* International Reference Ionosphere * IRInfo No. 19 *

The 2-day session reviewed modeling efforts in the topside ionosphere and plasmasphere with special emphasis on improvements of the International Reference Ionosphere (IRI) in these regions. As always these sessions organized by the IRI Working Group are also an opportunity to discuss other IRI-related topics. The session began with a talk by Bilitza (USA) describing the new IRI-2000 and detailing the many improvements over the previous version of the model. The first day of the session was primarily focused on the topside and plasmasphere. Leitinger (Austria) used theoretical considerations in conjunction with empirical models for the O⁺ scale height and gradient and the O⁺/H⁺ transition height based on the work of Titheridge (New Zealand). Pulinets (Russia) and Radicella (Italy) are using a model approach that is based on Epstein layer functions with a height-varying thickness parameter fitted to Interkosmos 19 topside sounder data. Gulyaeva (Russia) extrapolated ISIS data to plasmaspheric heights with the help of the SMI-96 model. A method for deducing Total Electron Content (TEC) from ionosonde measurements was presented by Reinisch (USA). He found good agreement with TOPEX-deduced TEC. Pavlov (Russia) using Millstone Hill data in conjunction with his theoretical model finds good agreement when applying the Carpenter-Anderson (1992) model for the boundary conditions at the magnetic equator. Ezquer (Argentina) found that IRI underestimated the measurements of the Japanese Hinotori satellite at 600 km altitude when using the CCIR or URSI F peak models and good agreement when using measured (ionosonde) peak parameters. Iwamoto (Japan) in comparisons with ISS-b measurements noted the unrealistically large values that IRI predicts at high latitudes during very large solar activities; this ‘high-density spot’ was also mentioned by several other speakers as one of the areas where future IRI improvements are essential. Gallagher (USA) presented his new Global Core Plasmasphere Model (GCPM) and discussed ways of connecting it to the IRI model. The GCPM is primarily based on data from the DE-1 spacecraft (RIMS, PWI data). The model merges with IRI at about 500 km altitude. One of the results of the European COST initiative is the PLES model for ionospheric characteristics developed by Stanislawska (Poland) and colleagues. Truhlik (Czech Republic) studied solar activity changes of the topside electron temperatures and density using data from the Interkosmos and Hinotori satellites. Comparing their Akebono plasmasphere temperature data (1989-1998) with simulations, Oyama and Abe (Japan) find that they need an additional energy source at the top of the magnetic field line to explain the measured values. From the huge amount of data accumulated since 1989, they made an animation showing the variation of electron temperature with solar activity as well as local time. Triskova (Czech Republic) utilized ion composition data from the Intercosmos 24 and Atmosphere Explorer satellites to propose improvements of the current IRI ion composition model. Bhuyan (India) found that H⁺ and O⁺ measurements of the Indian SROSS C2 satellite compared well with theoretical computations but differed considerably from the IRI predictions. He also pointed out shortcomings of the IRI electron and ion temperature models based on SROSS RPA data.

During the 2^nd day improvement for areas other than the topside/plasmasphere were discussed. For the D-region Friedrich (Austria) presented a new model that combines his auroral and non-auroral models. The high latitude D-region was also the subject of a paper by Danilov and Smirnova (Russia). Their model includes dependences on solar zenith angle and magnetic activity. Data from the German/Indian DEOS rocket campaign were reported by Thieman and Steigies (Germany) showing good agreement with the IRI predictions. The behaviour of F1 region parameters was studied with ionosonde data by Mosert (Argentina) and Scotto (Italy). Combining IRI and GPS data for space weather applications was the topic of a talk by Juan (Spain). Another interesting application of IRI is as background ionospheric model for testing different methods of deducing TEC from GPS and NNSS measurements (Ciraolo and Spalla,Italy). Gonzalez (USA) showed that the Arecibo and Jicamarca incoherent scatter radars are good tools for measuring topside parameters from the ground (300-2000km) and he highlighted the data sets and measurement programs that could be of help for the IRI modeling effort. Bradley (U.K.) reviewed the European PRIME (Prediction Regional Ionospheric Modelling over Europe) and the IITS (Improved Quality of Service in Ionospheric Telecommunication Systems Planning and Operation) projects promoted by the European Union as part of its COST (Co-operation in the Field of Scientific and Technical Research) program from 1990 to 1994 and 1995 to 1999 respectively.

