239 research outputs found
The relation between photospheric supergranular flows and magnetic flux emergence
A recent study carried out on high sensitivity SUNRISE/IMAX data has reported
about the existence of areas of limited flux emergence in the quiet Sun. By
exploiting an independent and longer (4 hours) data set acquired by HINODE/SOT,
we further investigate these regions by analysing their spatial distribution
and relation with the supergranular flow. Our findings, while confirming the
presence of these calm areas, also show that the rate of emergence of small
magnetic elements is largely suppressed at the locations where the divergence
of the supergranular plasma flows is positive. This means that the dead calm
areas previously reported in literature are not randomly distributed over the
solar photosphere but they are linked to the supergranular cells themselves.
These results are discussed in the framework of the recent literature.Comment: Accepted as A&A Lette
The spectrum of kink-like oscillations of solar photospheric magnetic elements
Recently, the availability of new high-spatial and -temporal resolution
observations of the solar photosphere has allowed the study of the oscillations
in small magnetic elements. Small magnetic elements have been found to host a
rich variety of oscillations detectable as intensity, longitudinal or
transverse velocity fluctuations which have been interpreted as MHD waves.
Small magnetic elements, at or below the current spatial resolution achieved by
modern solar telescopes, are though to play a relevant role in the energy
budget of the upper layers of the Sun's atmosphere, as they are found to cover
a significant fraction of the solar photosphere. Unfortunately, the limited
temporal length and/or cadence of the data sets, or the presence of
seeing-induced effects have prevented, so far, the estimation of the power
spectra of kink-like oscillations in small magnetic elements with good
accuracy. Motivated by this, we studied kink-like oscillations in small
magnetic elements, by exploiting very long duration and high-cadence data
acquired with the Solar Optical Telescope on board the Hinode satellite. In
this work we present the results of this analysis, by studying the power
spectral density of kink-like oscillations on a statistical basis. We found
that small magnetic elements exhibit a large number of spectral features in the
range 1-12 mHz. More interestingly, most of these spectral features are not
shared among magnetic elements but represent a unique signature of each
magnetic element itself.Comment: A&A accepted for publication. 8 pages, 5 figure
Non-linear propagation of kink waves to the solar chromosphere
Small-scale magnetic field concentrations (magnetic elements) in the quiet
Sun are believed to contribute to the energy budget of the upper layers of the
Sun's atmosphere, as they are observed to support a large number of MHD modes.
In recent years, kink waves in magnetic elements were observed at different
heights in the solar atmosphere, from the photosphere to the corona. However,
the propagation of these waves has not been fully evaluated. Our aim is to
investigate the propagation of kink waves in small magnetic elements in the
solar atmosphere. We analysed spectropolarimetric data of high-quality and long
duration of a photospheric quiet Sun region observed near the disk center with
the spectropolarimeter CRISP at the Swedish Solar Telescope (SST), and
complemented by simultaneous and co-spatial broad-band chromospheric
observations of the same region. Our findings reveal a clear upward propagation
of kink waves with frequency above mHz. Moreover, the signature of a
non-linear propagation process is also observed. By comparing photospheric to
chromospheric power spectra, no signature of an energy dissipation is found at
least at the atmospheric heights at which the data analysed originate. This
implies that most of the energy carried by the kink waves (within the frequency
range under study mHz) flows to upper layers in the Sun's atmosphere.Comment: A&A accepte
Multiple field-of-view MCAO for a Large Solar Telescope: LOST simulations
In the framework of a 4m class Solar Telescope we studied the performance of
the MCAO using the LOST simulation package. In particular, in this work we
focus on two different methods to reduce the time delay error which is
particularly critical in solar adaptive optics: a) the optimization of the
wavefront reconstruction by reordering the modal base on the basis of the
Mutual Information and b) the possibility of forecasting the wavefront
correction through different approaches. We evaluate these techniques
underlining pros and cons of their usage in different control conditions by
analyzing the results of the simulations and make some preliminary tests on
real data.Comment: 10 pages, 5 figures to be published in Adaptive Optics Systems II
(Proceedings Volume) Proceedings of SPI
Observational evidence for buffeting induced kink waves in solar magnetic elements
The role of diffuse photospheric magnetic elements in the energy budget of
the upper layers of the Sun's atmosphere has been the recent subject of many
studies. This was made possible by the availability of high temporal and
spatial resolution observations of the solar photosphere, allowing large
numbers of magnetic elements to be tracked to study their dynamics. In this
work we exploit a long temporal series of seeing-free magnetograms of the solar
photosphere to study the effect of the turbulent convection in the excitation
of kink oscillations in magnetic elements. We make use of the empirical mode
decomposition technique (EMD) in order to study the transverse oscillations of
several magnetic flux tubes. This technique permits the analysis of
non-stationary time series like those associated to the horizontal velocities
of these flux tubes which are continuously advected and dispersed by granular
flows.
