The importance of EUV indices for the international reference ionosphere

Phys. Chem. Earth (C), Vol. 25,No.5-6, pp. 515-521, 2000

Pergamon

0 2000Elsevier Science Ltd. All rights reserved 1464-1917/00/%- see front matter

PII: S1464-1917(00)00068-4

The Impqrtance of EW Indices for the International Reference Ionosphere D. Billtza Raytheon ITS& GSFC, Code 632, Greenbelt, MD 20771 Received 31 October 1999; accepted 15 March 2000

Abstract. The International Reference Ionosphere 0, a standard model for ionospheric densities and temperatures developed by a joint working group of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI), is widely used for specifications of the ionospheric environment. This paper will discuss the importance ofthe solar and/or ionospheric indices for driving the IRI model. The different existing indices will be reviewed and future needs will be outlined.Q 2000 Elsevier Science Ltd. All rights reserved

The model should be primarily based on experimental evidence using ali available

ground and space data sources; theoretical considerations can be helpful in bridging data gaps aud for internal consistency checks. Where discrepancies exist between different data sources the IRl team should promote critical discussion to establish the reliability ??

of the different a&a bases. RI should be updated as new data become

??

1. Introduction

available and as old data bases are fully evaluated and exploited. lRl is a joint working group of COSPAR and UW. COSPAR’s prime interest is in a general description of the ionosphere as part of the terrestrial environment for the evaluation of environmental effects on spacecrafl and experiments in space. URSI’s prime interest is in the electron density part of IRl for defining the background ionosphere for radiowave propagation studies and applications. ??

The International Reference Ionosphere (Bilitza, 1990) is a widely used standard for the specification of ionospheric parameters and is recommended for international use by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). Szuszczewiczet al. (1998), for example, evahrated several models (including first-principle models) for their global SUNDIAL/D%-IR campaign and find IRI to be the “best-of the-best”. At the 1999 URSI General Assembly in Toronto, Canada, the “URSI Commission G resolved that IRI be intemationally recognized as the standard for the ionosphere”. The latest version of the model, II&2000, is describedby Bilika (2000). IRI was developed and is being improved/updated by a joint Working Group of URSI and COSPAR The objectives are described in the terms of reference of the Working Group: The Task Group was established to develop and improve a standard model of the ionospheric plasma parameters (electron and ion densities, temperatures aud velocities). ??

Correspondence to: D. Bilitza 515

The Working Group consists of a team of experts representing different countries, different measurement techniques aud different aspects of the modeling problem. Currently the roster includes 43 members: M. Abdu (Brazil), J. Adeniyi (Nigeria), A. Alcayde (France), D. Anderson (Boulder, USA), K. Bib1 (UML, USA), D. Bilitza @SIX, USA., Chair), P. Bradley (UK), Y. Chasovitiu (Russia), A Dauilov (Russia), P. Dyson (LaTrobe, Australia), R Ezquer (Argentina), M. Friedrich (Austria), T. Fuller-Rowe11(SEL, USA), T. Gulyaeva (IZMIRAN,Russia), S. Gupta (Iudia), R. Hanbaba (France), X. Huang (UML, USA), K. Igarashi (CRL, Japan), G. Ivanov-Kholodny (Russia),

