Phase transitions in a disordered system in and out of equilibrium

The equilibrium and non--equilibrium disorder induced phase transitions are compared in the random-field Ising model (RFIM). We identify in the demagnetized state (DS) the correct non-equilibrium hysteretic counterpart of the T=0 ground state (GS), and present evidence of universality. Numerical simulations in d=3 indicate that exponents and scaling functions coincide, while the location of the critical point differs, as corroborated by exact results for the Bethe lattice. These results are of relevance for optimization, and for the generic question of universality in the presence of disorder.Comment: Accepted for publication in Phys. Rev. Let

Ground state optimization and hysteretic demagnetization: the random-field Ising model

We compare the ground state of the random-field Ising model with Gaussian distributed random fields, with its non-equilibrium hysteretic counterpart, the demagnetized state. This is a low energy state obtained by a sequence of slow magnetic field oscillations with decreasing amplitude. The main concern is how optimized the demagnetized state is with respect to the best-possible ground state. Exact results for the energy in d=1 show that in a paramagnet, with finite spin-spin correlations, there is a significant difference in the energies if the disorder is not so strong that the states are trivially almost alike. We use numerical simulations to better characterize the difference between the ground state and the demagnetized state. For d>=3 the random-field Ising model displays a disorder induced phase transition between a paramagnetic and a ferromagnetic state. The locations of the critical points R_c(DS), R_c(GS) differ for the demagnetized state and ground state. Consequently, it is in this regime that the optimization of the demagnetized stat is the worst whereas both deep in the paramagnetic regime and in the ferromagnetic one the states resemble each other to a great extent. We argue based on the numerics that in d=3 the scaling at the transition is the same in the demagnetized and ground states. This claim is corroborated by the exact solution of the model on the Bethe lattice, where the R_c's are also different.Comment: 13 figs. Submitted to Phys. Rev.

Remote sensing e spatial modelling per strategie di adattamento ai cambiamenti climatici: caso studio Valle Savio

Gli effetti indotti dai cambiamenti climatici (CC) su città e territori stanno ponendo seri interrogativi sulla necessità di definire nuove pratiche di governo del territorio. Sistemi insediativi e naturali sono chiamati ad aggiornare le proprie strategie di resilienza nei confronti di fenomeni come la convergenza spaziale di impatti e vulnerabilità di diversa natura. I domini della vulnerabilità richiedono la definizione di un nuovo paradigma della ‘conoscenza spaziale’ articolabile per diversi contesti ambientali. Il presente contributo guida la possibilità di testare questa ipotesi mediante tecniche di spatial modelling e remote sensing. L’obiettivo è quello di agevolare il riconoscimento delle correlazioni fisico-ambientali tra fattori esogeni ed endogeni, ossia: morfologie del terreno (DTM), hazard climatici (IPCC 2014), indici satellitari (Land Surface Temperature, Normalized Difference Moisture Index, Vegetation Health Index). Il presente contributo si inserisce nel progetto europeo Adriadapt Interreg Italia-Croazia coordinato dal CMCC (Centro Euro-Mediterraneo sui Cambiamenti Climatici) testando la associazione e la correlazione spaziale tra gli impatti Urban flooding (inondazioni urbane), Landslides (frane), Wildfire (rischio di incendio) e Drought (rischio di siccità e desertificazione). Il test è condotto nel territorio dell’Unione Valle del Savio (FC – Emilia-Romagna): area geografica interessata periodicamente da onde di calore, siccità ed eventi atmosferici estremi. I risultati dell’indagine evidenziano una significativa associazione tra variabili VHI e dinamiche idrogeologiche. Il metodo, di aiuto nell’elaborazione di nuove informazioni spaziali, orienta le regioni adriatiche verso nuove pratiche di governo del territorio in chiave climate proof

The Comparative Exploration of the Ice Giant Planets with Twin Spacecraft: Unveiling the History of our Solar System

