513 research outputs found
The spectrum of kink-like oscillations of solar photospheric magnetic elements
Recently, the availability of new high-spatial and -temporal resolution
observations of the solar photosphere has allowed the study of the oscillations
in small magnetic elements. Small magnetic elements have been found to host a
rich variety of oscillations detectable as intensity, longitudinal or
transverse velocity fluctuations which have been interpreted as MHD waves.
Small magnetic elements, at or below the current spatial resolution achieved by
modern solar telescopes, are though to play a relevant role in the energy
budget of the upper layers of the Sun's atmosphere, as they are found to cover
a significant fraction of the solar photosphere. Unfortunately, the limited
temporal length and/or cadence of the data sets, or the presence of
seeing-induced effects have prevented, so far, the estimation of the power
spectra of kink-like oscillations in small magnetic elements with good
accuracy. Motivated by this, we studied kink-like oscillations in small
magnetic elements, by exploiting very long duration and high-cadence data
acquired with the Solar Optical Telescope on board the Hinode satellite. In
this work we present the results of this analysis, by studying the power
spectral density of kink-like oscillations on a statistical basis. We found
that small magnetic elements exhibit a large number of spectral features in the
range 1-12 mHz. More interestingly, most of these spectral features are not
shared among magnetic elements but represent a unique signature of each
magnetic element itself.Comment: A&A accepted for publication. 8 pages, 5 figure
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
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
Super-diffusion versus competitive advection: a simulation
Magnetic element tracking is often used to study the transport and diffusion
of the magnetic field on the solar photosphere. From the analysis of the
displacement spectrum of these tracers, it has been recently agreed that a
regime of super-diffusivity dominates the solar surface. Quite habitually this
result is discussed in the framework of fully developed turbulence. But the
debate whether the super-diffusivity is generated by a turbulent dispersion
process, by the advection due to the convective pattern, or by even another
process, is still open, as is the question about the amount of diffusivity at
the scales relevant to the local dynamo process. To understand how such
peculiar diffusion in the solar atmosphere takes places, we compared the
results from two different data-sets (ground-based and space-borne) and
developed a simulation of passive tracers advection by the deformation of a
Voronoi network. The displacement spectra of the magnetic elements obtained by
the data-sets are consistent in retrieving a super-diffusive regime for the
solar photosphere, but the simulation also shows a super-diffusive displacement
spectrum: its competitive advection process can reproduce the signature of
super-diffusion. Therefore, it is not necessary to hypothesize a totally
developed turbulence regime to explain the motion of the magnetic elements on
the solar surface
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)
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