Papers by Luca Sorriso-Valvo
By using the maximum Tsallis entropy principle, it can be found that a simple relation generalize... more By using the maximum Tsallis entropy principle, it can be found that a simple relation generalizes the wall know Omori's law for aftershocks. We investigate on the occurrence of the above relation to both aftershock sequences and earthquake datasets.
Recent results obtained from the analysis of interplanetary observations recorded by Helios 2 at ... more Recent results obtained from the analysis of interplanetary observations recorded by Helios 2 at 0.3 AU showed, for the first time, that the Probability Distribution Function of magnetic field vector differences within the MHD frequency range of turbulence can be remarkably well fitted by a double log-normal. These two components of the total PDF have been ascribed to Alfvénic fluctuations and convected structures. This analysis, which needs high time resolution data to be performed, was carried on using only magnetic field data since plasma measurements were at much lower time resolution. Consequently, we lack of a sure proof of the Alfvénic nature of those fluctuations that we believe contribute to one of the two lognormals. The second lognormal should be due to convected structures but, whether these structures are locally generated by the turbulent evolution of the fluctuations or they rather come from the source regions of the wind, where they reflect the complicate topology of the magnetic field, is still unknown. High time resolution in-situ observations performed by Solar Orbiter at the perihelion, during the corotation phase, would certainly amplify the results already reached by Helios would finally give an answer to these problems. In particular, while in-situ measurements would allow to test the Alfvénic nature of the fluctuations, high spatial resolution remote sensing would help to disentangle spatial from temporal effects allowing to recognize whether convected structures come directly from the sun or they rather are a by-product of the turbulent evolution of interplanetary fluctuations.
The spatio-temporal dynamics of the solar photosphere is studied by performing a Proper Orthogona... more The spatio-temporal dynamics of the solar photosphere is studied by performing a Proper Orthogonal Decomposition (POD) of line of sight velocity fields computed from high resolution data coming from the MDI/SOHO instrument. Using this technique, we are able to identify and characterize the different dynamical regimes acting in the system. Low frequency oscillations, with frequencies in the range 20-130 µHz, dominate the most energetic POD modes (excluding solar rotation), and are characterized by spatial patterns with typical scales of about 3 Mm. Patterns with larger typical scales of 10 Mm, are associated to p-modes oscillations at frequencies of about 3000 µHz.
European Physical Journal E, 2009
The spontaneous formation of dendritic aggregates is observed in a two-dimensional confined layer... more The spontaneous formation of dendritic aggregates is observed in a two-dimensional confined layered system consisting of a film composed of liquid crystal, dye and solvent cast above a polymer substrate. The observed aggregates are promoted by phase separation processes induced by dye diffusion and solvent evaporation. The growth properties of the aggregates are studied through the temporal evolution of their topological properties (surface, perimeter, fractal dimension). The fractal dimension of the completely formed structures, when they are coexistent with different types of structures, is consistent with theoretical and experimental values obtained for Diffusion-Limited Aggregates. Under different experimental conditions (temperature and local dye concentration) the structure forms without interactions with other kinds of structures, and its equilibrium fractal dimension is smaller. The fractal dimension is thus not a universal property of the observed structures, but rather depends on the experimental conditions.
European Physical Journal E, 2011
The complex spatio-temporal dynamics generated by electrohydrodynamics instabilities in a nematic... more The complex spatio-temporal dynamics generated by electrohydrodynamics instabilities in a nematic liquid crystal under the action of a driving oscillating electric field is investigated. Quasi-stationary convective structures which are visible at large scales are broken into chaotic patterns at higher driving voltages, thus generating small-scale structures. Scaling analysis reveals that these small-scale structures self-organize in a network of subleading structures which are reminescent of convective rolls. This network persists well inside the chaotic regimes, disappearing only at very high voltages, where stochastic dynamical scattering mode takes place.
Physical Review Letters, 2011
Nonlinear straining and random sweeping spatiotemporal decorrelation properties, originally intro... more Nonlinear straining and random sweeping spatiotemporal decorrelation properties, originally introduced as the main processes for turbulent fluctuations decorrelation in usual fluid flows, have been observed experimentally in anisotropic electroconvective turbulence generated in a nematic liquid crystal under the action of an external oscillating electric field. A transition between both processes occurs when the instability is driven toward states of increasing complexity, thus showing that decorrelation mechanisms in turbulent media are more universal than naively expected. A model for both decorrelation mechanisms is introduced, its comparison with experimental results providing an estimate of the characteristic sweeping velocity.
