719 research outputs found
Occurrence and persistence of magnetic elements in the quiet Sun
Turbulent convection efficiently transports energy up to the solar
photosphere, but its multi-scale nature and dynamic properties are still not
fully understood. Several works in the literature have investigated the
emergence of patterns of convective and magnetic nature in the quiet Sun at
spatial and temporal scales from granular to global. Aims. To shed light on the
scales of organisation at which turbulent convection operates, and its
relationship with the magnetic flux therein, we studied characteristic spatial
and temporal scales of magnetic features in the quiet Sun. Methods. Thanks to
an unprecedented data set entirely enclosing a supergranule, occurrence and
persistence analysis of magnetogram time series were used to detect spatial and
long-lived temporal correlations in the quiet Sun and to investigate their
nature. Results. A relation between occurrence and persistence representative
for the quiet Sun was found. In particular, highly recurrent and persistent
patterns were detected especially in the boundary of the supergranular cell.
These are due to moving magnetic elements undergoing motion that behaves like a
random walk together with longer decorrelations ( h) with respect to
regions inside the supergranule. In the vertices of the supegranular cell the
maximum observed occurrence is not associated with the maximum persistence,
suggesting that there are different dynamic regimes affecting the magnetic
elements
Imaging Spectropolarimetry with IBIS: Evolution of Bright Points in the Quiet Sun
We present the results from first spectropolarimetric observations of the
solar photosphere acquired at the Dunn Solar Telescope with the Interferometric
Bidimensional Spectrometer. Full Stokes profiles were measured in the Fe I
630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions
for 53 minutes, with a Stokes V noise of 0.003 the continuum intensity level.
The dataset allows us to study the evolution of several magnetic features
associated with G-band bright points in the quiet Sun. Here we focus on the
analysis of three distinct processes, namely the coalescence, fragmentation and
cancellation of G-band bright points. Our analysis is based on a SIR inversion
of the Stokes I and V profiles of both Fe I lines. The high spatial resolution
of the G-band images combined with the inversion results helps to interpret the
undergoing physical processes. The appearance (dissolution) of high-contrast
G-band bright points is found to be related to the local increase (decrease) of
the magnetic filling factor, without appreciable changes in the field strength.
The cancellation of opposite-polarity bright points can be the signature of
either magnetic reconnection or the emergence/submergence of magnetic loops.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
HINODE Observations of Chromospheric Brightenings in the Ca II H Line during small-scale Flux Emergence Events
\ion{Ca}{2} H emission is a well-known indicator of magnetic activity in the
Sun and other stars. It is also viewed as an important signature of
chromospheric heating. However, the \ion{Ca}{2} H line has not been used as a
diagnostic of magnetic flux emergence from the solar interior. Here we report
on Hinode observations of chromospheric \ion{Ca}{2} H brightenings associated
with a repeated, small-scale flux emergence event. We describe this process and
investigate the evolution of the magnetic flux, G-band brightness, and
\ion{Ca}{2} H intensity in the emerging region. Our results suggest that energy
is released in the chromosphere as a consequence of interactions between the
emerging flux and the pre-existing magnetic field, in agreement with recent 3D
numerical simulations.Comment: 12 Pages, 6 Figures, Accepted for publication in ApJ Letter
Pair separation of magnetic elements in the quiet Sun
The dynamic properties of the quiet Sun photosphere can be investigated by
analyzing the pair dispersion of small-scale magnetic fields (i.e., magnetic
elements).
By using hr-long Hinode magnetograms at high spatial resolution
(), we tracked magnetic element pairs within a supergranular
cell near the disk center.
