894 research outputs found

    Free core nutation: new large disturbance and connection evidence with geomagnetic jerks

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    Variations in free core nutation (FCN) are connected with various processes in the Earth's fluid core and core-mantle coupling, which are also largely responsible for the geomagnetic field variations, particularly the geomagnetic jerks (GMJs). A previous study (Malkin, 2013) revealed that the epochs of the observed extremes in the FCN amplitude and phase variations are close to the GMJ epochs. In this paper, a new evidence of this connection was found. The large FCN amplitude and phase disturbance occurred at the epoch close to the newly revealed GMJ 2011. This event occurred to be the second large change in the FCN amplitude and phase after the 1999 disturbance that is also associated with the GMJ 1999. Moreover, the long-time FCN phase drift had changed suddenly in 1998--1999, immediately before the GMJ 1999, and seemed to change again at the epoch immediately preceding the GMJ 2011. The FCN amplitude showed a general long-time decrease before GMJ 1999, and it subsequently grew until GMJ 2011, and then seemed to decrease again. A smaller FCN change can be observed at the epoch around 2013, which is also suspected as the GMJ epoch. The latter confirms the suggestion that a rapid change in the FCN amplitude and/or phase can be used as an evidence of the GMJ that is not clearly detected from the geomagnetic observations

    Migration of the Antarctic Polar Front through the mid-Pleistocene transition: evidence and climatic implications

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    The Antarctic Polar Front is an important biogeochemical divider in the Southern Ocean. Laminated diatom mat deposits record episodes of massive flux of the diatom Thalassiothrix antarctica beneath the Antarctic Polar Front and provide a marker for tracking the migration of the Front through time. Ocean Drilling Program Sites 1091, 1093 and 1094 are the only deep piston cored record hitherto sampled from the sediments of the circumpolar biogenic opal belt. Mapping of diatom mat deposits between these sites indicates a glacial-interglacial front migration of up to 6 degrees of latitude in the early / mid Pleistocene. The mid Pleistocene transition marks a stepwise minimum 7 degree northward migration of the locus of the Polar Front sustained for about 450 kyr until an abrupt southward return to a locus similar to its modern position and further south than any mid-Pleistocene locus. This interval from a “900 ka event” that saw major cooling of the oceans and a ?13C minimum through to the 424 ka Mid-Brunhes Event at Termination V is also seemingly characterised by 1) sustained decreased carbonate in the subtropical south Atlantic, 2) reduced strength of Antarctic deep meridional circulation, 3) lower interglacial temperatures and lower interglacial atmospheric CO2 levels (by some 30 per mil) than those of the last 400 kyr, evidencing less complete deglaciation. This evidence is consistent with a prolonged period lasting 450 kyr of only partial ventilation of the deep ocean during interglacials and suggests that the mechanisms highlighted by recent hypotheses linking mid-latitude atmospheric conditions to the extent of deep ocean ventilation and carbon sequestration over glacial-interglacial cycles are likely in operation during the longer time scale characteristic of the Mid-Pleistocene Transition. The cooling that initiated the “900 ka event” may have been driven by minima in insolation amplitude related to eccentricity modulation of precession that also affected low latitude climates as marked by threshold changes in the African monsoon system. The major thresholds in earth system behaviour through the Mid-Pleistocene Transition were likely governed by an interplay of the 100 kyr and 400 kyr eccentricity modulation of precession

    Das Magnetfeld der Erde: Beobachtung und Modellierung von Globalen zu Regionalen Skalen