516

D. Bilitza: The Importance of EUV Indices

E. Kazmmovsky (SibIZMlRAN, Russia), P. Kishcha (Russia/Israel), E. Kopp (U Bern, Switzerland), I. Kutiev (Bulgaria),K. Mahajan (NPL, India), A Mikhailov (Russia), A. Mitra (NPL, India), M. Mosert de Gonzalez (Aregentina), K. Gyama @AS, Japan,fice-Chair COSTAR), A. Poole (South Africa), S. P&nets (IZMIRAN,Russia), S. Radicella (ICTP, Italy/Argentina), K. Rawer (Germany), B. Rein&h (UML, USA,Vice-Chair cI;IIsI), M. Rycroft (LJK),W.Singer (Germany), J. Sojka (USU, USA), I. Stanislavska (Poland), L. Triskova (Czech Rep), B. Ward (Australia), S. Watanabe (Japan), V. Wickwar (USA), P. Wilkinson (lPS, Australia), B. Zolesi (Italy). The main forum for the presentation and discussion of IRI shortcomings, improvements, new additions and applications are the annual workshops. The 1997 lRI Workshop was held jointly with the European Union Cooperation in Scientific and Technical Research (COST) project 251 “Improved Quality of Service in Ionospheric Telecommunication System Planning and Operation” at the Institute for Atmospheric Science in Ktihhmgsborn, Gemany. It’focused on ‘New Developments in Ionospheric Modeling and Prediction’and highlighted efforts to improve the bottomside and topside electron density profile and the description of ionospheric storm effects. Selected papers from this Workshop are published in Advances in Space Research (Rawer and Bradley, 1998). During the 1998 COSPAR General Assembly in Nagoya, Japan the IRI team organized a 3day session on ‘Data and Models for the Lower Ionosphere’. 32 papers/posters from this session were published as a special issue of Advances in Space Research (K. Rawer et al., 2000). The 1999 IRI Workshop was held at the University of Massachusetts in Lowell from August 8 to 12. The special emphasis was on the description of ionospheric variability and on my tracing through model ionospheres. A primary goal of the Nagoya and Lowell meetings were preparations for the IRI-2000 release of the model during the COSPAR General Assembly in Warsaw, Poland in July 2000. Summary reports from the IRI meetings can be found at: httn://nssdc.gsfc.nasa.~ov/soace/model/ ionos/iri/iri workshops.html. As an empirical model, IRI is based on the existing data record and the IRI WG is tasked to deduce the dominant variation patterns of ionospheric parameters from this data record. Variations with altitude, latitude, longitude, time of day, and season are evident in the data and are reproduced with appropriated functions in IRI. In addition a ll-year periodicity is observed that is caused by the 1l-year variation in the intensity of the solar irradiance. To reproduce these variations ionospheric models rely on solar and ionospheric indices. In the following chapters the most often used indices will be reviewed and their shortcomings will be highlighted.

The final chapter discusses EW-speciflc indices and their benefits for lRI. 2. Solar Indices Ideally ionospheric models for a certain region should rely on an index that reflects the solar cycle variation in the wavelength range that produces most of the ionospheric ions in that region. Figure 1 illustrates the ions produced in different altitude regimes and the responsible ionimtion source. The ionosphere above the D-region is mostly produced by solar irradiance in the EW wavelength range, whereas in the D-region X-rays and cosmic rays are dominant ionization sources. A good solar index for the ionosphere above 100 km would therefore be an index that tracks the solar cycle variation of solar photonfluxes in the EW wavelength range. But these fluxes cannot be observed at the ground, since they are absorbed in the atmosphere. Satellite measurements (from above the atmosphere), on the other hand, are only available for the relatively short time period for which EW experiments have operated in space. In the absence of longterm records for EW-specific iIldiiXS, ionospheric modelers have relied on two solar indices from a different wavelength range: the sunspot number R and the solar radio flux at 10.7 cm wavelength (F10.7). Both of these indices can be observed from the ground and long data records exist for these indices. R is a measure of the area of solar surface covered by spots. Yearly sunspot numbers are available since the telescope was invented in 1610. The R values issued by the World Data Centers in Boulder and Brussels are calculated as a weighted mean of the spots and spot groups reported by a network of solar observatories. The solar radio flux at 2800 MHz (F10.7)from the,entire solar disk has been recorded routinely by a radio telescope near Ottawa since 1947. The observed values are adjusted for the chan~g Sun-Earth distance and are given in units of 10qz Js-‘meI&-‘. These indices exhibit an 11-year cycle similar to the EW fluxes but the daily and monthly variations of the indices and fluxes often show considerable differences (e.g. Barth et al., 1990). Overall it was found that the correlation between solar indices and ionospheric parameters is best for a 12-months running mean and weaker for monthly and daily averages. Correlation studies between these indices and ionospheric pammeters have shown that F10.7 is the better index for the region of strong solar control below the F peak and R for the F region and above whete dynamic processes compete with the solar intluence. The R12 values from 1985 to the present are plotted in Figure 2 (solid line).

D. Bilitza: The Importance

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Fig. 1. Average daytime electron distribution

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of EUV Indices

in the ionosphere

with principle ions and ionization source in the different layers.

