90 research outputs found
Contribution to solving the orientation problem for an automatic magnetic observatory
The problem of the absolute calibration of a vectorial (tri-axial) magnetometer is addressed with the objective that the apparatus, once calibrated, gives afterwards, for a few years, the absolute values of the three components of the geomagnetic field (say the Northern geographical component, Eastern component and vertical component) with an accuracy on the order of 1 nT. The calibration procedure comes down to measure the orientation in space of the three physical axes of the sensor or, in other words, the entries of the transfer matrix from the local geographical axes to these physical axes. Absolute calibration follows indeed an internal calibration which provides accurate values of the three scale factors corresponding to the three axes – and in addition their relative angles. The absolute calibration can be achieved through classical absolute measurements made with an independent equipment. It is shown – after an error analysis which is not trivial – that, while it is not possible to get the axes absolute orientations with a high accuracy, the assigned objective (absolute values of the Northern geographical component, Eastern component and vertical component, with an accuracy of the order of 1 nT) is nevertheless reachable; this is because in the time interval of interest the field to measure is not far from the field prevailing during the calibration process
The six-month line in geomagnetic long series
International audienceDaily means of the horizontal components X (north) and Y (east) of the geomagnetic field are available in the form of long series (several tens of years). Nine observatories are used in the present study, whose series are among the longest. The amplitudes of the 6-month and 1-year periodic variations are estimated using a simple but original technique. A remarkably clear result emerges from the complexity of the geomagnetic data: the amplitude of the 6-month line presents, in all observatories, the same large variation (by a factor of 1.7) over the 1920–1990 time span, regular and quasi-sinusoidal. Nothing comparable comes out for the annual line. The 6-month line results from the modulation by an astronomical mechanism of a magnetospheric system of currents. As this latter mechanism is time invariant, the intensity of the system of currents itself must present the large variation observed on the 6-months variation amplitude. This variation presents some similarities with the one displayed by recent curves of reconstructed solar irradiance or the "Earth's temperature". Finally, the same analysis is applied to the aa magnetic index
Scenario-based earthquake hazard and risk assessment for Baku (Azerbaijan)
A rapid growth of population, intensive civil and industrial building, land and water instabilities (e.g. landslides, significant underground water level fluctuations), and the lack of public awareness regarding seismic hazard contribute to the increase of vulnerability of Baku (the capital city of the Republic of Azerbaijan) to earthquakes. In this study, we assess an earthquake risk in the city determined as a convolution of seismic hazard (in terms of the surface peak ground acceleration, PGA), vulnerability (due to building construction fragility, population features, the gross domestic product per capita, and landslide's occurrence), and exposure of infrastructure and critical facilities. The earthquake risk assessment provides useful information to identify the factors influencing the risk. A deterministic seismic hazard for Baku is analysed for four earthquake scenarios: near, far, local, and extreme events. The seismic hazard models demonstrate the level of ground shaking in the city: PGA high values are predicted in the southern coastal and north-eastern parts of the city and in some parts of the downtown. The PGA attains its maximal values for the local and extreme earthquake scenarios. We show that the quality of buildings and the probability of their damage, the distribution of urban population, exposure, and the pattern of peak ground acceleration contribute to the seismic risk, meanwhile the vulnerability factors play a more prominent role for all earthquake scenarios. Our results can allow elaborating strategic countermeasure plans for the earthquake risk mitigation in the Baku city
Scenario-based earthquake hazard and risk assessment for Baku (Azerbaijan)
A rapid growth of population, intensive civil and industrial building, land and water instabilities (e.g. landslides, significant underground water level fluctuations), and the lack of public awareness regarding seismic hazard contribute to the increase of vulnerability of Baku (the capital city of the Republic of Azerbaijan) to earthquakes. In this study, we assess an earthquake risk in the city determined as a convolution of seismic hazard (in terms of the surface peak ground acceleration, PGA), vulnerability (due to building construction fragility, population features, the gross domestic product per capita, and landslide’s occurrence), and exposure of infrastructure and critical facilities. The earthquake risk assessment provides useful information to identify the factors influencing the risk. A deterministic seismic hazard for Baku is analysed for four earthquake scenarios: near, far, local, and extreme events. The seismic hazard models demonstrate the level of ground shaking in the city: PGA high values are predicted in the southern coastal and northeastern parts of the city and in some parts of the downtown. The PGA attains its maximal values for the local and extreme earthquake scenarios. We show that the quality of buildings and the probability of their damage, the distribution of urban population, exposure, and the pattern of peak ground acceleration contribute to the seismic risk, meanwhile the vulnerability factors play a more prominent role for all earthquake scenarios. Our results can allow elaborating strategic countermeasure plans for the earthquake risk mitigation in the Baku city
On the Slow Drift of Solstices: Milankovic Cycles and Mean Global Temperature
The Earth's revolution is modified by changes in inclination of its rotation
axis. Despite the fact that the gravity field is central, the Earth's
trajectory is not closed and the equinoxes drift. Milankovic (1920) argued that
