arXiv subjects

Francesco Berrilli

Publications and source records attributed to Francesco Berrilli.

At least 19 recordsLinked to original sources

arXiv2026

Stochastic Resonance in a Thermally Driven Low-Dimensional Geodynamo Model

Geomagnetic field reversal sequences exhibit persistence times spanning a broad range, from a few $10^4$ years to superchrons lasting more than $10^7$ years. Despite extensive observational and theoretical work, the physical mechanisms governing how such reversals occur and how their broad temporal variability is organized are still not fully understood. Here we investigate the temporal variability of geomagnetic polarity in a thermally driven low-dimensional geodynamo model subject to a slow periodic modulation of the control parameter governing the large-scale induction, namely the $\alpha$-effect parameter. We find that the modulation generates a multipeaked probability density function of magnetic persistence times, with local maxima occurring at approximately integer multiples of the modulation timescale, as expected in a stochastic-resonance-like regime. The peak positions follow an approximately linear dependence on their index, showing that the characteristic timescales selected by the system are set by the imposed modulation period. These results provide a physically motivated numerical framework in which slow modulation of a geodynamo control parameter can organize reversal statistics through stochastic-resonance-like dynamics.

physics.geo-ph↗

arXiv2026

Correlation between active regions' spectra at high radio frequencies and solar flare occurrences

High radio frequencies observations with the Italian network of large single-dish radio telescopes resulted in ~450 solar images between 2018 and 2023 in K-band frequency range (18-26 GHz). Solar radio mapping at these frequencies allows the probing of the Active Regions (ARs) chromospheric magnetic field close to the Transition Region, where strong flares and coronal mass ejection events occur. Enhanced magnetic fields up to 1500-2000 G determine anomalous spectra in the ARs brightness compared to pure free-free emission, due to the addition of a steeper gyro-resonance component also associated with circular polarisation up to ~40%. When a significant AR spectral flattening is detected, the probability of a strong flare occurrence within ~30 hours is high (~89% in terms of statistical precision). Despite an approximate weekly cadence of our observations, only ~12% of strong flares are missed/unpredicted within this time interval. Through a correlation analysis, we assess the trade-off on the sensitivity and the robustness of this physics-based flare forecast method.

astro-ph.SR↗

arXiv2025

On the Temporal Variability in the Magnetic Dichotomy of Late M Dwarf Stars

Rapidly rotating late M dwarfs are observed in two different branches of magnetic activity, although they operate in the same stellar parameter range. Current empirical evidence indicates that M dwarfs with spectral types ranging from M3 / M4 to late-type M dwarfs, stellar masses smaller than 0.15 M$_\odot$, and rotational period shorter than four days display either a stable dipolar magnetic field or magnetic structures with significant time variability. The magnetic activity of fully convective M dwarfs is known to be regulated by a mechanism named the ${\alpha}^2$ dynamo. To further constrain the physics of this mechanism, we use a low-dimensional model for thermally driven magnetoconvection producing an ${\alpha}^2$ dynamo, specifically a modified magnetohydrodynamic (MHD) shell model. Although the model neglects density stratification, it captures the essential nonlinear dynamics of an $\alpha^2$ dynamo. Therefore, the results should be interpreted in a qualitative sense, highlighting possible trends rather than providing direct quantitative predictions for fully convective stars. The model is validated by comparing the statistical properties of magnetic polarity reversals with paleomagnetic data, since the geodynamo provides the only natural ${\alpha}^2$ dynamo with sufficiently rich reversal statistics. Our findings reveal that increased convective heat transport correlates with more frequent magnetic-polarity reversals, resulting in enhanced magnetic variability. This suggests that the observed magnetic dichotomy in late M dwarfs could be interpreted in terms of differences in global heat transport efficiency. However, additional models and observations of M dwarfs are needed to further constrain this interpretation.

astro-ph.SR↗

arXiv2025

Modeling Decadal and Centennial Solar UV Irradiance Changes

Reconstructions of solar spectral irradiance - especially in the ultraviolet (UV) range - are crucial for understanding Earth's climate system. Although total solar irradiance (TSI) has been thoroughly investigated, the spectral composition of solar radiation offers a deeper insight into its interactions with the atmosphere, biosphere, and climate. UV radiation, in particular, plays a key role in stratospheric chemistry and the dynamics of stratospheric ozone. Reconstructing solar irradiance over the past centuries requires accounting for both the cyclic modulation of active-region coverage associated with the 11-year solar cycle and the longer-term secular trends, including their centennial variability. This study utilizes an empirical framework, based on a 1000-year record of Open Solar Flux, to characterize the various temporal components of solar irradiance variability. We then combine these components to reconstruct Solar UV irradiance variations in spectral bands crucial for Earth's atmospheric studies.

