3,334 research outputs found
Discriminant analysis of solar bright points and faculae I. Classification method and center-to-limb distribution
While photospheric magnetic elements appear mainly as Bright Points (BPs) at
the disk center and as faculae near the limb, high-resolution images reveal the
coexistence of BPs and faculae over a range of heliocentric angles. This is not
explained by a "hot wall" effect through vertical flux tubes, and suggests that
the transition from BPs to faculae needs to be quantitatively investigated. To
achieve this, we made the first recorded attempt to discriminate BPs and
faculae, using a statistical classification approach based on Linear
Discriminant Analysis(LDA). This paper gives a detailed description of our
method, and shows its application on high-resolution images of active regions
to retrieve a center-to-limb distribution of BPs and faculae. Bright "magnetic"
features were detected at various disk positions by a segmentation algorithm
using simultaneous G-band and continuum information. By using a selected sample
of those features to represent BPs and faculae, suitable photometric parameters
were identified in order to carry out LDA. We thus obtained a Center-to-Limb
Variation (CLV) of the relative number of BPs and faculae, revealing the
predominance of faculae at all disk positions except close to disk center (mu >
0.9). Although the present dataset suffers from limited statistics, our results
are consistent with other observations of BPs and faculae at various disk
positions. The retrieved CLV indicates that at high resolution, faculae are an
essential constituent of active regions all across the solar disk. We speculate
that the faculae near disk center as well as the BPs away from disk center are
associated with inclined fields
Wave propagation and energy transport in the magnetic network of the Sun
We investigate wave propagation and energy transport in magnetic elements,
which are representatives of small scale magnetic flux concentrations in the
magnetic network on the Sun. This is a continuation of earlier work by Hasan et
al. (2005). The new features in the present investigation include a
quantitative evaluation of the energy transport in the various modes and for
different field strengths, as well as the effect of the boundary-layer
thickness on wave propagation. We carry out 2-D MHD numerical simulations of
magnetic flux concentrations for strong and moderate magnetic fields. Waves are
excited in the tube and ambient medium by a transverse impulsive motion of the
lower boundary. The nature of the modes excited depends on the value of beta.
Mode conversion occurs in the moderate field case when the fast mode crosses
the beta=1 contour. In the strong field case the fast mode undergoes conversion
from predominantly magnetic to predominantly acoustic when waves are leaking
from the interior of the flux concentration to the ambient medium. We also
estimate the energy fluxes in the acoustic and magnetic modes. The main
conclusions of our work are twofold: firstly, for transverse, impulsive
excitation, flux tubes/sheets with strong fields are more efficient than those
with weak fields in providing acoustic flux to the chromosphere. However, there
is insufficient energy in the acoustic flux to balance the chromospheric
radiative losses in the network, even for the strong field case. Secondly, the
acoustic emission from the interface between the flux concentration and the
ambient medium decreases with the width of the boundary layer.Comment: Accepted for publication in A&A, 13 pages, 10 figures. v2: improved
placement and quality of figures, acknowledgments, acceptance dat
Imaging Spectropolarimetry with IBIS II: on the fine structure of G-band bright features
We present new results from first observations of the quiet solar photosphere
performed through the Interferometric BIdimensional Spectrometer (IBIS) in
spectropolarimetric mode. IBIS allowed us to measure the four Stokes parameters
in the FeI 630.15 nm and FeI 630.25 nm lines with high spatial and spectral
resolutions for 53 minutes; the polarimetric sensitivity achieved by the
instrument is 0.003 the continuum intensity level. We focus on the correlation
which emerges between G-band bright feature brightness and magnetic filling
factor of ~ 1000 G (kG) fields derived by inverting Stokes I and V profiles.
More in detail, we present the correlation first in a pixel-by-pixel study of
an approximatively 3 arcsec wide bright feature (a small network patch) and
then we show that such a result can be extended to all the bright features
found in the dataset at any instant of the time sequence. The higher the kG
filling factor associated to a feature the higher the brightness of the feature
itself. Filling factors up to about 35 % are obtained for the brightest
features. Considering the values of the filling factors derived from the
inversion analysis of spectropolarimetric data and the brightness variation
observed in G-band data we put forward an upper limit for the smallest scale
over which magnetic flux concentrations in intergranular lanes produce a G-band
brightness enhancement (~ 0.1''). Moreover, the brightness saturation observed
for feature sizes comparable to the resolution of the observations is
compatible with large G-band bright features being clusters of sub-arcsecond
bright points. This conclusion deserves to be confirmed by forthcoming
spectropolarimetric observations at higher spatial resolution.Comment: 10 pages, 7 figures, 1 table - Accepted for publication on Ap
The continuum intensity as a function of magnetic field I. Active region and quiet Sun magnetic elements
Small-scale magnetic fields are major contributors to the solar irradiance
variations. Hence, the continuum intensity contrast of magnetic elements in the
quiet Sun (QS) network and in active region (AR) plage is an essential quantity
that needs to be measured reliably. By using Hinode/SP disk center data at a
constant, high spatial resolution, we aim at updating results of earlier
ground-based studies of contrast vs. magnetogram signal, and to look for
systematic differences between AR plages and QS network. The field strength,
filling factor and inclination of the field was retrieved by Milne-Eddington
inversion (VFISV). As in earlier studies, we performed a pixel-by-pixel study
of 630.2 nm continuum contrast vs. apparent (i.e. averaged over a pixel)
