Evaluation of the international reference ionosphere with the large AE-C and DE2 data bases

Adv. Space Res. Vol. 8, No. 4, pp. (4)209—(4)212, 1988 Printed in Great Britain. All rights reserved.

0273—1177/88 $0.00 + .50 Copyright © COSPAR

EVALUATION OF THE INTERNATIONAL REFERENCE IONOSPHERE WITH THE LARGE AE-C AND DE2 DATA BASES D. Bilitza,* W. R. Hoegy,** L. H. Brace** and R. F. Theis** *

Goddard Space Flight Center, National Space Science Data Center, Science

Applications Research, Code 633, Greenbelt, MD 20771, U.S.A. “Goddard Space Flight Center, Laboratory for Atmospheres, Planetary Atmospheres Branch, Code 614, Greenbelt, MD 20771, U.S.A. ABSTRACT Empirical models such as the International Reference Ionosphere (IRI) are synthesized from large data bases. They can be viewed as analytical tools to facilitate accessing information stored in the data banks. However, in establishing the models, one has to apply smoothing and averagingprocedures that in effect reduce the original information content. Our study evaluates the agreement between the data base and the model at two opposite extremes of time resolution. We compare electron densities and temperatures in the altitude range of 300 to 400 km predicted by the IRJ and measured by the AE-C and DE 2 satellites on the level of individual orbits as well as on the level of mission averages. Whereas the averages show excellent agreement, the comparison for individual measurements indicates the limitations ofempirical models. AE-C AND DE 2 DATA Our study is based on the Atmosphere Explorer C (AE-C) and Dynamics Explorer 2 (DE 2) Langmuir probe measurement. Almost identical instruments were flown on AE-C /1/ and on DE 2 /2/ covering a time period close to a whole solar cycle. The instrument provides the electron density and temperature at the satellite altitude. AE-C was launched on December 16, 1973, into a highly elliptical orbit (perigee 149 km; apogee 4294 km) with an inclination of 68.1 degrees. In December 1974, the orbit was circularized at about 300 km with the help ofthe onboard propulsion system. The satellite was kept at about 300 km orbit altitude until Februaiy 1977 and was than thrust into a circular orbit at about 400 km altitude until the end of its lifetime (reentry in December 1978). AE-C was part of a satellite series that continued with AE-D and -E. At this time, AE-C is the longest and most data-intensive ionospheric satellite mission. DE 2 was launched on August 3, 1981, into an elliptical orbit (perigee 309 km; apogee 1012 km) with an inclination of almost 90 degrees. DE 2 complemented the high-altitude DE 1 satellite. The two satellites were launched together and were placed in polar coplanar orbits, permitting simultaneous measurements at high and low altitudes in the same field line. INTERNATIONAL REFERENCE IONOSPHERE (IRI) IRI is a joint project by the Union of Radio Science (URSI) and the Committee on Space Research (COSPAR) /3,4/. It was established by an international body ofexperts and has undergone more than a decade of improvement and critical review. IRI describes the global and temporal variation of electron density, electron and ion temperatures, and ion composition based on a large collection of ground-based, rocket and satellite measurements. In the altitude range of interest for our study (300-400 km), the IRI electron density model is deduced from ground-based measurements /3,4/, and the IRI electron temperature model was established with AE-C measurements /5/. COMPARISON Our comparison is part ofan ongoing project of evaluating the accuracy and reliability of ionospheric models. Information about the ionospheric environment is needed for a wide variety of applications including satellite orbit determination, radio astronomy, geodesy, etc. The present study intends to visualize the capabilities and limitations of the IRI in reproducing the AE-C/DE 2 data base by starting from opposite ends of the time resolution scale, namely individual orbits and mission averages. The DE 2 instrument measures electron density and temperature every half second; however, for our purposes we consider the 16 seconds average data. In the altitude range of 300 to 400 km, the AE-C data base consists of 165,987 measurements and the DE 2 data base of 225,241 measurements. Whole Mission First we compare the average behavior as determined by the combined AE-C, DE 2 data base and by the IRI. For each satellite data point in the altitude range of 300 to 400 km. the IRI model value is calculated. Average profiles in invariant latitude, magnetic local time (MLT), and 10.7-cm solar radio flux (F10.7) are determined for the whole combined mission period (1974 - 1983). The resulting model (M) and data (D) average profiles (4)209

