Diurnal and seasonal variation of F2-layer ionospheric parameters at equatorial ionization anomaly crest region and their comparison with IRI-2001

Diurnal and seasonal variations of critical frequency of ionospheric F2-region ‘foF2’ and the height of peak density ‘hmF2’ are studied using modern digital ionosonde observations of equatorial ionization anomaly (EIA) crest region, Bhopal (23.2°N, 77.6°E, dip 18.5°N), during solar minimum period 2007. Median values of these parameters are obtained at each hour using manually scaled data during different seasons and compared with the International Reference Ionosphere-2001 model predictions. The observations suggest that on seasonal basis, the highest values of foF2 are observed during equinox months, whereas highest values of hmF2 are obtained in summer and lowest values of both foF2 and hmF2 are observed during winter. The observed median and IRI predicted values of foF2 and hmF2 are analyzed with upper and lower bound of inter-quartile range (IQR) and it is find out that the observed median values are well inside the inter-quartile range during the period of 2007. Comparison of the recorded foF2 and hmF2 values with the IRI-2001 output reveals that IRI predicted values exhibit better agreement with hmF2 as compared to foF2. In general, the IRI model predictions show some agreement with the observations during the year 2007. Therefore it is still necessary to implement improvements in order to obtain better predictions for EIA regions.     

Ionospheric F2-Layer critical frequency retrieved from COSMIC radio occultation: A statistical comparison with measurements from a meridional ionosonde chain over Southeast Asia

In this paper, we compared the F2-Layer critical frequency (foF2) derived from FORMOSAT-3/COSMIC radio occultation (RO) and ionosondes at Chiang Mai, Chumphon and Kototabang during the years 2008–2015 to evaluate the performance of COSMIC RO over Southeast Asia region. The results show that the time development of foF2 values derived from COSMIC RO generally agrees well with those from ionosonde measurements. However, the differences between the foF2 derived from COSMIC RO and that derived from ionosonde observations display latitudinal dependence. COSMIC RO tends to underestimate foF2 at Chiang Mai and Kototabang, which is near to the north EIA crest and the south one, respectively, while a little overestimate foF2 at Chumphon, which is close to the geomagnetic equator. COSMIC RO agrees best with ionosonde at Chumphon and worst at Chiang Mai. At each ionosonde station, the quality of COSMIC RO data degrades with the increase of solar activity. In addition, at the station Chiang Mai and Kototabang, COSMIC RO performs better in summer than in equinox and winter. Furthermore, the differences in foF2 derived from COSMIC RO and that from ionosonde measurements vary with local time, i.e., the differences in foF2 are generally smaller at night and larger in noontime when equatorial ionization anomaly (EIA) is well developed.     

Ionospheric variation at Thailand equatorial latitude station: Comparison between observations and IRI-2001 model predictions

In this paper, the F2-layer critical frequency (foF2) and peak height (hmF2) measured by the FM/CW ionosonde at Thailand equatorial latitude station, namely Chumphon (10.72°N, 99.37°E, dip 3.22) are presented. The measurement data during low solar activity from January 2004 to December 2006 are analyzed based on the diurnal, seasonal variation. The results are then compared with IRI-2001 model predictions. Our study shows that: (1) In general, both the URSI and CCIR options of the IRI model give foF2 close to the measured ones, but the CCIR option produces a smaller range of deviation than the URSI option. The agreement during daytime is generally better than during nighttime. Overestimation mostly occurs in 2004 and 2006, while underestimation is during pre-sunrise hours in June solstice in 2005. The peak foF2 around sunset is higher during March equinox and September equinox than the other seasons, with longer duration of maximum levels in March equinox than September equinox. Large coefficients of variability foF2 occur during pre-sunrise hours. Meanwhile, the best agreement between the observed foF2 and the IRI model is obtained in June solstice. (2) In general, The IRI (CCIR) model predicts the observed hmF2 well during daytime in June solstice from 2004–2006, but it overestimates during March equinox, September equinox and December solstice. For nighttime, the model overestimates hmF2 values for all seasons especially during March equinox and September equinox. However, the model underestimates hmF2 values during September equinox and for some cases during June solstice and December solstice at pre-sunrise. The agreement between the IRI model and the hmF2(M3000_OBS) is worst around noontime, post-sunset and pre-sunrise hours. All comparative studies give feedback for new improvements of CCIR and URSI IRI models.     

