Ionospheric Models

As in many areas of geophysical study, ionospheric modeling may assume a number of forms ranging from the purely theoretical to the totally empirical. Approaches may also include a combination of these forms, although empirical models dominate the field. Recent developments include allowance for adaptivity within the models to accommodate exploitation in the near-real-time environment for special applications. While physical or theoretical principles are the inspiration for a number of models, in fact most models in use today are largely specified on the basis of semi-empirical relationships derived from observational data.

Ionospheric models fulfill a variety of needs beyond basic research, with the most prominent application being radio system performance assessment and prediction. For example, ionospheric models are the engines that drive HF system performance models such as IONCAP [Teters et al., 1983]. Related models are supported by the U.S. Department of Commerce, including VOACAP, ICEPAC, and REC533. (The latter models may downloaded from an ITS website [Hand, 2004].) Other applications include evaluation of transionospheric signal parameters and errors in ranging or geolocation introduced by the electron content of the ionosphere. A general discussion of the status of ionospheric modeling in the context of HF communication systems has been covered by Goodman [1991], and recent information regarding telecommunication system planning has been published by the Commission of European Communities [Hanbaba, 1999]. We shall discuss activities of the European Union in the context of COST Actions in Section 3.12.3, Section 5.4.7, Section 5.4.9, and Section 6.6.1

Ionospheric profile models are based upon the superposition of various submodels of the ionospheric layers or regions (i.e., D, E, Es, Fl, and F2). The basic purpose of modeling is to represent the electron density profile under a variety of conditions (See Figure 3-4). These profile models may represent the respective layers as thin horizontal sheets (e.g. sporadic E), or quasi-parabolic regions in the vicinity of maximum ionization. The models are specified by the maximum electron density of the layer, the layer height, layer thickness and a functional representation of the layer shape. There are a number of models for the height profile, with the main differences being the manner in which the component layers are combined. Figure 3-34 depicts the general profile shape for the International Reference Ionosphere [Bilitza, 1990], and Figure 3-35 shows the ionospheric model contained in the computer program IONCAP.

There are also geographical, seasonal, and solar epochal variations in the specified ionospheric profiles, and the parameters upon which they are built. An example of the geographical variations in foF2 was shown in Figure 3-10, and the Global Atlas of Ionospheric Coefficients was discussed in Section 3.4. Ionospheric coefficients used to produce maps similar to Figure 3-10 are common to virtually every global empirical model of the ionosphere. Currently there are two sets of ionospheric coefficients, which may be specified; and these are the original CCIR (or ITU-R) set, which is sanctioned by the ITU-R, and the newer URSI set [Rush et al., 1989].

Sometimes it is good to keep models relatively simple in order to make subsequent applications more convenient. A sample application would be ray tracing. Simplicity may also be sufficient if the application is not demanding of precision in the profile. A simplistic model of the ionosphere consists of a parabolic E-layer, a linear increase in electron density in the Fl layer followed by a parabolic F2 layer [Bradley and Dudeney, 1973], At nighttime, the E and Fl layers effectively disappear. A newer ITU-R recipe consisting of multi-quasi-parabolic layers to provide continuity of the overall profile and its height derivatives [Dick and Bradley, 1992] has replaced the so-called Bradley-Dudeney profile model.

Figure 3-34: Depiction of the general profile shape for the International Reference Ionosphere.

Figure 3-34: Depiction of the general profile shape for the International Reference Ionosphere.

Figure 3-35: Ionospheric model contained in the computer program IONCAP.

Significant improvements in empirical ionospheric modeling have been promoted by military agencies around the world, including the U.S. Department of Defense, the U.K Ministry of Defence, and others. This is not surprising in view of the large number of applications of ionospheric specification in radiowave systems used by the military. The original ICED model was intended to be a northern hemispheric ionospheric specification model to serve the requirements of the US Air Force. It was only a regional model, descriptive of midlatitude behavior but extending into the auroral zone. It was designed to allow for recovery of some of the dynamic features embodied in auroral climatology which are smeared out in most mapping procedures. The model as described by Tascione et al.[1987a, 1987b] is driven by an effective sunspot number and an index derived from auroral oval imagery. The effective sunspot number is not based on solar data at all, but is derived from ionospheric data extracted from the US Air Force real-time ionosonde network. This effective sunspot number is similar to an ionospheric T-index developed by Australian workers, and the pseudoflux concept used by the U.S. Navy for HF predictions [Goodman, 1991]. The ICED model has been generalized to incorporate global considerations, while emphasizing near real-time applications.

