51 research outputs found
Spatial distribution and statistical properties of small-scale convective vortex-like motions in a quiet Sun region
High-resolution observations of a quiet Sun internetwork region taken with
the Solar 1-m Swedish Telescope in La Palma are analyzed. We determine the
location of small-scale vortex motions in the solar photospheric region by
computing the horizontal proper motions of small-scale structures on time
series of images. These plasma convectively-driven swirl motions are associated
to: (1) downdrafts (that have been commonly explained as corresponding to sites
where the plasma is cooled down and hence returned to the interior below the
visible photospheric level), and (2) horizontal velocity vectors converging
into a central point. The sink cores are proved to be the final destination of
passive floats tracing plasma flows towards the center of each vortex. We
establish the occurrence of these events to be 1.4 x 10^(-3) and 1.6 x 10^(-3)
vortices Mm^(-2) min^(-1) respectively for two time series analyzed here.Comment: 8 pages, 6 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Multiscale magnetic underdense regions on the solar surface: Granular and Mesogranular scales
The Sun is a non-equilibrium dissipative system subjected to an energy flow
which originates in its core. Convective overshooting motions create
temperature and velocity structures which show a temporal and spatial
evolution. As a result, photospheric structures are generally considered to be
the direct manifestation of convective plasma motions. The plasma flows on the
photosphere govern the motion of single magnetic elements. These elements are
arranged in typical patterns which are observed as a variety of multiscale
magnetic patterns. High resolution magnetograms of quiet solar surface revealed
the presence of magnetic underdense regions in the solar photosphere, commonly
called voids, which may be considered a signature of the underlying convective
structure. The analysis of such patterns paves the way for the investigation of
all turbulent convective scales from granular to global. In order to address
the question of magnetic structures driven by turbulent convection at granular
and mesogranular scales we used a "voids" detection method. The computed voids
distribution shows an exponential behavior at scales between 2 and 10 Mm and
the absence of features at 5-10 Mm mesogranular scales. The absence of
preferred scales of organization in the 2-10 Mm range supports the multiscale
nature of flows on the solar surface and the absence of a mesogranular
convective scale
Stokes Diagnostis of 2D MHD-simulated Solar Magnetogranulation
We study the properties of solar magnetic fields on scales less than the
spatial resolution of solar telescopes. A synthetic infrared
spectropolarimetric diagnostics based on a 2D MHD simulation of
magnetoconvection is used for this. We analyze two time sequences of snapshots
that likely represent two regions of the network fields with their immediate
surrounding on the solar surface with the unsigned magnetic flux density of 300
and 140 G. In the first region we find from probability density functions of
the magnetic field strength that the most probable field strength at logtau_5=0
is equal to 250 G. Weak fields (B < 500 G) occupy about 70% of the surface,
while stronger fields (B 1000 G) occupy only 9.7% of the surface. The magnetic
flux is -28 G and its imbalance is -0.04. In the second region, these
parameters are correspondingly equal to 150 G, 93.3 %, 0.3 %, -40 G, and -0.10.
We estimate the distribution of line-of-sight velocities on the surface of log
tau_5=-1. The mean velocity is equal to 0.4 km/s in the first simulated region.
The averaged velocity in the granules is -1.2 km/s and in the intergranules is
2.5 km/s. In the second region, the corresponding values of the mean velocities
are equal to 0, -1.8, 1.5 km/s. In addition we analyze the asymmetry of
synthetic Stokes-V profiles of the Fe I 1564.8 nm line. The mean values of the
amplitude and area asymmetry do not exceed 1%. The spatially smoothed amplitude
asymmetry is increased to 10% while the area asymmetry is only slightly varied.Comment: 24 pages, 12 figure
Comment on "Evidence from acoustic imaging for submarine volcanic activity in 2012 off the west coast of El Hierro (Canary Islands, Spain)" by Pérez NM, Somoza L, Hernández PA, González de Vallejo L, León R, Sagiya T, Biain A, González FJ, Medialdea T, Barrancos J, Ibáñez J, Sumino H, Nogami K and Romero C [Bull Volcanol (2014) 76:882-896]
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Inter-Network magnetic fields observed with sub-arcsec resolution
We analyze a time sequence of Inter-Network (IN) magnetograms observed at the
solar disk center. Speckle reconstruction techniques provide a good spatial
resolution (0.5" cutoff frequency) yet maintaining a fair sensitivity (some
20G). Patches with signal above noise cover 60% of the observed area, most of
which corresponds to intergranular lanes. The large surface covered by signal
renders a mean unsigned magnetic flux density between 17G and 21G (1G \equiv
1Mx cm). The difference depends on the spectral line used to generate
the magnetograms (Fe I 6302 or Fe I 6301). Such systematic difference can be