The presence of a background magnetic field induces anisotropy in hydromagnetic turbulence. Under... more The presence of a background magnetic field induces anisotropy in hydromagnetic turbulence. Understanding properties of anisotropy is important to characterize turbulence power spectrum. This work presents a statistical study of anisotropy of intermittent properties, by using in three different regions of the heliosphere, namely in the solar wind, and in the Earth's foreshock and magnetosheath behind a quasi parallel bow shock. The whole two-points structure function tensor is studied to point out the anisotropic effects on intermittency intensity in the different datasets. The results are discussed.
Statistical features of bursty behaviour observed in turbulence are compared to both intermittent... more Statistical features of bursty behaviour observed in turbulence are compared to both intermittent events in a GOY shell model, and in different models of Self-Organized Criticality (SOC). Applications to space plasmas are outlines.
Earth Moon and Planets, 2009
Direct evidence for the presence of an inertial energy cascade, the most characteristic signature... more Direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), is observed in the solar wind by the Ulysses spacecraft. A linear relation is indeed observed for the scaling of mixed third order structure functions involving Elsässer variables. This experimental result, confirming the prescription stemming from a theorem for MHD turbulence, firmly establishes the turbulent character of low-frequency velocity and magnetic field fluctuations in the solar wind plasma.
The earliest measurements of velocity and magnetic fields in the solar wind showed that the turbu... more The earliest measurements of velocity and magnetic fields in the solar wind showed that the turbulence can be strongly Alfvenic in the fast wind. This property is very important as the cross-helicity, that is the correlation between velocity and magnetic field is an invariant of the ideal MHD equations and can inhibit the turbulent energy cascade if it is completely realized. It is however very important to study the local values of the cross-helicity, and in particular its probability distribution function, to determine the statistics of the local turbulent energy transfer. Indeed, even if there is no global Alfvenicity, the fluctuations of the local correlation can be very strong and, as they are related to the cosine of the angle between the velocity and the magnetic field, their distribution will depend on the dimensionality of these vector fluctuations. We have determined the probability distribution of the angle and its cosine between random vectors for different values of the cross-correlation, and we have compared these distributions with those observed at different distances by the Ulysses spacecraft in high-latitude fast solar wind, and those measured by the Helios satellite in fast and slow wind in the ecliptic. The distributions are found to always differ from the casual ones, but to be closer to two-dimensional distributions in the fast solar wind, suggesting a possible link between Alfvenicity and dimensionality of the fluctuations.
Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exa... more Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exact relation for the energy flux through the scales. This Yaglom-like scaling law has been recently observed in some samples of data collected by the Ulysses spacecraft in the solar wind above solar poles, where turbulence is Alfvenic. An analogous scaling law, suitably modified to take into account compressible fluctuations, can be observed in a more extended fraction of the same dataset. Large scale density fluctuations, despite their low amplitude, play thus a crucial role in the basic scaling properties of turbulence. The compressive turbulent cascade, moreover, can supply the energy needed to account for the local heating of the non-adiabatic solar wind.
Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exa... more Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exact relation for the energy flux through the scales. This Yaglom-like scaling law has been recently observed in the solar wind above the solar poles observed by the Ulysses spacecraft, where the turbulence is in an Alfvenic state. An analogous phenomenological scaling law, suitably modified to take into account compressible fluctuations, is observed more frequently in the same data set. Large-scale density fluctuations, despite their low amplitude, thus play a crucial role in the basic scaling properties of turbulence. The turbulent cascade rate in the compressive case can, moreover, supply the energy dissipation needed to account for the local heating of the nonadiabatic solar wind.
Recent experimental results have shown that solar wind displays a clear scaling of the mixed thir... more Recent experimental results have shown that solar wind displays a clear scaling of the mixed third-order structure function, in the framework of magnetohydrodynamic turbulent cascade. Here we review the results from Ulysses data of ecliptic and polar solar wind, focusing on the accuracy of the moments estimate. We discuss the dataset size necessary for the convergence of the moments as a compromise among nonstationarity, local conditions and statistical requirements.
