arXiv subjects

Serena Criscuoli

Publications and source records attributed to Serena Criscuoli.

34 records · Page 2Linked to original sources

arXiv2017

Photometric Properties of Network and Faculae Derived from HMI Data Compensated for Scattered-Light

We report on the photometric properties of faculae and network as observed in full-disk, scattered-light corrected images from the Helioseismic Magnetic Imager. We use a Lucy-Richardson deconvolution routine that corrects an image in less than one second. Faculae are distinguished from network through proximity to active regions. This is the first report that full-disk observations, including center-to-limb variations, reproduce the photometric properties of faculae and network observed previously only in sub-arcsecond resolution, small field-of-view studies, i.e. that network, as defined by distance from active regions, exhibit higher photometric contrasts. Specifically, for magnetic flux values larger than approximately 300 G, the network is brighter than faculae and the contrast differences increases toward the limb, where the network contrast is about twice the facular one. For lower magnetic flux values, network appear darker than faculae. Contrary to reports from previous full-disk observations, we also found that network exhibits a higher center-to-limb variation. Our results are in agreement with reports from simulations that indicate magnetic flux alone is a poor proxy of the photometric properties of magnetic features. We estimate that the contribution of faculae and network to Total Solar Irradiance variability of the current Cycle 24 is overestimated by at least 11\% due to the photometric properties of network and faculae not being recognized as different. This estimate is specific to the method employed in this study to reconstruct irradiance variations, so caution should be paid when extending it to other techniques.

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arXiv2017

Polarised kink waves in magnetic elements: evidence for chromospheric helical waves

In recent years, new high spatial resolution observations of the Sun's atmosphere have revealed the presence of a plethora of small-scale magnetic elements down to the resolution limit of current cohort of solar telescopes ($\sim 100-120$ km on the solar photosphere). These small magnetic field concentrations, due to the granular buffeting, can support and guide several magneto-hydrodynamics (MHD) wave modes that would eventually contribute to the energy budget of the upper layers of the atmosphere.\\ In this work, exploiting the high spatial and temporal resolution chromospheric data acquired with the Swedish 1-meter Solar Telescope (SST), and applying the empirical mode decomposition (EMD) technique to the tracking of the solar magnetic features, we analyse the perturbations of the horizontal velocity vector of a set of chromospheric magnetic elements. We find observational evidence that suggests a phase relation between the two components of the velocity vector itself, resulting in its helical motion.

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arXiv2016

Lower solar atmosphere and magnetism at ultra-high spatial resolution

We present the scientific case for a future space-based telescope aimed at very high spatial and temporal resolution imaging of the solar photosphere and chromosphere. Previous missions (e.g., HINODE, SUNRISE) have demonstrated the power of observing the solar photosphere and chromosphere at high spatial resolution without contamination from Earth's atmosphere. We argue here that increased spatial resolution (from currently 70 km to 25 km in the future) and high temporal cadence of the observations will vastly improve our understanding of the physical processes controlling solar magnetism and its characteristic scales. This is particularly important as the Sun's magnetic field drives solar activity and can significantly influence the Sun-Earth system. At the same time a better knowledge of solar magnetism can greatly improve our understanding of other astrophysical objects.

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arXiv2016

A study of solar photospheric temperature gradient variation using limb darkening measurements

The variation in area of quiet magnetic network measured over the sunspot cycle should modulate the spatially averaged photospheric temperature gradient, since temperature declines with optical depth more gradually in magnetic flux tube atmospheres. Yet, limb darkening measurements show no dependence upon activity level, even at an rms precision of 0.04%. We study the sensitivity of limb darkening to changes in area filling factor using a 3D MHD model of the magnetized photosphere. The limb darkening change expected from the measured 11 yr area variation lies below the level of measured limb darkening variations, for a reasonable range of magnetic flux in quiet network and internetwork regions. So the remarkably constant limb darkening observed over the solar activity cycle is not inconsistent with the measured 11 year change in area of quiet magnetic network. Our findings offer an independent constraint on photospheric temperature gradient changes reported from measurements of the solar spectral irradiance from the Spectral Irradiance Monitor (SIM), and recently, from wavelength differential spectrophotometry using the Solar Optical Telescope (SOT) aboard the HINODE spacecraft.

