The URSI/COSPAR Task Group on the International Reference Ionosphere (IRI) conducted its annual scientific meeting during the World Space Congress (COSPAR/IAF) in Washington, D.C., USA on September 3rd, 1992. Four well attended sessions were held with participants from 11 countries presenting 15 talks and 10 posters (to be published in Advances in Space Research, edited by K. Rawer and W.R. Piggott). The following is a synopsis of meeting highlights, and final resolutions and recommendations.
After the release of IRI-90 (Version-No 12) the model is now mid-way through its next improvement cycle. The focus of this workshop was therefore on verification of IRI-90 with newer data and on the discussion of future improvements. Noteworthy is the number of presentations that compared the present IRI model with large data bases providing valuable information about IRI-90 shortcomings and indicating ways of improvement. Data sets used in this context included the data base of Arecibo incoherent scatter data (Mahajan et al.), the GSFC comprehensive data base of satellite insitu measurements (Hoegy and Grebowsky), ionosonde data from the worldwide network of digisondes (Reinisch), a compilation of close to 100 rocket observations (Danilov and Smirnova) and the data accumulated by the SUNDIAL program (Szuszczewicz et al.).
Electron density - topside:
Shortcomings of the IRI-90 topside model were noted in comparisons with DE Langmuir probe measurements (Hoegy and Grebowsky) and with electron content data from Paulista, Brazil (Batista et al.). All topside modelling to date depends in one way or another on the rather limited compilation of Alouette/ISIS topside sounder data done by Bent and his co-workers in the early seventies. Considering the importance of topside electron density for a wide range of IRI applications, high priority is given to an improvement of the IRI topside model with newer data. A topside modelling project is now in its planning stage utilizing as yet unused topside sounder data from the Alouette and ISIS missions (Bilitza); an important consideration will be the use of a field-aligned coordinate in place of the presently used vertical height.
Electron density - middle ionosphere:
A data intensive study of the E valley region was presented based on Arecibo incoherent scatter data base (Mahajan et al.). The task group decided to use the Mahajan results for a better representation of the E valley parameters at low latitudes. Incoherent scatter data (from Malvern, UK) were also used for an assessment of foE during nighttime (Bradley). Discrepancies were found for the dawn periods and will be corrected in the next version of IRI. Digisonde data were used for a look at the representation of the half-density point (Ne= 0.5 * NmF2) in IRI. A persistent discrepancy was observed in the early morning hours, which will need further investigation (Reinisch).
Electron density - lower ionosphere:
The limited pool of reliable data for this region remains the major road block towards a better representation of seasonal, diurnal, and solar cycle variations. Based on the study of a large amount of rocket data (compilations by Danilov and by Friedrich) it was also proposed to include the effect of the Winter Anomaly and of stratospheric warmings in the IRI D-region model (Danilov and Smirnova). Further studies are solicited for the 1993 IRI workshop, where a dedicated half-day session should lead to a group consensus on future improvements for this critical region.
Electron density - F peak mapping:
Results were presented of two international projects: VIM (Verification of Ionospheric Models, URSI G.4, Reinisch) and SUNDIAL (NSF, Szuszczewicz). Concerning the F peak density, in general, good agreement was found between these ionosonde data and IRI/CCIR. It is clear, however, that a better representation of the solar cycle variation would be of great benefit.
Comparisons with the large DE data base confirmed the overall excellent representation of electron and ion temperatures in IRI (Hoegy and Grebowsky). Insitu measurements of the Japanese Hinotori satellite show that the smoothed diurnal structure of the topside electron temperature omits important features, in particular the early morning peak ("morning overshoot") (Oyama). The task group decided that the Hinotori results should be incorporated into the next edition of IRI (Bilitza, Oyama). Data from the Japanese Akebono satellite could be of help in describing the plasmaspheric electron temperature (Oyama).
Several studies pointed to two of the major shortcomings of the present IRI ion composition model: (i) the height of the transition from O+ to H+ and He+ ions is too low (DE: Hoegy and Grebowsky; OGO-6, ISS-b, Taiyo: Kutiev and Stankov); (ii) cluster ions and negative ions are not yet included (Danilov and Smirnova). A modelling approach based on the characteristic transition heights (light ions <-> O+ <-> molecular ions <-> cluster ions <-> negative ions) may help to alleviate both of these shortcomings (Bilitza, Kutiev). Global maps exist for the upper two transition heights, but more data are needed for the lower two. Modelling efforts are also concentrating on the molecular ion ratio NO+/O2+ in the E region and on the cluster-to-molecular ion ratio in the D region (Kopp; Friedrich).
Membership and meetings:
Dieter Bilitza was elected as new Chairman of the IRI task group and K. Oyama as Vice-Chairman. T. Gulyaeva remains as the URSI Vice-Chairman. Several new members were accepted into the task group: M.A. Abdu, INPE, Brazil; S. Radicella, ICTP, Italy/Argentina; T. Maruyama, Hiraiso, CRL, Japan; R. Hanbaba, CNET, France; P. Kishcha, IZMIRAN, Russia and P. Wilkinson, IPS, Australia.
IRI workshops and related meetings: "Ionospheric Prediction and Modeling", Kyoto, Japan, URSI 1993 General Assembly; "Off-Median Phenomena and IRI", Trieste, Italy, October 19-22, 1993; "The High-Latitudes in the IRI", Hamburg, Germany, 2 days during the 1994 COSPAR General Assembly.