In the course of the selection of the scientific themes for the second and third L-class missions of the Cosmic Vision 2015-2025 program of the European Space Agency, the exploration of the ice giant planets Uranus and Neptune was defined "a timely milestone, fully appropriate for an L class mission". Among the proposed scientific themes, we presented the scientific case of exploring both planets and their satellites in the framework of a single L-class mission and proposed a mission scenario that could allow to achieve this result. In this work we present an updated and more complete discussion of the scientific rationale and of the mission concept for a comparative exploration of the ice giant planets Uranus and Neptune and of their satellite systems with twin spacecraft. The first goal of comparatively studying these two similar yet extremely different systems is to shed new light on the ancient past of the Solar System and on the processes that shaped its formation and evolution. This, in turn, would reveal whether the Solar System and the very diverse extrasolar systems discovered so far all share a common origin or if different environments and mechanisms were responsible for their formation. A space mission to the ice giants would also open up the possibility to use Uranus and Neptune as templates in the study of one of the most abundant type of extrasolar planets in the galaxy. Finally, such a mission would allow a detailed study of the interplanetary and gravitational environments at a range of distances from the Sun poorly covered by direct exploration, improving the constraints on the fundamental theories of gravitation and on the behaviour of the solar wind and the interplanetary magnetic field.Comment: 29 pages, 4 figures; accepted for publication on the special issue "The outer Solar System X" of the journal Planetary and Space Science. This article presents an updated and expanded discussion of the white paper "The ODINUS Mission Concept" (arXiv:1402.2472) submitted in response to the ESA call for ideas for the scientific themes of the future L2 and L3 space mission

(1) Ceres: Study of Thermal Convection in the Mantle and its Mechanical Effects

Ceres is the largest body of the Main Belt, which is characterized by a huge abundance of water ice in its interior. This feature is suggested by its relatively low bulk density (2162 kg m-3, Russell et al. 2016, Park et al. 2016) and by several geological and geochemical evidences (specific minerals or salts produced by acqueous alteration, icy patches on the surface, lobate morphologies interpretable as surface flows (De Sanctis et al. 2016, Carrozzo et al. 2018, Raponi et al. 2018, Zolotov 2017 and Schmidt et al., 2017).Ceres is partially differentiated as suggested by its normalized moment of inertia, 0.37 (Park et al. 2016). A typical internal structure proposed for Ceres is: a rocky core (300-350 km), an icy (or muddy) mantle (100-150 km) and a rocky crust some kilometers in depth (eg. Mc Cord & Sotin 2005, Neveu & Desch, 2015). The temperature gradient across the mantle, estimated through numerical modelling (e.g. McCord & Sotin 2005, Neveu & Desch 2015) would be large enough to initiate a thermal convection in the mantle. Since the mantle is not uniquely defined from a composition point of view, in this work we explore how the composition and, in particular the "degree" of muddiness of the mantle, can influence the characteristic of thermal convection. We also estimate the thickness of the top conductive boundary layer and the mechanical stress, which can cause its deformation. - De Sanctis, M., et al. (2015) doi:10.1038/nature16172.- Russell, C., et al. (2016), doi:10.1126/science.aaf4219.- Park, R., et al. (2016),Lunar and Planetary Science Conference, vol. 47, p. 1781.- Schmidt, B. E., et al. (2017), doi:doi:10.1038/ngeo2936- Zolotov, M. Y. (2017), doi:https://doi.org/10.1016 j.icarus.2017.06.018.- Carrozzo, F., et al. (2018), Nature, formation and distribution of carbonates on ceres, Science Advances.- Raponi, A., et al. (2018), Variations in the amount of water ice on ceres' surface suggest a seasonal water cycle, Science Advances.- McCord, T., and C. Sotin (2005), doi:10.1029/2004JE002244.- Neveu, M., and S. Desch (2015), Geochemistry, thermal evolution, and cryovolcanism on Ceres with a muddy ice mantle, Geophys. Res. Lett

Earthquake-like dynamics in ultrathin magnetic film

We study the motion of a domain wall on an ultrathin magnetic film using the magneto-optical Kerr effect (MOKE). At tiny magnetic fields, the wall creeps only via thermal activation over the pinning centers present in the sample. Our results show that this creep dynamics is highly intermittent and correlated. A localized instability triggers a cascade, akin to aftershocks following a large earthquake, where the pinned wall undergoes large reorganizations in a compact active region for a few seconds. Surprisingly, the size and shape of these reorganizations display the same scale-free statistics of the depinning avalanches in agreement with the quenched Kardar-Parisi-Zhang universality class.Comment: 5 pages, 4 figure