A method to ingest GPS-TEC into the NeQuick ionospheric model

This paper presents a method to ingest Total Electron Content measurements from ground-based GPS receivers into the empirical NeQuick model. The method here presented relies upon optimizing the parameter that primarily drives the NeQuick profile, i.e., the electron density of the F2 peak, NmF2. The effectiveness of the method is assessed in a rather benevolent ionospheric scenario: a midlatitude region and quiet geomagnetic days that cover solstices and equinoxes conditions during a medium-high solar activity year. Thus, the procedure demonstrated to be capable of improving the climatological value of NmF2 computed from the Radioscience Section of the International Telecommunication Union (ITU-R) database. This capability was assessed by comparing the ITU-R value and the corrected value produced by our method to the value measured with a Digisonde. The result of this comparison was an overall reduction of the error of the NmF2 parameter to approximately half of its original value.Facultad de Ciencias Astronómicas y Geofísica

Efecto del campo geomagnético en la predicción de la frecuencia de apantallamiento de las E-esporádicas

Se utiliza un estudio realizado anteriormente del comportamiento de la frecuencia de apantallamiento de las capas esporádicas (fbEs) para estaciones ionosféricas de la red americana de sondadores, para dos años de baja y alta actividad solar: 1964 ͞(R=10) y 1967 (͞R=94), respectivamente. Los resultados obtenidos indican la necesidad de tener en cuenta una variación con la latitud geomagnética en el modelo de predicción de fbEs, que en su fórmulación actual tiene en cuenta únicamente el ángulo cenital solar, la actividad solar y la latitud geográfica. La nueva expresión obtenida provee valores de predicción de fbEs con un error menor que con el modelo anterior, para latitudes geomagnéticas medias.A former study on the behavior of the sporadic-E blanketing frequency (fbEs) for the american chain of ionospheric station, for two years of low and high solar activity: 1964 (͞R=10) and 1967 (͞R=94) respectively, has been used. The results obtained indicate the need of taking into account the variation with the geomagnetic latitude in the prediction model for fbEs. In the present form it takes into consideration only: solar Zenith angle, solar activity and geographic latitude. The new expression obtained gives fbEs prediction values with a lower error than the former, for middle geomagnetic latitudes.Asociación Argentina de Geofísicos y Geodesta

Variations of foF2 and GPS total electron content over the Antarctic sector

This paper presents a preliminary analysis of the variations of the critical frequency of the F2 region (foF2) and the total electron content (TEC) derived from Global Positioning System (GPS) data. Hourly foF2 values were scaled from ionograms recorded at San Martin (68.1°S, 293.0°E) and the TEC values were derived from GPS observations at O'Higgins (63.3°S, 302.5 °E). The database includes measurements obtained under different seasonal and solar activity conditions. The study shows that the daily peak of foF2 occurs around local noon in winter and fall, and in spring a secondary peak is observed around midnight. In summer (January) foF2 reaches its minimum value around the noon sector while the maximum in the diurnal variation of foF2 is located in a time sector close to midnight. This behaviour is observed at low and high solar activity. The semiannual anomaly appears around noon at high and low solar activity and the winter anomaly is not observed. The effect of the solar activity is generally observed in every season. The analysis of the GPS TEC measurements in the same region indicates that the diurnal, seasonal and solar activity variations are similar to those observed in the foF2 values. An analysis of the performance of the IRI model to predict foF2 is also shown using the two IRI options (URSI and CCIR). The comparisons between the experimental values and the IRI predictions show some discrepancies.Facultad de Ciencias Astronómicas y Geofísica

Electron density profile modeling

The Base Point Model (BPM) is used to model the electron density (N) profile in the ionosphere, This model assumes two Chapman profile expressions one for the bottomside and one for the topside, and requires a characteristic point called "F region base point". The comparison among the modeled and experimental bottom-side N profiles obtained from Tucuman (26,9°S; 65.4°W) ionosonde shows that, in general, there is a very good agreement within 30 km below the height of the maximum N(hm). Cases with a very good agreement for the entire N-profile are observed. The study of the electron content below hm and the Total Electron Content (TEC) measured over Tucuman shows that, the difference among predicted and measured TEC is due to the disagreement in the topside N-profile more than that observed in the bottomside N-profile

