Ilaria Ermolli

Recent variability of the solar spectral irradiance and its impact on climate modelling

The lack of long and reliable time series of solar spectral irradiance (SSI) measurements makes an accurate quantification of solar contributions to recent climate change difficult. Whereas earlier SSI observations and models provided a qualitatively consistent picture of the SSI variability, recent measurements by the SORCE (SOlar Radiation and Climate Experiment) satellite suggest a significantly stronger variability in the ultraviolet (UV) spectral range and changes in the visible and near-infrared (NIR) bands in anti-phase with the solar cycle. A number of recent chemistry-climate model (CCM) simulations have shown that this might have significant implications on the Earths atmosphere. Motivated by these results, we summarize here our current knowledge of SSI variability and its impact on Earths climate. We present a detailed overview of existing SSI measurements and provide thorough comparison of models available to date. SSI changes influence the Earths atmosphere, both directly, through changes in shortwave (SW) heating and therefore, temperature and ozone distributions in the stratosphere, and indirectly, through dynamical feedbacks. We investigate these direct and indirect effects using several state-of-the art CCM simulations forced with measured and modelled SSI changes. A unique asset of this study is the use of a common comprehensive approach for an issue that is usually addressed separately by different communities. We show that the SORCE measurements are difficult to reconcile with earlier observations and with SSI models. Of the five SSI models discussed here, specifically NRLSSI (Naval Research Laboratory Solar Spectral Irradiance), SATIRE-S (Spectral And Total Irradiance REconstructions for the Satellite era), COSI (COde for Solar Irradiance), SRPM (Solar Radiation Physical Modelling), and OAR (Osservatorio Astronomico di Roma), only one shows a behaviour of the UV and visible irradiance qualitatively resembling that of the recent SORCE measurements. However, the integral of the SSI computed with this model over the entire spectral range does not reproduce the measured cyclical changes of the total solar irradiance, which is an essential requisite for realistic evaluations of solar effects on the Earths climate in CCMs. We show that within the range provided by the recent SSI observations and semi-empirical models discussed here, the NRLSSI model and SORCE observations represent the lower and upper limits in the magnitude of the SSI solar cycle variation. The results of the CCM simulations, forced with the SSI solar cycle variations estimated from the NRLSSI model and from SORCE measurements, show that the direct solar response in the stratosphere is larger for the SORCE than for the NRLSSI data. Correspondingly, larger UV forcing also leads to a larger surface response. Finally, we discuss the reliability of the available data and we propose additional coordinated work, first to build composite SSI data sets out of scattered observations and to refine current SSI models, and second, to run coordinated CCM experiments.

Comparison Among Ca II K Spectroheliogram Time Series with an Application to Solar Activity Studies

Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image quality and contents of three Ca II K spectroheliogram time series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives, in order to estimate their value for studies focusing on timescales longer than the solar cycle. We analyze the quality of these data and compare the results obtained with those achieved for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We also investigate whether image-segmentation techniques, such as those developed for identification of plage regions on present-day Ca II K observations, can be used to process historic series. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar present-day observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. The segmentation technique tested in this study gives reasonably consistent results for the three analyzed series after application of a simple photographic calibration. Although the plage areas measured from the three analyzed series differ somewhat, the difference to previously published results is larger.

Photometric properties of facular features over the activity cycle

Aims. We have analyzed the contrast of facular features identified in a large dataset of PSPT full-disk photometric images and SoHO/MDI magnetograms, obtained from 1998 to 2005. The aim of this work is to contribute to the improvement of semi-empirical atmospheric models and of irradiance studies and to understand the reasons for the controversial results of facular contrast already presented in the literature. Methods. We used different identification methods to analyze their effects upon the results obtained. We also analyzed the effects of the limited information content in the analyzed images. Results. We show that selection effects associated with the identification method may produce significant differences in the results. The facular contrast is not only a function of both selection methods and the heliocentric angle, but also of feature size, activity level, and content of the analyzed images. Comparisons of the results obtained with computations of the most recent semi-empirical atmospheric models of facular features show that these models reproduce limb-angle corrected contrast measurements with an offset up to ≈ 1% from the disk center to μ = 0.3.