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    The magnetic field of the Earth varies in space and time. Geomagnetism as research area that aims to describe and understand the sources of these variations is supported by two pillars: first, regular high-precision measurements in the global network of magnetic observatories and repeat stations are necessary to register the field and its variations at all. Second, mathematical methods are required in order to extract magnetic field models from this large data set. Methods applied to data offer insights in the mechanisms generating the magnetic field. This thesis covers both subjects. In a first part, following the description of the state of the art in observatory instrumentation, I explain in detail two instruments that have the potential to streamline the classical procedures: The Geomagnetic AUtomated SyStem GAUSS paves the way to automated absolute measurements, up to now only possible manually. The newly developed DI3 technique improves and simplifies the standard manual measurements significantly and thus reduces the requirements placed on observers. The second part deals with the mathematical tools available for geomagnetic field modelling. I focus on harmonic splines that can be derived from the classical approach of spherical harmonics. These base functions are interpolatory and have a localised shape while satisfying Laplaces equation. Hence, they are applicable to fit data regionally or globally. The harmonic splines are used with a data set made of repeat station and observatory measurements from Southern Africa. This region is of special interest because the field intensity is very low and and both spatial and temporal field gradients exist. Subdivided into an analysis of ancient (years 1961-2001) and recent (2005-2009) data, two continuous regional field models SAMS and X-SAMS are derived. From the analysis of the field models, a better understanding of the field behaviour is gained. Finally, the harmonic splines are used in a case study on globally distributed secular variation data. Rotating the data set in a system of coordinates aligned with the dipole axis and modelling it with the harmonic splines reveals the external origin of observed fast variations.Das Magnetfeld der Erde ändert sich in Raum und Zeit. Die Ursachen dieser Änderungen zu beschreiben und zu verstehen ist das Ziel der geomagnetischen Forschung. Sie ruht auf zwei Säulen: Erstens werden regelmäßig hochpräzise Messungen in einem Netz von Observatorien und Säkularpunkten durchgeführt, um das Feld mit seinen Variationen überhaupt aufzuzeichnen. Zweitens sind mathematische Methoden notwendig, um aus dieser Fülle von Daten magnetische Feldmodelle abzuleiten. Werden die Methoden auf die Daten angewandt, kann man Erkenntnisse gewinnen, wie die Felder erzeugt und Variationen hervorgerufen werden. In dieser Arbeit werden beide Herangehensweisen beschrieben. Im ersten Teil stelle ich zwei Instrumente vor, mit deren Hilfe die klassischen Messprozeduren in geomagnetischen Observatorien überholt werden können. Das Geomagnetic AUtomated SyStem GAUSS ebnet den Weg für automatisierte Absolutmessungen, die bisher nur händisch möglich sind. Die neu entwickelte DI3 Technologie verbessert und vereinfacht die manuelle Standardmethode und setzt somit die Anforderungen, die an Observatoren gestellt werden, herab. Der zweite Teil behandelt die mathematischen Methoden, die für geomagnetische Modellierung zur Verfügung stehen. Insbesondere hebe ich harmonische Splinefunktionen hervor, die sich aus dem klassischen Ansatz der sphärisch-harmonischen Analyse ableiten lassen. Diese Basisfunktion haben interpolierenden Charakter und eine lokalisierte Form, wobei sie gleichzeitig die Laplacebedingung erfüllen. Sie können sowohl für regionale als auch für globale Modellierungen eingesetzt werden. Im Anschluss werden die harmonischen Splines auf einen Datensatz angewendet, der aus Messungen in Observatorien und an Säkularpunkten im südlichen Afrika besteht. Diese Region ist geomagnetisch besonders interessant, da die Feldstärke hier verhältnismäßig klein ist und starke räumliche und zeitliche Änderungen beobachtet werden. Die Analyse wird unterteilt in eine Untersuchung älterer und neuerer Daten, aus denen ich die beiden kontinuierlichen Feldmodelle SAMS und X-SAMS ableite. Aus der näheren Untersuchung der Modelle gewinne ich ein besseres Verständnis des geomagnetischen Verhaltens dieser Region. Zum Schluss werden die harmonischen Splines in einem Fallbeispiel benutzt, um Schwankungen in global verteilten Säkularvariationsdaten genauer zu beschreiben. Die Drehung in ein geomagnetisches Koordinatensystem und die Modellierung mit Splinefunktionen deckt auf, dass beobachtete schnelle Variationen durch externe Einflüsse hervorgerufen werden

    An investigation in the use of satellite data to develop a geomagnetic secular variation model over Southern Africa.

