International Reference Ionosphere

Lore & Background

Karl Rawer, the first chairman of the URSI/COSPAR Task group on the IRI (1968–84), specified the goal of establishing a monthly average model of the terrestrial ionosphere based on reliably measured data from ground- and space-based methods. Contradictions between these data sets had to be resolved in critical discussions. After a decade of data collection, a first set of tables was issued in 1978. Computer source code in ALGOL and Fortran followed; in 1986 the code became available on floppy disk, and later on the Web. The model is improved yearly according to results obtained at task group meetings, often held at COSPAR meetings.

The IRI used (and still has an option to use) an ITU-R model developed for radio propagation via the ionosphere, specifying two parameters: one narrowly related to peak electron density and the other to peak altitude. Both have been regularly determined from ionograms at ionospheric sounding stations. The authors R. M. Gallet and W. B. Jones analyzed a wealth of such data worldwide by combining Fourier analysis in time with worldwide Legendre analysis of the Fourier coefficients. Regional models are often applied because they reach better local performance.

The IRI model specifies monthly averages of electron density, electron and ion temperature, and the relative percentage of several positive ions (O+, H+, He+, N+, NO+, O2+, and Cluster ions). It can represent variation with altitude, latitude, longitude, date, and time of day, and can use solar, ionospheric, and geomagnetic indices to refine the model. Vertical total electron content (TEC) may be derived. The IRI has been extended to the plasmasphere in the IRI-Plas model.

Reader's Guide

The International Reference Ionosphere serves as the international standard for the terrestrial ionosphere, a status it has held since 1999. Its significance lies in providing a consistent, empirical baseline for average ionospheric conditions, derived from decades of ground- and space-based measurements. The model's ability to specify electron density, temperatures, and ion composition across altitude, latitude, longitude, time, and date makes it a foundational tool for radio propagation studies and space weather applications. The inclusion of an ITU-R model for radio propagation parameters, based on peak electron density and peak altitude from global ionogram data, directly ties the IRI to practical telecommunications needs. The extension to the plasmasphere via IRI-Plas broadens its applicability to higher altitudes. While the IRI remains the global standard, the notes acknowledge that regional models often achieve better local performance, indicating a pragmatic recognition of the model's limitations. Its continuous yearly refinement through task group meetings ensures it evolves with new data and understanding, maintaining its relevance as a reference for both scientific research and operational use.

Did You Know?

• The IRI is a common permanent project of COSPAR and URSI, started in 1968/69.
• The first set of IRI tables was issued in 1978 after a decade of data collection.
• The IRI model can derive vertical total electron content (TEC).