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

rank ordering multifractal analysis of the auroral electrojet index

Abstract. In the second half of the 90s interest grew on the complex features of the magnetospheric dynamics in response to solar wind changes. An important series of papers were published on the occurrence of chaos, turbulence and complexity. Among them, particularly interesting was the study of the bursty and fractal/multifractal character of the high latitude energy release during geomagnetic storms, which was evidenced by analyzing the features of the Auroral Electrojet (AE) indices. Recently, the multifractal features of the small time-scale increments of AE-indices have been criticized in favor of a more simple fractal behavior. This is particularly true for the scaling features of the probability density functions (PDFs) of the AE index increments. Here, after a brief review of the nature of the fractal/multifractal features of the magnetospheric response to solar wind changes, we investigate the multifractal nature of the scaling features of the AE index increments PDFs using the Rank Ordering Multifractal Analysis (ROMA) technique. The ROMA results clearly demonstrate the existence of a hierarchy of scaling indices, depending on the increment amplitude, for the data collapsing of PDFs relative to increments at different time scales. Our results confirm the previous results by Consolini et al. (1996) and the more recent results by Rypdal and Rypdal (2010)

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

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

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)

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

A probabilistic approach to heterogeneity in space plasmas: the case of magnetic field intensity in solar wind

Abstract. Since the early 90s it was shown that Probability Distribution Functions (PDFs) of small scale differences (fluctuations) of several quantities in space plasmas display significant departures from Gaussianity. The non-Gaussian shape of PDFs was ascribed to intermittency and discussed in the framework of intermittent MHD turbulence. Here, we put the attention to the PDF of magnetic field intensity instead of its differences showing how the PDF of such quantity in a quiet solar wind can be related with the occurrence of heterogeneity. In detail, we derive the shape of the PDFs by simple statistical considerations based on the concept of subordination in probability theory. An interpretation and a discussion in terms of existing coherent magnetic structures in a mechanical near-equilibrium state are also presented

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