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    Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2011.Time variations of the geomagnetic field can be classified into two main categories of internal and external origin with respect to the surface of the Earth. It has been found that the variations that take place on longer time scales (~ 1 year and longer) are commonly known as secular variation (SV) and are of internal origin. There is a need to develop SV models using satellite data as the use of ground data is not always possible with many limitations including the limited data points and lack of data over ocean areas that are not easily accessible. Two regional geomagnetic field modelling techniques namely polynomial surface modelling (PolyM) and Spherical Cap Harmonic Analysis (SCHA) were applied to CHAMP satellite data recorded between 2001 and 2005 to investigate the use of satellite data to develop a geomagnetic SV model over southern Africa. The restricted area of investigation is between 10° and 40° South in latitude and between 10° and 40° East in longitude. The resulting regional models of this investigation were validated against the two widely used global field models IGRF 10 and CHAOS using the available ground survey data obtained during the same period over southern Africa. The results suggest that the regional field models can be derived based entirely on satellite data. However, the regional SV models can be improved by combining both high quality satellite and ground survey data, since they lack the high quality of a global field model like CHAOS

    Hermanus Magnetic Observatory: a historical perspective of geomagnetism in southern Africa

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    In this paper a brief summary will be given about the historical development of geomagnetism as a science in southern Africa and particularly the role played by Hermanus Magnetic Observatory in this regard. From a very modest beginning in 1841 as a recording station at the Cape of Good Hope, Hermanus Magnetic Observatory is today part of the South African National Space Agency (SANSA), where its geomagnetic field data are extensively used in international research projects ranging from the physics of the geo-dynamo to studies of the near-Earth space environment.</p

    Modelling extreme precipitation in hazardous mountainous areas. Contribution to landscape planning and environmental management

