Sandro Maria Radicella, | International Centre for Theoretical Physics (Trieste) (original) (raw)

Papers by Sandro Maria Radicella,

Research paper thumbnail of Validation of Tec Models Over an Equatorial Station

Research paper thumbnail of The NeQuick model genesis, uses and evolution

Research paper thumbnail of Electron density profile analysis at low latitudes

Advances in Space Research, 1988

ABSTRACT

Research paper thumbnail of Epstein function topside profile modeling on the basis of Intercosmos-19 topside sounding data (Disturbed conditions)

The topside daytime ionosphere reaction on the magnetic storm on 27-29 March 1979 was studied on ... more The topside daytime ionosphere reaction on the magnetic storm on 27-29 March 1979 was studied on the basis of topside sounding data from Intercosmos-19. Topside profiles were parameterized with the help of Epstein function approximation. 3 different longitudinal sectors were analyzed (0°E, 180°E, and 280°E) and strong longitudinal dependence was revealed in ionosphere behavior.

Research paper thumbnail of Plasma bubbles detection in the African Equatorial Ionosphere

Fisica De La Tierra, 2008

Research paper thumbnail of NeMars: An empirical model of the martian dayside ionosphere based on Mars Express MARSIS data

Research paper thumbnail of Effects of geomagnetic storms on the ionospheric regions over the Southwest of Europe. An approach to the influence  of the G condition on ionospheric values

Acta Geodaetica et Geophysica Hungarica, 2002

ABSTRACT

Research paper thumbnail of An investigation of the ionospheric F region near the EIA crest in India using OI 777.4 and 630.0 nm nightglow observations

Research paper thumbnail of Recent advances in topside profile modeling

Advances in Space Research, 2002

A parameterized model for topside profile was developed based on the Epstein function approximati... more A parameterized model for topside profile was developed based on the Epstein function approximation. Using the Intercosmos-19 database, model parameters were obtained for different geophysical conditions, including strong I magnetic storms. In some specific conditions the F3 layer was observed on topside ionograms. A physical explanation is proposed as well as results based on modeling approach. Topside ionogams from the sounder on MIR Space Station were studied. Some exotic cases are presented includin g oblique propagation, station position under the peak height etc. Peak height global distribution is described as well as neutral wind parameters derived from the topside peak height values. 0

Research paper thumbnail of Modelling bottom and topside electron density and TEC with profile data from topside ionograms

Advances in Space Research, 2001

The paper describes the technique that has been implemented to model the electron density distrib... more The paper describes the technique that has been implemented to model the electron density distribution above and below the F2 peak making use of only the profiles obtained from the INTERCOSMOStopside ionograms. Each single profile from the satellite height to the ionosphere peak has been fitted by a semi-Epstein layer function of the type used in the DGR model with shape factor variable with altitude. The topside above the satellite height has been extrapolated to match given values of plasmaspheric electron densities to obtain the full topside profile. The bottomside electron density has been calculated by using the maximum electron density and its altitude estimated from the topside ionogram as input for a modified version of the DGR derived profiler that uses model values for the foF1 and foE layers of the ionosphere. Total electron content has also been calculated. Longitudinal cross sections of vertical profiles from latitudes 50"~ to 50"s latitude are shown for low and high geomagnetic activity. These cross sections indicate the equatorial anomaly effect and the changes of the shape of low latitude topside ionosphere during geomagnetic active periods. These results and the potentiality of the technique are discussed.

Research paper thumbnail of The study of the electron density variability at fixed heights over San Juan and Tucuman

Advances in Space Research, 2002

The electron density relative variability at fixed heights is studied using electron density prof... more The electron density relative variability at fixed heights is studied using electron density profiles obtained from two Argentine ground ionosondes measurements. For midnight conditions, the results show that, in general, the variability increases with height reaching a maximum below the peak density. The highest variability was observed at 280 km of height for night equinox and high solar activity. Variability is considerably larger during night than during daytime at F region altitudes. An opposite but much smaller difference is seen al E region heights. At noon variability is larger at low solar activity than at high solar activity because of the smaller mean values.

