arXiv subjects

S. Criscuoli

Publications and source records attributed to S. Criscuoli.

15 recordsLinked to original sources

arXiv2025

Magnetic Reconnection in a Compact Magnetic Dome: Chromospheric Emissions and High-velocity Plasma Flows

Magnetic reconnection at small spatial scales is a fundamental driver of energy release and plasma dynamics in the lower solar atmosphere. We present observations of a brightening in an active region, captured in high-resolution data from the Daniel K. Inouye Solar Telescope (DKIST) using the Visible Broadband Imager (VBI) and the Visible Spectro-Polarimeter (ViSP). The event exhibits Ellerman bomb-like morphology in the H$β$ filter, associated with flux cancellation between a small negative polarity patch adjacent to opposite-polarity plage. Additionally, it displays enhanced emissions in Ca II K, hot elongated features containing Alfvénic plasma flows, and cooler blue-shifted structures. We employ multi-line, non-local thermodynamic equilibrium (non-LTE) inversions of the spectropolarimetric data to infer the stratification of the physical parameters of the atmosphere. Furthermore, we use the photospheric vector magnetogram inferred from the ViSP spectra as a boundary condition for nonlinear force-free field extrapolations, revealing the three-dimensional distribution of squashing factors. We find significant enhancements in temperature, velocity, and microturbulence confined to the upper photosphere and low chromosphere. Our findings provide observational evidence of low-altitude magnetic reconnection along quasi-separatrix layers in a compact fan-spine-type configuration, highlighting the complex interplay between magnetic topology, energy release, and plasma flows.

astro-ph.SR↗

arXiv2025

Emergence of magnetic flux sheets in the quiet Sun. I. Statistical properties

Small-scale magnetic flux emergence in the quiet Sun is crucial for maintaining solar magnetic activity. On the smallest scales studied so far, namely within individual granules, two mechanisms have been identified: emergence in tiny magnetic loops and emergence in the form of magnetic flux sheets covering the granule. While there are abundant observations of tiny magnetic loops within granules, the evidence for the emergence of granule-covering magnetic sheets is much more limited. This work aims to statistically analyze magnetic flux sheets, quantify their frequency on the solar surface and their potential contribution to the solar magnetic budget in the photosphere, and investigate the plasma dynamics and granular-scale phenomena associated with their emergence. Using spectro-polarimetric datasets from the solar optical telescope aboard the Hinode satellite and the ground-based Swedish Solar telescope, we developed a two-step method to identify magnetic flux sheet emergence events, detecting magnetic flux patches based on the calculation of the transverse and longitudinal magnetic flux density and associating them with their host granules based on velocity field analysis. We identified 42 events of magnetic flux sheet emergence and characterized their magnetic properties and the associated plasma dynamics of their host granules. Our results align with numerical simulations, indicating a similar occurrence rate. We investigated the relationship between magnetic flux emergence and granular phenomena, finding that flux sheets often emerge in association with standard nascent granules, as well as exploding granules, or granules with granular lanes. In particular, we highlight the potential role of recycled magnetic flux from downflow regions in facilitating flux sheet emergence.

astro-ph.SR↗

arXiv2023

Ground-based monitoring of the variability of visible Solar spectral lines for improved understanding of solar and stellar magnetism and dynamics

Long-term high-cadence measurements of stellar spectral variability are fundamental to better understand stellar atmospheric properties and stellar magnetism. These, in turn, are fundamental for the detectability of exoplanets as well as the characterization of their atmospheres and habitability. The Sun, viewed as a star via disk-integrated observations, offers a means of exploring such measurements while also offering the spatially resolved observations that are necessary to discern the causes of observed spectral variations. High-spectral resolution observations of the solar spectrum are fundamental for a variety of Earth-system studies, including climate influences, renewable energies, and biology. The Integrated Sunlight Spectrometer at SOLIS, has been acquiring daily high-spectral resolution Sun-as-a-star measurements since 2006.More recently, a few ground-based telescopes with the capability of monitoring the solar visible spectrum at high spectral resolution have been deployed (e.g. PEPSI, HARPS, NEID). However, the main scientific goal of these instruments is to detect exo-planets, and solar observations are acquired mainly as a reference. Consequently, their technical requirements are not ideal to monitor solar variations with high photometric stability, especially over solar-cycle temporal scales.The goal of this white paper is to emphasize the scientific return and explore the technical requirements of a network of ground-based spectrographs devoted to long-term monitoring of disk-integrated solar-spectral variability with high spectral resolution and high photometric stability, in conjunction with disk-resolved observations in selected spectral lines,to complement planet-hunter measurements and stellar-variability studies. The proposed network of instruments offers the opportunity for a larger variety of multidisciplinary studies.

