HAL (Le Centre pour la Communication Scientifique Directe), Dec 13, 2021
Rapid local directional alignment of different vector quantities has been observed in incompressi... more Rapid local directional alignment of different vector quantities has been observed in incompressible turbulence, either in MHD or in fluid turbulence, in both direct numerical simulations or in Solar wind data, and these alignments weaken the nonlinear terms with respect to those expected for Gaussian random fields. Evidence are based on the comparison of the probability distributions of the angle between vectors to those for independent isotropic three-dimensional vectors. We show that this apparent alignment can also be attributed to a local reduction of the dimensionality of the structures towards bidimensionality, leading to an increased probability of finding vectors close to alignment, compared to the spatial three-dimensional case. This interpretation is checked in Helios-2 Fast and Slow Solar wind data.
Automates de gaz sur réseaux. Aspects théoriques et simulations (Unpublished doctoral dissertatio... more Automates de gaz sur réseaux. Aspects théoriques et simulations (Unpublished doctoral dissertation). Université libre de Bruxelles, Faculté des sciences, Bruxelles.
In this short review we report about recent findings related to two fundamental points in the stu... more In this short review we report about recent findings related to two fundamental points in the study of solar wind turbulence: a) the verification of the equivalent of the -4/5 law in the solar wind and b) the estimate of the energy cascade along the spectrum and its comparison with the heating rate necessary to heat the solar wind during
In situ observations collected in the past decades, at different heliocentric distances and cover... more In situ observations collected in the past decades, at different heliocentric distances and covering almost the whole range of heliographic latitudes, represent a unique source of knowledge ready to be exploited to better understand the nature and the behavior of solar wind MHD turbulence. In fact, the solar wind can be used as an ideal wind tunnel in order to
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.
ABSTRACT A turbulent energy cascade has been recently identified in high-latitude solar wind data... more ABSTRACT A turbulent energy cascade has been recently identified in high-latitude solar wind data samples by using a Yaglom-like relation. However, analogous scaling law, suitably modified to take into account compressible fluctuations, has been observed in a much more extended fraction of the same data set recorded by the Ulysses spacecraft. Thus, it seems that large scale density fluctuations, despite their low amplitude, play a major role in the basic scaling properties of turbulence. The compressive turbulent cascade, moreover, seems to be able to supply the energy needed to account for the local heating of the non-adiabatic solar wind.
The detection of magnetic field variations as a signature of flaring activity is one of the main ... more The detection of magnetic field variations as a signature of flaring activity is one of the main goal in solar physics. Past efforts gave apparently no unambiguous observations of systematic changes. In the present study, we discuss recent results on observations that scaling laws of turbulent current helicity inside a given flaring active region change in response to large flares in that active region. Such changes can be related to the evolution of current structures by a simple geometrical argument, which had been tested using high Reynolds number direct numerical simulations of the MHD equations. Interpretation of the observed data within this picture indicates that the change in scaling behavior of the current helicity seems to be associated with a topological reorganization of the footpoint of the magnetic field loops, namely with the dissipation of small scales structures in turbulent media.
The occurrence of a nonlinear turbulent energy cascade in solar wind plasma has been recently est... more The occurrence of a nonlinear turbulent energy cascade in solar wind plasma has been recently established through the observation of an exact law from spacecraft measurements. The main results obtained in the fast, polar wind measured by Ulysses spacecraft are reviewed here. In particular, the turbulent cascade is seen as the mean to provide the energy necessary for the local heating in the non-adiabatic expansion of the solar wind. The importance of the density fluctuations in enhancing the turbulent energy transport is also evidenced. The ecliptic wind data measured by Ulysses are studied here in the same framework. This has been done by separating fast and slow streams, in order to avoid mixing of different physical conditions. The results further support the need for separate analysis of the two types of wind.