Research paper thumbnail of Equatorial F2-peak parameters in the IRI model

Advances in Space Research, 2003

We have used measurements of an ionosonde station near the magnetic equator in Ouagadougou, Burki... more We have used measurements of an ionosonde station near the magnetic equator in Ouagadougou, Burkina Faso to evaluate the ability of the International Reference Ionosphere (IRI) model to correctly represent ionospheric F2 peak parameters in this region. The data represent conditions of high and low solar activity. Comparing the URSI and CCIR option for the F2 plasma frequency, foF2, we find that for low solar activity both options agree quite well with the ionosonde foF2 values and overall the CCIR maps show a slightly better fit. During high solar activity discrepancies are found during nighttime and the URSI maps providing the overall better fit. The measured F2 peak height values, hmF2, are compared on one hand with IRI predictions that are obtained based on the CCIR model for the propagation factor M(3OOO)F2 and on the other hand with the ionosonde-measured M(3OOO)F2 values. As expected using the measured values results in more accurate predictions. It is important to note that with the measured M(3OOO)F2 values IRI predicts the characteristic post-sunset that is seen in the measurements but not in the IRI predictions with the CCIR-M(3OOO)F2 model. 0 2003 COSPAR. Published by Elsevier Science Ltd. All

Research paper thumbnail of Equatorial F2-layer peak height and correlation with vertical ion drift and M(3000)F2

Advances in Space Research, 2003

Ionosonde data recorded at Korhogo, Côte-d&am... more Ionosonde data recorded at Korhogo, Côte-d'Ivoire (Latitude +9.3; Longitude −5.4, Dip −0.67) during a year of low solar activity (1995) were used to investigate ways of improving the representation of equatorial F2 peak height (hmF2) in the International Reference Ionosphere (IRI). For this purpose we have studied the correlation between hmF2 and the equatorial F region vertical drift as given

Research paper thumbnail of Variation of bottomside profile parameters B0 and B1 at high solar activity for an equatorial station

Journal of Atmospheric and Solar-terrestrial Physics, 1998

ABSTRACT Diurnal and seasonal variations in experimental profile parameters B0 and B1 are examine... more ABSTRACT Diurnal and seasonal variations in experimental profile parameters B0 and B1 are examined, at high solar activity, for Ouagadougou, Burkina Faso, an equatorial station in Africa (latitude 12.4°N, longitude 1.5°W, dip 5.9°N). The diurnal variations for B0 indicate a solar zenith angle dependence that can be described as B0 = A cosn (χ) with n = 0.40, 0.51, 0.46, and 0.71 for Winter, Spring, Summer and Autumn, respectively. The seasonal effect is most pronounced from 10:00–18:00 LT. Within this period, B0 is highest in Spring and lowest in Winter. A comparison of experimental B0 with the IRI B0 shows that the greatest discrepancy occur from about 11:00 to about 18:00 LT. Both are closer during most of the night time hours and in the early hours of the daytime. There is no obvious solar zenith angle dependence in B1. The range of variation for all seasons, during the day time (10:00–18:00 LT) is 0.8. For the remaining part of the 24 h period, the range is about 1.8. Generally, there is an overestimation of B1 by the IRI model.

Research paper thumbnail of Study of ionospheric variability at fixed heights using data from South America

Advances in Space Research, 1995

A series of about 800 electron density profiles calculated by inversion of ionograms obtained at ... more A series of about 800 electron density profiles calculated by inversion of ionograms obtained at four stations in Argentina(low to middle geomagnetic latitudes) have been used to study the variability of electron density at fixed heights. The variability index used is the percentage ratio of the standard deviation over the mean. A set of groups of days has been selected representing different seasons and two years with different levels of solar activity .Also used were "composite" noon profiles of San Juan ( in three years 219 profiles each representative for the given hour in a group of five consecutive days).Around noon minimum variability is consistently found at 170-190 km. Variability increases with height in the F2 region both by day and night, reaching a maximum somewhere below 300 km. At greater heights the variability show a tendency to decrease. Night-time variability is apparently larger. By day the variability decreases with increasing solar activity. This behaviour is reversed by night above 220 km.A certain increase of the variability with latitude appears to exist.