astro-ph.SR↗

arXiv2025

Investigating the Temperature Sensitivity of UV Line Ratios in the 280 nm Region of Solar-like Stars

Stellar UV spectra are fundamental diagnostics of physical and magnetic properties of stars. For instance, lines like Mg II at 280 nm serve as valuable indicators of stellar activity, providing insights into the activity levels of Sun-like stars and their potential influence on the atmospheres of orbiting planets. On the other hand, the effective temperature (Teff) is a fundamental stellar parameter, critical for determining stellar properties such as mass, age, composition and evolutionary status. In this study, we investigate the temperature sensitivity of three lines in the mid-ultraviolet range (i.e., Mg II 280.00 nm, Mg I 285.20 nm, and Si I 286.15 nm). Using spectra from the International Ultraviolet Explorer (IUE), we analyze the behavior of the ratios of their corresponding indices (core/continuum) for a sample of calibrating solar-like stars, and find that the ration R = Mg II/Mg I best traces Teff through a log-log relation. The Teff estimated using this relation on a test-sample of solar-like stars agree with the Teff from the literature at the 95% confidence level. The observed results are interpreted making use of Response Functions as diagnostics. This study extends the well-established use of line depth ratio-temperature relationships, traditionally applied in the visible and near-infrared ranges, to the mid-UV spectrum. With the growing interest in stellar UV spectroscopy, results presented in this paper are potentially relevant for future missions as HWO, MANTIS and UVEX.

astro-ph.SR↗

arXiv2024

Reconstruction of the Total Solar Irradiance during the last Millenium

Solar irradiance variations across various timescales, from minutes to centuries, represents a potential natural driver of past regional and global climate cold phases. To accurately assess the Sun's effect on climate, particularly during periods of exceptionally low solar activity known as grand minima, an accurate reconstruction of solar forcing is essential. While direct measurements of Total Solar Irradiance (TSI) only began in the late 1970s with the advent of space radiometers, indirect evidence from various historical proxies suggests that the Sun's magnetic activity has undergone possible significant fluctuations over much longer timescales. Employing diverse and independent methods for TSI reconstruction is essential to gaining a comprehensive understanding of this issue. This study employs a semi-empirical model to reconstruct TSI over the past millennium. Our approach uses an estimated open solar magnetic field ($F_{o}$), derived from cosmogenic isotope data, as a proxy for solar activity. We reconstruct the cyclic variations of TSI, due to the solar surface magnetic features, by correlating $F_{o}$ with the parameter of active region functional form. Instead, we obtain the long-term TSI trend by applying the Empirical Mode Decomposition (EMD) algorithm to the reconstructed $F_{o}$ to filter out the 11-year and 22-year solar variability. We prepare a reconstructed TSI record, spanning 971 to 2020 CE. The estimated departure from modern TSI values occurred during the Sp\"{o}rer Minimum (around 1400 CE), with a decrease of approximately 2.3 $W m^{-2}$. A slightly smaller decline of 2.2 $W m^{-2}$ is reported during the Maunder Minimum, between 1645 and 1715 CE.

astro-ph.SR↗

arXiv2022

A new catalogue of solar flare events from soft x-ray GOES signal in the period 1986-2020

Solar flares, along with other sun-originated events such as Coronal Mass Ejections, fast solar wind streams, and solar energetic particles are among the most relevant events in Space Weather. Moreover, solar flares are the most energetic processes that occur in our solar system. The in-depth study of their occurrence statistics, both over extended periods or during individual solar cycles, allows us to improve and constrain the basic physical models of their origin. Increasing the number of detected events, especially those of lower intensity, and the number of physical parameters that describe the detected flares is, therefore, a mandatory goal. In this paper, we present a computationally efficient algorithm for the detection of solar flares in the soft-X solar flux provided by the GOES (NASA/NOAA) satellite constellation. Our code produces a new flare catalogue increasing the number of events with respect to the official GOES list. In addition to increasing the number of identified events, the catalogue contains information such as: an estimate of the total energy released, start and end time of the event, possible overlap with other events, background level of the GOES X-ray emission close to the revealed event. After a detailed description of the detection algorithm, we carry out a preliminary analysis of the flares reported in our catalogue and compare our results with the official list of GOES for the period from 1998 to 2020.

