347 research outputs found
The Water Factor: Hygiene, environment, ethics, economics
Water does many things: it questions our idea of sustainability; it forces us to rethink the word economics and extend its scope to environmental, as well as to all financial, accounting and political issues. Talking about water (in terms of availability, distribution, use…) means trying to focus on certain issues, first and foremost to promote the possibility of doing things: raising awareness about the need to act upon water resources to ensure their quality; recognizing water as a critical element which may alterate a delicate natural balance; considering everyday behaviors without underestimating emergencies or necessities
A New NF-ÎşB Inhibitor, MEDS-23, Reduces the Severity of Adverse Post-Ischemic Stroke Outcomes in Rats
High-Lundquist Number Scaling in Three-Dimensional Simulations of Parker's Model of Coronal Heating
Parker's model is one of the most discussed mechanisms for coronal heating
and has generated much debate. We have recently obtained new scaling results in
a two-dimensional (2D) version of this problem suggesting that the heating rate
becomes independent of resistivity in a statistical steady state [Ng and
Bhattacharjee, Astrophys. J., 675, 899 (2008)]. Our numerical work has now been
extended to 3D by means of large-scale numerical simulations. Random
photospheric footpoint motion is applied for a time much longer than the
correlation time of the motion to obtain converged average coronal heating
rates. Simulations are done for different values of the Lundquist number to
determine scaling. In the high-Lundquist number limit, the coronal heating rate
obtained so far is consistent with a trend that is independent of the Lundquist
number, as predicted by previous analysis as well as 2D simulations. In the
same limit the average magnetic energy built up by the random footpoint motion
tends to have a much weaker dependence on the Lundquist number than that in the
2D simulations, due to the formation of strong current layers and subsequent
disruption when the equilibrium becomes unstable. We will present scaling
analysis showing that when the dissipation time is comparable or larger than
the correlation time of the random footpoint motion, the heating rate tends to
become independent of Lundquist number, and that the magnetic energy production
is also reduced significantly.Comment: Accepted for publication in Astrophysical Journa
MHD consistent cellular automata (CA) models II. Applications to solar flares
In Isliker et al. (2000b), an extended cellular automaton (X-CA) model for
solar flares was introduced. In this model, the interpretation of the model's
grid-variable is specified, and the magnetic field, the current, and an
approximation to the electric field are yielded, all in a way that is
consistent with Maxwell's and the MHD equations. Here, we reveal which relevant
plasma physical processes are implemented by the X-CA model and in what form,
and what global physical set-up is assumed by this model when it is in its
natural state (SOC). The basic results are: (1) On large-scales, all variables
show characteristic quasi-symmetries. (2) The global magnetic topology forms
either (i) closed magnetic field lines, or (ii) an arcade of field lines above
the bottom plane line, if the model is slightly modified. (3) In case of the
magnetic topology (ii), loading can be interpreted as if there were a plasma
which flows predominantly upwards, whereas in case of the magnetic topology
(i), as if there were a plasma flow expanding from the neutral line. (4) The
small-scale physics in the bursting phase represent localized diffusive
processes. (5) The local diffusivity usually has a value which is effectively
zero, and it turns locally to an anomalous value if a threshold is exceeded,
whereby diffusion dominates the quiet evolution (loading). (6) Flares
(avalanches) are accompanied by the appearance of localized, intense electric
fields. (7) In a variant on the X-CA model, the magnitude of the current is
used directly in the instability criterion. First results indicate that the SOC
state persists. (8) The current-dissipation during flares is spatially
fragmented into a large number of dissipative current-surfaces of varying
sizes, which show a highly dynamic temporal evolution.Comment: 13 pages, 12 figures; in press at Astronomy and Astrophysics (2001
Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun
We search for persistent and quasi-periodic release events of streamer blobs
during 2007 with the Large Angle Spectrometric Coronagraph on the \textit{Solar
and Heliospheric Observatory} and assess the velocity of the slow solar wind
along the plasma sheet above the corresponding streamer by measuring the
dynamic parameters of blobs. We find 10 quasi-periodic release events of
streamer blobs lasting for three to four days. In each day of these events, we
observe three-five blobs. The results are in line with previous studies using
data observed near the last solar minimum. Using the measured blob velocity as
a proxy for that of the mean flow, we suggest that the velocity of the
background slow solar wind near the Sun can vary significantly within a few
hours. This provides an observational manifestation of the large velocity
variability of the slow solar wind near the Sun.Comment: 14 pages, 5 figures, accepted by Soalr Physic
Persistence of small-scale anisotropies and anomalous scaling in a model of magnetohydrodynamics turbulence
The problem of anomalous scaling in magnetohydrodynamics turbulence is
considered within the framework of the kinematic approximation, in the presence
of a large-scale background magnetic field. The velocity field is Gaussian,
-correlated in time, and scales with a positive exponent .
