No description available.

No description available.

No description available.

The purpose of the work reported here was to determine the feasibility of applying the nuclear induction technique of Bloch to the direct observation of nuclear magnetic resonance in the very weak magnetic field of the earth.

A geomagnetic storm is triggered in response to a disturbance in the solar wind. The earth's ring current gets energized during a geomagnetic storm, which leads to a decrease in the horizontal component of the geomagnetic field on the earth's surface. The Disturbance Storm Time (Dst) index, which is a measure of the intensity of the ring current, is calculated by taking the average of this decrease in the horizontal intensity across four low latitude magnetometer stations and removing the quiet time secular variations. The rate of decrease of the Dst index is an indicator of the deenergization of the ring current particles. But there are several issues with the Dst measurement as a proxy of the ring current energy. In particular, the percentage contribution of the tail current effect to the Dst index is still debated. In this work, an effort has been made to separate and quantify the possible contribution of the tail current to the Dst index. The relative contribution for a selected set of storms for which the interplanetary magnetic field turned northward abruptly after the peak in Dst was observed is estimated. The WINDMI model of the nightside magnetosphere is used to investigate the contributions of ring current, magnetotail current, and magnetopause current on the observed two-phase decay of the Dst index. The role of different solar wind magnetosphere coupling functions on the Dst index calculated by the WINDMI model is also investigated. The performance of four other coupling functions in addition to the rectified vBs is evaluated. These coupling functions emphasize different physical mechanisms to explain the energy transfer into the magnetosphere due to solar wind velocity, dynamic pressure, magnetic field, and Mach number. One coupling function is due to Siscoe, another by Borovsky, and two by Newell. The results indicate that for a majority of cases, at most only vx, By, and Bz are needed to sufficiently account for the supply of energy to the ring current and geotail current components that contribute to the Dst index. The capabilities of the WINDMI model to reliably determine the state of the global magnetosphere are improved by employing the the Magnetotail (MT) index as a measurement constraint during large geomagnetic storms. The MT index is used as a proxy for the strength of the magnetotail current in the magnetosphere. The inclusion of the MT index as an optimization constraint in turn increases our confidence that the ring current contribution to the Dst index calculated by the WINDMI model is correct during large geomagnetic storms. To improve the models prediction of AL index, we also modify the ionospheric conductivity and fit to two substorms. The rate of reduction of convection in the magnetotail for some of these storms is numerically simulated by using inner magnetospheric models like the Fok Ring Current (FRC) and the Rice Convection Model along with the global BATSRUS model at the community coordinated modeling center. Model results are compared against magnetometer data by creating movie maps from several low-latitude magnetometer stations. The results indicate the contribution from the tail current to the Dst is important. In addition, the reduction of the cross-tail current during substorm dipolarization is predicted by the measured isotropic boundary locations. Several well known phenomena are identified in the magnetometer movie maps.