Research paper thumbnail of Ionospheric variability at low latitude station: Hainan, China

Advances in Space Research, 2004

In this paper, the ionospheric characteristic parameters obtained from ionograms recorded by the ... more In this paper, the ionospheric characteristic parameters obtained from ionograms recorded by the DPS-4 digisonde at Hainan (19.4°N, 109.0°E), China are used to analyze the diurnal and seasonal behavior of the ionospheric variability at low latitude. Data used for the present study are hourly interval resolution data of the F2 layer critical frequency foF2, the F2 layer peak height hmF2, the thickness parameter B0 and the shape parameter B1. Time period coverage of the data used for the present study is from March 2002 to June 2003. The monthly inter-quartiles (D IntQT ) and monthly ranges (D Range ) as well as their percentage ratios to the monthly medians (D IntQT (%), D Range (%)) are used as quantitative measurements of the variability of the ionospheric parameters in the present study. Seasonal averages of these quantities are also obtained for summer, winter and equinox. Our study showed that: (1) for the foF2 parameter, the variability is much lower during daytime hours than during nighttime hours, with the maximum variability occurring at pre-sunrise hours -in terms of percentage inter-quartile values. The seasonal averages of D IntQT (%) are less than 15% by day and fluctuate between 12% and 40% at night; (2) for the hmF2 parameters, the variability has a minimum value at hours after sunrise and sunset, whereas it has a maximum value at about 4:00LT. The seasonal averages of D IntQT (%) fluctuate between 6.9% and 19.2%, with a slightly lower value at equinox than in the other two seasons; (3) for the B0 parameter, for the winter and equinox seasons, the variability has a minimum value at hours after sunrise and sunset, whereas it has maximum values at about 02:00-03:00LT and post-noon hours; for the summer season, the occurrence time of the maximum and minimum shifts to later compared with those observed in the other two seasons. The seasonal averages of D IntQT (%) range between 21% and 46.2% for winter and between 18.2% and 37.6% for the other two seasons; (4) for the B1 parameter, in terms of the absolute monthly inter-quartiles and monthly ranges, except for the hours near 06:00-07:00LT when the variability reaches a maximum value, it changes within a very limited extent. In terms of the percentage ratios to the monthly medians, the variability of B1 is higher during daytime than during nighttime. The seasonal averages of D IntQT (%) change between 11.6% and 33.2%, showing in general slightly higher values in the solstices (23.4-32.9%) than at equinox (15.0-23.1%) in the time period between 02LT and 15LT.

Research paper thumbnail of Improvements of the International Reference Ionosphere model for the topside electron density profile

Radio Science, 2006

1] Shortcomings of the representation of the topside electron density profile in the Internationa... more 1] Shortcomings of the representation of the topside electron density profile in the International Reference Ionosphere (IRI) model have been noted in comparison with recently analyzed topside sounder data and also with total electron content (TEC) data. Various studies have proposed corrections of the IRI formulas or have introduced a new formalism. This paper reviews the different approaches, their implications for IRI, and their current status. An important challenge for topside modeling is the truthful representation of profiles in the equatorial anomaly (EA) region over the whole range of the EA fountain. This means that the latitudinal representation has to reproduce the merging of the double-peak signature at F region heights into a single peak at the top of the fountain. In this context, special emphasis is given to the coupling between topside and plasmaspheric models.

Research paper thumbnail of New B0 and B1 models for IRI

Advances in Space Research, 2000

The electron density profile in the F region bottomside is described in the International Referen... more The electron density profile in the F region bottomside is described in the International Reference Ionosphere (IRI) by two parameters: a thickness parameter BO and a shape parameter Bl. The models used for BO and Bl in IRI are based on ionosonde data from magnetic mid-latitude stations. Comparisons with ionosonde data from several stations close to the magnetic equator show large discrepancies between the model and the data. We propose new models for BO and Bl based on data from several ionosondes including low and mid latitude stations. Close to the magnetic dip equator the new BO model provides an improvement over the current IRI model by a factor of up to 1 S.

Research paper thumbnail of Diurnal variation of ionospheric profile parameters B 0 and B 1 for an equatorial station at low solar activity

Journal of Atmospheric and Solar-terrestrial Physics, 1998

Research paper thumbnail of On the prediction of F1 ledge occurrence and critical frequency

Advances in Space Research, 1997

The probability of occurrence of Fl ledge is presented for different solar position and activity ... more The probability of occurrence of Fl ledge is presented for different solar position and activity conditions by using the hourly ionogram scaling information given by the monthly bulletins of ionospheric data. The information considered includes cases of L condition. An updated and selected data base is used to test the Du Charme et al. (1973) formula adopted in the IRI model taking into account alternative solutions for the particular restrictions imposed by the IRI for high values of solar zenith angle. Special attention is given to the critical frequency prediction for twilight hours. 01997 COSPAR. Published by Elsevier Science Ltd.