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arXiv2016

Angular dependence of the facular-sunspot coverage relation as derived by MDI magnetograms

Previous studies have shown that the variation over the solar magnetic activity cycle of the area of facular/network features identified on broad band and narrow band imagery is positively correlated with the sunspot area and number, the relation between the area coverages being described as either linear or quadratic. On the other hand, the temporal variation of the spatial distributions of faculae, network and sunspots follows patterns that are less obviously correlated, so that we expect the relation that describes variation of the area coverage of different types of magnetic features to vary with the position over the disk. In this work we employ MDI full-disk magnetograms acquired during Cycle 23 and at the beginning of Cycle 24 to investigate the relation between the coverage of magnetic elements characterized by different amounts of magnetic flux and located at different angular distances from disk center with the sunspot number. In agreement with some previous studies we find that daily data are best described by a quadratic function while data averaged over 6-months are best described by a linear function. In both cases the coefficients of the fits show large dependence on the position over the disk and the magnetic flux. We also find that toward disk center 6-months averaged data show asymmetries between the ascending and the descending phase. Implications for solar irradiance modeling are discussed.

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arXiv2015

Understanding the Fe I Line Measurements Returned by the Helioseismic and Magnetic Imager

The Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO) observes the Sun at the Fe I 6173 Å line and returns full-disk maps of line-of-sight (LOS) observables including the magnetic flux density, velocities, Fe I line width, line depth, and continuum intensity. These data are estimated through an algorithm (the MDI-like algorithm, hereafter), which combines observables obtained at six wavelength positions within the Fe I 6173 Å line. To properly interpret such data it is important to understand any effects of the instrument and the pipeline that generates these data products. We tested the accuracy of the line width, line depth, and continuum intensity returned by the MDI-like algorithm using various one-dimensional (1D) atmosphere models. It was found that HMI estimates of these quantities are highly dependent on the shape of the line, therefore on the LOS angle and the magnetic flux density associated with the model, and less to line shifts with respect to the central positions of the instrument transmission profiles. In general, the relative difference between synthesized values and HMI estimates increases toward the limb and with the increase of the field; the MDI-like algorithm seems to fail in regions with fields larger than approximately 2000 G. Instrumental effects were investigated by the analysis of HMI data obtained at daily intervals for a span of three years at disk center in the quiet Sun and hourly intervals for a span of 200 hours. The analysis revealed periodicities induced by the variation of the orbital velocity of the observatory with respect to the Sun, and long-term trends attributed to instrument adjustments, re-calibrations and instrumental degradation.

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arXiv2014

Interpretation of Solar Irradiance Monitor measurements through analysis of 3D MHD simulations

Measurements from the Solar Irradiance Monitor (SIM) onboard the SORCE mission indicate that solar spectral irradiance at Visible and IR wavelengths varies in counter phase with the solar activity cycle. The sign of these variations is not reproduced by most of the irradiance reconstruction techniques based on variations of surface magnetism employed so far, and it is not clear yet whether SIM calibration procedures need to be improved, or if instead new physical mechanisms must be invoked to explain such variations. We employ three-dimensional magneto hydrodynamic simulations of the solar photosphere to investigate the dependence of solar radiance in SIM Visible and IR spectral ranges on variations of the filling factor of surface magnetic fields. We find that the contribution of magnetic features to solar radiance is strongly dependent on the location on the disk of the features, being negative close to disk center and positive toward the limb. If features are homogeneously distributed over a region around the equator (activity belt) then their contribution to irradiance is positive with respect to the contribution of HD snapshots, but decreases with the increase of their magnetic flux for average magnetic flux larger than 50 G in at least two of the Visible and IR spectral bands monitored by SIM. Under the assumption that the 50 G snapshots are representative of quiet Sun regions we find thus that the Spectral Irradiance can be in counter-phase with the solar magnetic activity cycle.

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arXiv2013

The statistical distribution of magnetic field strength in G-band bright points

G-band bright points are small-sized features characterized by high photometric contrast. Theoretical investigations indicate that these features have associated magnetic field strengths between 1-2 kG. Results from observations instead lead to contradictory results, indicating magnetic fields of only kG strength in some and including hG strengths in others. In order to understand the differences between measurements reported in the literature, and to reconcile them with results from theory, we analyze the distribution of magnetic field strength of G-band bright features identified on synthetic images of the solar photosphere, and its sensitivity to observational and methodological effects. We investigate the dependence of magnetic field strength distributions of G-band bright points identified in 3D magnetohydrodynamic simulations on feature selection method, data sampling, alignment and spatial resolution. The distribution of magnetic field strength of G-band bright features shows two peaks, one at about 1.5 kG and one below 1 hG. The former corresponds to magnetic features,the second mostly to bright granules. Peaks at several hG are obtained only on spatially degraded or misalligned data. Simulations show that magnetic G-band bright points have typically associated field strengths of few kG. Field strengths in the hG range can result from observational effects, thus explaining the discrepancies presented in the literature. Our results also indicate that outcomes from spectro-polarimetric inversions with imposed unit filling-factor should be employed with great caution.