Observation of bipolar moving magnetic features streaming out from a naked spot

Context. Mechanisms responsible for active-region formation, evolution, and decay have been investigated by many authors and several common features have been identified. In particular, a key element in the dispersal of the magnetic field seems to be the presence of magnetic elements, called moving magnetic features (MMFs). Aims. We analyze the short-lived sunspot group NOAA 10977, which appeared on the solar disk between 2 and 8 December 2007, to study the details of its emergence and decay phases. Methods. We performed a multi wavelength analysis of the region using images at visible (G band and Hα) and near-IR (Ca ii) wavelengths acquired by both the IBIS instrument and SOT/HINODE, EUV images (17.1 nm) acquired by TRACE, and MDI and SOT magnetograms. Results. The observed region exhibits some peculiarities. During the emergence phase the formation of the f-pore was initially observed, while the p-polarity later formed a naked spot, i.e., a sunspot without a penumbra. We measured a moat flow around this spot, and observed some MMFs streaming out from it during the decay phase. The characteristics of these MMFs allowed us to classify them as type I (U-shaped) MMFs. They were also cospatial with sites of increased brightness both in the photosphere and the chromosphere. Conclusions. The presence of bipolar MMFs in a naked spot indicates that current interpretation of bipolar MMFs, as extensions of the penumbral filaments beyond the sunspot outer boundaries, should be revised, to take into account this observational evidence. We believe that our results provide new insights into improving models of sunspot evolution.

Solar Cycle Indices from the Photosphere to the Corona: Measurements and Underlying Physics

A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short- and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements and typical use, and for those that quantify physical observables, we describe the underlying physics.

The Prototype RISE-PSPT Instrument Operating in Rome

The breadboard prototype of the PSPT (Precision Solar Photometric Telescope), built by NSO at Sacramento Peak, has been operating in Rome since February 1996 to test observing procedures and future network operations. In this paper we briefly describe the kind of preliminary data we are deriving from the first observations concerning the contrast histogram and the fractal analysis of the ‘network cells.’

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 at the Mauna Loa Solar Observatory. 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 105 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.

On the Geometrical Properties of the Chromospheric Network

A sequence of Ca-K images obtained in a period of minimum solar activity, from July to November 1996, at the Rome Observatory with the PSPT (Precision Solar Photometric Telescope) prototype instrument have been used to analyze the geometrical properties of cells identified by the chromospheric network. In particular, we used 256 × 256 sub-arrays of the calibrated full-disk PSPT images. These sub-arrays, centered on the solar disk, are reduced to two-levels (binary) images by means of a suitable threshold after an FFT high-pass filtering. A medial axis transform, better known as skeleton, combined with a cellular automaton, is applied to the two-level images, in order to derive the cell boundaries. The regions corresponding to the cells are then filled by a growing algorithm. In this way we can derive a set of output parameters describing the cells geometry. The size distribution of the identified cells shows a continuous increase toward the smaller scales, rather than a small dispersion around a characteristic scale. Nevertheless the analysis of the inter-cell distances and of the area distribution pointed out a characteristic scale (square root of the area) of ± 24 Mm. To describe the cells irregularity and to probe the nature of solar turbulence, we apply a Mandelbrot fractal analysis to such irregularly shaped features. Examining the cell perimeter–area relationship we found the existence of a ‘critical’ area at which a change in the geometrical properties occurs. This area corresponds to the scale of ± 24 Mm. The estimated fractal dimension for cells with area greater than the ‘critical’ one is 1.35. This value, close to that predicted for isobars in the Kolmogorov 3-D turbulent theory, does not exclude a turbulent origin for such cells. The analysis seems to point to a common origin for solar granulation and supergranulation.

Radiative emission of solar features in the Ca II K line: comparison of measurements and models

Context. The intensity of the Ca II K resonance line observed with spectrographs and Lyot-type filters has long served as a diagnostic of the solar chromosphere. However, the literature contains a relative lack of photometric measurements of solar features observed at this spectral range. Aims. 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. Methods. We analysed full-disk 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 non-local thermal equilibrium (NLTE) spectral synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The synthesis was carried out by utilizing the partial frequency redistribution (PRD) with the most recent set of semi-empirical atmosphere models in the literature and some earlier atmosphere models. As the studied models were computed by assuming the complete redistribution formalism (CRD), we also performed simulations with this approximation for comparison. Results. We measured the center-to-limb variation 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 bandpasses 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. When moving towards the limb, PRD computations display closer agreement with the data than performed in CRD. Moreover, 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.

European Solar Telescope: project status

The European Solar Telescope (EST) is a project for a large aperture (3-5 meters) ground-based telescope, to be located in the Canary Islands. EST will be optimized for studies of magnetic coupling between the deep photosphere and upper chromosphere. This will require diagnostics of the thermal, dynamic and magnetic properties of the plasma over many scale heights, by using multiple wavelength imaging, spectroscopy and spectropolarimetry. The design of EST will strongly emphasize the use of a large number of visible and near-infrared instruments simultaneously. To achieve these goals, EST will specialize in high spatial and temporal resolution using instruments that can efficiently produce twodimensional spectral information. In this communication, the present situation of the design is outlined, as well as the expected future phases and scheduling.