astro-ph.IM↗

arXiv2022

Total Solar Irradiance during the Last Five Centuries

The total solar irradiance (TSI) varies on timescales of minute to centuries. On short timescales it varies due to the superposition of intensity fluctuations produced by turbulent convection and acoustic oscillations. On longer scale times, it changes due to photospheric magnetic activity, mainly because of the facular brightenings and dimmings caused by sunspots. While modern TSI variations have been monitored from space since 1970s, TSI variations over much longer periods can only be estimated using either historical observations of magnetic features, possibly supported by flux transport models, or from the measurements of the cosmogenic isotope (e.g., \textsuperscript{14}C or \textsuperscript{10}Be) concentrations in tree rings and ice cores. The reconstruction of the TSI in the last few centuries, particularly in the 17th/18th centuries during the Maunder minimum, is of primary importance for studying climatic effects. To separate the temporal components of the irradiance variations, specifically the magnetic cycle from secular variability, we decomposed the signals associated with historical observations of magnetic features and the solar modulation potential $Φ$ by applying an Empirical Mode Decomposition algorithm. Thus, the reconstruction is empirical and does not require any feature contrast or field transport model. The assessed difference between the mean value during the Maunder minimum and the present value is $\simeq2.5 Wm^{-2}$. Moreover it shows, in the first half of the last century, a growth of $\simeq 1.5 W m^{-2}$ which stops around the middle of the century to remain constant for the next 50 years, apart from the modulation due to the solar cycle.

astro-ph.SR↗

arXiv2021

Solar Atmosphere Radiative Transfer Model Comparison based on 3D MHD simulations

The reconstruction of the solar spectral irradiance (SSI) on various time scales is essential for the understanding of the Earth's climate response to the SSI variability. The driver of the SSI variability is understood to be the intensity contrast of magnetic features present on the Sun with respect to the largely non-magnetic quiet Sun. However, different spectral synthesis codes lead to diverging projections of SSI variability. In this study we compare three different radiative transfer codes and carry out a detailed analysis of their performance. We perform the spectral synthesis at the continuum wavelength of 665 nm with the Code for Solar Irradiance (COSI), and the Rybicki-Hummer (RH), and Max Planck University of Chicago Radiative MHD (MURaM) codes for three 3D MHD simulations snapshots, a non-magnetic case, and MHD simulations with 100 G, and 200 G magnetic field strength. We determine the intensity distributions, the intensity differences and ratios for the spectral synthesis codes. We identify that the largest discrepancies originate in the intergranular lanes where the most field concentration occurs. Overall, the applied radiative transfer codes give consistent intensity distributions. Also, the intensity variation as a function of magnetic field strength for the particular 100 G and 200 G snapshots agree within the 2-3% range.

astro-ph.SR↗

arXiv2019

Continuum emission enhancements and penumbral changes observed during flares by IRIS, ROSA, and Hinode

In this paper we describe observations acquired by satellite instruments (Hinode/SOT and IRIS) and ground-based telescopes (ROSA@DST) during two consecutive C7.0 and X1.6 flares occurred in active region NOAA 12205 on 2014 November 7. The analysis of these data show the presence of continuum enhancements during the evolution of the events, observed both in ROSA images and in IRIS spectra. Moreover, we analyze the role played by the evolution of the $δ$ sunspots of the active region in the flare triggering, indicating the disappearance of a large portion of penumbra around these sunspots.

astro-ph.SR↗

arXiv2017

The Signature of Flare Activity in Multifractal Measurements of Active Regions Observed by SDO/HMI