Das Magnetfeld der Erde wird durch Konvektionsströmungen im elektrisch leitfähigen, flüssigen eisenreichen äußeren Erdkern erzeugt. Eine drastische Ausprägung der dynamischen Prozesse im äußeren Erdkern sind sowohl Polaritätswechsel über geologische Zeiträume, als auch geomagnetische Feldexkursionen (kurze Umpolungen). Letztere sind in geologischen Archiven häufig unzureichend dokumentiert. Für ein verbessertes Verständnis über die Entwicklung des Erdmagnetfeldes in geologischer Vergangenheit benötigen wir Informationen über die Geometrie des gesamten Vektorfeldes, wofür neben der Bestimmung der Feldrichtungen auch die Bestimmung der absoluten Paläointensität und des Alters notwendig ist. Insbesondere Vulkanite bieten die Möglichkeit, Daten über die Richtung und vor allem auch die Intensität des Erdmagnetfeldes zur Zeit ihrer Platznahme zu gewinnen. Bisweilen ist eine genaue Charakterisierung der Entwicklung des Erdmagnetfeldes in Zeit und Raum schwer möglich, was sich in erster Linie auf den generellen Mangel an Paläointensitätsdaten zurückführen lässt. Ein Grund hierfür ist, dass die meisten Methoden zur absoluten Paläointensitätsbestimmung, auf Modifikationen der Thellier Methode basieren, welche nur auf magnetische Minerale im Einbereichs-Domänenzustand anwendbar ist und zudem hohe Ausschussraten liefert. Eine alternative Methode zur Bestimmung der absoluten Paläointensität ist die kürzlich entwickelte „multispecimen parallel differential pTRM“ (MS) Methode, welche im Vergleich zur Thellier Methode den Vorteil hat, dass sie theoretisch unabhängig ist vom Domänenzustand der magnetischen Minerale und somit auf alle Vulkanite anwendbar ist. Ein Schwerpunkt dieser Arbeit lag darauf, neue Informationen über das Auftreten und gegebenfalls die globale Gültigkeit von geomagnetischen Feldexkursionen zu gewinnen. Hierfür wurden etwa 75 Lavaflüsse des Transmexikanischen Vulkangürtels für paläomagnetische Studien beprobt. Eine Korrelation der mittleren Paläorichtungen von 56 mexikanischen Laven mit einer um Feldexkursionen ergänzten geomagnetischen Polaritätszeitskala, lieferte Hinweise auf 4 Exkursionen. Ein bedeutendes Ergebnis dieser Arbeit sind annährend komplett inversen Richtungen zweier Laven der Brunhes Chron. Dies gibt einen Hinweis darauf, dass diese Exkursionen kurze Zeitintervalle inverser Polarität mit globaler Gültigkeit repräsentieren könnten. Ein weiterer Schwerpunkt der vorliegenden Arbeit war, die neue MS Methode auf ihre Anwendbarkeit und Genauigkeit hin zu testen. Hierfür wurden Paläointensitätsexperimente an 11 historischen Laven aus Mexiko und Island durchgeführt. Ein Vergleich der Paläointensitäten mit Daten von magnetischen Observatorien ergab, dass die MS Methode einen generellen Trend zur Überschätzung der Paläointensität aufweisst, welcher anhand von komplementierenden gesteinsmagnetischen Daten mit magnetischen Mineralen im Mehrbereichsteilchen-Zustand in Verbindung gebracht werden konnte. Diese Beobachtung liefert demnach einen ersten Beweis dafür, dass die MS Methode möglicherweise nicht wie ursprünglich angenommen unabhängig vom Domänenzustand der Trägerminerale ist. Im weiteren wurde eine Komplementierung der Richtungsdaten mexikanischer Laven durch absolute Paläointensitätsbestimmungen angestrebt. Hierfür wurde die MS Methode herangezogen und zum ersten Mal in großem Umfang auf Vulkanite mit Altern von bis zu 3,5 Millionen Jahre angewendet. Ein Vergleich mit Rekonstruktionen des Dipol-Momentes, welche auf den Daten der gegenwärtigen globalen Paläointensitätsdatanbasis basieren, ergaben, dass diese MS Daten mit hoher statistischer Wahrscheinlichkeit im Mittel etwa 30% höher sind. Die generell zu hohen Paläonintensitäten nach der MS Methode bekräftigen daher die Ergebnisse von historischen Laven dieser Arbeit, sowie anderer experimenteller Studien an synthetischen Proben, bei denen Überschätzungen von MS Paläointensitäten von bis zu 30% festgestellt wurden. Der Process, aus dem diese Überschätzung der Paläointensität resultiert ist eine Asymetrie des Entmagnetisierungs- und Remagnetisierungsprozesses heisst, dass ein effektives Entmagnetisieren während der Remagnetisierung im angelegten Laborfeld erfolgt. Diese Asymetrie scheint besonders bei pseudo-Einbereichsteilchen ausgeprägt zu sein. Es wird allerdings davon ausgegangen, dass diese Überschätzung nicht größer ist, als was man bei einem Thellier Experiment an Proben mit ähnlicher magnetischer Korngröße erwarten würde. / The Earth’s magnetic field (EMF) is generated by convections in the electrically conducting liquid iron-rich outer core, modified by the Earth’s rotation. A drastic manifestation of the dynamics of this fluid body is the occurrence of geomagnetic field reversals in the Earth’s history but also geomagnetic excursions, which are more frequent features of otherwise stable polarity chrons, but often poorly constrained in the geological record. To better understand the origin of the field, we need to know how the field has varied on different geological timescales. This includes not only