astro-ph.SR↗

arXiv2022

The exoplanetary magnetosphere extension in Sun-like stars based on the solar wind -- solar UV relation

The Earth's magnetosphere extension is controlled by the solar activity level via solar wind properties. Understanding such a relation in the Solar System is important for predicting also the condition of exoplanetary magnetospheres near Sun-like stars. We use measurements of a chromospheric proxy, the Ca II K index, and solar wind OMNI parameters to connect the solar activity variations, on the decennial time scales, to the solar wind properties. The data span over the time interval 1965-2021, which almost entirely covers the last 5 solar cycles. Using both cross-correlation and mutual information analysis, a 3.2-year lag of the solar wind speed with respect to the Ca II K index is found. Analogously, a 3.6-year lag is found once considering the dynamic pressure. A correlation between the solar wind dynamic pressure and the solar UV emission is found and used to derive the Earth's magnetopause standoff distance. Moreover, the advantage of using a chromospheric proxy, such as the Ca II K index, opens the possibility to extend the relation found for the Sun to Sun-like stars, by linking stellar variability to stellar wind properties. The model is applied to a sample of Sun-like stars as a case study, where we assume the presence of an Earth-like exoplanet at 1 AU. Finally, we compare our results with previous estimates of the magnetosphere extension for the same set of Sun-like stars.

astro-ph.SR↗

arXiv2021

The Great Aurora of 4 February 1872 observed by Angelo Secchi in Rome

Observation of auroras at low latitudes is an extremely rare event typically associated with major magnetic storms due to intense Earth-directed Coronal Mass Ejections. Since these energetic events represent one of the most important components of space weather their study is of paramount importance to understand the Sun-Earth connection. Due to the rarity of these events, being able to access all available information for the few cases studied is equally important. Especially if we refer to historical periods in which current accurate observations from ground-based instruments or from space were not available. Certainly, among these events we must include the great aurora of February 4, 1872. An event whose effects have been observed in different regions of the Earth. What we could consider today a global event, especially for its effects on the communication systems of the time, such as the transatlantic cable that allowed a connection between the United States and Europe since 1866. In this paper we describe the main results of the observations and studies carried out by Angelo Secchi at the Observatory of the Roman College and described in his "Memoria sull'Aurora Elettrica del 4 Febbraio 1872" for the Notes of the Pontifical Academy of new Lincei. This note is extremely modern both in its multi-instrumental approach to the study of these phenomena and in its association between solar-terrestrial connection and technological infrastructures on the Earth. The Secchi's note definitely represents the first example of analysis and study of an event on a global scale, such as the Atlantic cable, affecting the Earth. What we nowadays call an extreme space weather event.

physics.hist-ph↗

arXiv2021

Prediction of sunspot and plage coverage for Solar Cycle 25

Solar variability occurs over a broad range of spatial and temporal scales, from the Sun's brightening over its lifetime to the fluctuations commonly associated with magnetic activity over minutes to years. The latter activity includes most prominently the 11-year sunspot solar cycle and its modulations. Space weather events, in the form of solar flares, solar energetic particles, coronal mass ejections, and geomagnetic storms, have long been known to approximately follow the solar cycle occurring more frequently at solar maximum than solar minimum. These events can significantly impact our advanced technologies and critical infrastructures, making the prediction for the strength of future solar cycles particularly important. Several methods have been proposed to predict the strength of the next solar cycle, cycle 25, with results that are generally not always consistent. Most of these methods are based on the international sunspot number time series, or other indicators of solar activity. We present here a new approach that uses more than 100 years of measured fractional areas of the visible solar disk covered by sunspots and plages and an empirical relationship for each of these two indices of solar activity in even-odd cycles. We anticipate that cycle 25 will peak in 2024 and will last for about 12 years, slightly longer than cycle 24. We also found that, in terms of sunspot and plage areas coverage, the amplitude of cycle 25 will be substantially similar or slightly higher than cycle 24.

astro-ph.SR↗

arXiv2021

Flare Forecasting Algorithms Based on High-Gradient Polarity Inversion Lines in Active Regions