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    Extreme precipitation episodes are very common in Mediterranean area and can lead to serious and catastrophic environmental hazards. They have special incidence during autumn months, September, October and November (SON) with important impacts on society, leading frequently to significant economic losses and mortality. These events have special impact in mountainous areas where steep slopes enhance the effects of extreme precipitation. In mountainous areas rain gauge stations are sparse and normally in lower amount. Due to these reasons it is very important to map with higher accuracy the distribution of extreme precipitation. Also, in mountainous environments precipitation patterns can change in small distances that make the prediction more difficult, but also more important. A better prediction of areas with higher values of extreme precipitation will contribute to a better land use planning and avoid the effects of flash floods, land‐slides and soil erosion recognized as environmental problems. The aim of this paper is testing several well‐known interpolation methods, Inverse Distance Weight (IDW) with weighs of 1, 2, 3, 4 and 5, Local Polynomial (LP) with order 1 and 2, Radial Basis Methods (RBS), particularly Spline With Tension (SPT) and Thin Plate Spline (TPS), and Kriging techniques, Ordinary Kriging (OK) and Ordinary CoKriging (COK) in order to identify the less‐biased method to interpolate extreme precipitation calculated from the 95th percentile (P95) of SON precipitation in a mountainous area located in Portugal.&nbsp;The results show that extreme precipitation increases with the altitude and there are important differences between stations located at higher and lower altitudes. This relation is observed in the omni‐directional semi‐variograms calculated where we identified two major P95 areas coincident with higher elevations. The first one occurred at 12.19 km and the second at 23.57 km. The higher values of P95 are identified at Southeast and Northeast. In contrast, the lower P95 values are identified at Northwest due to lower altitudes and in the Northeast corner as a consequence of rain shadow effect. Prediction with precision of precipitation patterns in mountainous areas is difficult due to lack of data and the complex effect of topography in rainfall, however, it is of major importance in order to identify vulnerable areas. The findings observed in this study are a fundamental contribution to landscape planning and environmental management in areas with higher occurrence and vulnerability to extreme precipitation. Article in English. Gausių kritulių pasiskirstymo poveikio pavojingose kalnų vietovėse modeliavimas. Pagalba planuojant kraštovaizdį, aplinkos valdymą Santrauka.&nbsp;Gausūs krituliai, ypač būdingi Viduržemio jūros regionui, gali kelti rimtų pavojų aplinkai, lemti katastrofas. Kritulių poveikio zona ypač išsiplečia rudens laikotarpiu, t. y. rugsėjo, spalio ir lapkričio (RSL) mėnesiais, neretai patiriama didelių ekonominių nuostolių, žūsta ar kitaip nukenčia žmonės. Statūs kalnų šlaitai sustiprina gausių kritulių poveikį. Kalnuotose vietovėse&nbsp;kritulių kiekio stebėjimo stočių nedaug, jos toli viena nuo kitos, todėl&nbsp;labai aktualu turėti žemėlapius, tiksliai nusakančius gausių kritulių pasiskirstymą. Kalnuotose vietovėse&nbsp;krituliu pobūdis skirtingų atstumu gali būti skirtingas, kritulių kieki numatyti sunkiau, tačiau tai itin svarbu. Tiksliau numatant vietoves, kur iškrenta&nbsp;daugiau kritulių, galima tikslingiau planuoti žemėtvarką, išvengti staigiu potvynių, nuošliaužų ir dirvožemio erozijos. Tyrimo tikslas buvo patikrinti kelis gerai žinomus interpoliacijos metodus – Inverse Distance Weight&nbsp;(IDW) su 1–5 svertinemis vertėmis,&nbsp;Local Polynomial&nbsp;(LP),&nbsp;Radial Basis&nbsp;(RBS), ypač&nbsp;Spline With Tension&nbsp;(SPT),&nbsp;Thin Plate Spline&nbsp;(TPS), – bei&nbsp;Kriging&nbsp;technikas ir&nbsp;Ordinary Kriging&nbsp;(OK) bei&nbsp;Ordinary CoKriging&nbsp;(COK) metodus, siekiant parinkti tiksliausią, kuris leistų interpoliuoti gausius (95 %) kritulius RSL mėnesiais kalnuotoje Portugalijos teritorijoje. Nustatyta, kad kritulių kiekis didėja didėjant altitudei. Reikšmingi kritulių skirtumai nustatyti stotyse, esančiose skirtinguose aukščiuose virš jūros lygio. Šis santykis pastebėtas visomis kryptimis apskaičiuotose pusinėse variogramose. Jose išsiskiria dvi teritorijos, kurių P95 vertes didžiausios, ir akivaizdi sąsaja su dildelėmis altitudemis. Pirmoji teritorija 12,19 km, o antroji 23,57 km aukštyje. Didžiausios P95 vertes pietryčių ir šiaurės rytų kryptimis, mažiausios – šiaurės vakarų kryptimi, kur altitudės mažos, ir šiaurės rytų teritorijos kampe dėl lietaus “šešėlio” efekto. Tiksliai numatyti kritulių pobūdį kalnuotose teritorijose apsunkina duomenų trūkumas ir sudėtinga topografijos įtaka krituliams. Rezultatai ypač parankūs planuojant ir valdant dažniems ir gausiems krituliams jautrias teritorijas. Reikšminiai žodžiai: gausūs krituliai, pavojus aplinkai, Viduržemio regionas, rugsėjis, spalis, lapkritis (RSL), kalnuotos vietovės, interpoliacijos metodai, kraštovaizdžio planavimas ir aplinkos valdymas. Моделирование влияния распределения обильных осадков в опасных горных местностях. Помощь в планировании ландшафта и управлении&nbsp;окружающей средой Резюме.&nbsp;Обильные осадки, особенно характерные для Средиземноморского региона, могут быть опасными для окружающей среды. Зона влияния осадков особенно увеличивается в осенний период – сентябре, октябре и ноябре. Осадки зачастую вызывают большие экономические убытки, гибнут или страдают люди. Влияние обильных осадков усиливают крутые горные склоны. В горных местностях станций наблюдения за количеством осадков немного и расположены они далеко друг от друга, поэтому необходимы карты, на которых было бы точно указано распределение обильных осадков. Характер осадков в горных местностях в зависимости от удаленности может значительно различаться, предвидеть количество осадков чрезвычайно сложно, однако это жизненно необходимо. Зная местности, на которых ожидается большее количество осадков, можно точнее планировать землеустройство, избежать внезапных наводнений, оползней и эрозии почв. Целью исследования было проверить несколько хорошо известных методов интерполяции – Inverse Distance Weight (IDW) с 1–5 значениями весомостей, Local Polynomial (LP), Radial Basis (RBS), особенно Spline With Tension (SPT), Thin Plate Spline (TPS), а также технику Kriging, методы Ordinary Kriging (OK) и Ordinary CoKriging (COK) с целью выявить наиболее точный, с помощью которого можно было бы интерполировать обильные (95%) осенние осадки в горных местностях Португалии. Установлено, что количество осадков увеличивается с увеличением амплитуды. Значительная разница в количестве осадков выявлена на станциях, находящихся на разной высоте от уровня моря. Это соотношение замечено на всех направлениях рассчитанных полувариограмм. На них выделяются две территории, на которых значения P95 наиболее велики и очевидна связь с большими амплитудами. Первая территория находится на высоте 12,19 км, вторая – 23,57 км. Наибольшие значения P95 отмечены в юго-восточном и северо-восточном, наименьшие – северо-западном направлениях, где амплитуды малы, и в углу северо-восточной территории из-за эффекта «тени» дождя. Точнее предвидеть характер осадков в горных местностях представляется затруднительным из-за недостатка данных и сложного влияния на осадки топографии. Результаты исследования окажутся особенно полезными при планировании и управлении территориями, подвергаемыми частым и обильным осадкам. Ключевые слова: обильные осадки, опасность для окружающей среды, Средиземноморский регион, осенний период, горная местность, методы интерполяции, планирование ландшафта и управление окружающей средой. First Publish Online: 10 Feb 201