Recent studies indicate that measurements of fractal and multifractal parameters of active regions (ARs) are not efficient tools to discriminate ARs on the basis of the flare activity, as well as to predict flare events. Attempting validation of this result on a large data set of higher spatial and temporal resolution, as well as higher flux sensitivity, observations than employed in previous studies, we analyzed high-cadence time series of line-of-sight magnetograms of 43 ARs characterized by different flare activity, which were observed with SDO/HMI from May 2010 to December 2013. On these data, we estimated four parameters, the generalized fractal dimensions $D_0$ and $D_8$, and the multifractal parameters $C_{\mathrm{div}}$ and $D_{\mathrm{div}}$. We found distinct average values of the parameters measured on ARs that have hosted flares of different class. However, the dispersion of values measured on ARs that have produced same class events is such that the parameters deduced from distinct classes of flaring regions can also largely overlap. Based on the results of our measurements, C- and M-class flaring ARs are practically indistinguishable, as well as M- and X-class flaring ARs. We found consistent changes on the time series of the measured parameters only on $\approx$ 50\% of the studied ARs and $\approx$ 50\% of the M- and X-class events considered. We show that these results hold for fractal and multifractal parameter estimates based on both total unsigned and signed flux data of the analyzed ARs.

astro-ph.SR↗

arXiv2016

Kinematics and Magnetic Properties of a Light Bridge in a Decaying Sunspot

We present the results obtained by analyzing high spatial and spectral resolution data of the solar photosphere acquired by the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope on 6 August 2011, relevant to a large sunspot with a light bridge (LB) observed in NOAA AR 11263. These data are complemented by simultaneous Hinode Spectropolarimeter (SP) observation in the Fe I 630.15 nm and 630.25 nm lines. The continuum intensity map shows a discontinuity of the radial distribution of the penumbral filaments in correspondence with the LB, which shows a dark lane (about 0.3" wide and about 8.0" long) along its main axis. The available data were inverted with the Stokes Inversion based on Response functions (SIR) code and physical parameters maps were obtained. The line-of-sight (LOS) velocity of the plasma along the LB derived from the Doppler effect shows motions towards and away from the observer up to 0.6 km/s, which are lower in value than the LOS velocities observed in the neighbouring penumbral filaments. The noteworthy result is that we find motions toward the observer up to 0.6 km/s in the dark lane where the LB is located between two umbral cores, while the LOS velocity motion toward the observer is strongly reduced where the LB is located between an umbral core at one side and penumbral filaments on the other side. Statistically, the LOS velocities correspond to upflows/downflows andcomparing these results with Hinode/SP data, we conclude that the surrounding magnetic field configuration (whether more or less inclined) could have a role in maintaining the conditions for the process of plasma piling up along the dark lane. The results obtained from our study support and confirm outcomes of recent magnetohydro-dynamic simulations showing upflows along the main axis of a LBs.

astro-ph.SR↗

arXiv2012

High cadence spectropolarimetry of moving magnetic features observed around a pore

Moving magnetic features (MMFs) are small-size magnetic elements that are seen to stream out from sunspots, generally during their decay phase. Several observational results presented in the literature suggest them to be closely related to magnetic filaments that extend from the penumbra of the parent spot. Nevertheless, few observations of MMFs streaming out from spots without penumbra have been reported. The literature still lacks of analyses of the physical properties of these features. We investigate physical properties of monopolar MMFs observed around a small pore that had developed penumbra in the days preceding our observations and compare our results with those reported in the literature for features observed around sunspots. We analyzed NOAA 11005 during its decay phase with data acquired at the Dunn Solar Telescope in the FeI 617.3$ nm and the CaII 854.2$ nm spectral lines with IBIS, and in the G-band. The field of view showed monopolar MMFs of both polarities streaming out from the leading negative polarity pore of the observed active region. Combining different analyses of the data, we investigated the temporal evolution of the relevant physical quantities associated with the MMFs as well as the photospheric and chromospheric signatures of these features. We show that the characteristics of the investigated MMFs agree with those reported in the literature for MMFs that stream out from spots with penumbrae. Moreover, observations of at least two of the observed features suggest them to be manifestations of emerging magnetic arches.

