International Reference Ionosphere Workshop 2006:

New Measurements for Improved IRI TEC Representation

 

Buenos Aires, Argentina, October 16-20, 2006

 

Report / Dieter Bilitza

 

The 2006 IRI Workshop was held at the hotel “El Conquistador” in the heart of Buenos Aires exceptionally well organized by Dr. M. Mosert with her team from CASLEO, San Juan and with help from the ionospheric groups from the Universidads Nacional of Tucuman and La Plata. The close to 60 participants represented many countries (Argentina, Czech Republic, Italy, USA, Spain, Russia, Austria, Peru, Cuba, South Africa, Brazil, Nigeria, and Mexico) and many different data sources (ionosondes, GPS, incoherent scatter radars, TIMED, DMSP, Hinotori, Akebono and a few other satellites).

The week long meeting was divided in sessions entitled “TEC Data and Modeling”, “Topside”,  “Lower Ionosphere”, “Disturbed Ionosphere”, “Data Sources for IRI”,  “F Peak and Bottomside”, “Temperatures”, and “Posters”.

The meeting was financially supported by the Committee on Space Research (COSPAR), the International Union of radio Science (URSI), the US National Science Foundation (NSF), the US Air Force  Office of Scientific Research ( AFOSR), the Abdus Salam International Center for Theoretical Physics (ICTP), the Italian Instituto Nazionali di Geofisica e Vulcanologia (INGV), the Italian Embassy in Argentina, and several Argentine institutions including Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Secretaria de Ciencia Tecnologia e Innovacion Productiva – Agencia Nacional de Promocion Cientifica y Technologica (SECYT-ANPCT), Complejo Astronomica “El  Leoncito” (CASLEO), Universidad Nacional de San Juan (UNSJ), Facultad Regional Tucuman, Universitad Tecnologica Nacional (FRT-UNT), Universidad Nacional de Tucuman (UNT), Facultad de Cienccias Astronomicas y Geofisicicas de la Plata - Universidad Nacional La Plata (FCAGLP-UNLP), Fundacion Para el Avance de la Ciencia Astronomica (FUPACA), Congreso de la Nacion Argentina, Gobierno de la Provincia de San Juan - Ministero de Infrastructura y Tecnologia, and  Ana Allende Trust. The workshop was also the backdrop for the signing of an Italian-Argentine technology transfer agreement, which will provide the ionospheric group in Tucuman with an Italian-build ionosonde. This complements a US Air Force Digisonde that is being installed in San Juan.  Memorable highlights were a tour of the city of Buenos Aires, the Workshop Banquet in one of Buenos Aires oldest Tango Dinner Theaters, and a visit to the Argentine Congress. More details are available on the workshop home page http://www.casleo.gov.ar/IRI2006/

 

Lower Ionosphere

 

McKinnell (South Africa) presented a new version of her NeuralNet IMAZ model for the auroral lower ionosphere. It now uses solar zenith angle and Ap index as input parameters and includes a special version that runs without the absorption value input that is required for the standard model. This version is now scheduled to be included in IRI-2006. Friedrich (Austria) introduced his first attempt at representing the electron density in the polar cap using the NN technique. The model reaches up to the F-region and describes variations with day-of-year, altitude, Kp, F10.7, and solar zenith angle based on 6 years of EISCAT Svalbard incoherent scatter measurements and on 371 profiles from Heiss Island rocket measurements. Comparisons with IRI indicate that the extrapolation of IRI into the polar cap produces values that correspond to fairly disturbed conditions.

 

Bottomside

 

A number of papers dealt with the three parameters (B0, B1, D1) that define the shape of the bottomside electron density profile using ionosonde data from several European and South African stations (Buresova, Czech Republic) and from Ilorin, Nigeria (Adeniyi, Nigeria). The long-term goal is to replace the current tabular form of the IRI model with appropriate mathematical functions similar to what had been proposed at the Beijing IRI session by Altadill (Spain) for the seasonal and diurnal variations of these parameters. Reinisch and Huang (UML, USA) noted abrupt changes in the F1 region electron density due to discontinuities in the model for the F1 layer thickness parameter D1 and suggested improvements. Coisson, Radicella, Nava (Italy), Adeniyi (Nigeria), and Savio (Cuba) note that in some cases the large bottomside profiles observed at low latitudes (Ascension Islands, Jicamarca, Ilorin) are severely underestimated by IRI even to the point of affecting the TEC. Correction of this problem will be an important element of the new B0 modeling initiative.