In recent years the occurrence of magnetohydrodynamic (MHD) turbulent cascade has been validated ... more In recent years the occurrence of magnetohydrodynamic (MHD) turbulent cascade has been validated in the solar wind. This has been done through the systematic observation of the exact scaling law of the mixed third order moment of the Elsasser variables, also known as Politano and Pouquet law. The scaling was observed in the ecliptic and in polar wind, the properties of the cascade depending on the local wind conditions, such as cross-helicity. In polar fast, Alfvénic wind measured by Ulysses spacecraft the observation of the turbulent cascade is not ubiquitous, the reason for this sporadic behavior being unknown so far. After a brief review of the main results obtained in this framework, we show that occurrence of cascade is related with the local properties of the velocity and magnetic field fluctuations. In particular, the alignment of the fields inhibits the nonlinear interactions, as suggested in the early 80' (dynamic alignment), whilst the cascade takes place when the alignment is locally perturbed. Dependence of the cascade rate with radial distance, latitude and solar activity is presented, corroborating the importance of local dynamic alignment, and confirming the non universal character of solar wind MHD turbulence.
The observations of magnetic field variations as a signature of flaring activity is one of the ma... more The observations of magnetic field variations as a signature of flaring activity is one of the main goal in solar physics. Some efforts in the past give apparently no unambigu- ous observations of changes. We observed that the scaling laws of the current helicity inside a given flaring active region change clearly and abruptly before the eruption of big flares at the top of that active region. Comparison with numerical simulations of MHD equations, indicates that the change of scaling behavior in the current helicity, seems to be associated to a topological reorganization of the footpoint of the magnetic field loop, namely to dissipation of small scales structures in turbulence. It is evident that the possibility of forecasting in real time high energy flares, even if partially, has a wide practical interest to prevent the effects of big flares on Earth and its environment.
The topological properties of the typical current structures in a turbulent magnetohydrodynamic f... more The topological properties of the typical current structures in a turbulent magnetohydrodynamic flow can be measured using the cancellations analysis. In two-dimensional numerical simulations, this reveals current filaments being the most typical current structures. The observations of the topology of photospheric current structures within active regions shows that modifications occur correspondingly with strong flares.
The topological properties of the typical current structures in a turbulent magnetohydrodynamic f... more The topological properties of the typical current structures in a turbulent magnetohydrodynamic flow can be measured using the cancellations analysis. In two-dimensional numerical simulations, this reveals current filaments being the most typical current structures. The observations of the topology of photospheric current structures within active regions shows that modifications occur correspondingly with strong flares.
Magnetic helicity Hm is one of the three quadratic invariants of the ideal MHD equations of motio... more Magnetic helicity Hm is one of the three quadratic invariants of the ideal MHD equations of motion. An important property is that it is a pseudoscalar and changes sign under coordinate inversion x→ -x. Then, Hm represents a measure of lack of mirror symmetry of the magnetic field or, in other words, it estimates the "knottedness" of magnetic field lines. A positive/negative Hm would indicate a right/left- hand sense of polarization. A standard analysis of reduced Hm, based on Fourier decomposition and performed in interplanetary space, would show that the handedness of the fluctuations in a given wave number is uncorrelated with the handedness of fluctuations at nearby wavenumbers. As a matter of fact, Hm would continuously fluctuate between positive and negative values throught the whole frequency range. However, this kind of analysis provides only part of the information needed to fully characterize the topology of magnetic field lines. A remarkable help comes out from a new technique, which uses a wavelet decomposition, able to unravel interesting features of the magnetic field fluctuations probably related to the filamentary structure of turbulence. Results from a recent Sun-Earth-Ulysses alignment and, more in general, statistical studies performed in fast and slow wind within the inner heliosphere will be discussed. Moreover, single case studies focussing on interplanetary flux ropes will be reported and possible implications with persistence and sharpness of observed energetic particles dropouts phenomenon at the Earth orbit will be addressed.
The Yaglom law for the mixt third order moment of the Elsasser fields fluctuations has been recen... more The Yaglom law for the mixt third order moment of the Elsasser fields fluctuations has been recently observed in the solar wind. This observation confirm the existence of a MHD turbulent cascade in the interplanetary plasma, in both ecliptic and polar wind. In the polar wind measured by Ulysses spacecraft, the cascade occasionally holds up to scales of days. The nature of such cascade, and in particular the role of magnetic field at large scales, are discussed. The different possible scenaries show that solar wind turbulent cascade is not universal.
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Papers by Luca Sorriso-Valvo