astro-ph.SR↗

arXiv2010

Line shape effects on intensity measurements of solar features: Brightness correction to SoHO MDI continuum images

Continuum intensity observations obtained with the Michelson Doppler Imager (MDI) on-board the SoHO mission provide long time series of filtergrams that are ideal for studying the evolution of large-scale phenomena in the solar atmosphere and their dependence on solar activity. These filtergrams, however, are not taken in a pure continuum spectral band, but are constructed from a proxy, namely a combination of filtergrams sampling the Ni I 676.8 nm line. We studied the sensitivity of this continuum proxy to the shape of the nickel line and to the degradation in the instrumental transmission profiles. We compared continuum intensity measurements in the nearby of nickel line with MDI proxy values in three sets of high resolution spectro-polarimetric data obtained with the Interferometric Bidimensional Spectrometer (IBIS), and in synthetic data, obtained from multi-dimensional simulations of magneto-convection and one-dimensional atmosphere models. We found that MDI continuum measurements require brightness corrections which depend on magnetic field strength, temperature and, to a smaller extent, plasma velocity. The correction ranges from 2% to 25% in sunspots, and is, on average, less than 2% for other features. The brightness correction also varies with position on the disk, with larger variations obtained for sunspots, and smaller variations obtained for quiet sun, faculae and micropores. Correction factors derived from observations agree with those deduced from the numerical simulations when observational effects are taken into account. Finally, we found that the investigated potential uncertainties in the transmission characteristics of MDI filters only slightly affect the brightness correction to proxy measurements.

astro-ph.SR↗

arXiv2010

Properties of G-band Bright Points derived from IBIS observations

We have investigated properties of photospheric Bright Points (BPs) observed in an Active Region during its decay phase and in a quiet Sun region. We have analyzed two sets of photospheric observations taken with IBIS (Interferometric Bidimensioal Spectrometer) at the NSO Dunn Solar Telescope. The first set consists of spectral data acquired in the Fe I 709.0 nm and Ca I 854.2 nm lines and simultaneous broad-band and of G-band observations. The second set consists of spectro-polarimetric observations in the Fe I 630.15 nm - 630.25 nm doublet and simultaneous white light and G-band observations. The relation between BP filling factor and RMS image contrast indicates that, on average, BPs cover up to 3% of the solar surface outside Active Regions. The relation between area and intensity values of the features identified on both data sets suggests that they are composed of aggregations of magnetic flux elements. The horizontal velocity values are as high as 2 km/s, thus supporting the scenario of BPs motion contributing to the coronal heating.

astro-ph.SR↗

arXiv2010

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.

astro-ph.SR↗

arXiv2010

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.

astro-ph.SR↗

arXiv2010

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.

astro-ph.SR↗

arXiv2009

Magnetic evolution of superactive regions. Complexity and potentially unstable magnetic discontinuities

In this work, we have studied the temporal evolution of some properties of a sample of superactive regions with the aim to single out the most significant for flare activity forecasting. We have investigated properties of 14 superactive regions, observed between January 1st 2000 and December 31st 2006 with MDI/SOHO instrument and characterized by a particularly intense flare activity during their passage on the solar disk. We have analyzed the temporal evolution of fractal and multifractal properties of photospheric magnetic fields, namely the generalized fractal dimension and the cntribution and dimensionality diversities, as well as the potential unstable volumes of magnetic discontinuities above the studied ARs. Correlations of these quantities with the flare index, which provides information about the flare activity of a region, have also been estimated. We found that in 50 % of our sample the generalized fractal dimension is correlated with the flare index computed over windows of 50 hours, while the contribution diversity and the dimensional diversity are anticorrelated with the same index. An increase of the potential unstable volume of magnetic discontinuities in the corona is observed before the phases characterized by more frequent and intense flares. We also found that the free energy distribution functions of unstable volumes of the analyzed superactive regions can be fitted with straight lines whose slope is larger than the values found in previous works for less active magnetic regions. The generalized fractal dimension and the potential unstable volume of magnetic discontinuities are the most suitable for statistical investigations of relations with flare activity over longer (50 hours) and shorter (few hours) time intervals, respectively.

astro-ph.SR↗