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arXiv2013

Comparison of physical properties of quiet and active regions through the analysis of MHD simulations of the solar photosphere

Recent observations have shown that the photometric and dynamic properties of granulation and of small-scale magnetic features depend on the amount of magnetic flux of the region they are embedded in. We analyze results from numerical Hydro and Magneto Hydrodynamic simulations characterized by different amount of average magnetic flux and find qualitatively the same differences as those reported from observations. We show that these different physical properties result from the inhibition of convection induced by the presence of the magnetic field, which changes the temperature stratification of both quiet and magnetic regions. Our results are relevant for solar irradiance variations studies, as such differences are still not properly taken into account in irradiance reconstruction models.

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arXiv2012

Effects of unresolved magnetic field on Fe I 617.3 and 630.2 nm line shapes

The contribution of the quiet Sun to solar irradiance variability, due either to changes of the solar effective temperature or to the presence of unresolved magnetic field, is still poorly understood. In this study we investigate spectral line diagnostics that are sensitive to both temperature variations and the presence of small scale unresolved magnetic features in these areas of the solar atmosphere. Specifically we study the dependence on the magnetic flux density of three parameters describing the shape of two magnetically sensitive FeI lines, at 630.2 nm and 617.3 nm, namely the line core intensity (IC), full width at half maximum (FWHM), and the equivalent width (EQW). To this aim we analyze observations of active region NOAA 11172, acquired with IBIS at the Dunn Solar Telescope, as well as results from numerical synthesis. Our results show that IC is sensitive to both temperature and magnetic flux density variations, FWHM is mostly affected by magnetic field changes, and EQW is mostly sensitive to temperature. Variations of a few percent of the measured line parameters are found in observational data that was spatially degraded to represent quiet-Sun, disk-centre, medium resolution observations. It is therefore possible to disentangle magnetic from pure thermodynamic effects by comparison of temporal variations of the EQW and the FWHM of either the two FeI lines.

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arXiv2011

Why one-dimensional models fail in the diagnosis of average spectra from inhomogeneous stellar atmospheres

We investigate the feasibility of representing a structured multi-dimensional stellar atmosphere with a single one-dimensional average stratification for the purpose of spectral diagnosis of the atmosphere's average spectrum. In particular we construct four different one-dimensional stratifications from a single snapshot of a magneto-hydrodynamic simulation of solar convection: one by averaging its properties over surfaces of constant height, and three different ones by averaging over surfaces of constant optical depth at 500 nm. Using these models we calculate continuum, and atomic and molecular line intensities and their center-to-limb variations. From analysis of the emerging spectra we identify three main reasons why these average representations are inadequate for accurate determination of stellar atmospheric properties through spectroscopic analysis. These reasons are: non-linearity in the Planck function with temperature, which raises the average emergent intensity of an inhomogeneous atmosphere above that of an average-property atmosphere, even if their temperature-optical depth stratification is identical; non-linearities in molecular formation with temperature and density, which raise the abundance of molecules of an inhomogeneous atmosphere over that in a one-dimensional model with the same average properties; the anisotropy of convective motions, which strongly affects the center-to-limb variation of line-core intensities. We argue therefore that a one-dimensional atmospheric model that reproduces the mean spectrum of an inhomogeneous atmosphere necessarily does not reflect the average physical properties of that atmosphere, and are therefore inherently unreliable.

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arXiv2010

A decline and fall in the future of Italian Astronomy?

On May 27th 2010, the Italian astronomical community learned with concern that the National Institute for Astrophysics (INAF) was going to be suppressed, and that its employees were going to be transferred to the National Research Council (CNR). It was not clear if this applied to all employees (i.e. also to researchers hired on short-term contracts), and how this was going to happen in practice. In this letter, we give a brief historical overview of INAF and present a short chronicle of the few eventful days that followed. Starting from this example, we then comment on the current situation and prospects of astronomical research in Italy.

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arXiv2008

Photometric properties of resolved and unresolved magnetic elements

We investigate the photometric signature of magnetic flux tubes in the solar photosphere. We developed two dimensional, static numerical models of isolated and clustered magnetic flux tubes. We investigated the emergent intensity profiles at different lines-of-sight for various spatial resolutions and opacity models. We found that both geometric and photometric properties of bright magnetic features are determined not only by the physical properties of the tube and its surroundings, but also by the particularities of the observations, including the line/continuum formation height, the spatial resolution and the image analyses techniques applied. We show that some observational results presented in the literature can be interpreted by considering bright magnetic features to be clusters of smaller elements, rather than a monolithic flux tube.

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arXiv2008

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.

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arXiv2007

Photometric properties of magnetic elements: resolved and unresolved features

We investigate, by numerical simulations, the photometric signature of magnetic flux tubes in the solar photosphere. We show that the observed contrast profiles are determined not only by the physical properties of the tube and its surroundings, but also by the peculiarities of the observations, including the line/continuum formation height and the spatial and spectral resolution. The aim is to understand these contributions well enough so that multi-wavelength observations can begin to disentangle them.

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arXiv2006

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.

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