Ionospheric variability at taiwan low latitude station: Comparison between observations and IRI-2001 model

This paper presents the observed ionospheric F-region critical frequency, foF2, and peak height, hmF2, at northern crest of equatorial ionization anomaly (EIA) area station, namely Chung-Li (24.9°N, 121.1°E, dip 35°), and to be compared with International Reference Ionosphere model (IRI-2001) predictions for the period from 1994 to 1999, corresponding to half of the 23rd solar cycle. The diurnal and seasonal variation of foF2 and hmF2 are analyzed for different solar phases, respectively. The result shows the largest discrepancies were observed during nighttime for foF2 and hmF2, respectively. The value of foF2 both CCIR and URSI selected in the IRI model produced a good agreement during the daytime and underestimated during the noon time for high solar activities. The underestimation at noon time is mainly caused by the fountain effect from equator. Further, the peak height hmF2 shows a larger variability around the midnight than daytime in the equinox and winter seasons and reserved in summer, respectively. The study shows that the monthly median values of observed hmF2 is somewhat lower than those predicated by the IRI model, at night time in all the seasons except the period of 04:00–06:00 LT and reverse at daytime in summer. In general the IRI model predictions with respect to the observed in hmF2 is much better than foF2. The percentage deviation of the observed foF2 (hmF2) values with respect to the IRI model varies from 5% to 80% (0–25%) during nighttime and 2–17% (0–20%) at daytime, respectively. In general, the model generates good results, although some improvements are still necessary to implement in order to obtain better simulations for ionospheric low-latitudes region.     

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 ionospheric F2 peak parameters foF2 and hmF2 with IRI2001 at Hainan

Monthly median values of foF2, hmF2 and M(3000)F2 parameters, with quarter-hourly time interval resolution for the diurnal variation, obtained with DPS4 digisonde at Hainan (19.5°N, 109.1°E; Geomagnetic coordinates: 178.95°E, 8.1°N) are used to investigate the low-latitude ionospheric variations and comparisons with the International Reference Ionosphere (IRI) model predictions. The data used for the present study covers the period from February 2002 to April 2007, which is characterized by a wide range of solar activity, ranging from high solar activity (2002) to low solar activity (2007). The results show that (1) Generally, IRI predictions follow well the diurnal and seasonal variation patterns of the experimental values of foF2, especially in the summer of 2002. However, there are systematic deviation between experimental values and IRI predictions with either CCIR or URSI coefficients. Generally IRI model greatly underestimate the values of foF2 from about noon to sunrise of next day, especially in the afternoon, and slightly overestimate them from sunrise to about noon. It seems that there are bigger deviations between IRI Model predictions and the experimental observations for the moderate solar activity. (2) Generally the IRI-predicted hmF2 values using CCIR M(3000)F2 option shows a poor agreement with the experimental results, but there is a relatively good agreement in summer at low solar activity. The deviation between the IRI-predicted hmF2 using CCIR M(3000)F2 and observed hmF2 is bigger from noon to sunset and around sunrise especially at high solar activity. The occurrence time of hmF2 peak (about 1200 LT) of the IRI model predictions is earlier than that of observations (around 1500 LT). The agreement between the IRI hmF2 obtained with the measured M(3000)F2 and the observed hmF2 is very good except that IRI overestimates slightly hmF2 in the daytime in summer at high solar activity and underestimates it in the nighttime with lower values near sunrise at low solar activity.     

Comparisons of foF2 with IRI model and equatorial vertical drifts

Measurements of the critical frequency, foF2 recorded over Ibadan: 7.4°N, 3.9°E (geographic), 6°S (dip angle) have been compared with the International Reference Ionosphere (IRI-2007) model for solar maximum geomagnetically quiet conditions, with a view to determining what modifications might bring about better predictions for the model. Our results reveal that the present version of IRI essentially reproduces diurnal trends and the general features of the experimental observations for all seasons, except for nighttime June solstice periods, which the model seriously overestimated. The model errors ranging from 50% to 125% over the four seasons considered in this study. It is also indicated that the percentage relative deviations between the observed and the modeled values vary approximately from −11% to 12% (March), −34% to 11% (June), −16% to 12% (September), and −10% to 13% (December). An unexpected feature of foF2 is obvious and remarkable reduction in values during nighttime June solstice periods compared to that in other seasons. Relationship between equatorial vertical drift and foF2 is also investigated. However, cross correlation analysis reveals strong anti-correlation between vertical drift and critical frequency during the daytime hours, but exceptionally opposite is the case for the nighttime sector. The discrepancies which are noted, particularly during June solstice season are attributed to processes most likely within the thermosphere and from meteorological influences during quiet magnetic conditions.     