Solar wind plasma is known to cool down more slowly while it is blown away from the Sun than expe... more Solar wind plasma is known to cool down more slowly while it is blown away from the Sun than expected from an adiabatic spherical expansion. Some source of heating is thus needed to explain the observed temperature radial profile. The presence of a nonlinear turbulent magnetohydrodynamic energy cascade has been recently observed in solar wind plasma. This provides for the first time a direct estimation of the turbulent energy transfer rate, which can contribute to the in situ heating of the wind. The value of such contribution is shown to represent an important fraction (from 5% to 100%) of the total heating, and is strongly correlated with the wind temperature.
Interplanetary magnetic field magnitude fluctuations are notoriously more intermittent than veloc... more Interplanetary magnetic field magnitude fluctuations are notoriously more intermittent than velocity fluctuations in both fast and slow wind. This behaviour has been interpreted in terms of the anomalous scaling observed in passive scalars in fully developed hydrodynamic turbulence. In this paper, the strong intermittent nature of the interplanetary magnetic field is briefly discussed comparing results performed during different phases of the solar cycle. The scaling properties of the interplanetary magnetic field magnitude show solar cycle variation that can be distinguished in the scaling exponents revealed by structure functions. The scaling exponents observed around solar maximum coincide, within the errors, to those measured for passive scalars in hydrodynamic turbulence. However, it is also found that the values are not universal in the sense that the solar cycle variation may be reflected in dependence on the structure of the velocity field.
A signed measure analysis of two-dimensional intermittent magnetohydrodynamic turbulence is prese... more A signed measure analysis of two-dimensional intermittent magnetohydrodynamic turbulence is presented. This kind of analysis is performed to characterize the scaling behavior of the sign-oscillating flow structures, and their geometrical properties. In particular, it is observed that cancellations between positive and negative contributions of the field inside structures, are inhibited for scales smaller than the Taylor microscale, and stop near the dissipative scale. Moreover, from a simple geometrical argument, the relationship between the cancellation exponent and the typical fractal dimension of the structures in the flow is obtained.
The nonlinear dynamics of two-dimensional electrostatic interchange modes in a magnetized plasma ... more The nonlinear dynamics of two-dimensional electrostatic interchange modes in a magnetized plasma is investigated through a simple model that replaces the instability mechanism due to magnetic field curvature by an external source of vorticity and mass. Simulations in a cylindrical domain, with a spatially localized and randomized source at the center of the domain, reveal the eruption of mushroom-shaped bursts that propagate radially and are absorbed by the boundaries. Burst sizes and the interburst waiting times exhibit power-law statistics, which indicates long-range interburst correlations, similar to what has been found in sandpile models for avalanching systems. It is shown from the simulations that the dynamics can be characterized by a Yaglom relation for the third-order mixed moment involving the particle number density as a passive scalar and the E×B drift velocity, and hence that the burst phenomenology can be described within the framework of turbulence theory. Statistical features are qualitatively in agreement with experiments of intermittent transport at the edge of plasma devices, and suggest that essential features such as transport can be described by this simple model of bursty turbulence.
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 Alfvénic state. An analogous phenomenological scaling law, suitably modified to take into account compressible fluctuations, is observed more frequently in the same dataset. Large scale density fluctuations, despite their low amplitude, play thus 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 non-adiabatic solar wind.
We show in this article direct evidence for the presence of an inertial energy cascade, the most ... more We show in this article direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), in the solar wind as observed by the Ulysses spacecraft. After a brief rederivation of the equivalent of Yaglom's law for MHD turbulence, we show that 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.
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
ABSTRACT The observations of magnetic field variations as a signature of flaring activity is one ... more ABSTRACT 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.
ABSTRACT The observations of magnetic field variations as a signature of flaring activity is one ... more ABSTRACT 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.
