A statistical investigation of polar auroras     

Maurizio Giorgi  Giovanni P. Gregori  Carlo Valenti 《Pure and Applied Geophysics》1968,71(1):42-50

Summary The magnetosphere depends on the astronomical orientation of the geomagnetic field with respect to the solar wind. The statistical distribution of polar auroras must therefore depend on the orientation of the geomagnetic field with respect to the ecliptic plane. We have investigated this peculiar feature of auroras that we call auroral astronomical geometry. We give here some preliminary results concerning a limited set of pre-IGY auroras. The criteria that we have chosen to prepare the auroral collection are also briefly summarized. The results conform to the hypothesis of the auroral origin from the magnetospheric neutral sheet. Auroral particles are found to impinge over the earth with low angles with respect to the ecliptic plane (40°). Only in a 4-hours interval around midnight they are found to impinge with angles up to 70°. Definite evidence of these facts requires further investigation with a larger amount of data. — We have also prepared a complex code for recording the morphological features of each aurora, namely: standard information, movement. intensity, color, sunlight illumination, period of pulsation, location in the sky among stars and planets, time evolution, duration and general features of the auroral display. It is well known in fact that the auroral morphology affects auroral heights and latitudes, etc.; presumably it should also affect its astronomical geometry, which we will investigate later.Presented at the Inter-Union Symposium on Solar-Terrestrial Physics — Belgrade 1966.  相似文献   

Astrofix en coordonnees rectangulaires: Methode de bhattacharji,methode de dollen et methode de dollen generalisee     

A. Vassallo 《Journal of Geodesy》1980,54(2):213-220

Résumé Dans la première partie de cette note on met en évidence l’efficacité et la simplicité de résolution que les coordonnées rectangulaires offrent dans les problèmes de l’astronomie géodésique. Dans la dernière partie on expose une nouvelle méthode pour la détermination astronomique du point.  相似文献   

Statistical analysis of the largest possible earthquake magnitudes on the Ecuadorian coast for selected return periods     

Sandra García-Bustos  Jimmy Landín  Ricardo Moreno  A.S.E. Chong  Maurizio Mulas  Mónica Mite 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2020,14(1):56-68

ABSTRACT

In this work, we have studied the largest earthquake magnitudes on the Ecuadorian coast by using the principles of Extreme Value Analysis based on its two approaches: Block Maxima and Peaks-over-Threshold. First, before modelling the recorded earthquakes, the K-means clustering technique was applied to determine a classification according to the level of magnitude of the earthquakes. Then, models based on the Extreme Value theory of earthquake magnitudes were developed for each of the four clusters that were found, and finally, the best-fitted models were those known as Fréchet and Gumbel ones. The zone with the greatest earthquake magnitudes on the Ecuadorian coast is located between the north of the province of Manabí and the south of the province of Esmeraldas, with a return period of 50 years for an earthquake with magnitude greater than 7.7 M_W.  相似文献   

Pressure-induced phase transition in Mg0.8Fe0.2O ferropericlase     

Innokenty Kantor  Leonid Dubrovinsky  Catherine McCammon  Anastasia Kantor  Sakura Pascarelli  Giuliana Aquilanti  Wilson Crichton  Maurizio Mattesini  Rajeev Ahuja  Jailton Almeida  Vadim Urusov 《Physics and Chemistry of Minerals》2006,33(1):35-44

Combined X-ray powder diffraction, Mössbauer, and XANES spectroscopy in situ experiments revealed the transformation of cubic (Mg_0.8Fe_0.2)O ferropericlase to a rhombohedrally distorted phase at 35(1) GPa and room temperature. The Mössbauer spectroscopy results show that the rhombohedral distortion does not involve magnetic ordering. Combined with data from the literature, our results imply that the cubic to rhombodedral transition occurs in (Mg,Fe)O under conditions of non-hydrostatic stress over a wide range of composition (0.2≤x _Fe≤1).  相似文献   

Evidence for gas influence on volcanic seismic signals recorded at Stromboli     

Maurizio Ripepe 《Journal of Volcanology and Geothermal Research》1996,70(3-4)