Checking the new IRI model: The bottomside B parameters

Electron density profiles obtained at Pruhonice (50.0, 15.0), El Arenosillo (37.1, 353.2) and Havana (23, 278) were used to check the bottom-side B parameters BO (thickness parameter) and B1 (shape parameter) predicted by the new IRI - 2000 version. The electron density profiles were derived from ionograms using the ARP technique. The data base includes daytime and nighttime ionograms recorded under different seasonal and solar activity conditions. Comparisons with IRI predictions were also done. The analysis shows that: a) The parameter B1 given by IRI 2000 reproduces better the observed ARP values than the IRI-90 version and b) The observed BO values are in general well reproduced by both IRI versions: IRI-90 and IRI-2000

VTEC behavior in the American sector during high solar activity

The behavior of the vertical total electron content (VTEC) obtained from GPS signals received during the high solar activity year 1999 at stations placed in the American sector, is reported. The considered latitude range extends from 18.4 to -64.7 and the longitude ranges from 281.3 to 297.7. Median, lower and upper quartiles are used to specify variability, because they have the advantage of being less affected by large deviations that can occur during magnetic storms. The results show that the VTEC values corresponding to equinox are greater than those of solstice and that, the highest VTEC values are observed at low latitude stations. In general, the variability during daylight hours is about 30% of median or less, and that observed for nighttime hours is greater than the mentioned percentage, particularly at last hours of the night near the northern peak of the equatorial anomaly

Day-to-day changes in experimental electron density profiles and their implications to IRI model

The electron density variability at fixed heights is studied for use in the International Reference Ionosphere IRI model. Monthly median, upper and lower quartile values were obtained f o F2, hmF2, B0 and B1 as deduced from ionograms. The IRI electron density profiles established with these values were then compared with the median and quartiles at fixed heights. Results are shown for the three ionospheric stations E1 Arenosillo (37.1 N, 353.3 E), Tucuman (26.9 S, 294.6 E) and San Juan (31.5 S, 290.4 E) as a function of solar activity, season and local time. As found by other authors the height of maximum variability, hvmax, is located below the peak electron density height (hmF2) in all the cases. Values of differences between hmF2 and hvmax. are analized. Variability defined as the interquartile difference and hvmax results are calculated from experimental electron density profiles measured at the three stations.Fil: Amarante, G. Miró. Centro Internacional Abdus Salam de Física Teórica​; ItaliaFil: Santamaría, M. Cueto. Universidad Complutense de Madrid; EspañaFil: Mosert, Marta Estela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "El Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "El Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "El Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "El Leoncito"; ArgentinaFil: Radicella, Sandro María. Centro Internacional Abdus Salam de Física Teórica​; ItaliaFil: Ezquer, Rodolfo Gerardo. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Ionósfera; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentin

Are drought and wilfires turning Mediterranean cork oak forests into persistent shrublands ?

In the Iberian Peninsula Mediterranean oak forests have been transformed into a mosaic landscape of four main patch-types: forests, savannas, shrublands and grasslands. We used aerial photographs over a period of 45 years (1958–2002) to quantify the persistence and rates of transitions between vegetation patch-types in southern Portugal, where cork oak is the dominant tree species. We used logistic regression to relate vegetation changes with topographical features and wildfire history. Over the 45 years, shrublands have been the most persistent patch-type (59%), and have been expanding; forests are also persistent (55%) but have been decreasing since 1985; savannas and grasslands were less persistent (33% and 15%, respectively). Shrublands persistence was significantly correlated with wildfire occurrence, particularly on southern exposures after 1995. In contrast, forest persistence decreased with wildfire occurrence, and forests were more likely to change into shrublands where wildfire had occurred after 1995