Hurricane Bret underwent a rapid intensification (RI) and subsequent weakening between 1200 UTC August 21 and 1200 UTC August 22, 1999, before it made landfall as a category 3 hurricane on the Texas coast 12 h later. Its minimum sea-level pressure dropped 35 hPa from 979 to 944 hPa within 24 h. During this period, aircraft of the National Oceanographic and Atmospheric Administration (NOAA) flew several research missions that sampled the environment and inner core of the storm. These data sets combined with gridded data from the National Centers for Environmental Prediction's (NCEP) Global Model and the National Center for Atmospheric Research (NCAR) reanalyses are used to document the atmospheric and oceanic environments of the tropical cyclone (TC) as well as their relation to the observed structural and intensity changes. Bret's RI was linked to movement over a warm ocean eddy and high sea surface temperatures (SSTs) in the Gulf of Mexico coupled with a simultaneous decrease in vertical wind shear. SSTs at the beginning of the storm?s RI were approximately 29 degrees Celcius and steadily increased to 30 degrees Celcius as it moved northward. The vertical wind shear relaxed to less than 10 kt during this time. Mean values of oceanic heat content (OHC) beneath the storm were about 20 % higher at the beginning of the RI period than 6 h before. Cooling of near-coastal shelf waters (to between 25 and 26 degrees Celcius) by pre-storm mixing combined with an increase in vertical wind shear were responsible for the weakening of the storm. The available observations suggested that intrusion of dry air into the circulation core did not contribute to the intensity evolution. In order to quantitatively describe the influence of environmental conditions on the intensity forecast, sensitivity studies with the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model were conducted. Four different cases with modified vertical wind shear and/or SSTs were studied. Differences between all cases were relatively small due to the model design, but much cooler prescribed SSTs resulted in the greatest intensity changes. Model runs with idealized environmental conditions demonstrated the model?s general lack of capability to forecast RIs and also stressed the need of more accurate SST observations in the coastal shelf regions when predicting the intensity of landfalling TCs.

Accurate prediction of Tropical Cyclones (TC) is vital for the protection of life and property in areas that are prone to their destructive forces. While significant improvements have been made in forecasting TC track, intensity remains a challenge. It is hypothesized that accurate TC intensity forecast requires, among other things, an adequate initial description of their inner-core region. Therefore, there must be reliable observations of the inner-core area of the TC and effective data assimilation (DA) methods to ingest these data into the Numerical Weather Prediction (NWP) models. However, these requirements are seldom met at the relatively low resolution of operational global prediction models and the lack of routine observations assimilated in the TC inner-core. This study tests the impacts of assimilating inner-core Airborne Doppler Radar (ADR) winds on the initial structure and subsequent intensity forecast of Hurricane Ike (2008). The 4-dimensional variational (4DVar) and the 3-dimensional variational (3DVar) methods are used to perform DA while the Weather Research and Forecasting (WRF) model is used to perform forecasts. It is found that assimilating data helps to initialize a more realistic inner-core structure using both DA methods. Additionally, the resulting short-term and long-term intensity forecasts are more accurate when data is assimilated versus cases when there is no DA. Additionally, it is found that in some cases the impact of DA lasts up to 12 hours longer with 4DVar versus 3DVar. It is shown that this is because the flow-dependent 4DVar method produces more dynamically and balanced analysis increments compared to the static and isotropic increments of 3DVar. However, the impact of using both methods is minimal in the long-range. The analyses show that at longer forecast range the dynamics of hurricane Ike was influenced more by outer environment features than the inner-core winds.

No description available.

