24 research outputs found
Arbitrary-order Hilbert spectral analysis and intermittency in solar wind density fluctuations
The properties of inertial and kinetic range solar wind turbulence have been
investigated with the arbitrary-order Hilbert spectral analysis method, applied
to high-resolution density measurements. Due to the small sample size, and to
the presence of strong non-stationary behavior and large-scale structures, the
classical structure function analysis fails to detect power law behavior in the
inertial range, and may underestimate the scaling exponents. However, the
Hilbert spectral method provides an optimal estimation of the scaling
exponents, which have been found to be close to those for velocity fluctuations
in fully developed hydrodynamic turbulence. At smaller scales, below the proton
gyroscale, the system loses its intermittent multiscaling properties, and
converges to a monofractal process. The resulting scaling exponents, obtained
at small scales, are in good agreement with those of classical fractional
Brownian motion, indicating a long-term memory in the process, and the absence
of correlations around the spectral break scale. These results provide
important constraints on models of kinetic range turbulence in the solar wind
Identification of the different magnetic field contributions during a geomagnetic storm in magnetospheric and ground observations
Abstract. We used the empirical mode decomposition (EMD) to investigate the time variation of the magnetospheric and ground-based observations of the Earth's magnetic field during both quiet and disturbed periods. We found two timescale variations in magnetospheric data which are associated with different magnetospheric current systems and the characteristic diurnal orbital variation, respectively. On the ground we identified three timescale variations related to the solar-wind–magnetosphere high-frequency interactions, the ionospheric processes, and the internal dynamics of the magnetosphere. This approach is able to identify the different physical processes involved in solar-wind–magnetosphere–ionosphere coupling. In addition, the large-timescale contribution can be used as a local index for the identification of the intensity of a geomagnetic storm on the ground
STOCHASTICITY AND PERSISTENCE OF SOLAR CORONAL MASS EJECTIONS
The study of the statistical properties of coronal mass ejections (CMEs) reveals that their properties depend on the period of solar activity. In particular, when investigating the origin of the waiting time distribution between CMEs, a significant departure from a Poisson process during periods of high solar activity has been found, thus suggesting the existence of at least two physical processes underlying the origin of CMEs. One acts continuously, perhaps related to randomly occurring magnetic reconfigurations of the solar corona at large scales. The other plays a role only during the solar maximum, probably due to the photospheric emergence of magnetic flux as a statistically persistent mechanism, which generates long correlation times among CME events strong enough not to be destroyed by the former random process
Study of Turbulence Associated with the Faraji Cyclone
The formation of a cyclonic region in which nonlinear interactions generate turbulence in the form of small-scale vortices can be observed because of the different rotating air masses. Turbulence dynamics in cyclones (specifically hurricanes) has been under-researched; therefore, assessing the shear term is crucial to identify the onset of cyclonic formation within a region of the atmosphere. Earth observation techniques are able to provide relevant information on this physical process. In this article, we propose a new framework that is useful for connecting the study of the dynamics of a cyclonic system with the observations generated by geostationary satellite facilities. In particular, we applied the proper orthogonal decomposition (POD), a technique widely used in turbulent analysis to decompose a generic scalar or vector field in empirical eigenfunction, to investigate a tropical cyclone, the Faraji hurricane, from a dynamic point of view, beginning from the temporal evolution of its temperature field. The latter was obtained by elaborating on data and images collected by the SEVIRI radiometer, installed on the Meteosat Second Generation-8 (IODC) satellite. Using the POD, the energy spectra of both the spatial and temporal components of the temperature field obtained through remote sensing techniques were studied separately. Important information was then extracted and used for an in-depth characterization of the properties of the turbulence in the non-linear evolution of this phenomenon
Model for the spatio-temporal intermittency of the energy dissipation in turbulent flows
Modeling the intermittent behavior of turbulent energy dissipation processes
both in space and time is often a relevant problem when dealing with phenomena
occurring in high Reynolds number flows, especially in astrophysical and space
fluids. In this paper, a dynamical model is proposed to describe the
spatio-temporal intermittency of energy dissipation rate in a turbulent system.