information about changes in the ancient field’s direction but also about the absolute intensity (palaeointensity) and the age. This palaeointensity record is needed for compiling a full-vector description of the field. A palaeomagnetic and palaeointensity study on lava flows allows gaining insights about the evolution of the EMF through time and space. However, constraining the EMF evolution over different geological timescales remains a difficult objective due to the paucity of available palaeointensity data. One new alternative approach in palaeointensity studies is the recently proposed multispecimen parallel differential pTRM (MS) method, which has potentially several advantages over the commonly used Thellier method, because it is in theory independent of magnetic domain state, less prone to biasing effects, such as thermal alteration and significantly faster to perform in the laboratory. A study of highly active volcanic regions, such as the Trans-Mexican Volcanic Belt, seems promising when attempting a full-vector reconstruction or when looking for field excursions. One aim of this thesis was to gain new information about the occurrence and global validity of geomagnetic excursions from the Brunhes- or Matuyama Chron. For this purpose some 75 lava flows from within the Trans-Mexican Volcanic Belt were sampled for palaeomagnetic analyses. The scatter of virtual geomagnetic poles from lavas younger than 1.7 Ma was used for estimating palaeosecular variation and was found to be consistent with latitude dependent Model G and other high quality palaeomagnetic data from Mexico. The palaeomagnetic mean-vectors of 56 lavas were correlated to the Geomagnetic Polarity Timescale supplemented with information on geomagnetic excursions. On the grounds of their associated radioisotopic ages, four lavas were tentatively correlated with known excursions from marine records. Two lava flows dating of Brunhes Chron were associated with the Big Lost and Delts/Stage 17 excursions, respectively. From further two flows dating of Matuyama Chron, one flow was associated with either the Santa Rosa- or Kamikatsura excursions, while the other could have been emplaced during the Gilsa excursion. The most significant outcome was the finding that both Brunhes excursional flows display nearly fully reversed directions that deviate almost 180°C from the expected normal polarity direction. This observation could indicate that in particular the Big Lost and Delta/Stage17 excursions may represent other short periods during which the field completed a full reversal for a short time, such as was previously found for other older cryptochrons or tiny wiggles. Another focus of this thesis was set on estimating the feasibility of the new MS method for routine palaeointensity determination. This was accomplished by applying the MS method to samples from 11 historical lava flows from Mexico and Iceland from which the actual field intensity was either known from contemporary observatory data, or deduced from magnetic field models. Comparing observed with expected intensity values allowed to test the accuracy of the MS method. It a was found that the majority of palaeointensity estimates after the MS method yielded results that were very close or indistinguishable within the range of uncertainty from the expected values. However, a general trend towards an overestimate in the palaeointensity was also observed, which, on the grounds of corroborating rock magnetic analyses, was associated with multidomain material. This observation was taken as first evidence that the MS method is not entirely independent of magnetic domain state, as was originally claimed. However, a second experiment in which a modification of the most widely used Thellier method was applied to sister samples from 5 Icelandic flows revealed that, in comparison to the MS method, the latter produced more accurate and statistically better defined palaeointensities. Thus, from these first results, the MS method appeared as a viable alternative for future palaeointensity studies. Subsequently it was attempted to corroborate the directional record from Mexican lavas with palaeointensity data. It was possible to acquire palaeointensity estimates for 32 out of 51 investigated lava flows. These new results revealed that the new MS palaeointensities for Mexico are, with a high degree of statistical significance, around 30% higher than expected. The generally high palaeointensities seem to corroborate the results obtained from historical lava flows in this study and other previous studies on synthetic samples where domain state effects were found to cause overestimates in the palaeointensity of up to 30 per cent in the MS method. The primary process that leads to this overestimate is assigned to an asymmetry in the demagnetisation and remagnetisation process. Yet, this overestimate is expected to be no larger than what might be expected from Thellier experiments performed on samples with a given degree of multidomain behaviour.

No description available.