The meaning of the large variety of seismic waveforms showing different frequencies associated with volcanic activity is still uncertain. No definitive model for source dynamics has been proposed yet. At present, seismic models explaining the origin of the spectral content of the seismic signal are mainly linked to geometrical features of the volcanic conduit and to resonant effects induced by pressure fluctuations in a fluid-filled conduit. Such models assume the physical system to be in a steady state over a long interval of time. At Stromboli, the seismicity produced by each of the three active vents is not stable and can suddenly change in time. Therefore, the application of stable resonator models appears to be unwarranted.On the basis of infrared image analyses, atmospheric pressure, and seismic wavefield signals, we show that different frequency contents and different explosive styles occur at Stromboli at the same time at different vents. Moreover, we give evidence to support the idea that seismicity and explosivity are both controlled by a variable gas flow regime released during explosions at the top of the magma column. We have recognized two main pressure regimes, controlling the explositivity at Stromboli: one is impulsive, short and possibly produced by a gas accumulation beneath a cap rock; the other has an intermittent regime feature, and lasts longer. We demonstrate how the first one generates a monochromatic low-frequency wavefield, while the second shows a high-frequency spectrum where the frequency content depends on pressure fluctuations and on pressure gradient. We suggest different pressure growth and gas flow in the magma as the common source for both explosive style and seismic wavefield.  相似文献   

Crystal chemical variations in Ba-rich biotites from gabbroic rocks of lower crust (Ivrea Zone,NW Italy)     

Simona Bigi  Maria Franca Brigatti  Maurizio Mazzuchelli  Giorgio Rivalenti 《Contributions to Mineralogy and Petrology》1993,113(1):87-99

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.282 (up to 7.14 and 9.32 wt%, respectively) with respect to those of the diorites (up to 1.26 and 6.26 wt%, respectively). Systematic compositional variations support the substitution model 2 ^IV Si+( ^IV R²⁺)2 ^IV Al+ ^VI Ti (R²⁺=Fe+Mg+Mn) in gabbros and ^IV Si+ ^VI Al ^IV Al+ ^VI Ti in diorites. A predominance of disordered stacking sequences, coexisting with 1M, 2M ₁ and 3T polytypes was observed in all biotites. It was possible to carry out structural refinements only on three biotites-2M ₁ from diorites (R-values between 2.68 and 3.77) and one biotite-1M from gabbros (R-value=3.09). It was shown that: (1) the reduced thickness of the tetrahedral sheet in Ba-rich biotites supports the coupled substitution ^IV Si+ ^XII K ^IV Al+ ^XII Ba; (2) the interlayer site geometry is affected by the whole layer chemistry and does not reflect only local chemical variations; (3) in two samples of the 2M ₁ polytype, the M(1) octahedral site is larger and more distorted than the M(2) sites because of the preferential ordering of Fe²⁺ in the M(1) site, whereas one sample shows complete cation disorder in the octahedral sites. Biotite-1M shows that Fe²⁺ can also be located in the M(2) site. Some of the differences between the biotites of gabbros and diorites (e.g. Ba concentration and exchange vectors) may be linked to the host rock composition and to its crystallization process. Biotite occurs in trace amounts in gabbros and its crystallization is related to the interstitial melt which contributed to the adcumulus growth of the main rock forming phases and became highly enriched in K, Ba and Ti. Diorites are the result of equilibrium crystallization of a residual melt rich in incompatible elements, where biotite is a major constituent.  相似文献   

Micro-Analytical Zircon and Monazite U-Pb Isotope Dating by Laser Ablation-Inductively Coupled Plasma-Quadrupole Mass Spectrometry     

Keegan E. Alagna  Maurizio Petrelli  Diego Perugini  Giampiero Poli 《Geostandards and Geoanalytical Research》2008,32(1):103-120

