Publication: Momento dipolar virtual del campo magnético terrestre (Últimos 3000 años). Aplicaciones en Paleoclimatología
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2013-09
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Abstract
Gallet et al. (2005) propusieron una relación entre la intensidad del campo magnético terrestre y el clima. Utilizaron como proxy paleoclimático el avance y retroceso de glaciares alpinos (épocas frías/cálidas) y datos de paleointensidad regionales (Francia). Otros autores (e.g. Carcalliet et al., 2004) también observaron correlaciones entre las variaciones del campo geomagnético (intensidad o momento magnético dipolar) y algunos proxies climáticos basados en el ritmo de producción de isótopos. Estos últimos estudios presentan un problema fundamental: algunos de estos isótopos, como el 10Be, están afectados por el campo geomagnético (los isótopos o radionúclidos cosmogénicos). Cuanto mayor sea la intensidad del campo, menor será la cantidad de rayos cósmicos que logren penetrar en la atmósfera y menor será la producción de estos isótopos (Snowball y Mucheler, 2007). Como esta producción está afectada también por la intensidad del campo magnético del Sol, la corrección por el campo geomagnético es indispensable para poder reconstruir la actividad solar (e.g. Snowball y Mucheler, 2007; Roth y Joos, 2013). Por tanto, tanto en el estudio de la posible relación entre el campo magnético terrestre y el clima, como en reconstrucciones de la actividad solar, el correcto conocimiento del campo geomagnético es crucial.
El parámetro a determinar es el momento dipolar. La determinación del mismo se puede llevar a cabo de dos maneras distintas: i) A partir de promedios de datos paleomagnéticos obteniendo el denominado momento dipolar virtual; y ii) a partir de los coeficientes dipolares de un modelo de campo geomagnético. Estas metodologías presentan dos limitaciones fundamentales: a) el efecto de la contribución no dipolar y b) el efecto regional asociado a la contribución no dipolar y a la inhomogénea distribución (tanto espacial como temporal) de las bases de datos. El objetivo de este trabajo es el análisis detallado de los actuales métodos de computación del momento dipolar y la cuantificación de los errores asociados.
En el último siglo el efecto no dipolar es pequeño, no supera el 5%, pero el efecto regional asociado a estos términos es muy notable, pudiendo alcanzar valores superiores al 20%. El efecto regional más elevado se ha observado en Europa y Sudamérica debido a los términos cuadrupolar y octupolar. Encontrando incluso variaciones regionales de tendencias opuestas a la variación global (caso de Europa).
El efecto regional asociado a la inhomogeneidad de la base de datos se enfoca siguiendo el esquema de regionalización de Genevey et al. (2008). Los resultados muestran que este método no consigue eliminar los efectos regionales: se siguen sobreestimando las regiones con mayor cantidad de datos.
Finalmente, se propone un primer protocolo que establece unos criterios de calidad para determinar el momento dipolar. Protocolo que debería emplearse para corregir los ritmos de producción de isótopos cosmogénicos y para investigar la posible relación entre el campo geomagnético y el clima pasado de la Tierra.