Les progrès de l'imagerie sismique ces trente dernières années ont permis de révéler la structure complexe de la graine : une anisotropie cylindrique de quelques pourcents dont la structure fine présente des variations radiales et latérales. Ce travail de thèse s’est concentré sur les différentes dynamiques susceptibles de provoquer une telle structure. [Il y a autre chose dans cette structure]Nous avons revisité la dynamique induite par la force de Lorentz, discutant les conditions aux limites, la croissance de la graine et la stratification en densité. La déformation n’est raisonnablement suffisante que pour des viscosités inférieures à 1012 Pa.s, dans la fourchette basse des estimations. Les modèles d'écoulements globaux dans la graine peuvent se classer en deux grandes catégories. Pour un profil de densité stable, seul un forçage extérieur, tel que la force de Lorentz, peut induire un écoulement. Dans le cas instable, la dynamique est contrôlée principalement par des instabilités de convection. Les nouvelles estimations de la diffusion thermique limitent les instabilités thermiques à des âges de graine de l'ordre de la centaine de millions d'années. En se focalisant sur deux paramètres, la viscosité et l'âge de la graine, nous avons construit un diagramme de régime qui compare quantitativement les différents modèles proposés dans la littérature. En croisant amplitude du taux de déformation et géométrie attendues, on peut raisonnablement restreindre les domaines qui pourraient engendrer la structure observée. Pourtant, aucun modèle n'est pour l'instant capable d'expliquer à la fois l'amplitude et la géométrie de l'anisotropie sismique et encore moins la dichotomie Est-Ouest. La couche F est une anomalie dans le noyau externe : d’une épaisseur de 200 km environ, à la base du noyau externe, elle présente des vitesses d’ondes sismiques plus faibles que celles prédites pour un noyau liquide parfaitement mélangé. Elle est interprétée comme une zone chimiquement appauvrie, en contradiction avec la cristallisation de la graine qui libère des éléments légers à la surface même de la graine. Nous étudions la possibilité d’une cristallisation en volume dans cette couche. Les particules de fer solides sédimentent en croissant dans un liquide de plus en plus appauvri en éléments légers. Cette neige de fer est stable sous certaines conditions, étudiées ici. / In the past thirty years, our understanding of the inner core structure has increased with the number of seismic studies. Observations reveal a global anisotropy with a cylindrical symmetry and radial and lateral heterogeneities. In this work, I have studied different hypothesis on the dynamics of the inner core to explain these observations. Revisiting the dynamics induced by the Lorentz force, we studied the effect of new boundary conditions, the effect of stratification and growth rate. However, the obtained flow is not strong enough to deform the media for viscosities larger than 1012 Pa.s, in the lower bound of the published inner core estimates. Deformation mechanisms can be subdivided between natural convection (arising from unstable thermal or compositional gradients) and externally forced flows, like the one induced by the Lorentz force. Recent estimates of the thermal diffusivity of iron at high pressure limit the possibility of thermal convection to an age of the inner core lower than a hundred of millions years. Two key parameters emerge for the inner core dynamics: the sign and strength of the density stratification and the viscosity of the inner core. We construct a regime diagram for the Earth’s inner core dynamics that compares the different published models in term of maximum instantaneous deformation rate. This diagram allows us to compare both expected strain rate and deformation geometry with the seismic observations. However, we find that no published model can explain all the seismic observations. The inner core anisotropy and the hemispherical dichotomy are especially difficult to reconcile with these models. The F-Layer is a 200km anomalous layer at the bottom of the outer that presents low P-Wave velocities compared to well-Mixed model. It has been interpreted as a layer depleted in light elements, whereas we usually consider that light elements are expelled at the surface of the inner core by freezing of the outer core alloy. We study the hypothesis of freezing in the bulk of the layer, with iron particles growing and settling in an increasingly depleted liquid.

The Earth's magnetic field is generated in its fluid outer core through dynamo action. In this process, convection and differential rotation of an electrically conducting fluid maintain the magnetic field against its ohmic decay. Using numerical models, we can investigate planetary dynamo processes and the importance of various core properties on the dynamo. In this thesis, I use numerical dynamo models in Earth-like geometry in order to understand the influence of inner core electrical conductivity and the choice of thermal and velocity boundary conditions on the resulting magnetic field. I demonstrate how an electrically conducting inner core can reduce the frequency of reversals and produce axial-dipolar dominated fields in our models. I also demonstrate that a strong planetary magnetic field intensity does not imply that the dynamo operates in the strong field regime as is usually presumed. Through a scaling law analysis, I find that irrespective of the choice of thermal or velocity boundary conditions, the available power determines the magnetic and velocity field characteristics like the field strength, polarity and morphology. Also, whether a dynamo model is in a dipolar, transitional or multipolar regime is dependent on the force balance in the model. I demonstrate that the Lorentz force is balanced by the Coriolis force in the dipolar dynamo regime models resulting in magnetostrophically balanced dynamos whereas the Lorentz force is balanced by the Inertial force (and not the Coriolis force) in the multipolar dynamo regime models resulting in a non-magnetostrophically balanced dynamo. The generation mechanism differs between the regimes and depends on the velocity boundary conditions. The zonal flows of the stress-free models are stronger than in the no-slip models, and bistability is more prominent when stress-free boundary conditions are used. A single scaling law may be feasible for all the models, but there does appear to be some variation for models with different thermal and velocity boundary conditions. The results presented in this thesis are not only applicable to the geodynamo, but will also aid in understanding the dynamos of other planets and exoplanets.

