833 research outputs found
Dependence of solar wind power spectra on the direction of the local mean magnetic field
(Abridged) Wavelet analysis can be used to measure the power spectrum of
solar wind fluctuations along a line in any direction with respect to the local
mean magnetic field. This technique is applied to study solar wind turbulence
in high-speed streams in the ecliptic plane near solar minimum using magnetic
field measurements with a cadence of eight vectors per second. The analysis of
nine high-speed streams shows that the reduced spectrum of magnetic field
fluctuations (trace power) is approximately azimuthally symmetric about B_0 in
both the inertial range and dissipation range; in the inertial range the
spectra are characterized by a power-law exponent that changes continuously
from 1.6 \pm 0.1 in the direction perpendicular to the mean field to 2.0 \pm
0.1 in the direction parallel to the mean field. The large uncertainties
suggest that the perpendicular power-law indices 3/2 and 5/3 are both
consistent with the data. The results are similar to those found by Horbury et
al. (2008) at high heliographic latitudes.Comment: Submitted to: The Astrophysical Journa
Upflows in the upper transition region of the quiet Sun
We investigate the physical meaning of the prominent blue shifts of Ne VIII,
which is observed to be associated with quiet-Sun network junctions (boundary
intersections), through data analyses combining force-free-field extrapolations
with EUV spectroscopic observations. For a middle-latitude region, we
reconstruct the magnetic funnel structure in a sub-region showing faint
emission in EIT-Fe 195. This funnel appears to consist of several smaller
funnels that originate from network lanes, expand with height and finally merge
into a single wide open-field region. However, the large blue shifts of Ne VIII
are generally not associated with open fields, but seem to be associated with
the legs of closed magnetic loops. Moreover, in most cases significant upflows
are found in both of the funnel-shaped loop legs. These quasi-steady upflows
are regarded as signatures of mass supply to the coronal loops rather than the
solar wind. Our observational result also reveals that in many cases the
upflows in the upper transition region (TR) and the downflows in the middle TR
are not fully cospatial. Based on these new observational results, we suggest
different TR structures in coronal holes and in the quiet Sun.Comment: 4 pages, 4 figures, will appear in the Proceedings of the Solar wind
12 conferenc
A Model to Explore Responses of Spruce Stands to Air-Pollution Stress in Europe
Systems analysis has proven to be a suitable instrument for describing the processes taking place in pollution-damaged forest stands and for simulating the ecosystem behavior in various environmental situations. In a preliminary model showing the behavior of pollution-damaged spruce stands, site, air pollution, stand structure and management are taken into consideration. The target of this model consists of simulating system behavior under variable pollutant stressors and management strategies, and of serving as a basis for decision-making. The basic model is subdivided into the following submodels: "Forest stand" with management as well as external ecological factors and international modifications; "Leaf quantity/leaf fall"; "Net assimilation"; and "Dendromass distribution". A soil model from other authors is being incorporated. After a comparison with similar models, reference is made to further possibilities of application
Computation of Kolmogorov's Constant in Magnetohydrodynamic Turbulence
In this paper we calculate Kolmogorov's constant for magnetohydrodynamic
turbulence to one loop order in perturbation theory using the direct
interaction approximation technique of Kraichnan. We have computed the
constants for various , i.e., fluid to magnetic energy ratios
when the normalized cross helicity is zero. We find that increases from
1.47 to 4.12 as we go from fully fluid case to a situation when , then it decreases to 3.55 in a fully magnetic limit .
When , we find that .Comment: Latex, 10 pages, no figures, To appear in Euro. Phys. Lett., 199
Gyrokinetic Simulations of Solar Wind Turbulence from Ion to Electron Scales
The first three-dimensional, nonlinear gyrokinetic simulation of plasma
turbulence resolving scales from the ion to electron gyroradius with a
realistic mass ratio is presented, where all damping is provided by resolved
physical mechanisms. The resulting energy spectra are quantitatively consistent
with a magnetic power spectrum scaling of as observed in \emph{in
situ} spacecraft measurements of the "dissipation range" of solar wind
turbulence. Despite the strongly nonlinear nature of the turbulence, the linear
kinetic \Alfven wave mode quantitatively describes the polarization of the
turbulent fluctuations. The collisional ion heating is measured at
sub-ion-Larmor radius scales, which provides the first evidence of the ion
entropy cascade in an electromagnetic turbulence simulation.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Emission heights of coronal bright points on Fe XII radiance map
We study the emission heights of the coronal bright points (BPs) above the
photosphere in the bipolar magnetic loops that are apparently associated with
them. As BPs are seen in projection against the disk their true emission
heights are unknown. The correlation of the BP locations on the Fe XII radiance
map from EIT with the magnetic field features (in particular neutral lines) was
investigated in detail. The coronal magnetic field was determined by an
extrapolation of the photospheric field to different altitudes above the disk.
