INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

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1 APRL/95/3 INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS ATMOSPHERIC PHYSICS AND RADIOPROPAGATION LABORATORY EXPERIMENTAL EQUATORIAL IONOSPHERIC PROFILES AND IRI MODEL J.O. Adeniyi INTERNATIONAL ATOMIC ENERGY AGENCY UNITED NATIONS EDUCATIONAL, SCIENTIFIC AND CULTURAL ORGANIZATION MIRAMARE-TRIESTE

3 International Atomic Energy Agency and United Nations Educational Scientific and Cultural Organization INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS APRL/95/3 ATMOSPHERIC PHYSICS AND RADIOPROPAGATION LABORATORY EXPERIMENTAL EQUATORIAL IONOSPHERIC PROFILES AND IRI MODEL J.O. Adeniyi 1 International Centre for Theoretical Physics, Trieste, Italy. MIRAMARE - TRIESTE December 1995 Permanent address: Physics Department, University of Ilorin, P.M.B. 1515, Ilorin, Nigeria. "T"

4 PREFACE The ICTP-APRL reports consist of preprints relevant to research and development work done at the Atmospheric Physics and Radiopropagation Laboratory of the International Centre for Theoretical Physics with the participation of visiting scientists. More information can be obtained by contacting: Prof. Sandro M. Radicella Atmospheric Physics and Radiopropagation P.O. Box Trieste Italy Phone: FAX: Laboratory

5 Abstract Ionospheric profiles below the F2 peak ionisation density are compared with those of the International Reference Ionosphere (IRI). The data used are those of Ibadan (Lat. 7.4 N, Long. 3.9 E). The results show that at low solar activity, the greatest difference between the model and experimental observation occurred in winter and September equinox, when the IRI model gives a thinner bottomside ionisation density. During the summer and March equinox of this solar epoch, the major difference occurs only around the Fl region, where the model gives a lower electron density than what is observed experimentally. The difference in summer is not as great as that observed during the March equinox. At high solar activity, the model is close to observed profile, from the F2 peak down to a height around the point where electron density is half of the F2 peak density (h 0.5). From the h 0.5 to the height of the E layer peak, the IRI model gives a lower electron density than what is observed experimentally. The discrepancy is least during the March equinox season. Some suggestions are made for the improvement of the model. 1. INTRODUCTION The International Reference Ionosphere (IRI) has been developed to produce a global reference model which is able to reproduce a number of ionospheric parameters. The model is very useful in the design of ionospheric radio propagation, the estimation of ionospheric effects on satellite signals and the design of other experiments where radio signals are involved. Over the years, efforts have been made to improve the model. One such effort is the improvement in the calculation of the thickness parameter for the bottomside ionosphere (BO), in the last version of IRI model; (IRI-90). The IRI-90 offers two choices for the calculation of the bottomside thickness parameter (Bilitza, 1990). It was pointed out that the new model, (Gulyaeva, 1987) for the half density height (h 0.5) is the better choice, especially for low latitudes. The half density height is the height below the F2 peak, where the ionisation density is half of that of the F2 peak density. Discussions on the short comings, limitations and ways of improving the model are still subjects of interest to the IRI working group (Bilitza et al., 1993). This paper investigates how well the ionospheric profile below the F2 peak, generated by the IRI model agrees with experimental profiles in an equatorial station. 2. ANALYSIS OF DATA The data used for this study are those of Ibadan (Lat. 7.4 N, Long. 3.9 E, Magnetic Dip 6.3 S). Samples of good ionograms were selected for the period of 1960 (R12=100); a T"

