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Duka, Bejo
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Duka, Bejo
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- PublicationRestrictedGeomagnetic jerks as chaotic fluctuations of the Earth’s magnetic field(2013-04-12)
; ; ; ; ; ; ;Qamili, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Isac, A.; Geological Institute of Romania, Bucharest, Romania ;Mandea, M.; Centre National d’Etudes Spatiales, Paris, France ;Duka, B.; Faculty of Natural Sciences, University of Tirana, Tirana, Albania ;Simonyan, A.; Institute of Geophysics, Academy of Science, Yerevan, Armenia; ; ; ; ; The geomagnetic field is chaotic and can be characterized by a mean exponential time scaleafter which it is no longer predictable. It is also ergodic, so time analyses can substitute the more difficult phase space analyses. Taking advantage of these two properties of the Earth’s magnetic field, a scheme of processing global geomagnetic models in time is presented, to estimate fluctuations of the time scale t. Here considering that the capability to predict the geomagnetic field is reduced over periods of geomagnetic jerks, we propose a method to detect these events over a long time span. This approach considers that epochs characterized by relative minima of fluctuations in time scale t, i.e., those periods when a geomagnetic field is less predictable, are possible jerk occurrence dates. We analyze the last 400 years of the geomagnetic field (covered by the Gufm1 model) to detect minima of fluctuations, i.e., epochs characterized by low values of the time scale.Most of the well known jerks are confirmed through this method and a few others have been suggested. Finally, we also identify some short periods when the field is less chaotic (more predictable) than usual, naming these periods as steady state geomagnetic regime, to underline their opposite behavior with respect to jerks.319 39 - PublicationOpen AccessComparison of different methods of analysis of satellite geomagnetic anomalies over Italy(1998-04)
; ;Duka, B.; Department of Physics, Faculty of Natural Sciences, University of Tirana, AlbaniaDifferent methods of analysis have been applied to satellite geomagnetic data that are claimed to be representative of the crustal geomagnetic field of the Italian area. The methods are compared with each other concluding that the SCHA is the best one. The downward continuation of the field by different methods gives different results, the most realistic are those of SCHA and RHA methods. Some remarks about each method and their results for the Italian area are presented.242 200 - PublicationOpen AccessA numerical experiment on the spherical harmonic analysis of core and crustal geomagnetic fields(1993)
; ;Duka, B.; Faculty of Natural Science,s, Department of Physics, University of Tirana, Albania152 183 - PublicationRestrictedUsing ‘‘domino’’ model to study the secular variation of the geomagnetic dipolar moment(2015-03-14)
; ; ; ; ;Duka, B.; Dept. of Physics, Faculty of Natural Sciences – University of Tirana, Albania ;Peqini, K.; Dept. of Physics, Faculty of Natural Sciences – University of Tirana, Albania ;De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Pavón-Carrasco, F. J.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; ; ; Aiming to understand the physical processes underneath the reversals events of geomagnetic field, different numerical models have been conceived. We considered here the so named ‘‘domino’’ model, an Ising–Heisenberg model of interacting magnetic macrospins aligned along a ring. This model was proposed by Mazaud and Laj (1989) and then applied by Mori et al. (2013) to study geomagnetic field reversals. The long series of the axial magnetic moment (dipolar moment or ‘‘magnetization’’) generated by the ‘‘domino’’ model are empirically studied by varying all model parameters. We present here some results which are slightly different from those given by Mori et al. (2013), and will provide our explanation on the presence of these differences. We also define the set of parameters that supply the longest mean time between reversals. Using this set of parameters, a large number of time series of axial magnetic moment are also generated. After de-noising the fluctuation of these time series and averaging them, we compared the resulting averaged series with the series of axial dipolar magnetic moment values supplied by CALS7k.2, and CALS10k.1b models, finding similar behavior for the all time series. In a similar way, we also compared the averaged 14,000 years long series of dipolar moment with the dipolar magnetic moment obtained by the model SHA.DIF.14k.279 97 - PublicationRestrictedOn the Modeling of a Geomagnetic Reference Field for the Balkan Region(2006)
; ; ; ;Duka, B.; Department of Physics, Faculty of Natural Sciences, University of Tirana, Albania ;De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Gaya-Piqué, L. R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; ; ; ; ; ;Rasson, J. L.; Institut Royal Météorologique de Belgique, Dourbes, Belgium ;Delipetrov, T.; Department of Geology and Geophysics, Faculty of Mining and Geology, 2000 Štip, Macedonia; This paper presents the situation of the geomagnetic measurements over Albanian territory, distinguishing the campaigns of 1942.0, 1961.0, 1990.4, 1994.75 and 2004.7 epochs. The results of the latest model of the geomagnetic field for Southern Italy and Albanian territory are therefore shown. This model was created using SCHA (Spherical Cap Harmonic Analysis) technique, applied on all ground data from three different epochs (1990.4, 1994.5 and 2003.7) and a selected dataset from Oersted satellite mission. The ground data were confined in a small Spherical Cap that we expect to enlarge with ground data from adjoining countries in order to compose an accurate Balkan Geomagnetic Reference Field Model.170 26 - PublicationOpen AccessA GEOMAGNETIC REFERENCE MODEL FOR ALBANIA, SOUTH-EAST ITALY AND THE IONIAN SEA FROM 1990 TO 2005 WITH PREDICTION TO 2008.(2006)