The Earth's magnetic field is generated in its fluid outer core through dynamo action. In this process, convection and differential rotation of an electrically conducting fluid maintain the magnetic field against its ohmic decay. Using numerical models, we can investigate planetary dynamo processes and the importance of various core properties on the dynamo. In this thesis, I use numerical dynamo models in Earth-like geometry in order to understand the influence of inner core electrical conductivity and the choice of thermal and velocity boundary conditions on the resulting magnetic field. I demonstrate how an electrically conducting inner core can reduce the frequency of reversals and produce axial-dipolar dominated fields in our models. I also demonstrate that a strong planetary magnetic field intensity does not imply that the dynamo operates in the strong field regime as is usually presumed. Through a scaling law analysis, I find that irrespective of the choice of thermal or velocity boundary conditions, the available power determines the magnetic and velocity field characteristics like the field strength, polarity and morphology. Also, whether a dynamo model is in a dipolar, transitional or multipolar regime is dependent on the force balance in the model. I demonstrate that the Lorentz force is balanced by the Coriolis force in the dipolar dynamo regime models resulting in magnetostrophically balanced dynamos whereas the Lorentz force is balanced by the Inertial force (and not the Coriolis force) in the multipolar dynamo regime models resulting in a non-magnetostrophically balanced dynamo. The generation mechanism differs between the regimes and depends on the velocity boundary conditions. The zonal flows of the stress-free models are stronger than in the no-slip models, and bistability is more prominent when stress-free boundary conditions are used. A single scaling law may be feasible for all the models, but there does appear to be some variation for models with different thermal and velocity boundary conditions. The results presented in this thesis are not only applicable to the geodynamo, but will also aid in understanding the dynamos of other planets and exoplanets.

VP6 is one of the minor structural core proteins of African horsesickness virus. The minor core proteins VP1, VP4 and VP6 are presumed to constitute the dsRNA dependent RNA polymerase transcription complex of the virus. In the Orbivirus prototype bluetongue virus (BTV), VP6 has a helicase activity. The aim of this investigation was to characterize the primary structure and nucleic acid binding function of the inner core protein VP6 of African horsesickness virus (AHSV). To characterize the primary structure of AHSV VP6, VP6 genes of serotypes 3 and 6 were cloned and sequenced. Both genes encode a 369 amino acid polypeptide. A comparison to the VP6 proteins of other Orbiviruses indicated that in all cases the proteins are rich in basic residues and in glycine. The proteins are highly conserved within serogroups but the conservation between serogroups is low. VP6 of AHSV-3 and AHSV-6 have 93.5% identity and 96% similarity in amino acid residues. AHSV-6 VP6 has 27% identical and 46% similar amino acid residues to BTV-10 VP6. Phylogenetic analysis of four orbivirus VP6 genes indicated that AHSV and BTV are most closely related to each other. Motifs characteristic of known helicases were identified by sequence analysis. Glycine rich protein motifs and a N-glycosylation signal were present. No nucleic acid binding motifs identified in other proteins were found in AHSV VP6. To characterize the VP6 protein of AHSV VP6, the genes were expressed using both a baculovirus and a bacterial expression system. Proteins were found to be soluble and the VP6 expressed in insect cells was found to be N-glycosylated. The nucleic acid binding function of AHSV VP6 was investigated. Bacterially expressed VP6 was demonstrated to bind nucleic acids by electrophoretic mobility shift assays. Baculovirus expressed VP6 bound double and single-stranded RNA and DNA in nucleic acid overlay protein blot assays. Competition assays indicated that VP6 may have a preference for binding to RNA rather than DNA. Glycosylation was found to play