Research paper thumbnail of Validation of Tec Models Over an Equatorial Station

Research paper thumbnail of The NeQuick model genesis, uses and evolution

Research paper thumbnail of Electron density profile analysis at low latitudes

Advances in Space Research, 1988

ABSTRACT

Research paper thumbnail of Epstein function topside profile modeling on the basis of Intercosmos-19 topside sounding data (Disturbed conditions)

The topside daytime ionosphere reaction on the magnetic storm on 27-29 March 1979 was studied on ... more The topside daytime ionosphere reaction on the magnetic storm on 27-29 March 1979 was studied on the basis of topside sounding data from Intercosmos-19. Topside profiles were parameterized with the help of Epstein function approximation. 3 different longitudinal sectors were analyzed (0°E, 180°E, and 280°E) and strong longitudinal dependence was revealed in ionosphere behavior.

Research paper thumbnail of Plasma bubbles detection in the African Equatorial Ionosphere

Fisica De La Tierra, 2008

Research paper thumbnail of NeMars: An empirical model of the martian dayside ionosphere based on Mars Express MARSIS data

Research paper thumbnail of Effects of geomagnetic storms on the ionospheric regions over the Southwest of Europe. An approach to the influence  of the G condition on ionospheric values

Acta Geodaetica et Geophysica Hungarica, 2002

ABSTRACT

Research paper thumbnail of An investigation of the ionospheric F region near the EIA crest in India using OI 777.4 and 630.0 nm nightglow observations

Research paper thumbnail of Recent advances in topside profile modeling

Advances in Space Research, 2002

A parameterized model for topside profile was developed based on the Epstein function approximati... more A parameterized model for topside profile was developed based on the Epstein function approximation. Using the Intercosmos-19 database, model parameters were obtained for different geophysical conditions, including strong I magnetic storms. In some specific conditions the F3 layer was observed on topside ionograms. A physical explanation is proposed as well as results based on modeling approach. Topside ionogams from the sounder on MIR Space Station were studied. Some exotic cases are presented includin g oblique propagation, station position under the peak height etc. Peak height global distribution is described as well as neutral wind parameters derived from the topside peak height values. 0

Research paper thumbnail of Modelling bottom and topside electron density and TEC with profile data from topside ionograms

Advances in Space Research, 2001

The paper describes the technique that has been implemented to model the electron density distrib... more The paper describes the technique that has been implemented to model the electron density distribution above and below the F2 peak making use of only the profiles obtained from the INTERCOSMOStopside ionograms. Each single profile from the satellite height to the ionosphere peak has been fitted by a semi-Epstein layer function of the type used in the DGR model with shape factor variable with altitude. The topside above the satellite height has been extrapolated to match given values of plasmaspheric electron densities to obtain the full topside profile. The bottomside electron density has been calculated by using the maximum electron density and its altitude estimated from the topside ionogram as input for a modified version of the DGR derived profiler that uses model values for the foF1 and foE layers of the ionosphere. Total electron content has also been calculated. Longitudinal cross sections of vertical profiles from latitudes 50"~ to 50"s latitude are shown for low and high geomagnetic activity. These cross sections indicate the equatorial anomaly effect and the changes of the shape of low latitude topside ionosphere during geomagnetic active periods. These results and the potentiality of the technique are discussed.

Research paper thumbnail of The study of the electron density variability at fixed heights over San Juan and Tucuman

Advances in Space Research, 2002

The electron density relative variability at fixed heights is studied using electron density prof... more The electron density relative variability at fixed heights is studied using electron density profiles obtained from two Argentine ground ionosondes measurements. For midnight conditions, the results show that, in general, the variability increases with height reaching a maximum below the peak density. The highest variability was observed at 280 km of height for night equinox and high solar activity. Variability is considerably larger during night than during daytime at F region altitudes. An opposite but much smaller difference is seen al E region heights. At noon variability is larger at low solar activity than at high solar activity because of the smaller mean values.

