IRI-2016) predicted the TEC similar to those predicted by IRI-2012 for all the seasons at all stations except for morning hours (0500 LT to 1000?LT). This shows that the effect of the updated version is seen only during morning hours and also that there is no change in TEC values by IRI-2016 from those predicted by IRI-2012 for the rest of the time of the day in the Indian low latitude region. The semiannual variations in the daytime maximum values of TEC are clearly observed from both GPS and model-derived TEC values with two peaks around March-April and September-October months of each year. Further, the Correlation of TEC derived by IRI-2016 and IRI-2012 with EUV and F10.7 shows similar results. This shows that the solar input to the IRI-2016 is similar to IRI 2012. There is no significant difference observed in TEC, bottom-side thickness (B0) and shape (B1) parameter predictions by both the versions of the IRI model. However, a clear improvement is visible in hmF2 and NmF2 predictions by IRI-2016 to that by IRI-2012. The SHU-2015 option of the IRI-2016 gives a better prediction of NmF2 for all the months at low latitude station Ahmedabad compare to AMTB 2013.     

Comparison of ionosphere characteristic parameters obtained by ionosonde with IRI-2007 model over Southeast Asia

In this work, the foF2 and hmF2 parameters at the conjugate points near the magnetic equator of Southeast Asia are studied and compared with the International Reference Ionosphere (IRI) model. Three ionosondes are installed nearly along the magnetic meridian of 100°E; one at the magnetic equator, namely Chumphon (10.72°N, 99.37°E, dip angle 3.0°N), and the other two at the magnetic conjugate points, namely Chiang Mai (18.76°N, 98.93°E, dip angle 12.7°N) and Kototabang (0.2°S, 100.30°E, dip angle 10.1°S). The monthly hourly medians of the foF2 and hmF2 parameters are calculated and compared with the predictions obtained from the IRI-2007 model from January 2004 to February 2007. Our results show that: the variations of foF2 and hmF2 predicted by the IRI-2007 model generally show the similar feature to the observed data. Both parameters generally show better agreement with the IRI predictions during daytime than during nighttime. For foF2, most of the results show that the IRI model overestimates the observed foF2 at the magnetic equator (Chumphon), underestimates at the northern crest (Chiang Mai) and is close to the measured ones at the southern crest of the EIA (Kototabang). For hmF2, the predicted hmF2 values are close to the hmF2(M3000F2_OBS) during daytime. During nighttime, the IRI model gives the underestimation at the magnetic equator and the overestimation at both EIA crests. The results are important for the future improvements of the IRI model for foF2 and hmF2 over Southeast Asia region.     

Monthly mean foF2 model for an African low-latitude station and comparison with IRI

We have employed the hourly values of the ionospheric F-region critical frequency (foF2) obtained from Ouagadougou ionosonde, Burkina Faso (geographic coordinates 12° N, 1.8° W) during the interval of 1985–1995 (solar cycle 22) and solar radio flux of 10 cm wavelength (F10.7) to develop a local model (LM) for the African low-latitude station. The model was developed from regression analysis method, using the two-segmented regression analysis. We validated LM with foF2 data from Korhogo observatory, Cote d’Ivorie (geographical coordinates 9.3° N, 5.4° W). LM as well as the International Reference Ionosphere (IRI) agrees well with observations. LM gave some improvement on the IRI-predicted foF2 values at the sunrise (06 LT) at all solar flux levels and in all seasons except June solstice. The performance of the models at the representing the salient features of the equatorial foF2 was presented. Considering daytime and nighttime performances, LM and IRI are comparable in low solar activity (LSA), LM performed better than IRI in moderate solar activity (MSA), while IRI performed better than LM in high solar activity (HSA). CCIR has a root mean square error (r.m.s.e), which is only 0.10 MHz lower than that of LM while LM has r.m.s.e, which is about 0.05 MHz lower than that of URSI. In general, our result shows that performance of IRI, especially the CCIR option of the IRI, is quite comparable with the LM. The improved performance of IRI is a reflection of the numerous contributions of ionospheric physicists in the African region, larger volume of data for the IRI and the diversity of data sources, as well as the successes of the IRI task force activities.     

Validate the IRI2007 model by the COSMIC slant TEC data during the extremely solar minimum of 2008

During 2008, the solar activity is extremely low. The satellite observations show that the ionospheric height and electron density is much lower than the predictions by the international reference ionosphere (IRI) model. In this paper, we compared the slant total electron content (TEC) observed by the COSMIC satellites during 2008 with the IRI model results. It is found that the IRI model with IRI2001 and IRI2001 Cor. topside options will always overestimate the electron density in both lower and higher altitudes. But the rest two topside options (NeQuick, and TTS) tend to overestimate the electron density in the F layer and underestimate it in the topside altitudes. The switch altitude between overestimation and underestimation and the latitude-local time distribution of the model deviation depend on the topside option. The current investigation might be useful for the model improvement as well as data assimilation work based on the IRI model and the LEO TEC data.