understood if the magnetic structures producing the polarization have intrinsic
field strengths exceeding 1 kG, and consequently, occupying only a very small
fraction of the surface (some 2%). We observe both, magnetic signals changing
in time scales smaller than 1 min, and a persistent pattern lasting longer than
the duration of the sequence (17 min). The pattern resembles a network with a
spatial scale between 5 and 10 arcsec, which we identify as the
mesogranulation. The strong dependence of the polarization signals on spatial
resolution and sensitivity suggests that much quiet Sun magnetic flux still
remains undetected.Comment: Accepted for publication in A&A. 17 pages with 15 figure
A Substantial Amount of Hidden Magnetic Energy in the Quiet Sun
Deciphering and understanding the small-scale magnetic activity of the quiet
solar photosphere should help to solve many of the key problems of solar and
stellar physics, such as the magnetic coupling to the outer atmosphere and the
coronal heating. At present, we can see only of the complex
magnetism of the quiet Sun, which highlights the need to develop a reliable way
to investigate the remaining 99%. Here we report three-dimensional radiative
tranfer modelling of scattering polarization in atomic and molecular lines that
indicates the presence of hidden, mixed-polarity fields on subresolution
scales. Combining this modelling with recent observational data we find a
ubiquitous tangled magnetic field with an average strength of G,
which is much stronger in the intergranular regions of solar surface convection
than in the granular regions. So the average magnetic energy density in the
quiet solar photosphere is at least two orders of magnitude greater than that
derived from simplistic one-dimensional investigations, and sufficient to
balance radiative energy losses from the solar chromosphere.Comment: 21 pages and 2 figures (letter published in Nature on July 15, 2004
Center-to-limb variation of the area covered by magnetic bright points in the quiet Sun
CONTEXT: The quiet Sun magnetic fields produce ubiquitous bright points (BPs)
that cover a significant fraction of the solar surface. Their contribution to
the total solar irradiance (TSI) is so-far unknown. AIMS: To measure the
center-to-limb variation (CLV) of the fraction of solar surface covered by
quiet Sun magnetic bright points. The fraction is referred to as 'fraction of
covered surface', or FCS. METHODS: Counting of the area covered by BPs in
G-band images obtained at various heliocentric angles with the 1-m Swedish
Solar Telescope on La Palma. Through restoration, the images are close to the
diffraction limit of the instrument (~0.1 arcsec). RESULTS: The FCS is largest
at disk center (~1 %), and then drops down to become 0.2 % at 'mu'= 0.3 (with
'mu' the cosine of the heliocentric angle. The relationship has large scatter,
which we evaluate comparing different subfields within our FOVs. We work out a
toy-model to describe the observed CLV, which considers the BPs to be
depressions in the mean solar photosphere characterized by a depth, a width,
and a spread of inclinations. Although the model is poorly constrained by
observations, it shows the BPs to be shallow structures (depth < width) with a
large range of inclinations. We also estimate how different parts of the solar
disk may contribute to TSI variations, finding that 90 % is contributed by BPs
having 'mu' > 0.5, and half of it is due to BPs with 'mu' > 0.8.Comment: Accepted for publication in A&A. 8 pages, 6 Figs., 1 Tabl
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
Magnetic Coupling in the Quiet Solar Atmosphere
Three kinds of magnetic couplings in the quiet solar atmosphere are
highlighted and discussed, all fundamentally connected to the Lorentz force.
First the coupling of the convecting and overshooting fluid in the surface
layers of the Sun with the magnetic field. Here, the plasma motion provides the
dominant force, which shapes the magnetic field and drives the surface dynamo.
Progress in the understanding of the horizontal magnetic field is summarized
and discussed. Second, the coupling between acoustic waves and the magnetic
field, in particular the phenomenon of wave conversion and wave refraction. It
is described how measurements of wave travel times in the atmosphere can
provide information about the topography of the wave conversion zone, i.e., the
surface of equal Alfv\'en and sound speed. In quiet regions, this surface
separates a highly dynamic magnetic field with fast moving magnetosonic waves
and shocks around and above it from the more slowly evolving field of high-beta
plasma below it. Third, the magnetic field also couples to the radiation field,
which leads to radiative flux channeling and increased anisotropy in the
radiation field. It is shown how faculae can be understood in terms of this
effect. The article starts with an introduction to the magnetic field of the
quiet Sun in the light of new results from the Hinode space observatory and
with a brief survey of measurements of the turbulent magnetic field with the
help of the Hanle effect.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
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