172 research outputs found
1D atmosphere models from inversion of Fe I 630 nm observations with an application to solar irradiance studies
Present-day semi-empirical models of solar irradiance (SI) variations employ
spectra computed on one-dimensional atmosphere models (1D models)
representative of various solar surface features to reconstruct SI changes
measured on timescales greater than a day. Various recent studies have,
however, pointed out that the spectra synthesized on 1D models do not reflect
the radiative emission of the inhomogenous atmosphere revealed by
high-resolution solar observations. We aimed to derive observational-based
atmospheres from such observations and test their accuracy for SI estimates. We
analysed spectro-polarimetric data of the Fe I 630 nm line pair on photospheric
regions representative of the granular, quiet Sun pattern (QS) and of small-
and large-scale magnetic features, both bright and dark with respect to the QS.
The data were taken on 2011 August 6, with the CRISP at the Swedish Solar
Telescope, under excellent seeing conditions. We derived atmosphere models of
the observed regions from data inversion with the SIR code. We studied the
sensitivity of results to spatial resolution and temporal evolution, and
discussed the obtained atmospheres with respect to several 1D models. The
atmospheres derived from our study agree well with most of the compared 1D
models, both qualitatively and quantitatively (differences are within 10%), but
for pore regions. Spectral synthesis computations on the atmosphere obtained
from the QS observations return SI between 400 nm and 2400 nm that agrees, on
average, within 2.2% with standard reference measurements, and within -0.14%
with the SI computed on the quiet Sun atmosphere employed by the most advanced
semi-empirical model of SI variations.Comment: Accepted for publication in The Astrophysical Journa
The Intensity Profile of the Solar Supergranulation
We have measured the average radial (cell center to network boundary) profile
of the continuum intensity contrast associated with supergranular flows using
data from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa
Solar Observatory (MLSO). After removing the contribution of the network flux
elements by the application of masks based on Ca II K intensity and averaging
over more than 10^5 supergranular cells, we find a ~ 0.1% decrease in red and
blue continuum intensity from the supergranular cell centers outward,
corresponding to a ~ 1.0 K decrease in brightness temperature across the cells.
The radial intensity profile may be caused either by the thermal signal
associated with the supergranular flows or a variation in the packing density
of unresolved magnetic flux elements. These are not unambiguously distinguished
by the observations, and we raise the possibility that the network magnetic
fields play an active role in supergranular scale selection by enhancing the
radiative cooling of the deep photosphere at the cell boundaries.Comment: Accepted to Ap
Radiative emission of solar features in Ca II K
We investigated the radiative emission of different types of solar features
in the spectral range of the Ca II K line.
We analyzed full-disk 2k x 2k observations from the PSPT Precision Solar
Photometric Telescope. The data were obtained by using three narrow-band
interference filters that sample the Ca II K line with different pass bands.
Two filters are centered in the line core, the other in the red wing of the
line. We measured the intensity and contrast of various solar features,
specifically quiet Sun (inter-network), network, enhanced network, plage, and
bright plage (facula) regions. Moreover, we compared the results obtained with
those derived from the numerical synthesis performed for the three PSPT filters
with a widely used radiative code on a set of reference semi-empirical
atmosphere models.Comment: In Proceedings of the 25th NSO Workshop: Chromospheric Structure and
Dynamic
Stray-light restoration of full-disk CaII K solar observations: a case study
AIMS: We investigate whether restoration techniques, such as those developed
for application to current observations, can be used to remove stray-light
degradation effects on archive CaII K full-disk observations. We analyze to
what extent these techniques can recover homogeneous time series of data.
METHODS:We develop a restoration algorithm based on a method presented by
Walton & Preminger (1999). We apply this algorithm to data for both present-day
and archive CaII K full-disk observations, which were acquired using the PSPT
mounted at the Rome Observatory, or obtained by digitization of Mt Wilson
photographic-archive spectroheliograms. RESULTS:We show that the restoring
algorithm improves both spatial resolution and photometric contrast of the
analyzed solar observations. We find that the improvement in spatial resolution
is similar for analyzed recent and archive data. On the other hand, the
improvement of photometric contrast is quite poor for the archive data, with
respect to the one obtained for the present-day images. We show that the
quality of restored archive data depends on the photographic calibration
applied to the original observations. In particular, photometry can be
recovered with a restoring algorithm if the photographic-calibration preserves
the intensity information stored in the original data, principally outside the
solar-disk observations.Comment: 10 pages; 9 figure
Recommended from our members
On the reliability of the fractal dimension measure of solar magnetic features and on its variation with solar cycle
Several studies have investigated the fractal and multifractal nature of magnetic features in the solar photosphere and its variation with the solar magnetic activity cycle. Here we extend those studies by examining the fractal geometry of bright magnetic features at higher atmospheric levels, specifically in the solar chromosphere. We analyze structures identified in CaIIK images obtained with the Precision Solar Photometric Telescopes (PSPTs) at Osservatorio Astronomico di Roma (OAR) and Mauna Loa Solar Observatory (MLSO). Fractal dimension estimates depend on the estimator employed, the quality of the images, and the structure identification techniques used. We examine both real and simulated data and employ two different perimeter-area estimators in order to understand the sensitivity of the deduced fractal properties to pixelization and image quality. The fractal dimension of bright 'magnetic' features in CaIIK images ranges between values of 1.2 and 1.7 for small and large structures respectively. This size dependency largely reflects the importance of image pixelization in the measurement of small objects. The fractal dimension of chromospheric features does not show any clear systematic variation with time over the period examined, the descending phase of solar cycle 23. These conclusions, and the analysis of both real and synthetic images on which they are based, are important in the interpretation of previously reported results
Plasma flows and magnetic field interplay during the formation of a pore
We studied the formation of a pore in AR NOAA 11462. We analysed data
obtained with the IBIS at the DST on April 17, 2012, consisting of full Stokes
measurements of the Fe I 617.3 nm lines. Furthermore, we analysed SDO/HMI
observations in the continuum and vector magnetograms derived from the Fe I
617.3 nm line data taken from April 15 to 19, 2012. We estimated the magnetic
field strength and vector components and the LOS and horizontal motions in the
photospheric region hosting the pore formation. We discuss our results in light
of other observational studies and recent advances of numerical simulations.