Anderson has developed a low-latitude ionospheric profile model, SLIM [Anderson et al., 1987], and a Fully Analytic Ionospheric Model, FAIM [Anderson et al., 1989], in order to eliminate the use of limiting simplifications in the driving parameters associated with prediction models. A discussion of SLIM and FAIM may be found in a paper by Bilitza [1992], Other developments supporting Air Force requirements include PIM [Daniell et al., 1995] and PRISM. The model PIM, or Parameterized Ionospheric Model, is a global model of theoretical and empirical climatology, which specifies the ionospheric electron and ion densities from 90 to 25,000 Km. The model PRISM, for Parameterized Real-Time Ionospheric Specification Model, uses ground-based and space-based data available in real time to modify PIM thereby providing a near real-time ionospheric specification. Another model (viz., RIBG) discussed by Reilly and Singh [1993] combines ICED and several other models with a general ray tracing utility. Current versions of these models and validation of PRISM is discussed by Doherty et al. [1999]. An earlier survey of computer-based empirical models of the ionosphere has been published by Secan [1989],

The International Reference Ionosphere (IRI), mentioned previously, is a global empirical model that specifies monthly averages of electron, ion, and neutral temperatures, in addition to electron and ion densities from about 50 Km to ~ 2000 Km [Bilitza, 1990; 2001], The IRI development is a joint project of URSI and COSPAR, and has proven to be a useful model for scientific research. In recent years the IRI has been used in a number of applications and has gained greater acceptability within the operational community. The IRI is continually updated, and the responsible group holds periodic Task Force Activity meetings. The year 2002 meeting of the URSI-COSPAR IRI Working Group was the 9th meeting; and the major actions were considerations of ionospheric variability and the better understanding of the topside ionosphere [Radicella, 2003], As was pointed out in Section 3.10.2, IRI2000 has incorporated the STORM model to better represent the ionospheric personality during ionospheric storms. Figure 3-36 gives two global maps of the parameter foF2 using the IRI model. The conditions are summertime solar maximum and minimum at 00 UTC. Its form is similar to maps found in the original CCIR Global Atlas [CCIR, 1966] and in current ITU-R publications. The IRI has become a defacto standard for a number of applications including HF communication predictions and GPS studies. There is also a proposal to the International Standardization Organization (ISO) to recognize IRI as a standard model for the ionosphere. More information about the IRI model may be found on the NASA-GSFC website. There is also a newsletter that can be useful to researchers, and it is distributed through the ISAS organization. The source code of the IRI is available from the National Space Science Data Center (NSSDC); the documentation is given in Bilitza [1990]; and online computations may be organized from the NASA-GSFC website and the University of Leicester website.

Figure 3-36: The International Reference Ionosphere. Global map of the parameter foF2 (MHz) for solar minimum (R=10), summertime (July) at 00 UTC. Map provided by courtesy of Dieter Bilitza [2004],

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Figure 3-36: The International Reference Ionosphere. Global map of the parameter foF2 (MHz) for solar minimum (R=10), summertime (July) at 00 UTC. Map provided by courtesy of Dieter Bilitza [2004],

A model of primary interest to workers studying transionospheric propagation effects is the so-called Bent Model, a profile model based upon topside and bottomside sounder data [Bent et al., 1976]. Simplicity is not always important in the age of sophisticated computers, but the Ching-Chiu model [1973] has found a number of scientific applications in cases for which detailed ionospheric specification is not paramount.

Aside from global modeling of the ionosphere, there have been attempts to model selected regions of the world more accurately. During the decade of the 1990s, European scientists affiliated with the COST program have taken a lead in regional modeling and mapping of the ionosphere [Bradley, 1999; Hanbaba, 1999], More information on COST-related activities is provided in Section 3.12.3.

Another region-specific model is University of Alaska-Fairbanks Eulerian Parallel Polar Ionospheric Model (EPPIM). The EPPIM is a physical model of the polar ionosphere, which uses as inputs current solar and geomagnetic activity supplied by NOAA-SEC using the FTP protocol. The primary real-time geomagnetic activity driver is the U.S. Air Force estimate of Kp-'mAex that is supplied hourly. Interplanetary Magnetic Field (IMF) data is obtained from the WIND or ACE satellites; and this is used in conjunction with the Weimer [1995, 1996] electric field model to determine ionospheric drift patterns. The model can be run in a forecast mode (i.e., ~ 1 hour in advance) using the fact that an average solar wind takes between 0.5 hr and 1.5 hr to traverse the ACE-to-earth distance, depending on the solar wind speed involved. Real-time data can be obtained by accessing the special UAF EPPIM web site. Real-time comparisons of the model output and observed values of the F2 layer critical frequency (i.e.,foF2) are found on the web site. Real-time data from twelve stations belonging to the global ionosonde network are used in the comparisons. The sounder data are found at the NOAA-SEC web site (i.e., daily files). The EPPIM model has been of value when estimating the impact on HF communication circuits in the polar regions. Radio Propagation Services, a provider of HF communication forecasting services has used (Î) UAF-EPPIM output, (ii) polar GPS-based TEC maps and (iii) raw ionosonde data from NOAA-SEC to evaluate the influence of the polar ionosphere on air-to-ground HF circuits within the auroral oval.

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