483 research outputs found
Principal components' features of mid-latitude geomagnetic daily variation
Abstract. The ionospheric and magnetospheric current systems are responsible of the daily magnetic field changes. Recently, the Natural Orthogonal Components (NOC) technique has been applied to model the physical system responsible of the daily variation of the geomagnetic field, efficiently and accurately (Xu and Kamide, 2004). Indeed, this approach guarantees that the number of parameters used to represent the physical process is small as much as possible, and consequently process control for such system becomes apparent. We focus our present study on the analysis of the hourly means of the magnetic elements H, D and Z recorded at L'Aquila observatory in Italy from 1993 to 2004. We apply to this dataset the NOC technique to reconstruct the 3-dimensional structures of the different ionospheric and magnetospheric current systems which contribute to the geomagnetic daily variations. To support our interpretation in terms of the different ionospheric and magnetospheric current systems, the spectral and statistical features of the time-dependent amplitudes associated to the set of natural orthogonal components are analyzed and compared to those of a set of descriptors of the magnetospheric dynamics and solar wind changes
Observational evidence for buffeting induced kink waves in solar magnetic elements
The role of diffuse photospheric magnetic elements in the energy budget of
the upper layers of the Sun's atmosphere has been the recent subject of many
studies. This was made possible by the availability of high temporal and
spatial resolution observations of the solar photosphere, allowing large
numbers of magnetic elements to be tracked to study their dynamics. In this
work we exploit a long temporal series of seeing-free magnetograms of the solar
photosphere to study the effect of the turbulent convection in the excitation
of kink oscillations in magnetic elements. We make use of the empirical mode
decomposition technique (EMD) in order to study the transverse oscillations of
several magnetic flux tubes. This technique permits the analysis of
non-stationary time series like those associated to the horizontal velocities
of these flux tubes which are continuously advected and dispersed by granular
flows.
Our primary findings reveal the excitation of low frequency modes of kink
oscillations, which are sub-harmonics of a fundamental mode with a minute periodicity. These results constitute a strong case for
observational proof of the excitation of kink waves by the buffeting of the
convection cells in the solar photosphere, and are discussed in light of their
possible role in the energy budget of the upper Sun's atmosphere.Comment: A&A accepte
An invariant of smooth 4-manifolds
We define a diffeomorphism invariant of smooth 4-manifolds which we can
estimate for many smoothings of R^4 and other smooth 4-manifolds. Using this
invariant we can show that uncountably many smoothings of R^4 support no Stein
structure. (Gompf has constructed uncountably many smoothings of R^4 which do
support Stein structures.) Other applications of this invariant are given.Comment: 19 pages. Published copy, also available at
http://www.maths.warwick.ac.uk/gt/GTVol1/paper6.abs.htm
Magnetic field fluctuation features at Swarm's altitude: a fractal approach
The ESA Swarm mission provides a qualitatively new level of observational geomagnetic data, \textbf{which allows us to study the spatial features of magnetic field fluctuations}, capturing their essential characteristics and at the same time establishing a correlation with the dynamics of the systems responsible for the fluctuations. Our study aims to characterize changes in the scaling properties of the geomagnetic field's spatial fluctuations by evaluating the local Hurst exponent, and to construct maps of this index \textbf{at the Swarm's altitude ( km)}. Since a signal with a larger Hurst exponent is more regular and less erratic than a signal with a smaller one, the maps permit us to localize spatial structures characterized by different scaling properties. This study is an example of the potential of Swarm data to give new insights into ionosphere-magnetosphere coupling; at the same time, it develops new applications where changes in statistical parameters can be used as a local indicator of overall magnetospheric-ionospheric coupling conditions
On the nonstationarity of the decadal periodicities of the length of day
The Earth's rotation rate is not constant, but changes on all observable timescales, from subdaily to decadal and longer. These variations are usually discussed in terms of variations in the length of the day (LoD) and are caused by processes acting within the interior, at the surface and outside of the Earth. Here, we investigate the presence of long-standing decadal variations in yearly LoD data covering the period from 1832 to 2009 by applying the Hilbert–Huang transform (HHT). The HHT has been slightly modified here to take into account the uncertainty of LoD values that has changed greatly in time due to the use of different LoD measurement techniques. The LoD time series has been completely decomposed into five intrinsic mode functions (IMF) and a residual trend. The estimation of instantaneous frequencies and related amplitudes of the obtained IMFs has allowed us to compute the Hilbert spectrum that has been used as the starting point for studying and discussing the stationarity of typical LoD timescale stationarity. The obtained results while showing the presence of multiple periodicities also indicate the absence of really stationary periodicities. Therefore, rather than considering the processes taking place in the Earth's core as the result of a superposition of oscillations (i.e. stationary mechanisms) occurring on a discrete number of different timescales, it would be better to think of a superposition of fluctuations that are intermittent in both frequency and amplitude