Our primary findings reveal the excitation of low frequency modes of kink
oscillations, which are sub-harmonics of a fundamental mode with a minute periodicity. These results constitute a strong case for
observational proof of the excitation of kink waves by the buffeting of the
convection cells in the solar photosphere, and are discussed in light of their
possible role in the energy budget of the upper Sun's atmosphere.Comment: A&A accepte
SFADI: the Speckle-Free Angular Differential Imaging method
We present a new processing technique aimed at significantly improving the
angular differential imaging method (ADI) in the context of high-contrast
imaging of faint objects nearby bright stars in observations obtained with
extreme adaptive optics (EXAO) systems. This technique, named "SFADI" for
"Speckle-Free ADI", allows to improve the achievable contrast by means of
speckles identification and suppression. This is possible in very high cadence
data, which freeze the atmospheric evolution. Here we present simulations in
which synthetic planets are injected into a real millisecond frame rate
sequence, acquired at the LBT telescope at visible wavelength, and show that
this technique can deliver low and uniform background, allowing unambiguous
detection of contrast planets, from to mas separations,
under poor and highly variable seeing conditions ( to arcsec FWHM)
and in only min of acquisition. A comparison with a standard ADI approach
shows that the contrast limit is improved by a factor of . We extensively
discuss the SFADI dependence on the various parameters like speckle
identification threshold, frame integration time, and number of frames, as well
as its ability to provide high-contrast imaging for extended sources, and also
to work with fast acquisitions.Comment: Accepted for publication in Ap
High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI
We characterize waves in small magnetic elements and investigate their
propagation in the lower solar atmosphere from observations at high spatial and
temporal resolution. We use the wavelet transform to analyze oscillations of
both horizontal displacement and intensity in magnetic bright points found in
the 300 nm and the Ca II H 396.8 nm passbands of the filter imager on board the
Sunrise balloon-borne solar observatory. Phase differences between the
oscillations at the two atmospheric layers corresponding to the two passbands
reveal upward propagating waves at high frequencies (up to 30 mHz). Weak
signatures of standing as well as downward propagating waves are also obtained.
Both compressible and incompressible (kink) waves are found in the small-scale
magnetic features. The two types of waves have different, though overlapping,
period distributions. Two independent estimates give a height difference of
approximately 450+-100 km between the two atmospheric layers sampled by the
employed spectral bands. This value, together with the determined short travel
times of the transverse and longitudinal waves provide us with phase speeds of
29+-2 km/s and 31+-2 km/s, respectively. We speculate that these phase speeds
may not reflect the true propagation speeds of the waves. Thus, effects such as
the refraction of fast longitudinal waves may contribute to an overestimate of
the phase speed.Comment: 14 pages, 7 figure
Three-minute wave enhancement in the solar photosphere
It is a well-known result that the power of five-minute oscillations is
progressively reduced by magnetic fields in the solar photosphere. Many authors
have pointed out that this fact could be due to a complex interaction of many
processes: opacity effects, MHD mode conversion and intrinsic reduced acoustic
emissivity in strong magnetic fields. While five-minute oscillations are the
dominant component in the photosphere, it has been shown that chromospheric
heights are in turn dominated by three-minute oscillations. Two main theories
have been proposed to explain their presence based upon resonance filtering in
the atmospheric cavity and non linear interactions. In this work we show,
through the analysis of IBIS observations of a solar pore in the photospheric
Fe I 617.3 nm line, that three-minute waves are already present at the height
of formation of this line and that their amplitude depends on the magnetic
field strength and is strictly confined in the umbral region.Comment: A&A accepte
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