154 research outputs found
Finite size scaling in the solar wind magnetic field energy density as seen by WIND
Statistical properties of the interplanetary magnetic field fluctuations can provide an important insight into the solar wind turbulent cascade. Recently, analysis of the Probability Density Functions (PDF) of the velocity and magnetic field fluctuations has shown that these exhibit non-Gaussian properties on small time scales while large scale features appear to be uncorrelated. Here we apply the finite size scaling technique to explore the scaling of the magnetic field energy density fluctuations as seen by WIND. We find a single scaling sufficient to collapse the curves over the entire investigated range. The rescaled PDF follow a non Gaussian distribution with asymptotic behavior well described by the Gamma distribution arising from a finite range Lévy walk. Such mono scaling suggests that a Fokker-Planck approach can be applied to study the PDF dynamics. These results strongly suggest the existence of a common, nonlinear process on the time scale up to 26 hours
Analysis of cancellation in two-dimensional magnetohydrodynamic turbulence
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.Comment: 21 pages, 5 figures (3 .jpg not included in the latex file
Scaling Properties of Atmospheric Wind Speed in Mesoscale Range
The scaling properties of turbulent flows are well established in the inertial sub-range. However, those of the synoptic-scale motions are less known, also because of the difficult analysis of data presenting nonstationary and periodic features. Extensive analysis of experimental wind speed data, collected at the Mauna Loa Observatory of Hawaii, is performed using different methods. Empirical Mode Decomposition, interoccurrence times statistics, and arbitrary-order Hilbert spectral analysis allow to eliminate effects of large-scale modulations, and provide scaling properties of the field fluctuations (Hurst exponent, interoccurrence distribution, and intermittency correction). The obtained results suggest that the mesoscale wind dynamics owns features which are typical of the inertial sub-range turbulence, thus extending the validity of the turbulent cascade phenomenology to scales larger than observed before
Reversals in nature and the nature of reversals
The asymmetric shape of reversals of the Earth's magnetic field indicates a
possible connection with relaxation oscillations as they were early discussed
by van der Pol. A simple mean-field dynamo model with a spherically symmetric
coefficient is analysed with view on this similarity, and a comparison
of the time series and the phase space trajectories with those of paleomagnetic
measurements is carried out. For highly supercritical dynamos a very good
agreement with the data is achieved. Deviations of numerical reversal sequences
from Poisson statistics are analysed and compared with paleomagnetic data. The
role of the inner core is discussed in a spectral theoretical context and
arguments and numerical evidence is compiled that the growth of the inner core
might be important for the long term changes of the reversal rate and the
occurrence of superchrons.Comment: 24 pages, 12 figure
Proper orthogonal decomposition of solar photospheric motions
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 microHz, 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 microHz.Comment: 8 figures in jpg in press on PR
Solar Wind Turbulence and the Role of Ion Instabilities
International audienc
First Solar Orbiter observation of the Alfvénic slow wind and identification of its solar source
Context: Turbulence dominated by large amplitude nonlinear Alfvén-like fluctuations mainly propagating away from the Sun is
ubiquitous in high speed solar wind streams. Recent studies have shown that also slow wind streams may show strong Alfvénic
signatures, especially in the inner heliosphere.
Aims: The present study focuses on the characterisation of an Alfvénic slow solar wind interval observed by Solar Orbiter on July
14-18, 2020 at a heliocentric distance of 0.64 AU.
Methods: Our analysis is based on plasma moments and magnetic field measurements from the Solar Wind Analyser (SWA) and
Magnetometer (MAG) instruments, respectively. We compare the behaviour of different parameters to characterise the stream in
terms of the Alfvénic content and magnetic properties. We perform also a spectral analysis to highlight spectral features and waves
signature using power spectral density and magnetic helicity spectrograms, respectively. Moreover, we reconstruct the Solar Orbiter
magnetic connectivity to the solar sources via both a ballistic and a Potential Field Source Surface (PFSS) model.
Results: The Alfvénic slow wind stream described in this paper resembles in many respects a fast wind stream. Indeed, at large scales,
the time series of the speed profile shows a compression region, a main portion of the stream and a rarefaction region, characterised by
different features. Moreover, before the rarefaction region, we pinpoint several structures at different scales recalling the spaghetti-like
flux-tube texture of the interplanetary magnetic field. Finally, we identify the connections between Solar Orbiter in situ measurements,
tracing them down to coronal streamer and pseudostreamer configurations.
Conclusions. The characterisation of the Alfvénic slow wind stream observed by Solar Orbiter and the identification of its solar
source are extremely important aspects to understand possible future observations of the same solar wind regime, especially as solar
activity is increasing toward a maximum, where a higher incidence of this solar wind regime is expected
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