no direct role in nucleic acid binding but the binding is strongly dependent on the NaCl concentration. A series of truncated VP6 peptides were produced to investigate the importance of localized regions in nucleic acid binding. Two partially overlapping peptides were found to bind dsRNA at pH 7.0, while other peptides with the same overlap did not. Binding appeared to be influenced by charge as reflected by the isoelectric points (pI) of the peptides and experiments indicating the effect of pH on the binding activity. However, only peptides containing amino acid residues 190 to 289 showed binding activity. This region corresponded to the region on BTV VP6 that contains two binding domains. It is proposed that the dsRNA binding domain in AHSV VP6 is a sequence of positively charged amino acids constituting a domain that determines the nucleic acid binding characteristics of the peptide. The mechanism of binding of baculovirus expressed VP6 in a nucleic acid overlay protein blot is proposed to be charge related. / Thesis (PhD (Genetics))--University of Pretoria, 2007. / Genetics / unrestricted

Ce travail de thèse se concentre sur la dynamique rotationnelle du système couplé graine, noyau externe et manteau. Notre modèle inclut en effet deux couples électromagnétiques directs aux limites du noyau fluide et un couple gravitationnel entre la graine et le manteau. La dynamique rotationnelle est décrite par quatre cisaillements typiques et étudiés dans des simulations numériques convectives de la géodynamo reproduisant les principales caractéristiques du champ magnétique terrestre et de sa variation séculaire. Celle-ci est principalement représentée par la dérive géomagnétique vers l’ouest de taches de flux magnétique à la CMB, concentrée à l’équateur de l’hémisphère Atlantique, et bien documentée pour les quatre derniers siècles. Nous fournissons des contraintes sur la rotation différentielle de la graine en exprimant son lien avec la dérive géomagnétique vers l’ouest. Ceci est réalisé par la formulation et la validation de modèles dynamiques de couples électromagnétiques. Au long terme, le cisaillement global dans le noyau fluide est réparti entre la dérive géomagnétique vers l’ouest et la rotation différentielle de la graine, dans des proportions contrôlées par l’état des couplages. Puisqu’une estimation actuelle de ce cisaillement est proche de la vitesse de la dérive géomagnétique vers l’ouest, nous concluons que la rotation différentielle moyenne de la graine est proche de zéro. En ce qui concerne ses fluctuations, l’intensité du couplage gravitationnel est le paramètre dominant. Cette observation place une limite sur les fluctuations décennales de la rotation différentielle de la graine, qui ne devraient pas excéder quelques centièmes de degré par an / This PhD work focuses on the rotational dynamics of the coupled inner core - outercore - mantle system. The conservation of the angular momentum our coupled Earth model indeed involves two direct electromagnetic torques at the fluid core boundaries and a remote gravitational torque between the inner core and the mantle. The rotational dynamics is described by four typical shears and studied in convective numerical simulations of the geodynamo which are able to reproduce the main characteristics of the geomagnetic field and its secular variation. This secular variation is mainly embodied by the westward drift of magnetic flux patches at the CMB, concentrated on the equator of the Atlantic hemisphere, and is well documented for the last four centuries. We provide constrains on the inner core differential rotation by expressing its link to the geomagnetic westward drift. This is performed through the formulation and the validation of dynamical electromagnetic torque models, which are then introduced in the conservation of the angular momentum of the system. In the long-term state, the global shear in the fluid outer core is distributed between the westward drift and the differential rotation of the inner core, in proportions controlled by the state of couplings. As a present day estimate of this shear is close to the observed westward drift, we conclude there is no differential rotation of the inner core on time-average. In the time-dependent state, we observed that the strength of gravitational coupling is the dominant parameter. This places limit on the decadal fluctuations of the inner core differential rotation, which should not exceed a few hundredths of degree per year