The pore formation occurs in less than 1 hour in the leading region of the AR.
The evolution of the flux patch in the leading part of the AR is faster (< 12
hour) than the evolution (20-30 hour) of the more diffuse and smaller scale
flux patches in the trailing region. During the pore formation, the ratio
between magnetic and dark area decreases from 5 to 2. We observe strong
downflows at the forming pore boundary and diverging proper motions of plasma
in the vicinity of the evolving feature that are directed towards the forming
pore. The average values and trends of the various quantities estimated in the
AR are in agreement with results of former observational studies of steady
pores and with their modelled counterparts, as seen in recent numerical
simulations of a rising-tube process. The agreement with the outcomes of the
numerical studies holds for both the signatures of the flux emergence process
(e.g. appearance of small-scale mixed polarity patterns and elongated granules)
and the evolution of the region. The processes driving the formation of the
pore are identified with the emergence of a magnetic flux concentration and the
subsequent reorganization of the emerged flux, by the combined effect of
velocity and magnetic field, in and around the evolving structure.Comment: Accepted for publication in Astronomy and Astrophysic
Radiative emission of solar features in the Ca II K line: comparison of measurements and models
We study the radiative emission of various types of solar features, such as
quiet Sun, enhanced network, plage, and bright plage regions, identified on
filtergrams taken in the Ca II K line. We analysed fulldisk images obtained
with the PSPT, by using three interference filters that sample the Ca II K line
with different bandpasses. We studied the dependence of the radiative emission
of disk features on the filter bandpass. We also performed a NLTE spectral
synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The
synthesis was carried out by utilizing both the PRD and CRD with the most
recent set of semi empirical atmosphere models in the literature and some
earlier atmosphere models. We measured the CLV of intensity values for various
solar features identified on PSPT images and compared the results obtained with
those derived from the synthesis. We find that CRD calculations derived using
the most recent quiet Sun model, on average, reproduce the measured values of
the quiet Sun regions slightly more accurately than PRD computations with the
same model. This may reflect that the utilized atmospheric model was computed
assuming CRD. Calculations with PRD on earlier quiet Sun model atmospheres
reproduce measured quantities with a similar accuracy as to that achieved here
by applying CRD to the recent model. We also find that the median contrast
values measured for most of the identified bright features, disk positions, and
filter widths are, on average, a factor 1.9 lower than those derived from PRD
simulations performed using the recent bright feature models. The discrepancy
between measured and modeled values decreases by 12% after taking into account
straylight effects on PSPT images. PRD computations on either the most recent
or the earlier atmosphere models of bright features reproduce measurements from
plage and bright plage regions with a similar accuracy.Comment: 14 pages, 18 figures, accepted by A&
The center to limb variation of photospheric facular contrast
Abstract. The center-to-limb variation (CLV) in the contrast of photospheric faculae is of importance both to the physics of magnetic flux tubes and to the understanding of variations in the total solar irradiance. Measurements of the CLV of faculae are difficult and have resulted in widely disparate results. We tried an accurate measurement of the photospheric facular contrast by using both PSPT and SOHO/MDI observations, with the aim of understanding the reasons of the different results of facular photospheric contrast measurements already presented in literature
Height dependence of the penumbral fine-scale structure in the inner solar atmosphere
We studied the physical parameters of the penumbra in a large and
fully-developed sunspot, one of the largest over the last two solar cycles, by
using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and
chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional
Spectrometer. Inverting measurements with the NICOLE code, we obtained the
three-dimensional structure of the magnetic field in the penumbra from the
bottom of the photosphere up to the middle chromosphere. We analyzed the
azimuthal and vertical gradient of the magnetic field strength and inclination.
Our results provide new insights on the properties of the penumbral magnetic
fields in the chromosphere at atmospheric heights unexplored in previous
studies. We found signatures of the small-scale spine and intra-spine structure
of both the magnetic field strength and inclination at all investigated
atmospheric heights. In particular, we report typical peak-to-peak variations
of the field strength and inclination of G and , respectively, in the photosphere, and of G and
in the chromosphere. Besides, we estimated the vertical
gradient of the magnetic field strength in the studied penumbra: we find a
value of G km between the photosphere and the middle
chromosphere. Interestingly, the photospheric magnetic field gradient changes
sign from negative in the inner to positive in the outer penumbra.Comment: 14 page, 9 figures, accepted for Ap
- …