6 year of high solar activity and 1964 (R12=10); a year of low solar activity. The ionograms were reduced, using the POLAN technique (Titheridge, 1985). In order to avoid days of magnetic storms, the days considered were restricted to days with magnetic index Ap less than 26. Only day time ionograms within the period 1000 to 1400 hours; local time were considered. The months of January, April, July and October were used to represent the Winter (December Solstice), March equinox, Summer (June Solstice) and September equinox respectively. The numbers of days used during each of these months ranged from 8 to 10. Two sets of profiles of the IRI model were generated for each of the cases considered, by using both the old and new options for the bottomside thickness parameters offered in the IRI-90 (Bilitza, 1990). The experimental values of the F2 layer critical frequencies (fof2) and the corresponding heights (HmF2) were input into the IRI model programme in each case for the purpose of a fair comparison. 3.1 Old option bottomside parameter. 3. RESULTS Generally, the IRI profile generated with the old option bottomside representation, consistently gave a much thinner bottomside thickness of the F region, than what is observed experimentally. This is true for all seasons and for both high and low solar activity periods (figures 1-4). 3.2 New option bottomside parameter. Generally, the IRI profiles obtained with the new option gave much better results than those generated with the old option. Some discrepancies however still exist when the profiles obtained with the new option is compared with experimental observations. The differences vary with season and solar activity Winter. Figures 1 a and b show typical profiles for the Winter season. Observed data show the presence of the Fl layer at low solar activity most of the time, during this season. The Fl layer is completely absent in the IRI model. The model gives a much thinner ionisation density, compared to what is observed experimentally (fig. 1 a). At high solar activity, Fl layer is not observed during this season. The model is close to experimental observation from the F2 peak to a point around the half density height. Below this height, down to the E layer peak, the model gives a lower ionisation density (fig. 1 b) Summer. Typical profiles for the summer season are shown in figures 2 a and b. The result of the comparison between the model and observation during high and low solar activity periods are similar for the summer season (fig. 2 a and b). The difference between the model and

7 observation is centred around the Fl region, where the model gives a lower ionisation density March equinox. Figure 3 a and b show typical profiles for this season. The Fl layer is present at low but absent at high solar activity. At low solar activity, the model does not reproduce the Fl region well. A close agreement is observed from the F2 peak to a height around the half point density height (fig. 3 a). At high solar activity, the model gives a profile closer to the experimental one than for low solar activity, but still exhibit a lower ionisation density between the region from around the half density point to the height of the E layer peak density (fig. 3 b) September equinox. Typical examples of profiles for the September equinox season are shown in figures 4 a and b. Like the winter and the March equinox season, the Fl layer is present at low solar activity but absent at high solar activity. The results of the comparison between the model and experimental observation are similar to those of the winter season. At low solar activity, the IRI model gives a thinner bottomside ionisation density. The discrepancy is greatest around the Fl region (fig 4 a). At high solar activity, there is a good agreement between the model and observation from the F2 peak down to a height around the half density point. From the half density point to the E layer peak density, the model gives a smaller ionisation density (fig 4 b). 4. CONCLUSION The BO parameter in the IRI-90 model determines the thickness of the ionisation profile below the F2 peak density (Bilitza and Rawer, 1990). From this study for an equatorial station, the results for high solar activity show that the IRI model profile agrees with observation from the F2 peak down to a height around the half density point. This suggests that the new BO parameter is good for the equatorial region at all seasons of high solar activity. At low solar activity, some modification is needed in order to obtain a correct representation of this part of the profile during the Winter and September equinox. A more detailed study of the seasonal and solar cycle effects of the half density point on which the BO parameter is based, (Gulyaeva, 1987) is likely to reduce the discrepancy observed in this study in the region between the F2 peak and the half density point. For the height between the half density point and the E layer, it was observed in this study that the IRI model gives a consistently lower ionisation density at all seasons of both high and low solar activity periods. The Fl peak density is omitted in IRI model during winter (Bilitza, 1990). The Fl layer is however observed to be present during winter season of low solar activity. The omission of the Fl density in winter in the IRI is the likely reason for the occurrence of the worst discrepancies between the model and observation during winter season of low solar activity. One of the steps in improving the model should be the inclusion of the Fl density during winter of low solar activity. This step is not "T"