the shortest precession period of solstices is 20,7kyr: the Summer solstice in
one hemisphere takes place alternately every 11kyr at perihelion and at
aphelion. We have submitted the time series for the Earth's pole of rotation,
global mean surface temperature and ephemeris to iterative Singular Spectrum
Analysis. iSSA extracts from each a trend, a 1yr and a 60yr component. Both the
apparent drift of solstices of Earth around the Sun and the global mean
temperature exhibit a strong 60yr oscillation. The "fixed dates" of solstices
actually drift. Comparing the time evolution of the Winter and Summer solstices
positions of the rotation pole and the first iSSA component (trend) of the
temperature allows one to recognize some common features. A basic equation from
Milankovic links the derivative of heat received at a given location on Earth
to solar insolation, known functions of the location coordinates, solar
declination and hour angle, with an inverse square dependence on the Sun-Earth
distance. We have translated the drift of solstices as a function of distance
to the Sun into the geometrical insolation theory of Milankovic. Shifting the
inverse square of the 60yr iSSA drift of solstices by 15 years with respect to
the first derivative of the 60yr iSSA trend of temperature, that is exactly a
quadrature in time, puts the two curves in quasi-exact superimposition. The
probability of a chance coincidence appears very low. Correlation does not
imply causality when there is no accompanying model. Here Milankovic's equation
can be considered as a model that is widely accepted. This paper identifies a
case of agreement between observations and a mathematical formulation
On the semiannual and annual variations of geomagnetic activity and components
International audienceThe semiannual and annual lines in a long series of magnetic observatories daily values, as well as in the aa-activity index series, are investigated. For both periods, amplitudes and phases of the lines corresponding to the different series present grossly common variations on decadal time scales; relative phases and amplitude ratios between the observatories change with the same time constants. The results are briefly discussed with regards to commonly received theories of the semiannual variation of magnetic activity, and some possible mechanisms for the observed geographical variability are suggested
Is the Earth's magnetic field a constant ? a legacy of Poisson
In the report he submitted to the Acad\'emie des Sciences, Poisson imagined a
set of concentric spheres at the origin of the Earth's magnetic field. It may
come as a surprise to many that Poisson as well as Gauss both considered the
magnetic field to be constant. We propose in this study to test this surprising
assertion for the first time evoked by Poisson (1826). First, we will present a
development of Maxwell's equations in the framework of a static electric field
and a static magnetic field in order to draw the necessary consequences for the
Poisson hypothesis. In a second step, we will see if the observations can be in
agreement with Poisson (1826). To do so, we have chosen to compare 1) the polar
motion drift and the secular variation of the Earth's magnetic field, 2) the
seasonal pseudo-cycles of day length together with those of the sea level
recorded by different tide gauges around the globe and those of the Earth's
magnetic field recorded in different magnetic observatories. We then propose a
mechanism, in the spirit of Poisson, to explain the presence of the 11-year in
the magnetic field. We test this mechanism with observations and finally we
study closely the evolution of the g10 coefficient of the IGFR over time
On variations of global mean surface temperature: When Laplace meets Milankovi\'c
In his mathematical theory, Milankovic finds a link between the heat received
by the Earth surface per unit time as a function of the solar ephemerids and
derives a model of climate changes at periods longer than a few thousand years
and more. In this paper, we investigate the potential connections of global
temperature and Earth rotation at much shorter periods, in the complementary
range of one to a few hundred years. For temperature, we select the HadCrut05.
For Earth rotation, defined by pole coordinates and length of day, we use the
IERS data sets. Using iterative Singular Spectrum Analysis (iSSA), we extract
the trend and quasi-periodic components of these time series. The first
quasi-periodic components (period ~80-90 years) are expressions of the
Gleissberg cycle and are identical (at the level of uncertainty of the data).
Taken together, the trend and Gleissberg components allow one to reconstruct
87% of the variance of the data for lod and 48% for temperature. The next four
iSSA components, with periods ~40, 22, 15 and 9 years. The Lagrange and Laplace
theories imply that the derivative of pole motion should be identical to lod
variations: this strong check is passed by the trend + Gleissberg
reconstructions. The annual oscillations of pole motion and lod are linked to
annual variations in Sun-Earth distance, in agreement with an astronomical, but
not a climatic origin. The results obtained in this paper for the observed
temperature/rotation couple add to the growing list of evidence of solar and
planetary forcings of gravitational nature on a number of geophysical processes
(including sea-level, sea-level pressure, sea-ice extent, oceanic climate
indices).Comment: 13 pages, 13 figure
On the tilt of the Earth's polar axis (climat): Some 'impressionist' remarks
In this lengthy letter, we wanted to discuss the concept of climate based on
definitions established for over a century and direct observations that we have
been collecting for more than a century as well. To do this, we present and
discuss the remarkably stable maps over time of the various physical parameters
that make up the climate corpus: solar temperature, atmospheric pressure,
winds, precipitation, temperature anomalies. This impressionistic tableau that
we are gradually sketching as our reflection unfolds leads us to the following
proposition: What if, as Laplace first proposed in 1799 and later
Milankovi\{'}c in 1920, ground temperature were merely a consequence of climate
and not a separate parameter of climate in its own right?Comment: 24 pages, 11 Figure
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