Das Ziel dieser Arbeit ist es, die Strukturen im äußeren Erdkern zu untersuchen und Rückschlüsse auf die sich daraus ergebenden Konsequenzen für geodynamische Modellvorstellungen zu ziehen. Die Untersuchung der Kernphasenkaustik B mit Hilfe einer kumulierten Amplituden-Entfernungskurve ist Gegenstand des ersten Teils. Dazu werden die absoluten Amplituden der PKP-Phasen im Entfernungsbereich von 142 &#176; bis 147 &#176; bestimmt und mit den Amplituden synthetischer Seismogramme verglichen. Als Datenmaterial dienen die Breitbandregistrierungen des Deutschen Seismologischen Re-gionalnetzes (GRSN 1 ) und des Arrays Gräfenberg (GRF). Die verwendeten Wellen-formen werden im WWSSN-SP-Frequenzbereich gefiltert. Als Datenbasis dienen vier Tiefherdbeben der Subduktionszone der Neuen Hebriden (Vanuatu Island) und vier Nuklearexplosionen, die auf dem Mururoa und Fangataufa Atoll im Südpazifik stattgefunden haben. Beide Regionen befinden sich vom Regionalnetz aus gesehen in einer Epizentraldistanz von ungefähr 145 &#176;. Die Verwendung eines homogen instrumentierten Netzes von Detektoren und die Anwendung von Stations- und Magnitudenkorrekturen verringern den Hauptteil der Streuung bei den Amplitudenwerten. Dies gilt auch im Vergleich zu Untersuchungen von langperiodischen Amplituden im Bereich der Kernphasenkaustik (Häge, 1981). Ein weiterer Grund für die geringe Streuung ist die ausschließliche Verwendung von Ereignissen mit kurzer impulsiver Herdzeitfunktion. Erst die geringe Streuung der Amplitudenwerte ermöglicht eine Interpretation der Daten. Die theoretischen Amplitudenkurven der untersuchten Erdmodelle zeigen im Bereich der Kaustik B einen gleichartigen Kurvenverlauf. Bei allen Berechnungen wird ein einheitliches Modell für die Güte der P- und S-Wellen verwendet, das sich aus den Q-Werten der Modelle CIT112 und PREM 2 zusammensetzt. Die mit diesem Q-Modell berechneten Amplituden liegen in geringem Maße oberhalb der gemessenen Amplituden. Dies braucht nicht berücksichtigt zu werden, da die kumulierte Amplituden-Entfernungskurve anhand der Lage des Maximums auf der Entfernungsachse ausgewertet wird. Folglich wird darauf verzichtet, ein alternatives Q-Modell zu entwickeln.<br /> Hinsichtlich der Lage des Kaustikmaximums lassen sich die untersuchten Erdmodelle in zwei Kategorien einteilen. Eine Gruppe besteht aus den Modellen IASP91 und 1066B, deren Maxima bei 144.6 &#176; und 144.7 &#176; liegen. Zur zweiten Gruppe von Modellen zählen AK135, PREM und SP6 mit den Maxima bei 145.1 &#176; und 145.2 &#176; (SP6). Die gemessene Amplitudenkurve hat ihr Maximum bei 145 &#176;. Alle Entfernungsangaben beziehen sich auf eine Herdtiefe von 200 km. Die Kaustikentfernung für einen Oberflächenherd ist jeweils um 0.454 &#176; größer als die angegeben Werte. Damit liegen die Maxima der Modelle AK135 und PREM nur 0.1 &#176; neben dem der gemessenen kumulierten Amplitudenkurve. Daher wird auf die Erstellung eines eigenen Modells verzichtet, da dieses eine unwesentlich verbesserte Amplitudenkurve aufweisen würde. Das Ergebnis der Untersuchung ist die Erstellung einer gemessenen kumulierten Amplituden-Entfernungskurve für die Kaustik B. Die Kurve legt die Position der Kaustik B für kurzperiodische Daten auf &#177; 0.15 &#176; fest und bestimmt damit, welche Erdmodelle für die Beschreibung der Amplituden im Entfernungsbereich der Kaustik B besonders geeignet sind. Die Erdmodelle AK135 und PREM, ergänzt durch ein einheitliches Q-Modell, geben den Verlauf der Amplituden am besten wieder. Da die Amplitudenkurven beider Modelle nahe beieinander liegen, sind sie als gleichwertig zu bezeichnen.