The computed pair separation spectrum, calculated on the whole set of
particle pairs independently of their initial separation, points out what is
known as a super-diffusive regime with spectral index , in
agreement with the most recent literature, but extended to unprecedented
spatial and temporal scales (from granular to supergranular). Furthermore, for
the first time, we investigated here the spectrum of the mean square
displacement of pairs of magnetic elements, depending on their initial
separation . We found that there is a typical initial distance above
(below) which the pair separation is faster (slower) than the average. A
possible physical interpretation of such a typical spatial scale is also
provided
Diffusion of magnetic elements in a supergranular cell
Small scale magnetic fields (magnetic elements) are ubiquitous in the solar photosphere. Their interaction can provide energy to the upper atmospheric layers, and contribute to heat the solar corona. In this work, the dynamic properties of magnetic elements in the quiet Sun are investigated. The high number of magnetic elements detected in a supergranular cell allowed us to compute their displacement spectrum lang(Δr)2rangvpropτγ (with γ > 0, and τ the time since the first detection), separating the contribution of the network (NW) and the internetwork (IN) regions. In particular, we found γ = 1.27 ± 0.05 and γ = 1.08 ± 0.11 in NW (at smaller and larger scales, respectively), and γ = 1.44 ± 0.08 in IN. These results are discussed in light of the literature on the topic, as well as the implications for the build-up of the magnetic network
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Numerical studies towards practical large-eddy simulation
Large-eddy simulation developments and validations are presented for an
improved simulation of turbulent internal flows. Numerical methods are proposed
according to two competing criteria: numerical qualities (precision and
spectral characteristics), and adaptability to complex configurations. First,
methods are tested on academic test-cases, in order to abridge with fundamental
studies. Consistent results are obtained using adaptable finite volume method,
with higher order advection fluxes, implicit grid filtering and "low-cost"
shear-improved Smagorinsky model. This analysis particularly focuses on mean
flow, fluctuations, two-point correlations and spectra. Moreover, it is shown
that exponential averaging is a promising tool for LES implementation in
complex geometry with deterministic unsteadiness. Finally, adaptability of the
method is demonstrated by application to a configuration representative of
blade-tip clearance flow in a turbomachine
Lac Blanc Pass: a natural wind-tunnel for studying drifting snow at 2700ma.s.l
International audienceThe investigation of the spatial variability of snow depth in high alpine areas is an important topic in snow hydrology, glacier and avalanche research and the transport of snow by wind is an important process for the distribution of snow in mountainous regions. That's why, for 25 years IRSTEA (previously Cemagref) and Météo France (Centre for the Study of Snow) have joined together in studying drifting snow at Col du Lac Blanc 2700 m a.s.l. near the Alpe d'Huez ski resort in the French Alps. Initially, the site was mainly equipped with conventional meteorological stations and a network of snow poles, in order to test numerical models of drifting snow Sytron (CEN) and NEMO (Cemagref). These models are complementary in terms of spatial and temporal scales: outputs of Sytron model will form the inputs of NEMO model. Then new sensors and technologies appeared which allow to develop new knowledge dealing with thresholds velocity according to morphological features of snow grains, snow flux profiles including parameters such as fall velocity and Schmidt number, histograms of particle widths, aerodynamic roughness, gust factors. More recently, the coupled snowpack/ atmosphere model Meso-NH/Crocus has been evaluated at the experimental site. At the same time, some tested sensors have been deployed in Adelie Land in Antarctica, where blowing snow accounts for a major component of the surface mass balance. Japanese and Austrian research teams have been accomodated at Lac Blanc Pass and new foreign teams are welcome. Initial observations continue. That's why Lac Blanc Pass is also a climatological reference for 25 years at 2700 m. Data are available
Small-scale flux emergence events observed by Sunrise/IMaX
AbstractThanks to the unprecedented combination of high spatial resolution (0″.2) and high temporal cadence (33 s) spectropolarimetric measurements, the IMaX magnetograph aboard the Sunrise balloon-borne telescope is revealing new insights about the plasma dynamics of the all-pervasive small-scale flux concentrations in the quiet Sun. We present the result of a case study concerning the appearance of a bipole, with a size of about 4″ and a flux content of 5 × 1017 Mx, with strong signal of horizontal fields during the emergence. We analyze the data set using the SIR inversion code and obtain indications about the three-dimensional shape of the bipole and its evolution with time
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