In this study we evaluated the capability of a 213 nm laser ablation system coupled to a quadrupole-based ICP-MS in delivering accurate and precise U-Pb ages on zircons and monazites. Four zircon samples ( ca. 50 Ma to ca. 600 Ma) and four monazite samples ( ca. 30 Ma to ca. 1390 Ma) of known ages were analysed utilising laser ablation pits with diameters of 20 μm and 60 μm. Instrument mass bias and laser induced time-dependent elemental fractionation were corrected for by calibration against a matrix-matched reference material. Tera-Wasserburg plots of the calculated U-Pb data were employed to assess, and correct for, common Pb contributions. The results indicated that the LA-ICP-MS technique employed in this study allowed precise and accurate U-Pb isotope dating of zircon and monazite on sample areas 20 μm in diameter. At this spot size, the precisions achieved for single spot ²⁰⁶Pb/²³⁸U ages, were better than 5% (2s) for monazites and zircons with ages down to 30 Ma and 50 Ma, respectively. The precisions reported are comparable to those generally reported in SIMS and LA-MC-ICP-MS U-Pb isotope determinations.  相似文献   

Planktonic foraminiferal distribution record productivity cycles: evidence from the Aptian-Albian Piobbico core (central Italy)     

lsabella Premoli Silva  Maurizio Ripepe  M. Emilia Tornaghi 《地学学报》1989,1(5):443-448

The Aptian-Albian 'Scisti a Fucoidi' varicoloured pelagic sediments in central Italy, show a 'couplet' alternation of carbonate-rich/carbonate-poor layers, which are interpreted as the sedimentary expression of precession (frequency 19–23 kyr). Carbonate content, chromatic variation, and planktonic foraminiferal abundance were analysed at a 1-cm spacing for a 10-m interval of the Piobbico core, specifically drilled through this formation. Spectral analysis of these parameters shows a prominent signal equated to the c . 100 kyr cycle of orbital eccentricity at a sedimentation rate of 5 mm kyr⁻¹. The coherency of the spectral response of each parameter suggests that a single mechanism controlled the whole sedimentary record. Detailed study of planktonic foraminiferal distribution of the same section at 1-mm scale resolves the Milankovitch frequencies of 41 kyr and 18 to 23 kyr, equated with the obliquity and precessional cycles. But foraminiferal abundance is not in phase with carbonate content, which was largely controlled by calcareous nannofossils, but peaked at intermediate carbonate values. The proposed model for explaining the discrepancy at the precessional level is that foraminifera thrived at intermediate values of the precession index, when the environment was only moderately fertile but stable, while during highs of the precession index, mixing of the water column increased fertility and caused calcareous nannofossil blooms and restriction of planktonic foraminifera to few and tolerant species. The resulting bimodality of foraminiferal abundance per precessional cycle appears to be recorded in the spectrum by peaks at the 11 and 14 kyr levels. Cross correlation of foraminiferal abundances with the calcium carbonate curve over 1–2 Myr intervals produces discrepant results (apparent phase lags) which we attribute to differences in the response to the fundamental eccentricity cycles.  相似文献   

Geophysical and structural aspects of fault mechanics—A brief historical review     

Maurizio Bonafede  Jürgen Strehlau  A. Reinier Ritsema 《地学学报》1992,4(4):458-463

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Application of High Spatial Resolution Laser Ablation ICP-MS to Crystal-Melt Trace Element Partition Coefficient Determination     

Maurizio Petrelli  Luca Caricchi  Peter Ulmer 《Geostandards and Geoanalytical Research》2007,31(1):13-25

In this contribution we evaluate the capabilities of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) using a 12 μm spot size. Precision, accuracy and detection limits were assessed on the USGS BCR-2G reference material. We demonstrate that the 12 μm LA-ICP-MS analyses of experimentally-grown amphibole and garnet are in excellent agreement with secondary ion mass spectrometry (SIMS) trace element determinations on the same crystals. The 12 μm spot size configuration was subsequently used to determine trace element crystal-melt partition coefficients (D_c/m) for a wide range of trace elements in amphibole in equilibrium with a basanitic melt. The following strategy to determine accurately and evaluate D_c/m is proposed. One or more major elements determined previously by electron probe microanalysis (EPMA) was used to ensure consistency between EPMA and the composition of the aerosol produced by the laser ablation. Measured D_c/m values were successively evaluated using the lattice strain model. The use of this strategy significantly improved the precision and accuracy of D_c/m determination when a LA-ICP-MS configuration with a high spatial resolution was employed.  相似文献