Solar flares emanate from solar active regions hosting complex and strong bipolar magnetic fluxes. Estimating the probability of an active region to flare and defining reliable precursors of intense flares is an extremely challenging task in the space weather field. In this work, we focus on two metrics as flare precursors, the unsigned flux R, tested on MDI/SOHO data and one of the most used parameters for flare forecasting applications, and a novel topological parameter D representing the complexity of a solar active region. More in detail, we propose an algorithm for the computation of the R value which exploits the higher spatial resolution of HMI maps. This algorithm leads to a differently computed R value, whose functionality is tested on a set of cycle 24th solar flares. Furthermore, we introduce a topological parameter based on the automatic recognition of magnetic polarity-inversion lines in identified active regions, and able to evaluate its magnetic topological complexity. We use both a heuristic approach and a supervised machine learning method to validate the effectiveness of these two descriptors to predict the occurrence of X- or M- class flares in a given solar active region during the following 24 hours period. Our feature ranking analysis shows that both parameters play a significant role in prediction performances. Moreover, the analysis demonstrates that the new topological parameter D is the only one, among 173 overall predictors, which is always present for all test subsets and is systematically ranked within the top-ten positions in all tests concerning the computation of the weighs with which each predictor impacts the flare forecasting.

astro-ph.SR↗

arXiv2021

3-D climate simulations for the detectability of Proxima Centauri b

The discovery of a planet orbiting around Proxima Centauri, the closest star to the Sun, opens new avenues for the remote observations of the atmosphere and surface of an exoplanet, Proxima b. To date, three-dimensional (3D) General Circulation Models (GCMs) are the best available tools to investigate the properties of the exo-atmospheres, waiting for the next generation of space and groundbased telescopes. In this work, we use the PlanetSimulator (PlaSim), an intermediate complexity 3D GCM, a flexible and fast model, suited to handle all the orbital and physical parameters of a planet and to study the dynamics of its atmosphere. Assuming an Earth-like atmosphere and a 1:1 spin/orbit configuration (tidal locking), our simulations of Proxima b are consistent with a day-side open ocean planet with a superrotating atmosphere. Moreover, because of the limited representation of the radiative transfer in PlaSim, we compute the spectrum of the exoplanet with an offline Radiative Transfer Code with a spectral resolution of 1 nm. This spectrum is used to derive the thermal phase curves for different orbital inclination angles. In combination with instrumental detection sensitivities, the different thermal phase curves are used to evaluate observation conditions at ground level (e.g., ELT) or in space (e.g., JWST). We estimated the exposure time to detect Proxima b (assuming an Earth-like atmosphere) thermal phase curve in the FIR with JWST with signal-to-noise ratio $\simeq$1. Under the hypothesis of total noise dominated by shot noise, neglecting other possible extra contribution producing a noise floor, the exposure time is equal to 5 hours for each orbital epoch.

astro-ph.EP↗

arXiv2020

A New Method for Detecting Solar Atmospheric Gravity Waves

Internal gravity waves have been observed in the Earth's atmosphere and oceans, on Mars and Jupiter, and in the Sun's atmosphere. Despite ample evidence for the existence of propagating gravity waves in the Sun's atmosphere, we still do not have a full understanding of their characteristics and overall role for the dynamics and energetics of the solar atmosphere. Here we present a new approach to study the propagation of gravity waves in the solar atmosphere. It is based on calculating the three-dimensional cross-correlation function between the vertical velocities measured at different heights. We apply this new method to a time series of co-spatial and co-temporal Doppler images obtained by SOHO/MDI and Hinode/SOT as well as to simulations of upward propagating gravity wave packets. We show some preliminary results and outline future developments.

astro-ph.SR↗

arXiv2020

Chromospheric Heating by Acoustic Waves Compared to Radiative Cooling: II -- Revised Grid of Models

Acoustic and magnetoacoustic waves are considered to be possible agents of chromospheric heating. We present a comparison of deposited acoustic energy flux with total integrated radiative losses in the middle chromosphere of the quiet Sun and a weak plage. The comparison is based on a consistent set of high-resolution observations acquired by the IBIS instrument in the Ca II 854.2 nm line. The deposited acoustic-flux energy is derived from Doppler velocities observed in the line core and a set of 1737 non-LTE 1D hydrostatic semi-empirical models, which also provide the radiative losses. The models are obtained by scaling the temperature and column mass of five initial models VAL B-F to get the best fit of synthetic to observed profiles. We find that the deposited acoustic-flux energy in the quiet-Sun chromosphere balances 30-50 % of the energy released by radiation. In the plage, it contributes by 50-60 % in locations with vertical magnetic field and 70-90 % in regions where the magnetic field is inclined more than 50 degrees to the solar surface normal.