 

F peak

 

IRI currently includes two options for the F2 peak frequency (foF2), CCIR-67 is recommended for the continents and URSI-88 for the ocean areas. A large volume of ionosonde data has accumulated since these models were build and the IRI team has encouraged efforts to establish an improved model for the whole globe. Oyeyemi and McKinnell (South Africa and Nigeria) presented first results of such a new modeling attempt. They trained a Neural Net (NN) with foF2  from 85 global stations covering the years 1976 to 1986 and 1995 to 2005. Data were obtained through NGDC-SPIDR, UML-DIDBase, and IPS-services. Input parameters are day-of-year, UT, solar zenith angle, geographic latitude, magnetic inclination and declination, and magnetic and solar index. First result are very promising and indicate better performance than the CCIR and URSI NmF2 models currently used in IRI. The authors asked for more data to further improve their model and were promised new inputs from the Brazilian ionosondes and from topside sounder satellites. One hindrance in earlier modeling efforts were data quality problems with the long-term ionosonde data record. The IRI team had contacted the NGDC to point out specific problems with ionosonde data from their SPIDR system. Denig and Redmon (USA) representing NGDC at the meeting reported that many of these problems have now been resolved or are being worked on actively. Ezquer (UNT, Argentina) studied data from 14 South and Middle American ionosonde stations and found that the sunrise minimum in foF2 is often shifted by 1 or even 2 hours compared to the CCIR and URSI foF2 model; fewer such cases are seen with the CCIR model. A likely cause could be the use of a sector Local Times instead of Solar Local Times for the ionosonde data that were used in developing the CCIR and URSI maps.

 

Souza, Abdu and Batista (Brazil) presented a new version of their spread F occurrence probability model for Brazilian longitudes. The new model uses Bernstein polynomials as base functions, a higher resolution with solar activity, and assumes latitudinal symmetry with respect to the magnetic equator. Comparisons with measurements during the Conjugate Point Equatorial Observational Campaign (COPEX) show good agreement. This latest version of the model will become part of IRI-2006. The group plans to extend their model to other longitudes using topside sounder data and other ionosonde data specifically from the Indian subcontinent.

 

Efforts continue to include a description of ionospheric variability (quartiles, deciles) in IRI. Ezquer (UNT, Argentina) extended these studies to Antarctic latitudes with data from the Argentine Ellsworth station finding largest relative variability in winter (dark ionosphere) and data distributions that are skewed towards lower values. The ionospheric group of the Instituto de Gefisica y Astronomia in Havanna, Cuba has been very active in this area using a global set of ionosonde data. Lazo (Cuba) presented an overview of the activities of this group and a good summary of their variability work. Here the improved data quality of the SPIDR data will be also of great benefit.

 

Topside

 

With the availability of real-time TEC from many GPS receivers on the ground and aboard satellites, e.g., the recently launched COSMIC constellation, updating of IRI with TEC measurements is a topic of great interest. The ingest procedure of Nava, Radicella and Coisson (Italy) adjusts the IRI slant TEC to measured values with the help of an effective solar index. Meza, Gularte, Brunini (UNLP, Argentina) and Mosert (CASLEO, Argentina) presented a scheme for deducing Vary-Chap parameters for the topside profile from combining GPS and ionosonde data. The Vary-Chap approach as introduced by Reinisch, Nsumei, and Huang (UML, USA) represents the topside profile with a modified Chapman-function assuming a variable scale-height. Reinisch (UML,USA) presented the most recent results of the UML group including seasonal and latitudinal variation of Vary-Chap parameters deduced from ISIS topside sounder data. One of the Vary-Chap parameters is the scale height at the F-peak. Altadill (Spain) presented an empirical model for this parameter based on one solar cycle worth of data from the Ebro Digisonde. Scale heights obtained with the Grahamstown ionosonde were studied by Nambala, McKinnel, and Oyeyemi (South Africa). The scale height is largest in summer and lowest in winter and reaches its diurnal maximum during mid-day and exhibits a close correlation with the bottomside thickness parameter B0 and with the slab thickness. A 4-D modeling of the ionospheric electron density based on IRI and GPS measurements was described by Schmidt (Germany), Bilitza (GSFC, USA), and Shum (OSU, USA). Garner (UTD, USA) is undertaking a very comprehensive analysis of the large DMSP data set of electron density measurements at 850 km altitude. Of the 3 DMSP instruments measuring electron density (Retarding Potential Analyzer, Scintillation Cup, Ion Drift Meter), the RPA seems to give the most reliable results. Depuev (Russia) and Pulinets (Mexico) described a data base of more than 8,000 manually scaled topside ionograms from the Intercosmos-19 satellite covering the high solar activity years 1979 to 1981 and of about 2000 ionograms from Cosmos 1809 for the low solar activity year 1989. This will be an excellent data source for the foF2 modeling effort of the South African group (see previous section)

 

TEC and GPS

 