Explicit inertial-range expressions for the magnetic correlation functions are
obtained; they are represented by superpositions of power laws with
non-universal amplitudes and universal (independent of the anisotropy and
forcing) anomalous exponents. The complete set of anomalous exponents for the
pair correlation function is found non-perturbatively, in any space dimension
, using the zero-mode technique. For higher-order correlation functions, the
anomalous exponents are calculated to using the renormalization group.
The exponents exhibit a hierarchy related to the degree of anisotropy; the
leading contributions to the even correlation functions are given by the
exponents from the isotropic shell, in agreement with the idea of restored
small-scale isotropy. Conversely, the small-scale anisotropy reveals itself in
the odd correlation functions : the skewness factor is slowly decreasing going
down to small scales and higher odd dimensionless ratios (hyperskewness etc.)
dramatically increase, thus diverging in the limit.Comment: 25 pages Latex, 1 Figur
Simulating Flaring Events in Complex Active Regions Driven by Observed Magnetograms
We interpret solar flares as events originating from active regions that have
reached the Self Organized Critical state, by using a refined Cellular
Automaton model with initial conditions derived from observations. Aims: We
investigate whether the system, with its imposed physical elements,reaches a
Self Organized Critical state and whether well-known statistical properties of
flares, such as scaling laws observed in the distribution functions of
characteristic parameters, are reproduced after this state has been reached.
Results: Our results show that Self Organized Criticality is indeed reached
when applying specific loading and relaxation rules. Power law indices obtained
from the distribution functions of the modeled flaring events are in good
agreement with observations. Single power laws (peak and total flare energy) as
well as power laws with exponential cutoff and double power laws (flare
duration) are obtained. The results are also compared with observational X-ray
data from GOES satellite for our active-region sample. Conclusions: We conclude
that well-known statistical properties of flares are reproduced after the
system has reached Self Organized Criticality. A significant enhancement of our
refined Cellular Automaton model is that it commences the simulation from
observed vector magnetograms, thus facilitating energy calculation in physical
units. The model described in this study remains consistent with fundamental
physical requirements, and imposes physically meaningful driving and
redistribution rules.Comment: 14 pages; 12 figures; 6 tables - A&A, in pres
Turbulence in the Solar Atmosphere: Manifestations and Diagnostics via Solar Image Processing
Intermittent magnetohydrodynamical turbulence is most likely at work in the
magnetized solar atmosphere. As a result, an array of scaling and multi-scaling
image-processing techniques can be used to measure the expected
self-organization of solar magnetic fields. While these techniques advance our
understanding of the physical system at work, it is unclear whether they can be
used to predict solar eruptions, thus obtaining a practical significance for
space weather. We address part of this problem by focusing on solar active
regions and by investigating the usefulness of scaling and multi-scaling
image-processing techniques in solar flare prediction. Since solar flares
exhibit spatial and temporal intermittency, we suggest that they are the
products of instabilities subject to a critical threshold in a turbulent
magnetic configuration. The identification of this threshold in scaling and
multi-scaling spectra would then contribute meaningfully to the prediction of
solar flares. We find that the fractal dimension of solar magnetic fields and
their multi-fractal spectrum of generalized correlation dimensions do not have
significant predictive ability. The respective multi-fractal structure
functions and their inertial-range scaling exponents, however, probably provide
some statistical distinguishing features between flaring and non-flaring active
regions. More importantly, the temporal evolution of the above scaling
exponents in flaring active regions probably shows a distinct behavior starting
a few hours prior to a flare and therefore this temporal behavior may be
practically useful in flare prediction. The results of this study need to be
validated by more comprehensive works over a large number of solar active
regions.Comment: 26 pages, 7 figure
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