<br /> Im zweiten Teil der Arbeit wird die Struktur der Übergangszone in den inneren Erdkern anhand des spektralen Abklingens der Phase PKP(BC)diff am Punkt C der Laufzeitkurve untersucht. Der physikalische Prozeß der Beugung ist für die starke Abnahme der Amplituden dieser Phase verantwortlich. Die Diffraktion beeinflußt das Abklingverhalten verschiedener Frequenzanteile des seismischen Signals auf unterschiedliche Weise. Eine Deutung des Verhaltens erfordert die Berechnung von Abklingspektren. Dabei wird die Abschwächung des PKP(BC)diff Signals für acht Frequenzen zwischen 6.4 s und 1.25 Hz ermittelt und als Spektrum dargestellt. Die Form des Abklingspektrums ist charakteristisch für die Beschaffenheit der Geschwindigkeitsstruktur direkt oberhalb der Grenze zum inneren Erdkern (GIK). Die Beben, deren Kernphasen im Regionalnetz als diffraktierte Kernphasen BCdiff registriert werden, liegen in einem Entfernungsbereich jenseits von 150 &#176;. In dieser Distanz befinden sich die Erdbebenherde der Tonga-Fidschi-Subduktionszone, deren Breitbandaufzeichnungen verwendet werden. Die Auswertung unkorrigierter Wellenformen ergibt Abklingspektren, die mit plausiblen Erdmodellen nicht in Einklang zu bringen sind. Aus diesem Grund werden die Daten einer spektralen Stationskorrektur unterzogen, die eigens zu diesem Zweck ermittelt wird. Am Beginn der Auswertung steht eine Prüfung bekannter Erdmodelle mit unterschiedlichen Geschwindigkeitsstrukturen oberhalb der GIK. Zu den untersuchten Modellen zählen PREM, IASP91, AK135Q, PREM2, SP6, OICM2 und eine Variante des PREM. Die Untersuchung ergibt, daß Modelle, die einen verringerten Gradienten oberhalb der GIK aufweisen, eine bessere Übereinstimmung mit den gemessenen Daten zeigen als Modelle ohne diese Übergangszone. Zur Verifikation dieser These wird ein Erdmodell, das keinen verringerten Gradienten oberhalb der GIK besitzt (PREM), durch eine Reihe unterschiedlicher Geschwindigkeitsverläufe in diesem Bereich ergänzt und deren synthetische Seismogramme berechnet. Das Resultat der Untersuchung sind zwei Varianten des PREM, deren Frequenzanalyse eine gute Übereinstimmung mit den Daten zeigt. Das Abklingspektrum des Erdmodells PD47, das in einer 380 km mächtigen Schicht einen negativen Gradienten besitzt, zeigt eine große Ähnlichkeit mit den gemessenen Spektren. Dennoch kann es nicht als realistisches Modell angesehen werden, da der Punkt C in einer zu großen Entfernung liegt. Darüber hinaus müßte die zu kurze Differenzlaufzeit zwischen PKP(AB) und PKP(DF) beziehungsweise PKIKP durch eine größere Änderung der Geschwindigkeitsstruktur im inneren Kern kompensiert werden. Es wird deshalb das Modell PD27a favorisiert, das diese Nachteile nicht aufweist. PD27a besitzt eine Schicht konstanter Geschwindigkeit oberhalb der GIK mit einer Mächtigkeit von 150 km. Die Art des Geschwindigkeitsverlaufs steht im Einklang mit der geodynamischen Modellvorstellung, nach der eine Anreicherung leichter Elemente oberhalb der GIK vorliegt, die als Ursache für die Konvektion im äußeren Erdkern anzusehen ist. / In this thesis the structure of the outer core is investigated with PKP core phases. The knowledge of the physical properties of the earth&prime;s deep interior in this region is important for the understanding of geodynamical processes like the convective flow in the liquid outer core and the differential rotation of the earth&prime;s inner core.<br /> The first part of this thesis describes the investigation of the PKP caustic point B near 145 &#176;. For this purpose a cumulative amplitude distance curve is determined and compared with theoretical amplitude distance curves of different standard earth models. The data are broadband seismograms of the German Regional Seismic Network (GRSN) and the Gräfenberg Array (GRF). In order to measure the absolute amplitudes of the PKP phases, a WWSSN-SP filter is applied to the seismograms. The source regions are located in the South Pacific near Vanuatu Island (4 earthquakes) and on the French atolls Mururoa and Fangataufa (4 explosions). The advantage of a standardized network of seismic stations and the usage of station and magnitude corrections is a reduction of the scatter of the amplitude data. There is even less scatter than in studies with long period amplitude data (Häge, 1981). Another reason for the reduced scattering is the use of events with an impulsive source time function. Only the low scattering of the amplitude values makes it possible to interpret the data. More scattering of the data would have prevented an interpretation. The theoretical amplitude curves are similar in the caustic B distance range. The Q depth distribution for P and S waves used for calculating the synthetic seismograms is a combination of the values of the models CIT112 and PREM. The amplitudes determined with the help of this kind of model are slightly higher than the actually measured amplitudes. However, this needs not be taken into account because the interpretation is based on the position of the caustic peak. Therefore I rejected the computation of an improved Q model.