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Fig. 2. Variation of the 12-months-running

mean of sunspot number

(solid line) and ionospheric

IG index (broken line) since 1958.

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D. Bilitza: The Importance of EUV Indices

3. Ionospheric Indices

A serious problem with the use of the R12 index for ionospheric modeling was soon discovered at very high solar activities.For R12 greater than 150 it was found that electron densities in the F region and above increase much slower than R12. Kane (1992) observed this ‘saturation’effect with F peak plasmas frequencies foF2, Balan (1994a,b) with electron content data, and Benkova et al. (1984) with Intercosmos 19 topside sounder data The foF2 model of the InternationalRadio Consultative Committee (CCIR, 1991)of the International Telecommunication Union (ITU) is widely used for radio wave propagation studies and telecommunication applications and is also part of the IRI model. For this model CCIR recommends to keep R12 at a value of 150 even for higher solar activities to artificially simulate the ‘salutation’effect seen in the foF2 observations. As an alternative CCIR also allows the use of the Ionospheric Global index G112 (instead of Rz12) that was introduced by Liu et al. (1983). The first to experiment with an ionospheric index was Minnis (1955). He took foF2 measurements from three representative ionosonde stations and establishedthe regression line with R3 (3-month-running mean of R) R3= a*foF2 - b for each station For a specific month he could than use the measured monthly average foF2 and the regression equation to obtain an adjusted index R3’. Averaging over the R3’values for the three stations he than obtained his index IF2. Minnis and Bazzard (1960) later elaboratedtheir scheme to be based on data from 12 stations. In building the IF2 index Minnis (1955) used only noontime data. Turner (1968) used data from all hours to develop the T index for the Australian Ionospheric Prediction Service (IRS). Liu et al. (1983) noted that the regression equations used for IF2 did not agree with the regression equations on which the CCIR maps are based because data from different time periods were used in developing the IF2 index and the CCIR maps. Since many telecommunication applications were utilizing the CCIR maps, Liu et al. (1983) proposed a new index IG based on the regression equations of the CCIR maps. Many studies have proven the superiority of these ionospheric indices over the solar indices (& F10.7) in representing the solar cycle changes of ionospheric parameters, IF2 and IG were adopted by CCIR as basic indices for ionospheric propagation and monthly values were published regularly since 1964 in the Telecommunication Journal. The IG12 values from 1958 to the present are shown in Figure 2 (broken line). The comparison with R12 shows that both indices track the solar cycle with about the same phase but with different minima and maxima values and many differences in structural details. The overall plot of R12 versus IG12 in Figure 3 shows that the IG12 indices exhibits a ‘samrationeffect’at high solar activities similar to the ionospheric data. It also shows that for very low and very

high solar activities the R12 values exceed the IG12 values whereas for intermediate levels the IG12 index is higher. Using R12 will therefore lead to an underestimation or overestimation of ionospheric values depending on the level of solar activity. In comparing ionospheric and solar indices one has to keep in mind that the ionospheric index is not only alEcted by the sun’s radiation but also by dynamical processes in the ionosphere (diffusion and bulk trausport) since these processes together with production (ionization) and loss (recombination) terms determine the distribution of ionization in the ionosphere. Ionospheric indices therefore might exhibit latitudinal and diurnal variations not seen in solar indices. Averaging over all globally available stations and time periods during a month is necessary to obtain a global index like IG or lF2. But this will smooth out latitudinal differences and degrade the goodness of the ionospheric index. Regional indices provide better results and are in operational use in various parts of the world, e.g., the Ic index for the Chinese subcontinent (Wu et al., 1996). ‘,,

,

,

,

,

I50

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150

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Fig. 3. The R12 index versus the IG12 index 6om 1958 to 1999. The solid line indicates the approximation recommended for the CCIR (199 1) foF2 maps.

Such indices provide better predictions than global indices for the ionosphere above the specific country or continent for which they were designed. C&herglobal and regional (daily, monthly, even weekly) ionospheric indices have been computed for specific time periods (e.g. Bilitza, 1997) or are being computed on a regular basis (e.g., Secan and Wilkinson, 1997). But even using all available ionosonde data from the NGDC CD-ROM an improvement of only a few percent was achieved @l&a, 1997); such indices are strongly biased towards the northern mid-latitudes since most of the data were recorded there To obtain better results one needs to consider a more localized updating procedure, e.g. updating the model foF2 with a weighted mean of the closest foF2 measurements.