Gallet et al. (2005) proposed a relationship between the intensity of the geomagnetic field and the past climate. They used the advance and retreat of Alpine glaciers like climate proxy and regional paleointensity data (France). This kind of correlations between the geomagnetic field (through the intensity or magnetic dipole moment) and some climate proxies, like the rate of production of isotopes, seems also observe (e.g. Carcalliet et al., 2004). However, these studies have a problem. Some of these isotopes, such as 10Be, are affected by the geomagnetic field. They are cosmogenic isotopes or radionuclides. If the intensity of the geomagnetic field is high, then less cosmic rays achieve go into atmosphere and, therefore, less rate of production of cosmogenic isotopes (Snowball and Mucheler, 2007). On the other hand, this rate of production of cosmogenic isotopes is also affected by the solar magnetic field. So, the correction of the rate of production of cosmogenic isotopes by the geomagnetic field can provide reconstructions of the solar activity (e.g. Snowball and Mucheler, 2007; Roth and Joos, 2013). Thus, as in the study of the possible relationship of the geomagnetic field and past climate as in the reconstructions of solar activity in paleoclimatology, the well-knowledge of the geomagnetic field variability is fundamental. This is possible through the dipole moment. The computation of the dipole moment is achieved from two different methods: i) from paleomagnetic data directly. The result is known as the virtual dipole moment; ii) from a model of the geomagnetic field. This method employs the dipole coefficients of the model to calculate the dipole moment. These methodologies to compute the dipole moment have two main limitations: a) the non-dipole contribution effect and b) the regional effect due both non-dipole contribution and the inhomogeneous, temporal and spatial, distribution of the database. The objective of the present work is carry out a comprehensive analysis of the methods of dipole moment computation, and quantify the errors introduced in this calculation due to these limitations. In the last century, the non-dipole effect is less of 5%, but the regional effect associated with this non-dipole contribution is very important (20% or more), especially in Europe and South America, associated mainly with the quadrupole and octupole contribution. The regional effect due to the inhomogeneous database is studied from the scheme proposed by Genevey et al. (2008). Thus, we will see that the overestimation of the areas with more available data is not resolved with this kind of scheme. Finally, we will provide a first protocol with quality criteria to carry out the reliable computation of the dipole moment in the works about the reconstructions of solar activity from correction of the geomagnetic field and the possible relationship between this field and the past climate on the Earth.
Gallet et al. (2005) proposed a relationship between the intensity of the geomagnetic field and the past climate. They used the advance and retreat of Alpine glaciers like climate proxy and regional paleointensity data (France). This kind of correlations between the geomagnetic field (through the intensity or magnetic dipole moment) and some climate proxies, like the rate of production of isotopes, seems also observe (e.g. Carcalliet et al., 2004). However, these studies have a problem. Some of these isotopes, such as 10Be, are affected by the geomagnetic field. They are cosmogenic isotopes or radionuclides. If the intensity of the geomagnetic field is high, then less cosmic rays achieve go into atmosphere and, therefore, less rate of production of cosmogenic isotopes (Snowball and Mucheler, 2007). On the other hand, this rate of production of cosmogenic isotopes is also affected by the solar magnetic field. So, the correction of the rate of production of cosmogenic isotopes by the geomagnetic field can provide reconstructions of the solar activity (e.g. Snowball and Mucheler, 2007; Roth and Joos, 2013). Thus, as in the study of the possible relationship of the geomagnetic field and past climate as in the reconstructions of solar activity in paleoclimatology, the well-knowledge of the geomagnetic field variability is fundamental. This is possible through the dipole moment. The computation of the dipole moment is achieved from two different methods: i) from paleomagnetic data directly. The result is known as the virtual dipole moment; ii) from a model of the geomagnetic field. This method employs the dipole coefficients of the model to calculate the dipole moment. These methodologies to compute the dipole moment have two main limitations: a) the non-dipole contribution effect and b) the regional effect due both non-dipole contribution and the inhomogeneous, temporal and spatial, distribution of the database. The objective of the present work is carry out a comprehensive analysis of the methods of dipole moment computation, and quantify the errors introduced in this calculation due to these limitations. In the last century, the non-dipole effect is less of 5%, but the regional effect associated with this non-dipole contribution is very important (20% or more), especially in Europe and South America, associated mainly with the quadrupole and octupole contribution. The regional effect due to the inhomogeneous database is studied from the scheme proposed by Genevey et al. (2008). Thus, we will see that the overestimation of the areas with more available data is not resolved with this kind of scheme. Finally, we will provide a first protocol with quality criteria to carry out the reliable computation of the dipole moment in the works about the reconstructions of solar activity from correction of the geomagnetic field and the possible relationship between this field and the past climate on the Earth.
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Máster en Geofísica y Meteorología. Facultad de CC Físicas. Curso 2012-2013