Turbulent fluctuations within solar wind spectrum follows nearly Kolmogorov's power law s... more Turbulent fluctuations within solar wind spectrum follows nearly Kolmogorov's power law spectrum below the ion cyclotron frequency fci. Above this frequency, the observed steeper power law is believed to be a 'dissipative range' of the solar wind turbulence. The inertial range is studied here in terms of the pseudo-energy flux, which can be shown to have a linear scaling law
HAL (Le Centre pour la Communication Scientifique Directe), Dec 13, 2021
Rapid local directional alignment of different vector quantities has been observed in incompressi... more Rapid local directional alignment of different vector quantities has been observed in incompressible turbulence, either in MHD or in fluid turbulence, in both direct numerical simulations or in Solar wind data, and these alignments weaken the nonlinear terms with respect to those expected for Gaussian random fields. Evidence are based on the comparison of the probability distributions of the angle between vectors to those for independent isotropic three-dimensional vectors. We show that this apparent alignment can also be attributed to a local reduction of the dimensionality of the structures towards bidimensionality, leading to an increased probability of finding vectors close to alignment, compared to the spatial three-dimensional case. This interpretation is checked in Helios-2 Fast and Slow Solar wind data.
Automates de gaz sur réseaux. Aspects théoriques et simulations (Unpublished doctoral dissertatio... more Automates de gaz sur réseaux. Aspects théoriques et simulations (Unpublished doctoral dissertation). Université libre de Bruxelles, Faculté des sciences, Bruxelles.
In this short review we report about recent findings related to two fundamental points in the stu... more In this short review we report about recent findings related to two fundamental points in the study of solar wind turbulence: a) the verification of the equivalent of the -4/5 law in the solar wind and b) the estimate of the energy cascade along the spectrum and its comparison with the heating rate necessary to heat the solar wind during
In situ observations collected in the past decades, at different heliocentric distances and cover... more In situ observations collected in the past decades, at different heliocentric distances and covering almost the whole range of heliographic latitudes, represent a unique source of knowledge ready to be exploited to better understand the nature and the behavior of solar wind MHD turbulence. In fact, the solar wind can be used as an ideal wind tunnel in order to
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.
ABSTRACT A turbulent energy cascade has been recently identified in high-latitude solar wind data... more ABSTRACT A turbulent energy cascade has been recently identified in high-latitude solar wind data samples by using a Yaglom-like relation. However, analogous scaling law, suitably modified to take into account compressible fluctuations, has been observed in a much more extended fraction of the same data set recorded by the Ulysses spacecraft. Thus, it seems that large scale density fluctuations, despite their low amplitude, play a major role in the basic scaling properties of turbulence. The compressive turbulent cascade, moreover, seems to be able to supply the energy needed to account for the local heating of the non-adiabatic solar wind.
The detection of magnetic field variations as a signature of flaring activity is one of the main ... more The detection of magnetic field variations as a signature of flaring activity is one of the main goal in solar physics. Past efforts gave apparently no unambiguous observations of systematic changes. In the present study, we discuss recent results on observations that scaling laws of turbulent current helicity inside a given flaring active region change in response to large flares in that active region. Such changes can be related to the evolution of current structures by a simple geometrical argument, which had been tested using high Reynolds number direct numerical simulations of the MHD equations. Interpretation of the observed data within this picture indicates that the change in scaling behavior of the current helicity seems to be associated with a topological reorganization of the footpoint of the magnetic field loops, namely with the dissipation of small scales structures in turbulent media.
The occurrence of a nonlinear turbulent energy cascade in solar wind plasma has been recently est... more The occurrence of a nonlinear turbulent energy cascade in solar wind plasma has been recently established through the observation of an exact law from spacecraft measurements. The main results obtained in the fast, polar wind measured by Ulysses spacecraft are reviewed here. In particular, the turbulent cascade is seen as the mean to provide the energy necessary for the local heating in the non-adiabatic expansion of the solar wind. The importance of the density fluctuations in enhancing the turbulent energy transport is also evidenced. The ecliptic wind data measured by Ulysses are studied here in the same framework. This has been done by separating fast and slow streams, in order to avoid mixing of different physical conditions. The results further support the need for separate analysis of the two types of wind.