Hernandez-Pajares, Juan, and Sanz (Spain) described their technique for estimating medium-scale (period<20 min) traveling ionospheric disturbances (MSTIDs) from GPS measurements and a first assessment of the occurrence probability of MSTIDs. Maps of VTEC for South America are produced hourly by the GESA laboratory of La Plata University (Brunini, Meza, Gends, Azpilicueta, UNLP, Argentina) using data from all available GNSS receivers and applying a special de-biasing procedure. Ionosonde and GPS data from Brazil show that IRI underestimates the EA intensity and the TEC during nighttime in the Brazilian sector (Abdu et al., Brazil) for all levels of solar activity. Fuller-Rowell, Araujo-Pradere, and Codrescu (SEC, USA) estimate that even with a dense network of GPS dual-frequency ground receivers (like the more than 600 receivers in the US) an uncertainty of 2-4 TECU remains in the determination of real-time TEC. Garner (UTD, USA) presented the ARL:UT GPS toolkit (GPSTk) and described ARL’s Ionospheric Data Assimilation Three Dimensional (IDA3D) model that accepts data from ground and satellite GPS receivers, from satellite beacons, from TOPEX, from insitu measurements, and from ionosondes. He also presented first results from the recently launched COSMIC beacons (CERTO) and cautioned that all TEC measurements contain an unknown bias and are best used for studying changes in TEC. Pulinets (Mexico) studied the response of GPS-TEC to positive D_st pulses (solar flares, magnetopause currents) noting strong positive deviations up to 50%. 

 

TOPEX ionospheric data are a valuable data source for studying VTEC (Azpilicueta, Meza, and Brunini, UNLP, Argentina), however, it is important to agree on a common method for averaging and grouping the TOPEX (and other satellite altimeters, like Jason and Envisat) data before applying this analysis to IRI (Radicella, Italy). Migoya, Ezquer (UNT, Argentina), and Radicella, Coisson (Italy) used TOPEX data to evaluate the new IRI-NeQuick option against the older IRI-2001 topside model. Both models show good agreement with the data with IRI-2001 producing slightly better results.

 

ITEC measurements by Digisondes based on the assumption of a Chapman topside layer were compared with GPS TEC measurements for stations in Spain (Mosert, CASLEO, Argentina) and South Africa (Paradza, McKinnell, Opperman). The difference between the two is the plasmaspheric electron content which is about 2 -3 TECU and can reach up to 20-30% of the TEC at nighttime during low solar activity.

 

Temperatures

 

Bilitza (GSFC/Raytheon, USA), Richards (NASA, USA), Truhlik and Triskova (Czech Republic) studied the solar activity variation of the topside electron temperature and found discrepancies between DMSP measurement, the Millstone Hill ISR model, and the FLIP model, most importantly unrealistically high temperatures at low solar activities and ISR data that are consistently lower than DMSP and FLIP values. In a follow on talk by the same authors, Truhlik described a method for including solar activity variations in a new IRI electron temperature model. Rocket measurements of abnormally large electron temperatures in the equatorial ionosphere over Brazil were reported by Muralikrishna (Brazil).

 

Data Sources for IRI

 

Representatives from the Jicamarca and Arecibo Incoherent Scatter Radar (ISR) facilities reviewed the capabilities of their ISR systems and the potential use of their data for IRI improvements. Ilma and Chau (Peru) find that IRI underestimates Jicamarca ISR topside electron densities during daytime and overestimates the ISR densities during nighttime. Aponte (NAIC, USA) described recent developments at the Arecibo ISR and presented examples of comparisons with IRI. IRI reproduces qualitatively the general features of the topside ionosphere over Arecibo with best results for the ion composition and ion temperature. A comprehensive evaluation of the IRI-2006 model with the many years of Jicamarca and Arecibo data is highly encouraged and would surely result in major improvements of the IRI model. Rich (AFGL, USA) announced the availability of a number of data sets from his website https://swx.plh.af.mil (password required) including DMSP ionospheric data from 1987 to present, CHAMP Ni, Te at 400 km, Apex Ni, Te from 1994 to 1997.   

 

Denig and Redmon (NGDC, USA) reported on the restructuring of the ionosonde program within NGDC and on efforts to restore the confidence in the quality of the SPIDR ionospheric data. They will work closely with the IRI team, who had brought up these data quality problems at its 2003 Workshop. Several of these issues are now corrected.

 

Krause (USAFA, USA) described a project that the US Air Force Academy (USAFA) is in the process of developing to investigate the southern crest of the Equatorial Anomaly in the region near San Juan, Argentina. It consists of a series of ground based (Digisonde, GPS receivers) and space based (FalconSAT-3, FalconSAT-4) experiments. The use of TIMED/GUVI and DMSP/SSUSI electron density measurements for improvements of the IRI model was discussed by DeMajistre, Paxton, and Kil (JHU/APL, USA), with special emphasis on the F peak height and the O⁺ topside content.