The International Reference Ionosphere (IRI) is the standard model for ionospheric densities and temperatures developed and updated by a joint Working Group of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). NSSDC provides access to on-line IRI computations through its World Wide Web (WWW) models interface at http://nssdc.gsfc.nasa.gov/space/model/models_home.html.
In response to user requests three new input options have been implemented in the IRI Web interface. Quite often users have asked for ways to enter their own solar activity index rather than using the IRI-provided index; the IRI program finds the sunspot number for a given date from its internal data base. Such an input option lets users investigate solar cycle changes (i.e., changing only the solar index and leaving all other conditions the same), and it lets users also compute IRI parameters for dates beyond September 1998. (The IRI sunspot number data base covers the time period from 1958 to September 1998; typically, the last few months prior to the current date are based on predicts.) The new IRIWeb now provides this input capability. To exercise the option, users enter their sunspot number instead of the year in the year-input-window.
The second new option concerns the input of measured F2 peak parameters. The F2 peak is of special importance since it is the point of highest plasma density in the ionosphere. The IRI program takes a user-provided peak density and/or height and updates the whole density profile with these values. Users thus get a more reliable density prediction at altitudes where they did not have measured data available. Typically, a user might get measured peak parameters from an ionosonde, but the ionosonde measures densities only up to the F2 peak. Using the new IRIWeb, the user is now able to update the IRI density predictions above the F2 peak with the ionosonde measurements. Instead of the peak density a user can also enter the correlated plasma frequency, which is the primary parameter measured by the ionosonde. Similarly, a user can enter the propagation factor M(3000)F2 instead of the peak height since both parameters are correlated and since propagation factors can be deduced from ionograms.
Finally, as a third new input option users can now decide if they would like to use the International Radio Consultative Committee (CCIR) or URSI model for the F2 peak density. The CCIR model is the model officially recommended for international use by the CCIR, which is part of the International Telecommunication Union (ITU). It is based on the long record of ionosonde observations and on a representation in special spherical harmonics. It was developed in the mid-1960s, and shortcomings have been long noted, primarily above the ocean areas because of the non-availability of ionosonde data in these regions. An URSI Working Group was established in the early 1980s to develop better model maps. The URSI team used results from a theoretical model to bridge the data gaps and produced a new model (the URSI model) in the mid-1980s. Comparisons with Ionospheric Sounding Satellite (ISS) -b topside sounder data showed that the URSI model provided better results than the older CCIR model over the oceans but was slightly worse over land. Since overall the URSI model was found to provide closer agreement with the ISS-b data, the IRI team decided to recommend the URSI model as the standard option. Users of the new IRIWeb can now choose if they would like to use the CCIR or URSI model (e.g., a user primarily interested in data above land areas might choose CCIR, whereas the URSI model clearly would be the model of choice for someone using IRI for ionospheric corrections of sea surface heights measured by a spacecraft altimeter).
In addition to the enhanced functionalities provided by the three new input options, users now also have the option to view the results of the model calculations graphically in addition to the tabular listings that have been provided for the past two years.
Erin D. Gardner, firstname.lastname@example.org, (301) 286-0163
Raytheon STX, Code 633, NASA Goddard Space Flight Center
Greenbelt, MD 20771, U.S.A.