@article{ChambodutMandea2005,
  author    = {Chambodut, Aude and Mandea, Mioara},
  title     = {Evidence for geomagnetic jerks in comprehensive models},
  issn      = {1343-8832},
  year      = {2005},
  abstract  = {The rate of secular variation occasionally undergoes a sudden, sharp change, called a geomagnetic jerk. Such jerks have been detected in geomagnetic time series, centered-over the last four decades-around 1971, 1980, 1991, and 1999; others have been inferred from historical records. The geomagnetic jerks represent a reorganization of the secular variation, implying an internal origin, as established through spherical harmonic and wavelet analysis. However, some characteristics of jerks are not well understood. Here we estimate the occurrence dates for geomagnetic jerks, as they can be detected from a global geomagnetic model. This choice makes the present study novel, for two reasons. First, utilizing the comprehensive modelling approach allows for the use of a secular variation signal free of time-varying external fields and their corresponding induced counterpart, and observatory biases. Second, the model utilizes satellite data when available, in addition to observatory data. Indeed, POGO (1967 to 1971), MAGSAT (1979 to 1980), Orsted (1999 to present time) and CHAMP (2000 to present time) satellite measurements help to separate the different magnetic sources. In this study the CM4 comprehensive model is used for a global search of geomagnetic jerks and their occurrence dates. Our first result indicates that found geomagnetic jerks might not have been worldwide in occurrence. Moreover, the obtained dates suggest that jerks detected in the CM4 model over the last four decades occurred not simultaneously but at slightly different times around 1971, 1980 and 1991},
  language  = {en}
}
@article{ChambodutPanetMandeaetal.2005,
  author    = {Chambodut, Aude and Panet, I. and Mandea, Mioara and Diament, M. and Holschneider, Matthias and Jamet, O.},
  title     = {Wavelet frames : an alternative to spherical harmonic representation of potential fields},
  issn      = {0956-540X},
  year      = {2005},
  abstract  = {Potential fields are classically represented on the sphere using spherical harmonics. However, this decomposition leads to numerical difficulties when data to be modelled are irregularly distributed or cover a regional zone. To overcome this drawback, we develop a new representation of the magnetic and the gravity fields based on wavelet frames. In this paper, we first describe how to build wavelet frames on the sphere. The chosen frames are based on the Poisson multipole wavelets, which are of special interest for geophysical modelling, since their scaling parameter is linked to the multipole depth (Holschneider et al.). The implementation of wavelet frames results from a discretization of the continuous wavelet transform in space and scale. We also build different frames using two kinds of spherical meshes and various scale sequences. We then validate the mathematical method through simple fits of scalar functions on the sphere, named 'scalar models'. Moreover, we propose magnetic and gravity models, referred to as 'vectorial models', taking into account geophysical constraints. We then discuss the representation of the Earth's magnetic and gravity fields from data regularly or irregularly distributed. Comparisons of the obtained wavelet models with the initial spherical harmonic models point out the advantages of wavelet modelling when the used magnetic or gravity data are sparsely distributed or cover just a very local zone},
  language  = {en}
}
@article{GeeseMandeaLesuretal.2010,
  author    = {Geese, Anne and Mandea, Mioara and Lesur, Vincent and Hayn, Michael},
  title     = {Regional modelling of the Southern African geomagnetic field using harmonic splines},
  issn      = {0956-540X},
  doi       = {10.1111/j.1365-246X.2010.04575.x},
  year      = {2010},
  abstract  = {Over the southern African region the geomagnetic field is weak and changes rapidly. For this area series of geomagnetic field measurements exist since the 1950s. We take advantage of the existing repeat station surveys and observatory annual means, and clean these data sets by eliminating jumps and minimizing external field contributions in the original time-series. This unique data set allows us to obtain a detailed view of the geomagnetic field behaviour in space and time by computing a regional model. For this, we use a system of representation similar to harmonic splines. Initially, the technique is systematically tested on synthetic data. After systematically testing the method on synthetic data, we derive a model for 1961-2001 that gives a detailed view of the fast changes of the geomagnetic field in this region.},
  language  = {en}
}
@article{ZaourarHamoudiHolschneideretal.2013,
  author    = {Zaourar, Naima and Hamoudi, Mohamed and Holschneider, Matthias and Mandea, Mioara},
  title     = {Fractal dynamics of geomagnetic storms},
  series = {Arabian journal of geosciences},
  volume    = {6},
  journal   = {Arabian journal of geosciences},
  number    = {6},
  publisher = {Springer},
  address   = {Heidelberg},
  issn      = {1866-7511},
  doi       = {10.1007/s12517-011-0487-0},
  pages     = {1693 -- 1702},
  year      = {2013},