longitudinal magnetic field over large fields of view in ARs and in the QS. The
contrast of magnetic elements reaches larger values in the QS (on average 3.7%)
than in ARs (1.3%). This could not be attributed to any systematic difference
in the chosen contrast references. At Hinode's spatial resolution, the
relationship between contrast and apparent longitudinal field strength exhibits
a peak at around 700 G in both the QS and ARs, whereas earlier lower resolution
studies only found a peak in the QS and a monotonous decrease in ARs. We
attribute this discrepancy both to our careful removal of the pores and their
close surroundings affected by the telescope diffraction, as well as to the
enhanced spatial resolution and very low scattered light of the Hinode Solar
Optical Telescope. According to our inversions, the magnetic elements producing
the peak of the contrast curves have similar properties (field strength,
inclination, filling factor) in ARs and in the QS, so that the larger
brightness of magnetic elements in the QS remains unexplained.Comment: 8 figures, 14 page
The Lantern Vol. 8, No. 1, December 1939
• Christmas Resurrection • Autumn\u27s Song • Henry Cavendish • The Mystery of Loon Cove • All Hail, Fair Modesty • Mischall • Gift of the Magi • Camera-Phobia • One Envying a Poet • Sonnetshttps://digitalcommons.ursinus.edu/lantern/1019/thumbnail.jp
On the intensity contrast of solar photospheric faculae and network elements
Sunspots, faculae and the magnetic network contribute to solar irradiance
variations. The contribution due to faculae and the network is of basic
importance, but suffers from considerable uncertainty. We determine the
contrasts of active region faculae and the network, both as a function of
heliocentric angle and magnetogram signal. To achieve this, we analyze
near-simultaneous full disk images of photospheric continuum intensity and
line-of-sight magnetic field provided by the Michelson Doppler Interferometer
(MDI) on board the SOHO spacecraft. Starting from the surface distribution of
the solar magnetic field we first construct a mask, which is then used to
determine the brightness of magnetic features, and the relatively field-free
part of the photosphere separately. By sorting the magnetogram signal into
different bins we are able to distinguish between the contrasts of different
concentrations of magnetic field. We find that the contrasts of active region
faculae (large magnetogram signal) and the network (small signal) exhibit a
very different CLV, showing that the populations of magnetic flux tubes are
different. This implies that these elements need to be treated separately when
reconstructing variations of the total solar irradiance with high precision. We
have obtained an analytical expression for the contrast of photospheric
magnetic features as a function of both position on the disk and magnetic field
strength, by performing a 2-dimensional fit to the observations.Comment: 12 pages, 8 figures, uses aa.cl
Small-scale solar magnetic fields
As we resolve ever smaller structures in the solar atmosphere, it has become
clear that magnetism is an important component of those small structures.
Small-scale magnetism holds the key to many poorly understood facets of solar
magnetism on all scales, such as the existence of a local dynamo, chromospheric
heating, and flux emergence, to name a few. Here, we review our knowledge of
small-scale photospheric fields, with particular emphasis on quiet-sun field,
and discuss the implications of several results obtained recently using new
instruments, as well as future prospects in this field of research.Comment: 43 pages, 18 figure
AMR, stability and higher accuracy
Efforts to achieve better accuracy in numerical relativity have so far
focused either on implementing second order accurate adaptive mesh refinement
or on defining higher order accurate differences and update schemes. Here, we
argue for the combination, that is a higher order accurate adaptive scheme.
This combines the power that adaptive gridding techniques provide to resolve
fine scales (in addition to a more efficient use of resources) together with
the higher accuracy furnished by higher order schemes when the solution is
adequately resolved. To define a convenient higher order adaptive mesh
refinement scheme, we discuss a few different modifications of the standard,
second order accurate approach of Berger and Oliger. Applying each of these
methods to a simple model problem, we find these options have unstable modes.
However, a novel approach to dealing with the grid boundaries introduced by the
adaptivity appears stable and quite promising for the use of high order
operators within an adaptive framework
Observed Effect of Magnetic Fields on the Propagation of Magnetoacoustic Waves in the Lower Solar Atmosphere
We study Hinode/SOT-FG observations of intensity fluctuations in Ca II H-line
and G-band image sequences and their relation to simultaneous and co-spatial
magnetic field measurements. We explore the G-band and H-line intensity
oscillation spectra both separately and comparatively via their relative phase
differences, time delays and cross-coherences. In the non-magnetic situations,
both sets of fluctuations show strong oscillatory power in the 3 - 7 mHz band
centered at 4.5 mHz, but this is suppressed as magnetic field increases. A
relative phase analysis gives a time delay of H-line after G-band of 20\pm1 s
in non-magnetic situations implying a mean effective height difference of 140
km. The maximum coherence is at 4 - 7 mHz. Under strong magnetic influence the
measured delay time shrinks to 11 s with the peak coherence near 4 mHz. A
second coherence maximum appears between 7.5 - 10 mHz. Investigation of the
locations of this doubled-frequency coherence locates it in diffuse rings
outside photospheric magnetic structures. Some possible interpretations of
these results are offered.Comment: 19 pages, 6 figure
Generalized Parton Distributions at x->1
Generalized parton distributions at large are studied in perturbative QCD
approach. As and at finite , there is no dependence for the
GPDs which means that the active quark is at the center of the transverse
space. We also obtain the power behavior: for pion; and
for nucleon, where
represents the additional dependence on .Comment: 7 pages, 2 figure
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