<br /> Regarding the position of the caustic point there are two categories of earth models. The first group consists of the models IASP91 and 1066B with their maxima at 144.6 &#176; and 144.7 &#176; respectively. AK135, PREM and SP6 belong to a second group of models with caustic peaks at 145.1 &#176; and 145.2 &#176; (SP6). The measured curve has its maximum at 145 &#176;. All distances refer to a source depth of 200 km. For a surface focus the increase in distance is 0.454 &#176;. Therefore the peaks of the models AK135 and PREM are only 0.1 &#176; beside the maximum of the measured amplitude curve. The main result of this investigation is the amplitude distance curve in the vicinity of the cusp B. The curve determines the position of this point with an accuracy of &#177; 0.15 &#176; and points to earth models which would be good for modeling the amplitudes in the distance range of the PKP caustic B. The synthetic seismograms calculated for AK135 and PREM together with a standardized Q model fit the measured amplitude curve equally well.<br /> In the second part of this study the structure of the transition zone to the earth&prime;s inner core is investigated by using the spectral decay of the diffracted wave PKP(BC)diff at point C of the travel time curve. The physical process of diffraction is responsible for the strong reduction in amplitude of this wave. The influence of the diffraction on the seismic signal strongly depends on frequency. The interpretation of this phenomenon requires a calculation of decay spectra. In practice the attenuation of the PKP(BC)diff signal for eight frequencies between 6.4 s and 1.25 Hz are measured and visualized as a decay spectrum. The shape of a spectrum is characteristic of the velocity gradient above the inner core boundary (ICB). Those earthquakes whose core phases are recorded as diffracted core phases BCdiff lie beyond 150 &#176;. In this distance range there are the epicenters of the Tonga-Fiji slab. The broadband waveform data of the earthquakes in this region is used in this study. Decay spectra of waveform data which are not corrected for station site effects are incompatible with standard earth models. Therefore a spectral station correction is applied, which was especially determined for this purpose. The investigation starts with a review of a number of well-known earth models like PREM, IASP91, AK135Q, PREM2, SP6, OICM2 and a version of PREM. All these models have different velocity structures at the ICB. It is shown that models with a reduced velocity gradient above the ICB agree with the data rather than models without such a transition zone. For verification purposes a model without such a reduced gradient (PREM) is completed with different kinds of gradient zones to calculate synthetic seismograms. Two variants of the PREM correspond with the measured decay constants. The decay constants of model PD47 are very close to the measured ones. This model has a 380 km thick negative gradient above the ICB. Nevertheless it is not a realistic model because point C lies in a unrealistic great distance. As a result of the low velocity zone above the inner core there is a differential travel time between the PKP(AB) and the PKP(DF) phase (also PKIKP) which is too short. This would have to be compensated by a correction of the velocities in the inner core. Thus PD27a is the most suitable model which does not have the above mentioned disadvantages. PD27a has a 150 km thick layer of constant velocity above the ICB. This kind of velocity model is compatible with geodynamical theories according to which an enrichment of light elements above the ICB is present and powers the convection in the outer earth core by its buoyancy.

No description available.