Research paper thumbnail of Equatorial F2-peak parameters in the IRI model

Advances in Space Research, 2003

We have used measurements of an ionosonde station near the magnetic equator in Ouagadougou, Burki... more We have used measurements of an ionosonde station near the magnetic equator in Ouagadougou, Burkina Faso to evaluate the ability of the International Reference Ionosphere (IRI) model to correctly represent ionospheric F2 peak parameters in this region. The data represent conditions of high and low solar activity. Comparing the URSI and CCIR option for the F2 plasma frequency, foF2, we find that for low solar activity both options agree quite well with the ionosonde foF2 values and overall the CCIR maps show a slightly better fit. During high solar activity discrepancies are found during nighttime and the URSI maps providing the overall better fit. The measured F2 peak height values, hmF2, are compared on one hand with IRI predictions that are obtained based on the CCIR model for the propagation factor M(3OOO)F2 and on the other hand with the ionosonde-measured M(3OOO)F2 values. As expected using the measured values results in more accurate predictions. It is important to note that with the measured M(3OOO)F2 values IRI predicts the characteristic post-sunset that is seen in the measurements but not in the IRI predictions with the CCIR-M(3OOO)F2 model. 0 2003 COSPAR. Published by Elsevier Science Ltd. All

Research paper thumbnail of Equatorial F2-layer peak height and correlation with vertical ion drift and M(3000)F2

Advances in Space Research, 2003

Ionosonde data recorded at Korhogo, Côte-d&am... more Ionosonde data recorded at Korhogo, Côte-d'Ivoire (Latitude +9.3; Longitude −5.4, Dip −0.67) during a year of low solar activity (1995) were used to investigate ways of improving the representation of equatorial F2 peak height (hmF2) in the International Reference Ionosphere (IRI). For this purpose we have studied the correlation between hmF2 and the equatorial F region vertical drift as given

Research paper thumbnail of Variation of bottomside profile parameters B0 and B1 at high solar activity for an equatorial station

Journal of Atmospheric and Solar-terrestrial Physics, 1998

ABSTRACT Diurnal and seasonal variations in experimental profile parameters B0 and B1 are examine... more ABSTRACT Diurnal and seasonal variations in experimental profile parameters B0 and B1 are examined, at high solar activity, for Ouagadougou, Burkina Faso, an equatorial station in Africa (latitude 12.4°N, longitude 1.5°W, dip 5.9°N). The diurnal variations for B0 indicate a solar zenith angle dependence that can be described as B0 = A cosn (χ) with n = 0.40, 0.51, 0.46, and 0.71 for Winter, Spring, Summer and Autumn, respectively. The seasonal effect is most pronounced from 10:00–18:00 LT. Within this period, B0 is highest in Spring and lowest in Winter. A comparison of experimental B0 with the IRI B0 shows that the greatest discrepancy occur from about 11:00 to about 18:00 LT. Both are closer during most of the night time hours and in the early hours of the daytime. There is no obvious solar zenith angle dependence in B1. The range of variation for all seasons, during the day time (10:00–18:00 LT) is 0.8. For the remaining part of the 24 h period, the range is about 1.8. Generally, there is an overestimation of B1 by the IRI model.

Research paper thumbnail of Study of ionospheric variability at fixed heights using data from South America

Advances in Space Research, 1995

A series of about 800 electron density profiles calculated by inversion of ionograms obtained at ... more A series of about 800 electron density profiles calculated by inversion of ionograms obtained at four stations in Argentina(low to middle geomagnetic latitudes) have been used to study the variability of electron density at fixed heights. The variability index used is the percentage ratio of the standard deviation over the mean. A set of groups of days has been selected representing different seasons and two years with different levels of solar activity .Also used were "composite" noon profiles of San Juan ( in three years 219 profiles each representative for the given hour in a group of five consecutive days).Around noon minimum variability is consistently found at 170-190 km. Variability increases with height in the F2 region both by day and night, reaching a maximum somewhere below 300 km. At greater heights the variability show a tendency to decrease. Night-time variability is apparently larger. By day the variability decreases with increasing solar activity. This behaviour is reversed by night above 220 km.A certain increase of the variability with latitude appears to exist.

