A different interpretation of the annual and semiannual anomalies on the magnetic activity over the Earth

The H component of the magnetic field measured at the terrestrial surface presents several periodic signals caused by changes in the ring current that flows within the terrestrial magnetosphere. One of the most important of them is associated to the phenomenon known as the Semiannual Anomaly which produces two significant minima during the equinoxes. This phenomenon is global, i.e., every observatory registers a similar effect independently of the hemisphere where it is located. A second important signal is due to the phenomenon known as the Annual Anomaly that produces significant different values for solstices, with a particular feature: the effect depends on the hemisphere where the observatory is located, with maximum during local summer. In spite of the time since their discoveries (more than a hundred years ago) the physical processes behind them are still open to discussion. In this work we present a new physical interpretation for the combined effects of both anomalies. The main concept developed is that along the year the shape of the magnetospheric cavities within which the ring current flows is deformed according to the geometric configuration between the solar wind and the magnetosphere.Fil: Azpilicueta, Francisco Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Brunini, Claudio Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentin

Sistema y marco de referencia terrestre

El Sistema de Referencia Terrestre Internacional (ITRS) resulta del esfuerzo mancomunado y sostenido en el tiempo de la comunidad geodésica internacional. Su última realización, el ITRF 2014, proporciona la mejor referencia geodésica global para el estudio de numerosos problemas geofísicos y para la operación de muchas misiones espaciales.Facultad de Ciencias Astronómicas y Geofísica

A different interpretation of the annual and semiannual anomalies on the magnetic activity over the Earth

The H component of the magnetic field measured at the terrestrial surface presents several periodic signals caused by changes in the ring current that flows within the terrestrial magnetosphere. One of the most important of them is associated to the phenomenon known as the Semiannual Anomaly which produces two significant minima during the equinoxes. This phenomenon is global, i.e., every observatory registers a similar effect independently of the hemisphere where it is located. A second important signal is due to the phenomenon known as the Annual Anomaly that produces significant different values for solstices, with a particular feature: the effect depends on the hemisphere where the observatory is located, with maximum during local summer. In spite of the time since their discoveries (more than a hundred years ago) the physical processes behind them are still open to discussion. In this work we present a new physical interpretation for the combined effects of both anomalies. The main concept developed is that along the year the shape of the magnetospheric cavities within which the ring current flows is deformed according to the geometric configuration between the solar wind and the magnetosphere.Facultad de Ciencias Astronómicas y Geofísica

El Marco de Referencia Geodésico Global (GGRF)

Sumario: El rol del GGRF en la IDE ¿Qué implica el GGRF? Materialización del GGRF ¿Cómo se accede al GGRF? El Sistema de Referencia Geocéntrico para las Américas (SIRGAS) Infraestructura SIRGAS Resolución de UN sobre el GGRF Los instrumentos de AGGO HistoriaInfraestructura de Datos Espaciales de la República Argentina (IDERA)Facultad de Ciencias Astronómicas y Geofísica

GPS slant total electron content accuracy using the single layer model under different geomagnetic regions and ionospheric conditions

The use of observations from the Global Positioning System (GPS) has significantly impacted the study of the ionosphere. As it is widely known, dual-frequency GPS observations can provide very precise estimation of the slant Total Electron Content (sTEC?the linear integral of the electron density along a ray-path) and that the precision level is bounded by the carrier-phase noise and multi-path effects on both frequencies. Despite its precision, GPS sTEC estimations can be systematically affected by errors in the estimation of the satellites and receivers by Inter-Frequency Biases (IFB) that are simultaneously determined with the sTEC. Thus, the ultimate accuracy of the GPS sTEC estimation is determined by the errors with which the IFBs are estimated. This contribution attempts to assess the accuracy of IFBs estimation techniques based on the single layer model for different ionospheric regions (low,mid and high magnetic latitude); different seasons (summer and winter solstices and spring and autumn equinoxes); different solar activity levels (high and low); and different geomagnetic conditions (quiet and very disturbed). The followed strategy relies upon the generation of a synthetic data set free of IFB, multi-path, measurement noise and of any other error source. Therefore, when a data set with such properties is used as the input of the IFB estimation algorithms, any deviation from zero on the estimated IFBs should be taken as indications of the errors introduced by the estimation technique. The truthfulness of this assessment work is warranted by the fact that the synthetic data sets resemble, as realistically as possible, the different conditions that may happen in the real ionosphere. The results of this work show that during the high solar activity period the accuracy for the estimated sTEC is approximately of ±10 TECu for the low geomagnetic region and of ±2.2 TECu for the mid-latitude. During lowsolar activity the accuracy can be assumed to be in the order of ±2 TECu. For the geomagnetic high-disturbed period, the results show that the accuracy is degraded for those stations located over the region where the storm has the strongest impact, but for those stations over regions where the storm has a moderate effect, the accuracy is comparable to that obtained in the quiet period.Fil: Brunini, Claudio Antonio. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Azpilicueta, Francisco Javier. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentin

The geomagnetic semiannual anomaly on the four <i>Dst</i>-fundamental observatories: Dependences with Sun-Earth physical parameters

The semiannual anomaly (also known as semiannual variation) on the magnetic activity is a phenomenon that produces clear minima during March and September and maxima in June and December on the horizontal components of the geomagnetic field. This phenomenon has been known since the middle of the nineteenth century, but in spite of the accumulation of measurements and the development of three theoretical models, a conclusive physical explanation for it has not been developed. The usual approach to study the semiannual anomaly is by means of geomagnetic indices like the disturbance storm time, Dst, which is based on combining measurements registered on four magnetic observatories. This work follows a different approach based on the raw horizontal components registered at the four observatories. The analyses performed aimed to study and assess the impact of several external parameters, characteristics of the Sun-Earth environment, on the semiannual anomaly. The influence of the global geomagnetic activity level, the solar activity level, the solar magnetic polarity, and the rising/declining phase of the solar radiation cycle is analyzed in detail. The most important finding is that the semiannual anomaly is always present and that none of the previously mentioned parameters significantly favor the development of it. A second result is the presence of a 27 day signal superposed to the semiannual anomaly which is significantly affected by the solar activity level.Facultad de Ciencias Astronómicas y Geofísica

The geomagnetic semiannual anomaly on the four Dst-fundamental observatories: Dependences with Sun-Earth physical parameters

The semiannual anomaly (also known as semiannual variation) on the magnetic activity is a phenomenon that produces clear minima during March and September and maxima in June and December on the horizontal components of the geomagnetic field. This phenomenon has been known since the middle of the nineteenth century, but in spite of the accumulation of measurements and the development of three theoretical models, a conclusive physical explanation for it has not been developed. The usual approach to study the semiannual anomaly is by means of geomagnetic indices like the disturbance storm time, Dst, which is based on combining measurements registered on four magnetic observatories. This work follows a different approach based on the raw horizontal components registered at the four observatories. The analyses performed aimed to study and assess the impact of several external parameters, characteristics of the Sun-Earth environment, on the semiannual anomaly. The influence of the global geomagnetic activity level, the solar activity level, the solar magnetic polarity, and the rising/declining phase of the solar radiation cycle is analyzed in detail. The most important finding is that the semiannual anomaly is always present and that none of the previously mentioned parameters significantly favor the development of it. A second result is the presence of a 27 day signal superposed to the semiannual anomaly which is significantly affected by the solar activity level.Fil: Azpilicueta, Francisco Javier. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Brunini, Claudio Antonio. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Camilion, Emilio. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Primeros veinte años de SIRGAS

Se sintetizan los acontecimientos que jalonaron los primeros veinte años del “Sistema de Referencia Geocéntrico para las Américas” (SIRGAS), a la vez que se rinde un tributo a las instituciones y personalidades que los han hecho posibles. El relato, articulado en forma cronológica, abarca: los episodios que condujeron al establecimiento de SIRGAS y en el contexto en que aquellos se desarrollaron; la implementación de SIRGAS95 como primer marco de referencia continental compatible con la tecnología GPS y las densificaciones nacionales que desarrollaron los países del continente; las acciones encaminadas al mantenimiento de SIRGAS, incluyendo la medición de SIRGAS2000 y la instalación de la red SIRGAS de operación continua; los esfuerzos orientados a la definición y realización de un sistema de referencia vertical continental, compatible con las modernas tecnologías; la creación permanente de capacidades en los países de la región, abarcado la puesta en funcionamiento de centros de análisis de SIRGAS en instituciones latinoamericanas y las Escuelas SIRGAS; diversas actividades que contribuyen al cumplimento de la iniciativa “Global Geodetic Observing System” (GGOS) de la Asociación Internacional de Geodesia (IAG), a la “Agenda Panamericana 2010- 2020” del IPGH y al “Plan de Acción Conjunto 2013-2015 para Acelerar el Desarrollo de la Infraestructura de Datos Espaciales de las Américas” del IPGH, SIRGAS, CPIDEA y GEOSUR.This contribution describes the cornerstones of the first twenty years of the “Geocentric Reference System for the Americas” (in Spanish SIRGAS), while it pays tribute to the institutions and personalities that made it possible. In chronological order the story includes: the episodes that led to the establishment of SIRGAS and the context in those days; the establishment of SIRGAS95 as the first continentalwide reference frame compatible with the GPS, and the national densifications implemented by the Latin American countries; the actions aimed at maintaining SIRGAS, including the establishment of SIRGAS2000 and the continuously operating SIRGAS network; the efforts to establish a continental-wide vertical reference system compatible with the modern technologies; the building of capacities by the establishment of SIRGAS analysis centres in Latin American institutions; and various activities oriented to contribute to the International Association of Geodesy (IAG) “Global Geodetic Observing System”, the PAIGH “2010-2020 Pan American Agenda”, and the “2013-2015 PAIGH-SIRGASCPIDEA-GEOSUR Action Plan to Expedite the Development of Spatial Data Infrastructure of the Americas”.Facultad de Ciencias Astronómicas y Geofísica

