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
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 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.
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
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
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 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.
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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Papers by Alain Noullez