    Numerical simulation of subcontinent lithosphere dynamics: craton stability, evolution and formation

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    Through geodynamical modelling, two hypotheses about the craton stability and evolution were revisited and an important process of cratonization is investigated. Unlike most previous, related numerical studies, non-Newtonian rheology with composition dependence was used in these studies, and the rheological parameters are thus directly comparable with laboratory experiment of mantle. The first hypothesis, that the cratonic lithosphere is “isopycnic”, is found to be not strictly necessary for craton stability and longevity. The high viscosity of the cratonic litho- sphere due to compositional effects on the mantle rheology is found to be essential to maintain a thickness difference between cratonic and non-cratonic lithosphere for over billions of years and it allows a modest negative buoyancy of the cratonic root, depending on the strengthening factor due to the compositional effects. The second hypothesis to be tested is that mantle plume im- pingements cause rapid, significant removal of subcontinental lithosphere. The results presented in this thesis show that the erosion caused by a plume impact on a continent that is strong enough to have survived billions of years of Earth’s history is rather limited. A special weaken- ing mechanism of such highly viscous and buoyant roots is required to reactivate this cratonic lithosphere and thus cause significant thinning within 10s of Myrs. The fluid/melt-rock interac- tion during mantle metasomatism is probably the most likely mechanism to modify and weaken depleted cratonic lithosphere. Therefore, metasomatic weakening is essential for the significant thinning of subcontinental lithosphere observed, e.g.at North China Craton and Namibia, south- ern African, no matter whether caused by a plume impact or another tectonic event. Using the reasonable compositional effects on the buoyancy and rheology of mantle rocks from the above studies, numerical experiments are performed to study the formation of thick cratonic lithosphere from a layered, depleted mantle material. In this scenario, substantial tec- tonic shortening and thickening of previously depleted material seems to be an essential ingre- dient to initiate the cratonization process. Afterwards, gravitational self-thickening will cause further thickening. Compositional buoyancy resists Rayleigh-Taylor instability collapse and stabilizes the thick cratonic root, while the secular cooling also has a stabilizing effect on the cratonic root by reducing the thermal buoyancy contrast between lithosphere and asthenosphere and increasing mantle viscosity. The presented numerical results are consistent with the vertical movement of cratonic peridotite as suggested on petrological grounds
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