Analysis of the bias between TOPEX and GPS vTEC determinations

The TOPEX/Poseidon satellite was jointly developed and deployed by the National Aeronautics and Space Administration (NASA), USA, and the Centre National d’Etudes Spatiales (CNES), France (for details see Chelton et al. In: Fu L-L, Cazenave A (eds) International geophysics series, vol 69, ISBN 0-12-269545-3, Academic Press, CA, pp 1–131, 2001), with the main scientific goal of sea surface height monitoring. The process that ends with the TOPEX main observable (the range between the satellite and the sea surface) involves the measurement of several parameters of the radar pulses reflected by the sea surface and the computation of several other corrections. After several calibration campaigns performed by the Calibration/Validation team of the mission, it was found that TOPEX range determinations were systematically shorter than expected and it was decided to add an empirical correction of +15 mm to the TOPEX range-computation algorithm. As a by-product, TOPEX provides vertical total electron content (vTEC) determinations which have turned out to be a very important data source for the ionospheric research community. Since TOPEX vTEC measurements became available, several comparison studies have detected a constant bias, from +2 to +5 TECu, when TOPEX is compared to other vTEC sources, e.g., Global Positioning System (GPS), Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS), (TOPEX always greater than the others). In this work, we show that miscalibration of the corrections used in the TOPEX processing algorithm can cause the shortening effect of TOPEX ranges and at the same time the constant bias on the TOPEX vTEC values. It is also shown that changes on TOPEX System Biases of less than 10 mm for the Ku-band and between 40 and 70 mm for the C-band, can make both effects disappear. The analyzed hypothesis is supported by theoretical considerations and data analysis available in the specialized literature.Facultad de Ciencias Astronómicas y Geofísica

Influencia de la excitación hidrológica en la variación anual al movimiento del polo

Es sabido que los cambios de la redistribución de masas en la Tierra provocan variaciones en el tensor de inercia terrestre, causa no al mismo tiempo fluctuaciones en su campo gravitatorio. Además, la dinámica de los procesos geofísicos internos que redistribuyen masa globalmente es también responsable del cambio en la orientación de nuestro planeta en el espacio. Un gran número de procesos geofísicos a escala global causan variaciones a la rotación de la Tierra evidenciadas a través de los cambios en los parámetros de la rotación terrestre (Earth Rotation Parameters, ERP). Según el mecanismo de excitación involucrado, estos procesos suelen distinguirse como efectos (o términos) de movimiento (vientos y corrientes oceánicas) y efectos de masa (diferencias en los valores de presión sobre el lecho de los océanos o cambios en la presión atmosférica). Este trabajo se enfoca en la redistribución global de aguas subterráneas y en sus consecuencias como efecto excitatriz de las variaciones observadas del movimiento del polo. Los resultados de esta investigación se dividen en tres partes: Primero se evaluaron las funciones de excitación hidrológicas al movimiento del polo usyo el modelo LaD. A continuación, se comparó la respuesta anual de este modelo con respecto a varias contribuciones anteriores. Luego, se analizó y comparó la efectividad de tres modelos hidrológicos (NCEP, LDAS, LaD) en el cierre del balance al movimiento del polo para el periodo anual y para ambas componentes, prógrada y retrógrada, respectivamente. Para ello se consideraron la contribuciones atmosférica y oceánica teniendo en cuenta los modelos y series de datos recomendados por el Global Geophysical Fluids Center (GGFC) dependiente del International Earth Rotation y Reference Systems Service (IERS).It is known that the Earth mass redistribution changes provoke variations in the terrestrial inertia tensor, causing thus fluctuations in its gravitational field. In addition, the dynamics ofthe geophysical internal processes that redistribute mass globally is also responsible for the change in the orientation of our planet in the space. A great number of geophysical global processes cause variations to the rotation of the Earth showed as changes in the Earth Rotation Parameters (ERP). According to the mechanism of excitation involved, these processes are known as motionterms (winds y oceanic currents) y mass terms (differences in pressure values at the ocean bottom or atmospheric pressure changes). In this work we focus in the global continental waterredistribution y in its consequences as excitation ofobserved polar motion. The results ofthis investigationwere divided in three parts: First, we evaluated the hydrological excitation functions ofpolar motion by using the Ly Dynamic Model (LaD). Second, we compared the annual response of this model with respect to several previous contributions. Then, we analyzed y compared the efficiency ofthree hydrological models (NCEP, LDAS, LaD) in closing the polar motion budget for the annual period y for both components: prograde y retrograde, respectively. It was taken into accountthe annual atmospheric y oceanic contributions but considering the models y data series recommendedby the Global GeophysicalFluids Center(GGFC) dependent on the International EarthRotationy Reference Systems Service (IERS).Material digitalizado en SEDICI gracias a la colaboración de la Facultad de Ciencias Astronómicas y Geofísicas (UNLP).Asociación Argentina de Geofísicos y Geodesta