During the ‘Final Discussion’ session at the end of the 2^nd day the status and progress of the various IRI sub-tasks was reviewed and decisions were made regarding the post-2000 updates of IRI.

D-Region: The relatively small data volume remains one of the biggest hurdles in modeling this region. The IRI team has assembled all available, reliable rocket data and is at the lookout for more. Modeling efforts are continuing in Graz, Austria (Friedrich), Moscow, Russia (Danilov), and Bern, Switzerland (Kopp).

E-Region: Based on the approach presented at this meeting Fuller-Rowell (USA) proposed a parameterized model of the E region enhancement due to precipitating energetic electrons in the auroral oval. He plans to present a candidate model at the next IRI workshop. Efforts to improve the description of the E peak density at night and the E peak height especially at dawn and dusk continue (Titheridge, New Zealand; Mahajan, India using radar data). It was decided that it would be a worthwhile goal to include Sporadic E occurrence statistics in IRI for VHF users. The ‘older’ CCIR maps and recent ITU-R recommendations will be reviewed for this purpose (Bradley, U.K.; Radicella, Italy).

F-Region: IRI-2000 is going an important step forward with the inclusion of the storm-time updating algorithm for the F peak density developed by Fuller-Rowell et al. (USA). It describes quite well the Ap response of the F peak density during mid-latitude summer. The next steps are now the representation of storm effects on the F peak height and improvements of the peak density algorithm (Fuller-Rowell, Condrescu, USA; Pavlov, Karpachev, Russia). The average behavior of the quiet-time mid-latitude F peak parameters is well represented in IRI. The focus is now on a better representation of the specific features at low latitudes (equator anomaly) and at high latitudes (mid-latitude trough, polar hole etc.). A special task group is working on the development of a Spread-F occurrence statistics model for IRI (Abdu, Brazil). This will be one of the main topics of the 2001 IRI Workshop in Brazil.

Topside: Several modeling efforts are underway as reported at this meeting and should lead to a new topside model for IRI in the near future (Bilitza, USA; Leitinger, Austria; Radicella, Italy; Pulinets, Russia). Meanwhile it was decided to restrict the current IRI model to a value of Rz12=100 to avoid the unrealistic steep topside gradients that were reported at high latitudes during very high solar activities.

Plasmasphere: The merging of Gallagher’s GCPM model with IRI will be studied to provide IRI users with the possibility of a plasmaspheric extension (Gallagher, Bilitza, USA).

Variability: A quantitative description of the variability of ionospheric parameters is high on the wish-list of many users; a user wants not only to know the average value s/he can expect but also the standard deviation from this average. This will be one of the main topics of the IRI Task Force Activity that is organized annually by S. Radicella at the International Center for Theoretical Physics (ICTP) in Trieste, Italy. First steps were made by Kouris (Greece) and Ezquer (Argentina) as reported at this meeting .

Plasma temperatures: Inclusion of solar activity effects especially at higher altitudes is of primary interest for the IRI temperature models. Work towards this goal is progressing with data from the Intercosmos, ISIS, Akebono, and Hinotori satellites and with theoretical models (Truhlik, Czech Republic; Oyama, Japan).

Ion composition: The primary goal is the development of global models for the ion transition heights (from light ions down to O⁺ ions, to molecular ions, to Cluster ions, and finally to negative ions. These transition heights will then be used as anchor points for the new IRI ion composition model. (Triskova, Czech Republic; Grebowsky, USA; Danilov, Russia).

Miscellaneous: The IRI Workshop is now planned for the week of June 25 to 29, 2001 at the Instituto Nacional de Pesquisas Espaciais (INPE) in Brazil. The topic will be the ‘Representation of the Equatorial Region in IRI’ and the Local Organizer will be M. Abdu. Prof Kouris from the University of Thessaloniki, Greece was accepted as a new member into the IRI Working Group. The IRI team thanks K-I. Oyama and the Institute for Space and Aeronautics (ISAS, Japan) for the continued efforts and support in publishing the quarterly IRI Newsletter.