8 however enough for the elimination of the observed discrepancies, since the discrepancies are still present at other seasons. The high correlation between fofl and solar zenith angle, (Ducharme et al., 1971; 1973), suggests that the Fl region is predominantly under solar control, in contrast to the F2 region where transport terms play very significant role, thus making the F2 parameters exhibit very high degree of variability. It should therefore be possible to obtain a predictable characteristic point in the region of the FI heights. Defining a characteristic point between the half point density and the E region peak; which essentially is the region of the Fl is likely to eliminate the discrepancies observed around this height range. Acknowledgements-The interaction with Professor S.M. Radicella of ICTP when this paper was being prepared is appreciated. The author would like to thank the International Atomic Agency and UNESCO for hospitality at the International Centre for Theoretical Physics, Trieste, where this paper was finalised. He would also want to thank the Swedish Agency for Research Co-operation with Developing Countries; SAREC, for financial support during his visit at the ICTP under the associateship scheme. The support of the Unilorin Senate Research Grant is acknowledged. REFERENCES Bilitza D. (Ed) 1990 International Reference Ionosphere, NSSDC 90-2, World Data Centre A Rockets and Satellites, Greenbelt, USA. Bilitza D. and Rawer K New options for IRI electron density in the middle ionosphere, Adv. Space Res. 10(11), Bilitza D., Rawer K., Bossy L., 1993 International Reference Ionosphere- Past, and Gulyaeva T.L Present and Future: I. Electron Density. Adv. Space Res. 13(3), Ducharme E.D., Petrie L.E., 1971 A method for predicting the Fl layer critical and Eyfrig R. frequency. Radio Sci. 6, Ducharme E.D., Petrie L.E., 1973 A method for predicting the Fl layer critical and Eyfrig R. frequency based on the Zurich smoothed sunspot number. Radio Sci. 8, Gulyaeva T.L Progress in ionospheric informatic based on electron density profile analysis of ionograms. Adv. Space Res. 7(6) Titheridge J.E Ionogram analysis with the generalised program POLAN, Report UAG-39, World Data Centre A for STP, NOAA, Boulder, USA. Mailing address: Dr. Jacob O. Adeniyi I.C.TP. (Associate) P.O. Box Trieste, ITALY.

9 Figure Captions Figure 1. Typical profile for the Winter season at (a) low solar activity. (b) high solar activity. Figure 2. Typical profile for the Summer season at (a) low solar activity. (b) high solar activity. Figure 3. Typical profile for the March equiox season at (a) low solar activity. (b) high solar activity. Figure 4. Typical profile fo the September equinox season at (a) low solar activity. (b) high solar activity.

10 600 -i C a) CD 100 * OBS IRI OLD ( ) C 1200 hr LT) 0 I I I I I I! I I! I I T'TT T"["l ["T'r n X Electron Density 10**C 1 0) /m**c 3) OBS IRI OLD ( ) C 1200 hr LT) i i i r Electron Density 10**C 1 0) /m**( 3) 7 Fig.l

11 C a) en Q) 50 T t M i \\ i r [ i r i I T I i r i i i i i i i i i i OBS IRI OLD C ) ( 1300 hr LT) ,0 6.0 Electron Density 10**(10)/m**C3) = O) 0) OBS 50- IRI OLD (250760) C 1200 hr LT) 50 1 I II I M I I M II I I M I I I I I I I I I I = Electron Density 1 0* * ( 1 0) /m* * ( 3) Fig.2 r

12 > 50 OBS IRI OLD (160464) C 1200 hn i i [ i r i i i i r T i i i i T r T J I I i i i i i i i \ ElQctron DQnslLy 10** ( 10) / m **( 3) LT) cn ^ OBS IRI OLD (190460) C 1200 hn LT) I i! I I I I I I ] I I I I I M i I I I ' ] '!! I! I!! M Electron Density 10**(10)/m**(3) 9 Fig.3

13 350- cn ^ OBS OLD ( ) C 1200 hr LT) [ I I ] I I N I I I M I I If Electron Density 1 0* * ( 1 0) /m* * ( 3) 550 OBS IRI OLD C ) C 1200 hr LT) 50 i \ i i i i i i i i r i i T i i i i i i i i r i n i r Electron Density 10* * C 10) /m* * C 3) 10 Fig.4

The bottomside parameters B0, B1 obtained from incoherent scatter measurements during a solar maximum and their comparisons with the IRI-2001 model

The bottomside parameters B0, B1 obtained from incoherent scatter measurements during a solar maximum and their comparisons with the IRI-2001 model The bottomside parameters B, B obtained from incoherent scatter measurements during a solar maximum and their comparisons with the - model N. K. Sethi, K. K. Mahajan To cite this version: N. K. Sethi,