D. Bilitza: The Importance of EUV Indices

Figure 4 shows IG12’ indices deduced from adjusting IRl predictions of electron content to GPS measurements. It is clear that a global adjustment will only provide small improvements, whereas a regional adjustment (positive in the

519

northern mid-latitudes and negative everywhere else) could provide a considerable improvements and closest-neighbour adjustments even greater improvements in regions near to GPS ground stations.

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-60 -80

Fig. 4. The difference between adjusted IG12’index and standardIGl2 index. IG12’is obtained by adjustingthe ionospheric electron content computed with RI to GPS measurements. Plus signs indicate an upward adjustmentand diamonds a downward adjustment(units are degrees). Note the di&rence between data points Fip; 4. The difference between adiusted IG12’index and standardIGl2 index. in the northernmid-latitudesand the rest of the data points. (Coukesy M. knandn-Pajares) -

4. EUV-specifK indices and benefits for IRI The current IRI model utilizes the 12-months running mean

of the sunspot number R12 as provided by the Brussels World Data Center and the IG12 index as provided by the UK World Data Center. As recommended, the IG12 index is used with the CCIR (1991) maps for the F2 peak plasma frequency. For the height of the F2 peak and for the densities of the other layers solar cycle variations are represented with R12. For temperamres and ion densities the 12-monthrunning mean of the solar radio flux F10.7 is in use. The IRI has long supported efforts to establish EW-specsc solar indices that track the changes in the wavelength range of greatest importance for the production of ionospheric electrons and ions. It is clear that such indices would lead to a much better representation of the solar cycle changes in the region below the F peak since this region is under strong solar control (transport processes are negligible because of the large neutral densities and collision frequencies). In the region above the peak where transport processes become important, statistical studies have to be undertaken to

evaluate the advantages of an EW-specific index over the ionospheric indices. Balan et al. (1994a,b) could show that the ionospheric electron content is well correlated to the solar EW fluxes but not to Rz. In particular they could show that the slower increase at high solar activities (‘saturation’ effect) is also seen in the relevant EW photon fluxes. In the following we will discuss a statistical study by Tai et al. (1983), who used AEROS-A and -B EW measurements to study the correlation between the EW fluxes of different wavelengths and wavelength ranges. Their goal was to establish one or two EW indices that would represent the changes in the whole EW wavelength range. The Figures from this study are reproduced here since the original study was published in a Germanjournal of limited circulation In Figure 5 the correlation is shown between the commonly used solar indices (R F10.7) and the photon fluxes for the different wavelengths. In Figure 6 the fluxes are correlated with the Lyman-8 flux. Both Figures indicate the considerable wavelength-specific differences in the correlation coefficient. Figure 5 underlines the poor correlation between the EW fluxes and the solar indices R and

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D. Bilk:

The Importance of EUV Indices

F10.7. OverallF10.7 is strongercorrelatedthan R but both indices have much lower correlationcoefficients than a Lyman-8 specific index. But Figure 6 also shows tbat a Lyman-Bspecific index is representative for the solar cycle changesin the upperpartof the EW spectrumbut not in the lowerpart 3

Figures7 and 8 show that the intermediateand lower partof the EW wavelengthrangecould be repre-sentedby indices thatfollow the variationof the He I andII lines, reqectively.

a

Fig. 7. Correlation coefficknts between the solar photon fluxes for different wavelengths measwed by AEROS-A and the He I (58.4 nm) flux* Fig. 5. Correlation coeffkients between the Solar photon fluxes measured by AEROS-A in different wavelengths ranges and the sunspot number R (plus signs) and the solar radio flux F10.7 (squares).*

0

10

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Naoao40#0701#lootlo

WAVELENGTH Fig. 6. Correlation coefficients between the solar photon fluxes for different wavelengths measured by AEROS-Aand the Lyman-B flux.*

Fig. 8. Correlation coefficients between the solar photon fluxes for diierent wavelengths measured by AEROS-A and the He II (30.4 nm) flux.*

*Fig 5 - 8 are taken from Tai et al (1983). .