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.     

Longitudinal variation of the equatorial ionosphere: Modeling and experimental results

We describe a new version of the Parameterized Regional Ionospheric Model (PARIM) which has been modified to include the longitudinal dependences. This model has been reconstructed using multidimensional Fourier series. To validate PARIM results, the South America maps of critical frequencies for the E (foE) and F (foF2) regions were compared with the values calculated by Sheffield Plasmasphere-Ionosphere Model (SUPIM) and IRI representations. PARIM presents very good results, the general characteristics of both regions, mainly the presence of the equatorial ionization anomaly, were well reproduced for equinoctial conditions of solar minimum and maximum. The values of foF2 and hmF2 recorded over Jicamarca (12°S; 77°W; dip lat. 1°N; mag. declination 0.3°) and sites of the conjugate point equatorial experiment (COPEX) campaign Boa Vista (2.8°N; 60.7°W; dip lat. 11.4°; mag. declination −13.1°), Cachimbo (9.5°S; 54.8°W; dip lat. −1.8°; mag. declination −15.5°), and Campo Grande (20.4°S; 54.6°W; dip lat. −11.1°; mag. declination −14.0°) have been used in this work. foF2 calculated by PARIM show good agreement with the observations, except during morning over Boa Vista and midnight-morning over Campo Grande. Some discrepancies were also found for the F-region peak height (hmF2) near the geomagnetic equator during times of F₃ layer occurrences. IRI has underestimated both foF2 and hmF2 over equatorial and low latitude sectors during evening-nighttimes, except for Jicamarca where foF2 values were overestimated.     

Comparison of observed TEC values with IRI-2007 TEC and IRI-2007 TEC with optional foF2 measurements predictions at an equatorial region,Chumphon, Thailand

In this research, as part of working towards improving the IRI over equatorial region, the total electron content (TEC) derived from GPS measurements and IRI-2007 TEC predictions at Chumphon station (10.72°N, 99.37°E), Thailand, during 2004–2006 is analyzed. The seasonal variation of the IRI-2007 TEC predictions is compared with the TEC from the IRI-2007 TEC model with the option of the actual F2 plasma frequency (foF2) measurements as well as the TEC from the GPS and International GNSS service (IGS). The Chumphon station is located at the equatorial region and the low latitude of 3.22°N. For a declining phase of the solar cycle (2004–2006), the study shows that the IRI-2007 TEC underestimates the IRI-2007 TEC with the foF2 observation at the nighttime by about 5 TECU. The maximum differences are about 15 TECU during daytime and 5 TECU during nighttime. The overestimation is more evident at daytime than at nighttime. When compared in terms of the root-mean square error (RMSE), we find that the highest RMSE between GPS TEC and IRI 2007 TEC is 14.840 TECU at 1230 LT in 2004 and the lowest average between them is 1.318 TECU at 0630 LT in 2006. The noon bite-out phenomena are clearly seen in the IRI-2007 TEC with and without optional foF2 measurements, but not on the GPS TEC and IGS TEC. The IRI TEC with optional foF2 measurements gives the lowest RMSE values between IRI TEC predicted and TEC measurement. However, the TEC measurements (GPS TEC and IGS TEC) are more correct to use at Chumphon station.     

The variability and IRI2007-predictability of hmF2 over South Africa

This paper presents an investigation into the variability and predictability of the maximum height of the ionospheric F2 layer, hmF2 over the South African region. Data from three South African stations, namely Madimbo (22.4°S, 26.5°E, dip angle: −61.47°), Grahamstown (33.3°S, 26.5°E, dip angle: −64.08°) and Louisvale (28.5°S, 21.2°E, dip angle: −65.44°) were used in this study. The results indicate that hmF2 shows a larger variability around midnight than during the daytime for all seasons. Monthly median hmF2 values were used in all cases and were compared with predictions from the IRI-2007 model, using the URSI (Union Radio-Scientifique Internationale) coefficient option. The analysis covers the diurnal and seasonal hourly hmF2 values for the selected months and time sectors e.g. January, July, April and October for 2003 and 2005. The time ranges between (03h00–23h00 UT; LT = UT + 2h) representing the local sunrise, midday, sunset and midnight hours. The time covers sunrise, midday, sunrise, and midnight hours (03–06h00 UT, 07–11h00 UT, sunrise 16–18h00 UT and 22–23h00 UT; LT = UT + 2h). The dependence of the results on solar activity levels was also investigated. The IRI-2007 predictions follow fairly well the diurnal and seasonal variation patterns of the observed hmF2 values at all the stations. However, the IRI-2007 model overestimates and underestimates the hmF2 value during different months for all the solar activity periods.     

