arXiv subjects

F. Berrilli

Publications and source records attributed to F. Berrilli.

At least 37 records · Page 2Linked to original sources

arXiv2014

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 $7.6 \pm 0.2$ 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.

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arXiv2014

Diffusion of magnetic elements in a supergranular cell

Small scale magnetic fields (magnetic elements) are ubiquitous in the solar photosphere. Their interaction can provide energy to the upper atmospheric layers, and contribute to heat the solar corona. In this work, the dynamic properties of magnetic elements in the quiet Sun are investigated. The high number of magnetic elements detected in a supegranular cell allowed us to compute their displacement spectrum $\langle(Δr)^2\rangle\proptoτ^γ$ (being $γ>0$, and $τ$ the time since the first detection), separating the contribution of the network (NW) and the internetwork (IN) regions. In particular, we found $γ=1.27\pm0.05$ and $γ=1.08\pm0.11$ in NW (at smaller and larger scales, respectively), and $γ=1.44\pm0.08$ in IN. These results are discussed in light of the literature on the topic, as well as the implications for the build up of the magnetic network.

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arXiv2013

Dynamics of the solar atmosphere above a pore with a light bridge

Context: Solar pores are small sunspots lacking a penumbra that have a prevailing vertical magnetic field component. They can include light bridges at places with locally reduced magnetic field. Like sunspots, they exhibit a wide range of oscillatory phenomena. Aims: A large isolated pore with a light bridge (NOAA 11005) is studied to obtain characteristics of a chromospheric filamentary structure around the pore, to analyse oscillations and waves in and around the pore, and to understand the structure and brightness of the light bridge. Methods: Spectral imaging observations in the line Ca II 854.2 nm and complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS spectrometer and HINODE/SOT spectropolarimeter, were used to measure photospheric and chromospheric velocity fields, oscillations, waves, the magnetic field in the photosphere, and acoustic energy flux and radiative losses in the chromosphere. Results: The chromospheric filamentary structure around the pore has all important characteristics of a superpenumbra: it shows an inverse Evershed effect and running waves, and has a similar morphology and oscillation character. The granular structure of the light bridge in the upper photosphere can be explained by radiative heating. Acoustic waves leaking up from the photosphere along the inclined magnetic field in the light bridge transfer enough energy flux to balance the total radiative losses of the light-bridge chromosphere. Conclusions: The presence of a penumbra is not a necessary condition for the formation of a superpenumbra. The light bridge is heated by radiation in the photosphere and by acoustic waves in the chromosphere.

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arXiv2013

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.

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arXiv2013

Diffusion of solar magnetic elements up to supergranular spatial and temporal scales

The study of spatial and temporal scales on which small magnetic structures (magnetic elements) are organized in the quiet Sun may be approached by determining how they are transported on the solar photosphere by convective motions. The process involved is diffusion. Taking advantage of Hinode high spatial resolution magnetograms of a quiet Sun region at the disk center, we tracked 20145 magnetic elements. The large field of view (~50 Mm) and the long duration of the observations (over 25 hours without interruption at a cadence of 90 seconds) allowed us to investigate the turbulent flows at unprecedented large spatial and temporal scales. In the field of view, in fact, an entire supergranule is clearly recognizable. The magnetic elements displacement spectrum shows a double-regime behavior: superdiffusive (gamma=1.34 +/- 0.02) up to granular spatial scales (~1500 km), and slightly superdiffusive (gamma=1.20 +/- 0.05) up to supergranular scales.

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arXiv2012

On the asymmetry of velocity oscillation amplitude in bipolar active regions

The velocity field in the lower solar atmosphere undergoes strong interactions with magnetic fields. Many authors have pointed out that power is reduced by a factor between two and three within magnetic regions, depending on frequency, depth, the radius and the magnetic strength of the flux tube. Many mechanisms have been proposed to explain the observations. In this work, SDO dopplergrams and magnetograms of 12 bipolar active regions ($β$ARs) at a 45 second cadence, are used to investigate the relation between velocity fluctuations and magnetic fields. We show that there is an asymmetry within $β$ARs, with the velocity oscillation amplitude being more suppressed in the leading polarities compared to the trailing polarities. Also, the strongest magnetic fields do not completely suppress the five-minute oscillation amplitude, neither in the spot innermost umbrae.

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arXiv2012

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.

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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.

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arXiv2012

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.

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arXiv2011

Solar magnetism eXplorer (SolmeX)

The magnetic field plays a pivotal role in many fields of Astrophysics. This is especially true for the physics of the solar atmosphere. Measuring the magnetic field in the upper solar atmosphere is crucial to understand the nature of the underlying physical processes that drive the violent dynamics of the solar corona -- that can also affect life on Earth. SolmeX, a fully equipped solar space observatory for remote-sensing observations, will provide the first comprehensive measurements of the strength and direction of the magnetic field in the upper solar atmosphere. The mission consists of two spacecraft, one carrying the instruments, and another one in formation flight at a distance of about 200m carrying the occulter to provide an artificial total solar eclipse. This will ensure high-quality coronagraphic observations above the solar limb. Solmex integrates two spectro-polarimetric coronagraphs for off-limb observations, one in the EUV and one in the IR, and three instruments for observations on the disk. The latter comprises one imaging polarimeter in the EUV for coronal studies, a spectro-polarimeter in the EUV to investigate the low corona, and an imaging spectro-polarimeter in the UV for chromospheric studies. SOHO and other existing missions have investigated the emission of the upper atmosphere in detail (not considering polarization), and as this will be the case also for missions planned for the near future. Therefore it is timely that SolmeX provides the final piece of the observational quest by measuring the magnetic field in the upper atmosphere through polarimetric observations.