  abstract  = {We explore fluctuations of the horizontal component of the Earth's magnetic field to identify scaling behaviour of the temporal variability in geomagnetic data recorded by the Intermagnet observatories during the solar cycle 23 (years 1996 to 2005). In this work, we use the remarkable ability of scaling wavelet exponents to highlight the singularities associated with discontinuities present in the magnetograms obtained at two magnetic observatories for six intense magnetic storms, including the sudden storm commencements of 14 July 2000, 29-31 October and 20-21 November 2003. In the active intervals that occurred during geomagnetic storms, we observe a rapid and unidirectional change in the spectral scaling exponent at the time of storm onset. The corresponding fractal features suggest that the dynamics of the whole time series is similar to that of a fractional Brownian motion. Our findings point to an evident relatively sudden change related to the emergence of persistency of the fractal power exponent fluctuations precedes an intense magnetic storm. These first results could be useful in the framework of extreme events prediction studies.},
  language  = {en}
}
@article{HaynPanetDiamentetal.2012,
  author    = {Hayn, Michael and Panet, I. and Diament, M. and Holschneider, Matthias and Mandea, Mioara and Davaille, A.},
  title     = {Wavelet-based directional analysis of the gravity field evidence for large-scale undulations},
  series = {Geophysical journal international},
  volume    = {189},
  journal   = {Geophysical journal international},
  number    = {3},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0956-540X},
  doi       = {10.1111/j.1365-246X.2012.05455.x},
  pages     = {1430 -- 1456},
  year      = {2012},
  abstract  = {In the eighties, the analysis of satellite altimetry data leads to the major discovery of gravity lineations in the oceans, with wavelengths between 200 and 1400 km. While the existence of the 200 km scale undulations is widely accepted, undulations at scales larger than 400 km are still a matter of debate. In this paper, we revisit the topic of the large-scale geoid undulations over the oceans in the light of the satellite gravity data provided by the GRACE mission, considerably more precise than the altimetry data at wavelengths larger than 400 km. First, we develop a dedicated method of directional Poisson wavelet analysis on the sphere with significance testing, in order to detect and characterize directional structures in geophysical data on the sphere at different spatial scales. This method is particularly well suited for potential field analysis. We validate it on a series of synthetic tests, and then apply it to analyze recent gravity models, as well as a bathymetry data set independent from gravity. Our analysis confirms the existence of gravity undulations at large scale in the oceans, with characteristic scales between 600 and 2000 km. Their direction correlates well with present-day plate motion over the Pacific ocean, where they are particularly clear, and associated with a conjugate direction at 1500 km scale. A major finding is that the 2000 km scale geoid undulations dominate and had never been so clearly observed previously. This is due to the great precision of GRACE data at those wavelengths. Given the large scale of these undulations, they are most likely related to mantle processes. Taking into account observations and models from other geophysical information, as seismological tomography, convection and geochemical models and electrical conductivity in the mantle, we conceive that all these inputs indicate a directional fabric of the mantle flows at depth, reflecting how the history of subduction influences the organization of lower mantle upwellings.},
  language  = {en}
}
@article{QuesnelWeckmannRitteretal.2009,
  author    = {Quesnel, Yoann and Weckmann, Ute and Ritter, Oliver and Stankiewicz, Jacek and Lesur, Vincent and Mandea, Mioara and Langlais, Benoit and Sotin, Christophe and Gald{\´e}ano, Armand},
  title     = {Simple models for the Beattie Magnetic Anomaly in South Africa},
  issn      = {0040-1951},
  doi       = {10.1016/j.tecto.2008.11.027},
  year      = {2009},
  abstract  = {The origin of the approximately 1000 km-long Beattie Magnetic Anomaly (BMA) in South Africa remains unclear and contentious. Key issues include the width, depth and magnetization of its source. In this study, we use uniformly magnetized spheres, prisms and cylinders to provide the simplest possible models which predict the 1 km-altitude aeromagnetic measurements along a profile across the BMA. The source parameters are adjusted by forward modeling. In case of a sphere, an inversion technique is applied to refine the parameters. Our results Suggest that two similarly magnetized and adjacent sources. With a vertical offset, can explain the observed magnetic anomaly. The best fitting model corresponds to two highly-magnetized (>5 A m(-1)) sheet-like prisms, extending from 9 to 12 kill depth, and from 13 to 18 kill depth, respectively, and with a total width reaching 80 km. Other less-preferred models show thicker and deeper magnetized volumes. Associated magnetizations seem to be mostly induced, although a weak remanent component is required to improve the fit. We also compare our results With the interpretation of independent magnetotelluric and seismic experiments along the same profile. It suggests that the geological sources for the BMA are mostly located in the middle crust and may be displaced by a shear zone or a fault. Contrary to previous models suggesting a serpentinized sliver of paleo-oceanic crust within the Natal-Namaqua Mobile Belt, we propose that granulite-facies mid-crustal rocks within this belt may cause the BMA.},
  language  = {en}
}