astro-ph.SR↗

arXiv2019

Observed local dispersion relations for magneto-acoustic-gravity waves in the Sun's atmosphere: Mapping the acoustic cut-off frequency

We present the observed local dispersion relations for magneto-acoustic-gravity waves in the Sun's atmosphere for different levels of magnetic field strength. We model these data with a theoretical local dispersion relation to produce spatial maps of the acoustic cut-off frequency in the Sun's photosphere. These maps have implications for the mechanical heating of the Sun's upper atmosphere, by magneto-acoustic-gravity waves, at different phases of the solar magnetic activity cycle.

astro-ph.SR↗

arXiv2018

The correlation of synthetic UV color vs Mg II index along the solar cycle

Modeling of planets' climate and habitability requires as fundamental input the UV emission of the hosting star. \citet{lovric2017} employed SORCE/SOLSTICE solar observations to introduce a UV color index which is a descriptor of the UV radiation that modulates the photochemistry of planets atmospheres. After correcting the SOLSTICE data for residual instrumental effects that produced asymmetric signals during different phases of the cycles analyzed, the authors found that the UV color index is linearly correlated with the Mg II index.In this paper we employ an irradiance reconstruction technique to synthetize the UV color and Mg II index with the purpose of investigating whether the correction applied by \citet{lovric2017} to SORCE/SOLSTICE data might have compensated for solar variations, and to investigate the physical mechanisms that produce such a strong correlation between the UV color index and the solar activity. Reconstructed indices reproduce very well the observations and present the same strong linear dependence. Moreover our reconstruction, which extends back to 1989, shows that the UV color - Mg II index relation can be described by the same linear relation for almost three cycles, thus ruling out an overcompensation of SORCE/SOLTICE data in the analysis of \citet{lovric2017}. We suggest that the strong correlation between the indices results from the fact that most of the Far- and Middle- UV radiation originates in the chromosphere, where atmosphere models of quiet and magnetic features present similar temperature and density gradients.

astro-ph.SR↗

arXiv2018

A Probabilistic Approach to the Drag-Based Model

The forecast of the time of arrival of a coronal mass ejection (CME) to Earth is of critical importance for our high-technology society and for any future manned exploration of the Solar System. As critical as the forecast accuracy is the knowledge of its precision, i.e. the error associated to the estimate. We propose a statistical approach for the computation of the time of arrival using the drag-based model by introducing the probability distributions, rather than exact values, as input parameters, thus allowing the evaluation of the uncertainty on the forecast. We test this approach using a set of CMEs whose transit times are known, and obtain extremely promising results: the average value of the absolute differences between measure and forecast is 9.1h, and half of these residuals are within the estimated errors. These results suggest that this approach deserves further investigation. We are working to realize a real-time implementation which ingests the outputs of automated CME tracking algorithms as inputs to create a database of events useful for a further validation of the approach.

physics.space-ph↗

arXiv2017

JP3D compression of solar data-cubes: photospheric imaging and spectropolarimetry

Hyperspectral imaging is an ubiquitous technique in solar physics observations and the recent advances in solar instrumentation enabled us to acquire and record data at an unprecedented rate. The huge amount of data which will be archived in the upcoming solar observatories press us to compress the data in order to reduce the storage space and transfer times. The correlation present over all dimensions, spatial, temporal and spectral, of solar data-sets suggests the use of a 3D base wavelet decomposition, to achieve higher compression rates. In this work, we evaluate the performance of the recent JPEG2000 Part 10 standard, known as JP3D, for the lossless compression of several types of solar data-cubes. We explore the differences in: a) The compressibility of broad-band or narrow-band time-sequence; I or V stokes profiles in spectropolarimetric data-sets; b) Compressing data in [x,y,$\lambda$] packages at different times or data in [x,y,t] packages of different wavelength; c) Compressing a single large data-cube or several smaller data-cubes; d) Compressing data which is under-sampled or super-sampled with respect to the diffraction cut-off.

astro-ph.IM↗