Author(s)De Michelis, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia
Cafarella, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia
Meloni, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia
04. Solid Earth::04.05. Geomagnetism::04.05.02. Geomagnetic field variations and reversals
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AbstractA recent examination of the geomagnetic annual mean values for the European magnetic observatories has shown the existence of a sudden change in the secular acceleration in about 1991 (Cafarella & Meloni 1995; Macmillian 1996). Using first differences of the Y (east geomagnetic field component) mean values from 74 observatories, the worldwide character of the 1991 impulse has been determined (De Michelis et al. 1998). Using data from 109 observatories widely distributed all over the world, the structure of the secular variation for the X (north) and Z (vertical) magnetic field intensities around 1990 was investigated, and evidence of this most recent jerk was found. External effects were removed from the annual mean data by comparing the long-term variations of the geomagnetic field components at individual observatories with the long-term variations of two geomagnetic indices, aa and Dst, and of a solar index, the Wolf number R. A careful analysis has been carried out on the amplitude of the external disturbance, on its dependence on latitude, and on the weights of the geomagnetic indices in the evaluation of the resulting external field. The secular variation has been evaluated from the corrected annual means. Around 1990, the secular variation can be fitted at many observatories by two straight lines with a sudden and marked change in slope. In this manner the jerk occurrence time and the intensity of the step in the second time derivative (∆X", ∆Y" and ∆Z") were computed. Maps of ∆X", ∆Y" and ∆Z" provide information on the worldwide intensity distribution of the examined event. Maps of the jerk occurrence-time distributions are also given. The mean jerk occurrence time is 1990.1±0.6. Finally, a spherical harmonic analysis was used to complete the quantitative description of this phenomenon in order to study the trend of the energy density spectrum as a function of the harmonic degree n.
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Improving total field geomagnetic secular variation modeling from a new set 3 of cross-over marine dataPavón-Carrasco, F. J.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Torta, J. M.; Observatori de l'Ebre (Spain); Catalán, M.; Geophysical Dpt., Royal Spanish Naval Observatory (Spain); Talarn, A.; Observatori de l'Ebre; Ishihara, T.; Institute of Geology and Geoinformation, AIST (Japan) (Elsevier Science Limited, 2013-03)A new set of cross-over marine data has been used to generate a regional model for the secular variation of the total geomagnetic field, showing the potential of the suggested approach for gaining a better knowledge of the field over oceanic regions. The model, which is valid for the Northern Atlantic region during the temporal interval 1960–2000, was obtained using spherical cap harmonic analysis (SCHA) in space and penalized splines in time. The maximum spatial expansion is equivalent to degree 9 in ordinary spherical harmonic analysis. Annual mean intensity data from different geomagnetic observatories have been used to improve the spatial and temporal resolution of the original dataset. Results indicate that the regional model improves, in terms of the root mean square error, the prediction given by the 11th generation of IGRF and CM4 global models, especially for the geomagnetic observatories considered. We also provide the uncertainty of the model coefficients and the secular variation prediction given by a bootstrap algorithm. The model is available in the EarthRef. org Digital Archive at http://earthref.org/ERDA/1728/.
Geomagnetic jerks in the polar regionsTozzi, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; De Michelis, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Meloni, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia (AGU, 2009-08-08)The occurrence of geomagnetic jerks over the Arctic and Antarctic regions is here investigated. Maps of geomagnetic secular acceleration over the polar regions are produced from the CM4 and CHAOS models and the occurrence of geomagnetic jerks is associated with jumps in secular acceleration. The obtained results confirm that in Antarctica geomagnetic jerks systematically follow geomagnetic jerks in the Arctic region with a time delay from one to three years. Evidence is found of an abrupt change in secular acceleration in both polar regions around 1985, suggesting that the 1985 local jerk could actually be a worldwide event. Combining our results with the results previously obtained on the occurrence of a geomagnetic jerk at low-mid latitudes around 2003, we support the hypothesis of a global extension of an event occurred at the beginning of the 21st century.
A Holocene paleosecular variation record from the northwestern Barents Sea continental marginSagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Macrì, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Lucchi, R.; Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Borgo Grotta Gigante 42/c, Sgonico, Trieste I‐34010, Italy; Rebesco, M.; Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Borgo Grotta Gigante 42/c, Sgonico, Trieste I‐34010, Italy; Camerlenghi, A.; Istitució Catalana de Recerca i Estudis Avançats, E‐08028 Barcelona, Spain (American Geophysical Union, 2011-11-01)A high‐resolution paleomagnetic and rock magnetic study has been carried out on sediment cores collected in glaciomarine silty‐clay sequences from the continental shelf and slope of the southern Storfjorden trough‐mouth fan, on the northwestern Barents Sea continental margin. The Storfjorden sedimentary system was investigated during the SVAIS and EGLACOM cruises, when 10 gravity cores, with a variable length from 1.03 m to 6.41 m, were retrieved. Accelerator mass spectrometry (AMS) 14C analyses on 24 samples indicate that the cores span a time interval that includes the Holocene, the last deglaciation phase and in some cores the last glacial maximum. The sediments carry a well‐defined characteristic remanent magnetization and have a valuable potential to reconstruct the paleosecular variation (PSV) of the geomagnetic field, including relative paleointensity (RPI) variations. The paleomagnetic data allow reconstruction of past dynamics and amplitude of the geomagnetic field variations at high northern latitudes (75°–76° N). At the same time, the rock magnetic and paleomagnetic data allow a high‐resolution correlation of the sedimentary sequences and a refinement of their preliminary age models. The Holocene PSV and RPI records appear particularly sound, since they are consistent between cores and they can be correlated to the closest regional stacking curves (UK PSV, FENNOSTACK and FENNORPIS) and global geomagnetic model for the last 7 ka (CALS7k.2). The computed amplitude of secular variation is lower than that outlined by some geomagnetic field models, suggesting that it has been almost independent from latitude during the Holocene.