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arXiv2011

MHD wave transmission in the Sun's atmosphere

MHD wave propagation inside the Sun's atmosphere is closely related to the magnetic field topology. For example, magnetic fields are able to lower the cutoff frequency for acoustic waves thus allowing waves which would otherwise be trapped below the photosphere to propagate into the upper atmosphere. Another example is that MHD waves can be transmitted or converted into other forms of waves at altitudes where the sound speed equals the Alfvén speed. We take advantage of the large field-of-view provided by the IBIS experiment to study the wave propagation at two heights in the solar atmosphere, as sampled by the photospheric Fe 617.3 nm spectral line and the chromospheric Ca 854.2 nm spectral line, and its relationship to the local magnetic field. Among other things, we find substantial leakage of waves with 5-minute periods in the chromosphere at the edges of a pore and in the diffused magnetic field surrounding it. By using spectro-polarimetric inversions of Hinode SOT/SP data, we also find a relationship between the photospheric power spectrum and the magnetic field inclination angle; in particular, well-defined transmission peaks around 25\degree for 5 minutes waves and around 15\degree for 3 minutes waves. We propose a very simple model based upon wave transmission theory to explain this behavior. Finally, an analysis of both the power spectra and chromospheric amplification spectra suggests the presence of longitudinal acoustic waves along the magnetic field lines.

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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.

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arXiv2010

Interpretation of HINODE SOT/SP asymmetric Stokes profiles observed in quiet Sun network and internetwork

We present the first interpretation of the Stokes profile asymmetries measured in the FeI 630 nm lines by SOT/SP, in both quiet Sun internetwork (IN) and network regions. The inversion is carried out under the hypothesis of MISMA, where the unresolved structure is assumed to be optically thin. We analyze a 29.52"x31.70" subfield carefully selected to be representative of the properties of a 302"x162" quiet Sun field-of-view at disk center. The inversion code is able to reproduce the observed asymmetries in a very satisfactory way. The inversion code interprets 25% of inverted profiles as emerging from pixels in which both positive and negative polarities coexist. kG field strengths are found at the base of the photosphere in both network and IN; in the case of the latter, both kG fields and hG fields are admixed. When considering the magnetic properties at the mid photosphere most kG fields are gone, and the statistics is dominated by hG fields. We constrain the magnetic field of only 4.5% of the analyzed photosphere (and this percentage reduces to 1.3% when referred to all pixels, including those with low polarization not analyzed). The rest of the plasma is consistent with the presence of weak fields not contributing to the detected polarization signals. The average flux densities derived in the full subfield and in IN regions are higher than the ones derived from the same dataset by Milne-Eddington inversion. The existence of large asymmetries in SOT/SP polarization profiles is uncovered. These are not negligible in quiet Sun data. The MISMA inversion code reproduces them in a satisfactory way, and provides a statistical description of the magnetized IN and network which partly differs and complements the results obtained so far. From this it follows the importance of having a complete interpretation of the line profile shapes.

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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.

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arXiv2009

MCAO for the European Solar Telescope: first results

We analise the efficiency of wavefront reconstruction in the MultiConjugate Adaptive Optics system for the European Solar Telescope (EST). We present preliminary results derived from numerical simulations. We study a 4 meter class telescope with multiple deformable mirrors conjugated at variable heights. Along with common issues, difficulties peculiar to the solar case have to be considered, such as the low contrast and extended nature of the natural guide features. Our findings identify basic requirements for the EST Adaptive Optics system and show some of its capabilities.

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arXiv2009

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.

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arXiv2009

Imaging Spectropolarimetry with IBIS: Evolution of Bright Points in the Quiet Sun

We present the results from first spectropolarimetric observations of the solar photosphere acquired at the Dunn Solar Telescope with the Interferometric Bidimensional Spectrometer. Full Stokes profiles were measured in the Fe I 630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions for 53 minutes, with a Stokes V noise of 0.003 the continuum intensity level. The dataset allows us to study the evolution of several magnetic features associated with G-band bright points in the quiet Sun. Here we focus on the analysis of three distinct processes, namely the coalescence, fragmentation and cancellation of G-band bright points. Our analysis is based on a SIR inversion of the Stokes I and V profiles of both Fe I lines. The high spatial resolution of the G-band images combined with the inversion results helps to interpret the undergoing physical processes. The appearance (dissolution) of high-contrast G-band bright points is found to be related to the local increase (decrease) of the magnetic filling factor, without appreciable changes in the field strength. The cancellation of opposite-polarity bright points can be the signature of either magnetic reconnection or the emergence/submergence of magnetic loops.

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arXiv2008

Magnetic field distribution in the quiet Sun: a simplified model approach

We simulate the dynamics and the evolution of quiet Sun magnetic elements to produce a probability density function of the field strengths associated with such elements. The dynamics of the magnetic field are simulated through a numerical model in which magnetic elements are passively driven by an advection field presenting spatio-temporal correlations which mimicks the granulation and the mesogranulation scales observed on the solar surface. The field strength can increase due to an amplification process which takes place where the magnetic elements converge. Starting from a delta-like probability density function centered on B=30 G, we obtain magnetic field strengths up to 2 kG (in absolute value). To derive the statistical properties of the magnetic elements several simulation runs are performed. The model is able to produce kG magnetic fields in a time interval of the order of the granulation time scale. The mean unsigned flux density and the mean magnetic energy density of the synthetic quiet Sun reach respectively 100 G and 350 G in the stationary regime. The derived probability density function of the magnetic field strength decreases rapidly from B=30 G to B=100 G and presents a secondary maximum for B=2 kG. From this result it follows that magnetic fields >700 G dominate the unsigned flux density and magnetic energy density although the probability density function of the field strength presents a maximum for B=30 G.

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