; ; ; ; ; ; ; ; ;Section Roma 2, Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy; Email: desantisag@ingv.it ;Section Roma 2, Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy; Email: desantisag@ingv.it ;Science Faculty, Tirana University, Tirana, Albania;Email: bejo_duka@yahoo.com ;Section Roma 2, Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy; Email: desantisag@ingv.it ;Academy of Sciences of Albania, Tirana, Albania; Email: sbushati@akad.edu.al ;Section Roma 2, Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy; Email: desantisag@ingv.it ;UMBC/GEST at NASA/GSFC,Planetary Geodynamics, Greenbelt, USA;Email: gaya@geomag.gsfc.nasa.gov ;Qamili, E. ;De Santis, A. ;Duka, B. ;Dominici, G. ;Bushati, S. ;Cafarella, L. ;Miconi, M.Gaya-Pique, L.From the three-component magnetic field observations taken from 1988 to 2005 during the measurement of Albanian and Italian magnetic repeat station network, as well as from a selected set of Ørsted satellite total field measurements, a geomagnetic reference model is determined for the region comprising the Albanian territory, the south-east part of the Italian Peninsula and the Ionian Sea. This model is designed to represent the components X,Y,Z and the total intensity F of the main geomagnetic field for epochs between 1990 and 2005 with prediction to 2008. The model is based on an expansion of the magnetic potential in terms of spherical cap harmonics applied to a cap of semi angle 8≅ . The model can be used as a reference model to reduce magnetic surveys undertaken in the area during the time of validity of the model, including also epochs up to 2008 for which the model is predictive.396 187 - PublicationOpen AccessUsing neural networks to study the geomagnetic field evolution(2008-10)
; ; ;Duka, B.; Department of Physics, Faculty of Natural Sciences, University of Tirana ;Hyka, N.; Department of Physics, Faculty of Natural Sciences, University of Tirana; study their time evolution in years. In order to find the best NN for the time predictions, we tested many different kinds of NN and different ways of their training, when the inputs and targets are long annual time series of synthetic geomagnetic field values. The found NN was used to predict the values of the annual means of the geomagnetic field components beyond the time registration periods of a Geomagnetic Observatory. In order to predict a time evolution of the global field over the Earth, we considered annual means of 105 Geomagnetic Observatories, chosen to have more than 30 years registration (1960.5-2005.5) and to be well distributed over the Earth. Using the NN technique, we created 137 «virtual geomagnetic observatories» in the places where real Geomagnetic Observatories are missing. Then, using NN, we predicted the time evolution of the three components of the global geomagnetic field beyond 2005.5.348 247 - PublicationRestrictedThe Albanian Geomagnetic Repeat Station Network at 1994.75(1997)
; ; ; ; ; ; ; ; ; ; ;A new national geomagnetic network of repeat stations for total field F, horizontal component H, vertical component Z and declination D has been established in a collaboration between the Albanian Geological Society (Center for Geochemistry and Geophysics), the Tirana University (Albania) and the Istituto Nazionale di Geofisica (Italy). From the observed magnetic field elements all repeat stations were referred to Epoch 1994.75 and normal reference fields were computed in the form of 1st order polynomials in latitude and longitude. An overview of all work, including a brief history of the Albanian magnetic measurements, magnetic survey results, normal fields and secular variation estimates is presented.243 5 - PublicationOpen AccessCrustal field recovery and secular variation from regional and global models over AlbaniaThe static geomagnetic field of crustal origin is optionally calculated bythe recent global geomagnetic field models. However, their description in global scale tends to miss some local characteristics. The same can be inferred for the rate of the geomagnetic field changes i.e. secular variation (SV). In order to depict some particularity of crustal field in local scale for small region like as Albania, two regional models are constructed: one based on the Legendre’s polynomials and the other based on a linear approximation. Both models use data from different measurement campaigns in the Albanian repeat stations and few data from neighbourhood countries. The residuals produced by these models and by the recent global models: EMM2015, POMME – 9 and CM5 are calculated and compared. The SV from regional and global models following standard procedures are also calculated. Substantial differences between SV calculated by global models and regional models are observed.
155 49 - PublicationRestrictedA discussion of the uniqueness of a Laplacian potential when given only partial field information on a sphere(1995)
; ; ; ;Lowes, F. J.; Physics Department, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK ;De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Duka, B.; Faculty of Natural Sciences, Department of Physics, University of Tirana, Tirana, Albania; ; For a vector field defined by a scalar potential outside a surface enclosing all the sources, it is well known that the potential is defined uniquely if either the potential itself, or its derivative normal to the surface, is known everywhere on the surface. For a spherical surface, the normal derivative is the radial component of the field; the horizontal (vector) component of the field also gives uniqueness (except for any monopole contribution). This paper discusses the way other partial information of the field on the spherical surface can give a unique, or almost unique, knowledge of the external potential/field, bringing together and correcting previous work. For convenience the results are given in the context of the geomagnetic field B. This is often expressed in terms of its local Cartesian components (X, Y, Z), equivalent to (-Bθ, BΦ, -Br); it can also be expressed in terms of Z and the vector horizontal component H = (X, Y). Alternatively, local "spherical polar" components (F, I, D) are used, where F = |B|, the inclination I is the angle in the vertical plane downward from H to B, and the declination D is the angle in the horizontal plane eastward from north to H. Knowledge of X over the sphere gives a complete knowledge of the potential, apart from that of any monopole (which is zero in geomagnetism), and Y gives the potential except for any axially symmetric part (which can be provided by a knowledge of X along a meridian, or of H along any path from pole to pole). In terms of (F, I, D) the situation is more complicated; either F or the total angle (I, D) needs to be known throughout a finite volume; for the latter, this paper shows how, in principle, the actual potential can be determined (except for an unknown scaling factor). Similarly D on the sphere also needs a knowledge of |H| on a line from (magnetic) pole to pole. We also discuss how these various properties affect the determination, by surface integration, of the Gauss coefficients of the field representation in terms of spherical harmonics.266 26