An evaluation of the IRI-2007 storm time model at low latitude stations

This paper discusses the ability of the International Reference Ionosphere IRI-2007 storm time model to predict foF2 ionospheric parameter during geomagnetic storm periods. Experimental data (based on availability) from two low latitude stations: Vanimo (geographic coordinates, 2.7 °S, 141.3 °E, magnetic coordinates, 12.3 °S, 212.50 °E) and Darwin (geographic coordinates, 12.45 °S, 130.95 °E, magnetic coordinates, 22.9 °S, 202.7 °E) during nine storms that occurred in 2000 (Rz₁₂ = 119), 2001(Rz₁₂ = 111) and 2003 (Rz₁₂ = 64) are compared with those obtained by the IRI-2007 storm model. The results obtained show that the percentage deviation between the experimental and IRI predicted foF2 values during these storm periods is as high as 100% during the main and recovery phases. Based on the values of “relative deviation module mean” (RDMM) obtained (i.e. between 0.08 and 0.60), it is observed that there is a reasonable to poor agreement between measured foF2 values and the IRI-storm model prediction values during main and recovery phases of the storms under investigation. As a result, in addition to other studies that have been carried out from different sectors, more studies are required to be carried out. This will enable IRI community to improve on the present performance of the model. In general the IRI-storm model predictions follow normal trend of the foF2 measured values but does not reproduce well the measured values.     

TEC variability near northern EIA crest and comparison with IRI model

Monthly median values of hourly total electron content (TEC) is obtained with GPS at a station near northern anomaly crest, Rajkot (geog. 22.29°N, 70.74°E; geomag. 14.21°N, 144.9°E) to study the variability of low latitude ionospheric behavior during low solar activity period (April 2005 to March 2006). The TEC exhibit characteristic features like day-to-day variability, semiannual anomaly and noon bite out. The observed TEC is compared with latest International Reference Ionosphere (IRI) – 2007 model using options of topside electron density, NeQuick, IRI01-corr and IRI-2001 by using both URSI and CCIR coefficients. A good agreement of observed and predicted TEC is found during the daytime with underestimation at other times. The predicted TEC by NeQuick and IRI01-corr is closer to the observed TEC during the daytime whereas during nighttime and morning hours, IRI-2001 shows lesser discrepancy in all seasons by both URSI and CCIR coefficients.     

Variation of hmF2 and NmF2 deduced from DPS-4 over Multan (Pakistan) and their comparisons with IRI-2012 & IRI-2016 during the deep solar minimum between cycles 23 & 24

We report the results of ionospheric measurements from DPS-4 installed at Multan (Geog coord. 30.18°N, 71.48°E, dip 47.4°). The variations in F2-layer maximum electron density NmF2 and its peak height hmF2 are studied during the deep solar minimum between cycles 23 & 24 i.e 2008–2009 with comparisons conducted with the International Reference Ionosphere (IRI) versions 2012 & 2016. We find that the hmF2 observations peak around the pre-sunrise and sunrise hours depending on the month. Seasonally, the daytime variation of NmF2 is higher in the Equinox and Summer, while daytime hmF2 are slightly higher in the Equinox and Winter. High values of hmF2 around midnight are caused by an increase of upward drifts produced by meridional winds. The ionosphere over Multan, which lies at the verge of low and mid latitude, is affected by both $E\times B$ drifts and thermospheric winds as evident from mid-night peaks and near-sunrise dips in hmF2. The results of the comparison of the observed NmF2 and hmF2 for the year 2008–2009 with the IRI-2012 (both NmF2 and hmF2) and IRI-2016 (only hmF2) estimates indicate that for NmF2, IRI-2012 with Consultative Committee International Radio (CCIR) option produces values in better agreement with observed data. Whereas, for hmF2, IRI-2016 with both International Union of Radio Science (URSI) and CCIR SHU-2015 options, predicts well for nighttime hours throughout the year. However, the IRI-2012 with CCIR option produces better agreement with data during daytime hours. Furthermore, IRI-2012 with CCIR option gives better results during Equinox months, whereas, IRI-2016 with both URSI and CCIR SHU-2015 options predict well for Winter and Summer.     

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.     