It was found that most BPs sit on or near a photospheric neutral line, but that
the emission occurs at a height of about 5 Mm. Some BPs, while being seen in
projection, still seem to coincide with neutral lines, although their emission
takes place at heights of more than 10 Mm. Such coincidences almost disappear
for emissions above 20 Mm. We also projected the upper segments of the 3-D
magnetic field lines above different heights, respectively, on to the x-y
plane. The shape of each BP was compared with the respective field-line segment
nearby. This comparison suggests that most coronal BPs are actually located on
the top of their associated magnetic loops. Finally, we calculated for each
selected BP region the correlation coefficient between the Fe XII intensity
enhancement and the horizontal component of the extrapolated magnetic field
vector at the same x-y position in planes of different heights, respectively.
We found that for almost all the BP regions we studied the correlation
coefficient, with increasing height, increases to a maximal value and then
decreases again. The height corresponding to this maximum was defined as the
correlation height, which for most bright points was found to range below 20
Mm.Comment: 7 pages, 4 figures, 1 tabl
A Unified Picture of the FIP and Inverse FIP Effects
We discuss models for coronal abundance anomalies observed in the coronae of
the sun and other late-type stars following a scenario first introduced by
Schwadron, Fisk & Zurbuchen of the interaction of waves at loop footpoints with
the partially neutral gas. Instead of considering wave heating of ions in this
location, we explore the effects on the upper chromospheric plasma of the wave
ponderomotive forces. These can arise as upward propagating waves from the
chromosphere transmit or reflect upon reaching the chromosphere-corona
boundary, and are in large part determined by the properties of the coronal
loop above. Our scenario has the advantage that for realistic wave energy
densities, both positive and negative changes in the abundance of ionized
species compared to neutrals can result, allowing both FIP and Inverse FIP
effects to come out of the model. We discuss how variations in model parameters
can account for essentially all of the abundance anomalies observed in solar
spectra. Expected variations with stellar spectral type are also qualitatively
consistent with observations of the FIP effect in stellar coronae.Comment: 25 pages, 4 figures, submitted to Ap
A nonextensive entropy approach to solar wind intermittency
The probability distributions (PDFs) of the differences of any physical
variable in the intermittent, turbulent interplanetary medium are scale
dependent. Strong non-Gaussianity of solar wind fluctuations applies for short
time-lag spacecraft observations, corresponding to small-scale spatial
separations, whereas for large scales the differences turn into a Gaussian
normal distribution. These characteristics were hitherto described in the
context of the log-normal, the Castaing distribution or the shell model. On the
other hand, a possible explanation for nonlocality in turbulence is offered
within the context of nonextensive entropy generalization by a recently
introduced bi-kappa distribution, generating through a convolution of a
negative-kappa core and positive-kappa halo pronounced non-Gaussian structures.
The PDFs of solar wind scalar field differences are computed from WIND and ACE
data for different time lags and compared with the characteristics of the
theoretical bi-kappa functional, well representing the overall scale dependence
of the spatial solar wind intermittency. The observed PDF characteristics for
increased spatial scales are manifest in the theoretical distribution
functional by enhancing the only tuning parameter , measuring the
degree of nonextensivity where the large-scale Gaussian is approached for
. The nonextensive approach assures for experimental studies
of solar wind intermittency independence from influence of a priori model
assumptions. It is argued that the intermittency of the turbulent fluctuations
should be related physically to the nonextensive character of the
interplanetary medium counting for nonlocal interactions via the entropy
generalization.Comment: 17 pages, 7 figures, accepted for publication in Astrophys.
Previous cytomegalovirus infection and the risk of restenosis after a strategy of provisional stenting
The Heating of Test Particles in Numerical Simulations of Alfvenic Turbulence
We study the heating of charged test particles in three-dimensional numerical
simulations of weakly compressible magnetohydrodynamic (MHD) turbulence
(``Alfvenic turbulence''); these results are relevant to particle heating and
acceleration in the solar wind, solar flares, accretion disks onto black holes,
and other astrophysics and heliospheric environments. The physics of particle
heating depends on whether the gyrofrequency of a particle is comparable to the
frequency of a turbulent fluctuation that is resolved on the computational
domain. Particles with these frequencies nearly equal undergo strong
perpendicular heating (relative to the local magnetic field) and pitch angle
scattering. By contrast, particles with large gyrofrequency undergo strong
parallel heating. Simulations with a finite resistivity produce additional
parallel heating due to parallel electric fields in small-scale current sheets.
Many of our results are consistent with linear theory predictions for the
particle heating produced by the Alfven and slow magnetosonic waves that make
up Alfvenic turbulence. However, in contrast to linear theory predictions,
energy exchange is not dominated by discrete resonances between particles and
waves; instead, the resonances are substantially ``broadened.'' We discuss the
implications of our results for solar and astrophysics problems, in particular
the thermodynamics of the near-Earth solar wind. We conclude that Alfvenic
turbulence produces significant parallel heating via the interaction between
particles and magnetic field compressions (``slow waves''). However, on scales
above the proton Larmor radius, Alfvenic turbulence does not produce
significant perpendicular heating of protons or minor ions.Comment: Submitted to Ap
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