Solar wind plasma is known to cool down more slowly while it is blown away from the Sun than expe... more Solar wind plasma is known to cool down more slowly while it is blown away from the Sun than expected from an adiabatic spherical expansion. Some source of heating is thus needed to explain the observed temperature radial profile. The presence of a nonlinear turbulent magnetohydrodynamic energy cascade has been recently observed in solar wind plasma. This provides for the first time a direct estimation of the turbulent energy transfer rate, which can contribute to the in situ heating of the wind. The value of such contribution is shown to represent an important fraction (from 5% to 100%) of the total heating, and is strongly correlated with the wind temperature.
Interplanetary magnetic field magnitude fluctuations are notoriously more intermittent than veloc... more Interplanetary magnetic field magnitude fluctuations are notoriously more intermittent than velocity fluctuations in both fast and slow wind. This behaviour has been interpreted in terms of the anomalous scaling observed in passive scalars in fully developed hydrodynamic turbulence. In this paper, the strong intermittent nature of the interplanetary magnetic field is briefly discussed comparing results performed during different phases of the solar cycle. The scaling properties of the interplanetary magnetic field magnitude show solar cycle variation that can be distinguished in the scaling exponents revealed by structure functions. The scaling exponents observed around solar maximum coincide, within the errors, to those measured for passive scalars in hydrodynamic turbulence. However, it is also found that the values are not universal in the sense that the solar cycle variation may be reflected in dependence on the structure of the velocity field.
A signed measure analysis of two-dimensional intermittent magnetohydrodynamic turbulence is prese... more A signed measure analysis of two-dimensional intermittent magnetohydrodynamic turbulence is presented. This kind of analysis is performed to characterize the scaling behavior of the sign-oscillating flow structures, and their geometrical properties. In particular, it is observed that cancellations between positive and negative contributions of the field inside structures, are inhibited for scales smaller than the Taylor microscale, and stop near the dissipative scale. Moreover, from a simple geometrical argument, the relationship between the cancellation exponent and the typical fractal dimension of the structures in the flow is obtained.
The nonlinear dynamics of two-dimensional electrostatic interchange modes in a magnetized plasma ... more The nonlinear dynamics of two-dimensional electrostatic interchange modes in a magnetized plasma is investigated through a simple model that replaces the instability mechanism due to magnetic field curvature by an external source of vorticity and mass. Simulations in a cylindrical domain, with a spatially localized and randomized source at the center of the domain, reveal the eruption of mushroom-shaped bursts that propagate radially and are absorbed by the boundaries. Burst sizes and the interburst waiting times exhibit power-law statistics, which indicates long-range interburst correlations, similar to what has been found in sandpile models for avalanching systems. It is shown from the simulations that the dynamics can be characterized by a Yaglom relation for the third-order mixed moment involving the particle number density as a passive scalar and the E×B drift velocity, and hence that the burst phenomenology can be described within the framework of turbulence theory. Statistical features are qualitatively in agreement with experiments of intermittent transport at the edge of plasma devices, and suggest that essential features such as transport can be described by this simple model of bursty turbulence.
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 Alfvénic state. An analogous phenomenological scaling law, suitably modified to take into account compressible fluctuations, is observed more frequently in the same dataset. Large scale density fluctuations, despite their low amplitude, play thus 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 non-adiabatic solar wind.
We show in this article direct evidence for the presence of an inertial energy cascade, the most ... more We show in this article direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), in the solar wind as observed by the Ulysses spacecraft. After a brief rederivation of the equivalent of Yaglom's law for MHD turbulence, we show that 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.
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
ABSTRACT The observations of magnetic field variations as a signature of flaring activity is one ... more ABSTRACT 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.
ABSTRACT The observations of magnetic field variations as a signature of flaring activity is one ... more ABSTRACT 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.
Turbulent fluctuations within solar wind spectrum follows nearly Kolmogorov's power law s... more Turbulent fluctuations within solar wind spectrum follows nearly Kolmogorov's power law spectrum below the ion cyclotron frequency fci. Above this frequency, the observed steeper power law is believed to be a 'dissipative range' of the solar wind turbulence. The inertial range is studied here in terms of the pseudo-energy flux, which can be shown to have a linear scaling law
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