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

Relative ordering in the radial evolution of solar wind turbulence: the S-Theorem approach

Abstract. Over the past few decades scientists have shown growing interest in space plasma complexity and in understanding the turbulence in magnetospheric and interplanetary media. At the beginning of the 1980s, Yu. L. Klimontovich introduced a criterion, named S-Theorem, to evaluate the degree of order in far-from-equilibrium open systems, which applied to hydrodynamic turbulence showed that turbulence flows were more organized than laminar ones. Using the same theorem we have evaluated the variation of the degree of self-organization in both Alfvénic and non-Alfvénic turbulent fluctuations with the radial evolution during a long time interval characterized by a slow solar wind. This analysis seems to show that the radial evolution of turbulent fluctuations is accompanied by a decrease in the degree of order, suggesting that, in the case of slow solar wind, the turbulence decays with radial distance

Relative ordering in the radial evolution of solar wind turbulence: the S-Theorem approach

Over the past few decades scientists have shown growing interest in space plasma complexity and in understanding the turbulence in magnetospheric and interplanetary media. At the beginning of the 1980s, Yu. L. Klimontovich introduced a criterion, named S-Theorem, to evaluate the degree of order in far-from-equilibrium open systems, which applied to hydrodynamic turbulence showed that turbulence flows were more organized than laminar ones. Using the same theorem we have evaluated the variation of the degree of self-organization in both Alfvénic and non-Alfvénic turbulent fluctuations with the radial evolution during a long time interval characterized by a slow solar wind. This analysis seems to show that the radial evolution of turbulent fluctuations is accompanied by a decrease in the degree of order, suggesting that, in the case of slow solar wind, the turbulence decays with radial distance

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 $7.6 \pm 0.2$ 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

On the local Hurst exponent of geomagnetic field fluctuations: spatial distribution for different geomgnetic activity levels

This study attempts to characterize the spatial distribution of the scaling features of the short time scale magnetic field fluctuations obtained from 45 ground based geomagnetic observatories distributed in the northern hemisphere. We investigate the changes of the scaling properties of the geomagnetic field fluctuations by evaluating the local Hurst exponent and reconstruct maps of this index as a function of the geomagnetic activity level. These maps permit us to localize the different latitudinal structures responsible for disturbances and related to the ionospheric current systems. We find that the geomagnetic field fluctuations associated with the different ionospheric current systems have different scaling features, which can be evidenced by the local Hurst exponent. We also find that, in general, the local Hurst exponent for quiet magnetospheric periods is higher than that for more active periods suggesting that the dynamical processes that are activated during disturbed times are responsible for changes in the nature of the geomagnetic field fluctuations

Fractal time statistics of AE-index burst waiting times: evidence of metastability

Recent observations and analyses evidenced that the magnetotail, as well as the magnetospheric dynamics are characterised by a scale-free behaviour and intermittence. These results, along with numerical simulations on cellular automata, suggest that the observed scale-invariance may be due to forced and/or self-organised criticality (FSOC), meaning that the magnetotail operates near a marginally stable state (Chang, 1999). On the other hand, it was underlined that a complex magnetic field topology in the geotail regions may play a relevant role in the impulsive energy relaxation (Consolini and Chang, 2001)

Time intermittency and spectral features of the geomagnetic field

In the field of geomagnetism a number of studies have been devoted to the investigation of turbulence and intermittency in the outer core fluid motions. Here, in order to obtain information on such phenomena we study the time spectral and self-similarity features of the main geomagnetic field fluctuations as measured on the Earth’s surface. The existence of a power law spectrum, characterised by an exponent a˜-11/3, and an anomalous scaling of q-th order structure functions on time scales longer than 5 years, suggests the occurrence of intermittent turbulence rather than classical Kolmogorov turbulence in the fluid core motions. These results are briefly discussed in connection with the existence of a strong magnetic field and drift-wave turbulence

Relative ordering in the radial evolution of solar wind turbulence: the S-Theorem approach

Over the past few decades scientists have shown growing interest in space plasma complexity and in understanding the turbulence in magnetospheric and interplanetary media. At the beginning of the 1980s, Yu. L. Klimontovich introduced a criterion, named S-Theorem, to evaluate the degree of order in far-from-equilibrium open systems, which applied to hydrodynamic turbulence showed that turbulence flows were more organized than laminar ones. Using the same theorem we have evaluated the variation of the degree of self-organization in both Alfv´enic and non-Alfv´enic turbulent fluctuations with the radial evolution during a long time interval characterized by a slow solar wind. This analysis seems to show that the radial evolution of turbulent fluctuations is accompanied by a decrease in the degree of order, suggesting that, in the case of slow solar wind, the turbulence decays with radial distance

Some new approaches to the study of the Earth's magnetic field reversals

Paleomagnetic studies clearly show that the polarity of the magnetic fi eld has been subject to reversals. It is generally assumed that polarity intervals are exponentially distributed. Here, the geomagnetic polarity reversal record, for the past 166 Ma, is analysed and a new approach to the study of the reversals is presented. In detail, the occurrence of 1/f-noise in the Power Spectral Density (PSD), relative to geomagnetic fi eld reversals, the existence of a Zipf's law behaviour for the cumulative distribution of polarity intervals, and the occurrence of punctuated equilibrium, as shown by a sort of Devil's staircase for the reversal time series, are investigated. Our results give a preliminary picture of the dynamical state of the geomagnetic dynamo suggesting that the geodynamo works in a marginally stable out-of-equilibrium confi guration, and that polarity reversals are equivalent to a sort of phase transition between two metastable states

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 ($\sim460$ 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