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for electron density and temperature are shown in Figures 1, 2, and 3. The standard deviationsare between 15 and 25% for the data as well as the model. Figures ito 3 show excellent agreement between data and model averages. Latitudinal Profile. It should be noted that the characteristic features in Figures 1 to 3 represent fundamental variation patterns remaining after extensive averaging. Figure 1, for example, shows the typical latitudinal variation of electron density and temperature averaged over local time, season, and solar cycle. We recognize the equator anomaly and the strong anticorrelation between electron density and temperature. IRI is recommended for non-auroral latitudes. Therefore, it is not surprising that we find the largest deviations between data and model for latitudes poleward of 60 degrees. Diurnal Profile. Figure 2 shows us the typical day-night variation averaged over all latitudes, seasons, and almost a whole solar cycle. The early morning and late afternoon peaks in electron temperature and the post sunset increase in electron density remain as most consistent features and are well reproduced by IRI. Solar Activity Profile. Figure 3 shows that on average the electron density increases with solar activity, whereas the electron temperature remains at almost the same level. IRI represents this average behavior fairly well. The flU electron temperature model in our altitude range is independent of solar activity. Therefore, the small variation visible in Figure 3 is a result of the averaging procedure. The good agreement between IRI and AE-C averages at low solar activities is expected because the IRI electron temperature model is based on AE-C data. But the IRI averages also agree surprisingly well with the DE 2 averages at high solar activities, indicating that in a first order approximation the dependence on solar activity can be neglected in models describing the mean electron temperature in the altitude range of 300 to 400 km.

Evaluation of the IRI

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Fig. 3. Electron density and temperature variation with the 10.7-cm solar radio flux averaged over all longitudes, latitudes, local times, and seasons based on AE-CIDE 2 measurements CD) and on the liRI model (M). The average electron densities in Figure 3 indicate a saturation effect at very high solar activities in agreement with ground-based observations /6/. The IRE represents this variation pattern correctly but overestimates the data at low solar activities. The IRE electron density value in the altitude range of 300 to 400 km is strongly effected by the F2 peak density model. IRI presently applies the internationally recommended CCIR model, but in the future newer and better models promise a considerable improvement in this region (see [7/ for review). Individual Orbits The IRI is a monthly average model. Therefore, in comparing the IRE with individual measurement, one has to consider the high day-to-dayvariability(up to 30%) of the ionosphere. In addition, the simulation of plasma densities and temperatures along a satellite path has to include an accurate description of longitudinal, latitudinal, altitudinal, and diurnal variations because all these parameters can change along the orbit. In Figures 4 and 5, the electron densities and temperatures measured along the AE-C orbit 3366 (Figure 4) and DE 2 orbit 759 (Figure 5) are compared with IRE predictions. IRI represents the AE-C values fairly well; most of the measurements fall within the ±30% expectance band.

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Fig. 5. Electron density and temperature along the DE 2 orbit 759 (solid line) in comparison with IRE simulations (broken line). The DE 2 orbit in Figure 5 shows us shortcomings of IRE that were smoothed out in our earlier mission averages. The descrepancies between the AE-C based WI electron temperature model and the DE 2 data are probably caused by the difference in solar activity. AE-C measured during low solar activities, whereas during DE 2’s mission time very high solar activities were reached. Direct comparison of AE-C and DE 2 measurements indicated that the electron temperature increases with solar activity at low latitudes and decreases at mid-latitudes /8/, similar to what we find in Figure 5. Averaging over all latitudes as in Figure 3 disguises this dependency and produces an almost constant solar activity profile. The WIelectron density profilein Figure 5 exhibits a much stronger dip close to the magnetic equator than the DE 2 measurements. As was mentioned earlier, newer and better F peak models will considerably improve the predictions in our altitude range. It should be noted that these improvements are expected particularly above the oceans (the dip in Figure 5 is above the Pacific Ocean) where the newer models employ theoretical values instead of simple extrapolations (see [7/ for references). REFERENCES 1.

L. H. Brace, R. F. Theis, and A. Dalgarno, The cylindrical electrostatic probes for Atmosphere Explorer -C, -D, and -E, Radio Science 8, 341-348 (1973)

2.

1. B. Krehbiel, L. H. Brace, R. F. Theis, W. H. Pinkus, and R. B. Kaplan, The Dynamics Explorer Langmuir Probe Instrument, Snace Science Instrumentation 5, 493-502 (1981)

3.

International Reference Ionosphere Report UAG-82, World Data Center A for Solar-Terrestrial Physics, Boulder (1981)

4.

K. Rawer, D. Biitza, and S. Ramakrishnan, Goals and Status of the International Reference Ionosphere, Rev. Geophys. Space Phys. 16, 177-181 (1978)

5.

D. Bilitza, L. H. Brace, and R. F. Theis, Modelling of ionospheric temperature profiles, Adv. Space Res. 5, #7, 53-58 (1985)

6.

R. Y. Liu, P. A. Smith, and T. W. King, A new solar index which leads to improved foF2 predictions using the CCIR atlas, Telecomm. 1. 50, 408-414 (1983)

7.

D. Bilitza, K. Rawer, S. Pallaschke, C. M. Rush, N. Matuura, and W. R. Hoegy, Progress in modeling the ionospheric peak and topside electron density, Adv. Space Res., 7, #6, 5-12 (1987)

8.

L. H. Brace, R. F. Theis, and W. R. Hoegy, A global view of F-region electron density and temperature at solar maximum, Geophys. Res. Lett. 9, 989-992 (1982)