This is done by using a shell model to simulate the turbulent cascade and
introducing some heuristic rules, partly inspired by the well known -model,
to construct a spatial structure of the energy dissipation rate. In order to
validate the model and to study its spatially intermittency properties, a
series of numerical simulations have been performed. These show that the level
of spatial intermittency of the system can be simply tuned by varying a single
parameter of the model and that scaling laws in agreement with those obtained
from experiments on fully turbulent hydrodynamic flows can be recovered. It is
finally suggested that the model could represent a useful tool to simulate the
spatio-temporal intermittency of turbulent energy dissipation in those high
Reynolds number astrophysical fluids where impulsive energy release processes
can be associated to the dynamics of the turbulent cascade.Comment: 22 pages, 9 figure
RELAXATION PROCESSES WITHIN FLUX ROPES IN SOLAR WIND
Flux ropes are localized structures in space plasma whose tube-like organized magnetic configuration can be well approximated by a force-free field model. Both numerical simulations and simple models suggest that the ideal magnetohydrodynamics (MHD) can relax toward a minimum energy state, where magnetic helicity is conserved, characterized by force-free magnetic fields (Taylor relaxation). In this paper, we evaluate MHD rugged invariants within more than 100 flux ropes identified in the solar wind at 1 AU, showing that the magnetic and cross-helicity content carried out by these structures tend to be "attracted" toward a particular subphase in the parameter plane. The final configuration of the MHD rugged invariants in the parameter plane suggests indeed that flux ropes represent well-organized structures coming from the dynamical evolution of MHD turbulent cascade. These observational results, along with a simple model based on a truncated set of nonlinear ordinary differential equations for both the velocity and magnetic field Fourier coefficients, thus, support a scenario in which the flux ropes naturally come out from the ideal MHD decay to large-scale magnetic field in space plasmas, probably governed by relaxation processes similar to those observed in laboratory plasmas
Particle Acceleration in an Evolving Network of Unstable Current Sheets
We study the acceleration of electrons and protons interacting with
localized, multiple, small-scale dissipation regions inside an evolving,
turbulent active region. The dissipation regions are Unstable Current Sheets
(UCS), and in their ensemble they form a complex, fractal, evolving network of
acceleration centers. Acceleration and energy dissipation are thus assumed to
be fragmented. A large-scale magnetic topology provides the connectivity
between the UCS and determines in this way the degree of possible multiple
acceleration. The particles travel along the magnetic field freely without
loosing or gaining energy, till they reach a UCS. In a UCS, a variety of
acceleration mechanisms are active, with the end-result that the particles
depart with a new momentum. The stochastic acceleration process is represented
in the form of Continuous Time Random Walk (CTRW), which allows to estimate the
evolution of the energy distribution of the particles. It is found that under
certain conditions electrons are heated and accelerated to energies above 1 MeV
in much less than a second. Hard X-ray (HXR) and microwave spectra are
calculated from the electrons' energy distributions, and they are found to be
compatible with the observations. Ions (protons) are also heated and
accelerated, reaching energies up to 10 MeV almost simultaneously with the
electrons. The diffusion of the particles inside the active region is extremely
fast (anomalous super-diffusion). Although our approach does not provide
insight into the details of the specific acceleration mechanisms involved, its
benefits are that it relates acceleration to the energy release, and it well
describes the stochastic nature of the acceleration process.Comment: 37 pages, 10 figures, one of them in color; in press at ApJ (2004
Arbitrary-order Hilbert Spectral Analysis and Intermittency in Solar Wind Density Fluctuations
C.H.K.C. is supported by an STFC Ernest Rutherford
Fellowship. J.S. and Z.N. acknowledge support of the Czech
Science Foundation under Contract 16-04956S