A preliminary study of the single crest phenomenon in total electron content (TEC) in the equatorial anomaly region around 120°E longitude between 1999 and 2012

The diurnal variations in total electron content (TEC) in the equatorial ionisation anomaly (EIA) region are not always represented by two crests on both sides of the magnetic equator. Sometimes, only an obvious single crest is evident at equatorial and low latitudes. In this paper, we focus on analysis of the morphological features of the single crest phenomenon in TEC around 120°E longitude during geomagnetic quiet days (Kp < 4₋). The variations in TEC are also compared with morphological parameters (foF2 and hmF2) derived from the International Reference Ionosphere extended to Plasmasphere (IRI–Plas) model. Our results show that the single crest phenomenon occurs mainly on days with extremely low solar activity, while the corresponding F2 layer critical frequency showed obvious asymmetry, or even only a single peak.     

On the variabilities of the Total Electron Content (TEC) over the Indian low latitude sector

The Total Electron Content (TEC) from four locations in the Indian sector namely, Trivandrum (8.47°N, 76.91°E, Geomag.0.63°S, 0.3° dip), Waltair (17.7° N, 83.3°E, Geomag. 6.4°N, 20° dip), Bhopal (23.28°N, 77.34°E, Geomag.14.26°N, 33.2° dip), and Delhi (28.58°N, 77.21°E, Geomag.19.2°N, 43.4° dip) during a low sunspot year of 2004 are used to study the variabilities of the TEC. The day time TEC values are higher over Waltair and Bhopal compared to those at Trivandrum and Delhi. Considerable day-to-day variations in the diurnal values of TEC are observed at the anomaly crest locations. The observed GPS-TEC has been compared with the IRI-2007 model derived TEC considering three different options (IRI-2001, IRI-2001 corrected and Ne-Quick) available in the model for the topside electron density. The TEC derived with Ne-Quick and IRI-01 corrected options show better agreement with GPS-TEC while the TEC from IRI-01 method shows larger deviations. From the correlation analysis carried out between TEC value at 1300 h LT and solar indices parameters namely sunspot number (SSN), F10.7 and EUV, it is observed that the correlation is more during equinoctial months and less during summer months. The correlation coefficients observed over the anomaly locations, Bhopal and Delhi are lower compared to those at Trivandrum and Waltair.     

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.     

Towards a new reference model of hmF2 for IRI

A numerical model of the peak height of the F2 layer, hmF2_top, is derived from the topside sounding database of 90,000 electron density profiles for a representative set of conditions provided by ISIS1, ISIS2, IK19 and Cosmos-1809 satellites for the period of 1969–1987. The model of regular hmF2 variations is produced in terms of local time, season, geomagnetic latitude, geodetic longitude and solar radio flux. No geomagnetic activity trends were discernible in the topside sounding data. The nighttime peak of hmF2_top evident for mid-latitudes disappears near the geomagnetic equator where a maximum of hmF2_top occurs at sunset hours when it can exceed 500 km at solar maximum. The hmF2 given by the IRI exceeds hmF2_top at the low solar activities. The hmF2_top, obtained by extrapolation of the first derivative of the topside profile to zero shows saturation similar to foF2 the greater the solar activity. The proposed model differs from hmF2 given by IRI based on M(3000)F2 to hmF2 conversion by empirical relationships in terms of foF2, foE and R12 with these quantities mapped globally by the ITU-R (former CCIR) from ground-based ionosonde data. The differences can be attributed to the different techniques of the peak height derivation, different epochs and different global distribution of the source data as well as the different mathematical functions involved in the maps and the model presentation.     

Variability of foF2 in the African equatorial ionosphere

This paper presents the impact of diurnal, seasonal and solar activity effects on the variability of ionospheric foF2 in the African equatorial latitude. Three African ionospheric stations; Dakar (14.8°N, 17.4°W, dip: 11.4°N), Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N) and Djibouti (11.5°N, 42.8°E, dip: 7.2°N) were considered for the investigation. The overall aim is to provide African inputs that will be of assistance at improving existing forecasting models. The diurnal analysis revealed that the ionospheric critical frequency (foF2) is more susceptible to variability during the night-time than the day-time, with two peaks in the range; 18–38% during post-sunset hours and 35–55% during post-midnight hours. The seasonal and solar activity analyses showed a post-sunset September Equinox maximum and June Solstice maximum of foF2 variability in all the stations for all seasons. At all the stations, foF2 variability was high for low solar activity year. Overall, we concluded that equatorial foF2 variability increases with decreasing solar activity during night-time.