D. Bilitza: The Impeltance of EUV Indices

5. Conclusion The IRImodelandionosphericmodelingin generalcurrently relyonthesolarsunspotIuunberR,thesolarradioflux F10.7, and ionospheric indeces to describe solar cycle changes of ionosphericparameters.Theseareproxyindices that follow the changes in solar irradiancein a wavelength rangedifferentfromthe one thataffectsthe ionosphere,or in the case of ionospheric indices follow the ionospheric changesdirectlytherebymixing the solarinfluencewith the contribution from dynamical processes. Preferably, ionosphericmodelers and the IRI team would like to use indices that track the solar cycle changes in the EW wavelengthrange since this is the partof the solar qxctrnm that is the source of ionospheric ionization. Such indices should best be based on satellite measurements.The IRI team therefore strongly supports the TIGER project and encouragesa continuousmonitoringof EW fluxes in space. Acknowledgement. The author acknowledges support throughNSF grantATM-9713469thatmadehis contribution to the TIGEReffortandworkshoppossible. References EMan, N., G. Bsiley, B. Jenkins, P. Rao, R. Moffett, Variations of ionospheric ioniution end related solar fluxes during an intense solsr cycle, J. Geophys. Res. 99,2243-2253,1994s. Balm, N., G. Bailey, R. Moffett, Modeling studies of ionospheric variations

duringsn intense solsr cycle, J. Geophys. Res. 99,17467_17475,1994b. Barth, C., W. Tobisks, G. Rottman, and 0. White, Compsrison of 10.7 cm radio flux with SME solar Lyman alpha flux, Geophys. Res. Lett. 17, 571-574.1990.

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Benkova, N., N. Kochenovs, A Legenks, M. Fstkullin, M. Fligel, Model representation of mid-latitude electron density by means of Interkosmos-19data, Adv. Space Res. 4, #l, 51,1984. Bilii D., lntemationsl Reference Ionosphere, HU-90, National Space Science Data Center,Report90-22, Greenbelt,Maryland,4990. Bilitza, D., W-2000, submittedto Radio Science, 2000. Kane, R., Sunspots, solar radio noise, solar EW and ionospheric foF2, J. Atmos. Terr.Phys. 54,463,1992. Minnis, C., A new index of solar activity based on ionospheric measurements,J. Atmos. Tar. Phys. 7,310321,1955. Minnis, C. and G. Bszard, A monthly ionospheric index of solar activity based on F2-laya ionization st eleven stations, J. Atmos. Terr.Phys. 18, 297-305.1960. Rswer, K. end P. Bradley (eds.), W 1997 Symposium: New Developments in IonosphericModelling and Prediction,Adv. Spscs Res. 22, #6,1998. Rawer, K., D. Bilitu, K. Gyams, W. Singer (eds.), Lower Ionosphere Measurementsand Models, Adv. SF Res. 24, #l, 2000. Secan, J. and P. Wilkinson, Statistical studi= of sn effective sunspot number, ia: 1996 Ionospheric Effects Symposium, J. Goodman (ed.), NTIS #PB97-100101, Springfield,virginia, 1997. Szuszaewin, E., P. Blanchard, P. Wilkinson, 0. Crowley, T. FullerRowell, P. Richards, M. Abdu, T. Bullet&R lisnbabs, J. Lebretson,M. Lester, M. Lockwood, 0. Millward, M. Wild, S. Pulmets, B. Reddy, I. Staniilawsks, G. Vannaroni, and B. Zolesi, The fti real-time worldwide ionospheric predicitions networkz An advsnce in support of spaceborne ‘experimentation, on-line model validation, and space weather, Geophys. Res. L&t. 25.449-452.199s. Tai, H., K. RJIWW, D. Biitza, snd N. Von der Mtkhle, EUV-Indizes fhr niedrige Sonnenaktivit&t(German), Kleinheuhacher Berichte 26, 531535,1983. Turner, J., The development of the ionospheric index T%Ionspheric PredictionService, Repolt IPS Rll, 1968. WU,J., K. Quan, K. Dai, F. Luo, X. Sun, Z. Li, C. Cao, R. Liu, snd C. Shen, Progressin the study of the Chinese Reference Ionosphere, Adv. Space Res. 18, #6,187-190,1996.