on the nonstationarity of the decadal periodicities of the length of day
Abstract. The Earth's rotation rate is not constant, but changes on all observable timescales, from subdaily to decadal and longer. These variations are usually discussed in terms of variations in the length of the day (LoD) and are caused by processes acting within the interior, at the surface and outside of the Earth. Here, we investigate the presence of long-standing decadal variations in yearly LoD data covering the period from 1832 to 2009 by applying the Hilbert–Huang transform (HHT). The HHT has been slightly modified here to take into account the uncertainty of LoD values that has changed greatly in time due to the use of different LoD measurement techniques. The LoD time series has been completely decomposed into five intrinsic mode functions (IMF) and a residual trend. The estimation of instantaneous frequencies and related amplitudes of the obtained IMFs has allowed us to compute the Hilbert spectrum that has been used as the starting point for studying and discussing the stationarity of typical LoD timescale stationarity. The obtained results while showing the presence of multiple periodicities also indicate the absence of really stationary periodicities. Therefore, rather than considering the processes taking place in the Earth's core as the result of a superposition of oscillations (i.e. stationary mechanisms) occurring on a discrete number of different timescales, it would be better to think of a superposition of fluctuations that are intermittent in both frequency and amplitude
On the multi-scale nature of large geomagnetic storms: an empirical mode decomposition analysis
Abstract. Complexity and multi-scale are very common properties of several geomagnetic time series. On the other hand, it is amply demonstrated that scaling properties of geomagnetic time series show significant changes depending on the geomagnetic activity level. Here, we study the multi-scale features of some large geomagnetic storms by applying the empirical mode decomposition technique. This method, which is alternative to traditional data analysis and is designed specifically for analyzing nonlinear and nonstationary data, is applied to long time series of Sym-H index relative to periods including large geomagnetic disturbances. The spectral and scaling features of the intrinsic mode functions (IMFs) into which Sym-H time series can be decomposed, as well as those of the Sym-H time series itself, are studied considering different geomagnetic activity levels. The results suggest an increase of dynamical complexity and multi-scale properties for intermediate geomagnetic activity levels
On the multi-scale nature of large geomagnetic storms: an empirical mode decomposition analysis
Complexity and multi-scale are very common
properties of several geomagnetic time series. On the other hand, it is amply demonstrated that scaling properties of geomagnetic time series show significant changes depending on the geomagnetic activity level. Here, we study the multiscale
features of some large geomagnetic storms by applying the empirical mode decomposition technique. This method, which is alternative to traditional data analysis and is designed
specifically for analyzing nonlinear and nonstationary data, is applied to long time series of Sym-H index relative to periods including large geomagnetic disturbances. The
spectral and scaling features of the intrinsic mode functions(IMFs) into which Sym-H time series can be decomposed, as well as those of the Sym-H time series itself, are studied considering different geomagnetic activity levels. The results
suggest an increase of dynamical complexity and multi-scale properties for intermediate geomagnetic activity levels
Geomagnetic storms, dependence on solar and interplanetary phenomena: a review
Geomagnetic storms are probably the most intensively measured perturbations of the Earth’s magnetic field. They are multi-faceted phenomena that result as a final element of a chain of processes that starts on the Sun, a ects the solar wind and the interplanetary medium, and ends on the Earth.
At present, one of the key questions in the scientific community is the ability to predict the occurrence of geomagnetic storms on the basis of solar and interplanetary space observations. For these reasons, in recent years a number of investigations have been carried out to understand the solar-terrestrial relationships and to ascertain those factors that are ultimately
responsible for geomagnetic storms. Here a brief review of published results on the
geomagnetic storm e ectiveness from CMEs, solar flares, as well as interplanetary event
observations, is presented
On the terms of geomagnetic daily variation in Antarctica
The target of this work is to investigate the nature of magnetic perturbations produced by ionospheric and magnetospheric
currents as recorded at high-latitude geomagnetic stations. In particular, we investigate the effects of these
currents on geomagnetic data recorded in Antarctica. To this purpose we apply a mathematical method, known as Natural
Orthogonal Composition, to analyze the magnetic field disturbances along the three geomagnetic field components (X, Y and Z) recorded at Mario Zucchelli Station (IAGA
code TNB; geographic coordinates: 74.7° S, 164.1° E) from 1995 to 1998. Using this type of analysis, we characterize the dominant modes of the geomagnetic field daily variability through a set of empirical orthogonal functions (EOFs).
While such mathematically independent EOFs do not necessarily represent physically independent modes of variability,
we find that some of them are actually related to well known current patterns located at high latitudes
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