Research paper thumbnail of Ionospheric variability at low latitude station: Hainan, China

Advances in Space Research, 2004

In this paper, the ionospheric characteristic parameters obtained from ionograms recorded by the ... more In this paper, the ionospheric characteristic parameters obtained from ionograms recorded by the DPS-4 digisonde at Hainan (19.4°N, 109.0°E), China are used to analyze the diurnal and seasonal behavior of the ionospheric variability at low latitude. Data used for the present study are hourly interval resolution data of the F2 layer critical frequency foF2, the F2 layer peak height hmF2, the thickness parameter B0 and the shape parameter B1. Time period coverage of the data used for the present study is from March 2002 to June 2003. The monthly inter-quartiles (D IntQT ) and monthly ranges (D Range ) as well as their percentage ratios to the monthly medians (D IntQT (%), D Range (%)) are used as quantitative measurements of the variability of the ionospheric parameters in the present study. Seasonal averages of these quantities are also obtained for summer, winter and equinox. Our study showed that: (1) for the foF2 parameter, the variability is much lower during daytime hours than during nighttime hours, with the maximum variability occurring at pre-sunrise hours -in terms of percentage inter-quartile values. The seasonal averages of D IntQT (%) are less than 15% by day and fluctuate between 12% and 40% at night; (2) for the hmF2 parameters, the variability has a minimum value at hours after sunrise and sunset, whereas it has a maximum value at about 4:00LT. The seasonal averages of D IntQT (%) fluctuate between 6.9% and 19.2%, with a slightly lower value at equinox than in the other two seasons; (3) for the B0 parameter, for the winter and equinox seasons, the variability has a minimum value at hours after sunrise and sunset, whereas it has maximum values at about 02:00-03:00LT and post-noon hours; for the summer season, the occurrence time of the maximum and minimum shifts to later compared with those observed in the other two seasons. The seasonal averages of D IntQT (%) range between 21% and 46.2% for winter and between 18.2% and 37.6% for the other two seasons; (4) for the B1 parameter, in terms of the absolute monthly inter-quartiles and monthly ranges, except for the hours near 06:00-07:00LT when the variability reaches a maximum value, it changes within a very limited extent. In terms of the percentage ratios to the monthly medians, the variability of B1 is higher during daytime than during nighttime. The seasonal averages of D IntQT (%) change between 11.6% and 33.2%, showing in general slightly higher values in the solstices (23.4-32.9%) than at equinox (15.0-23.1%) in the time period between 02LT and 15LT.

Research paper thumbnail of Improvements of the International Reference Ionosphere model for the topside electron density profile

Radio Science, 2006

1] Shortcomings of the representation of the topside electron density profile in the Internationa... more 1] Shortcomings of the representation of the topside electron density profile in the International Reference Ionosphere (IRI) model have been noted in comparison with recently analyzed topside sounder data and also with total electron content (TEC) data. Various studies have proposed corrections of the IRI formulas or have introduced a new formalism. This paper reviews the different approaches, their implications for IRI, and their current status. An important challenge for topside modeling is the truthful representation of profiles in the equatorial anomaly (EA) region over the whole range of the EA fountain. This means that the latitudinal representation has to reproduce the merging of the double-peak signature at F region heights into a single peak at the top of the fountain. In this context, special emphasis is given to the coupling between topside and plasmaspheric models.

Research paper thumbnail of New B0 and B1 models for IRI

Advances in Space Research, 2000

The electron density profile in the F region bottomside is described in the International Referen... more The electron density profile in the F region bottomside is described in the International Reference Ionosphere (IRI) by two parameters: a thickness parameter BO and a shape parameter Bl. The models used for BO and Bl in IRI are based on ionosonde data from magnetic mid-latitude stations. Comparisons with ionosonde data from several stations close to the magnetic equator show large discrepancies between the model and the data. We propose new models for BO and Bl based on data from several ionosondes including low and mid latitude stations. Close to the magnetic dip equator the new BO model provides an improvement over the current IRI model by a factor of up to 1 S.

Research paper thumbnail of Diurnal variation of ionospheric profile parameters B 0 and B 1 for an equatorial station at low solar activity

Journal of Atmospheric and Solar-terrestrial Physics, 1998

Research paper thumbnail of On the prediction of F1 ledge occurrence and critical frequency

Advances in Space Research, 1997

The probability of occurrence of Fl ledge is presented for different solar position and activity ... more The probability of occurrence of Fl ledge is presented for different solar position and activity conditions by using the hourly ionogram scaling information given by the monthly bulletins of ionospheric data. The information considered includes cases of L condition. An updated and selected data base is used to test the Du Charme et al. (1973) formula adopted in the IRI model taking into account alternative solutions for the particular restrictions imposed by the IRI for high values of solar zenith angle. Special attention is given to the critical frequency prediction for twilight hours. 01997 COSPAR. Published by Elsevier Science Ltd.