Conditions for electron-cyclotron maser emission in the solar corona

Context. The Sun is an active source of radio emission ranging from long duration radio bursts associated with solar flares and coronal mass ejections to more complex, short duration radio bursts such as solar S bursts, radio spikes and fibre bursts. While plasma emission is thought to be the dominant emission mechanism for most radio bursts, the electron-cyclotron maser (ECM) mechanism may be responsible for more complex, short-duration bursts as well as fine structures associated with long-duration bursts. Aims. We investigate the conditions for ECM in the solar corona by considering the ratio of the electron plasma frequency {\omega}p to the electron-cyclotron frequency {\Omega}e. The ECM is theoretically possible when {\omega}p/{\Omega}e < 1. Methods. Two-dimensional electron density, magnetic field, plasma frequency, and electron cyclotron frequency maps of the off- limb corona were created using observations from SDO/AIA and SOHO/LASCO, together with potential field extrapolations of the magnetic field. These maps were then used to calculate {\omega}p/{\Omega}e and Alfven velocity maps of the off-limb corona. Results. We found that the condition for ECM emission ({\omega}p/{\Omega}e < 1) is possible at heights < 1.07 R_sun in an active region near the limb; that is, where magnetic field strengths are > 40 G and electron densities are greater than 3x10^8 cm-3. In addition, we found comparatively high Alfv\'en velocities (> 0.02 c or > 6000 km s-1) at heights < 1.07 R_sun within the active region. Conclusions. This demonstrates that the condition for ECM emission is satisfied within areas of the corona containing large magnetic fields, such as the core of a large active region. Therefore, ECM could be a possible emission mechanism for high-frequency radio and microwave bursts.Comment: 4 pages, 3 figure

Radio halos in merging clusters of galaxies

We present the preliminary results of 235 MHz, 327 MHz and 610 MHz observations of the galaxy cluster A3562 in the core of the Shapley Concentration. The purpose of these observations, carried out with the Giant Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo located at the centre of A3562 and determine the shape of its radio spectrum at low frequencies, in order to understand the origin of this source. In the framework of the re--acceleration model, the preliminary analysis of the halo spectrum suggests that we are observing a young source (few $10^8$ yrs) at the beginning of the re--acceleration phase.Comment: 3 pages, 2 figures. Proceedings of IAU Colloquium 195 - Outskirts of Galaxy Clusters: intense life in the suburb

Equality of Corecursive Streams Defined by Finitary Equational Systems

In recent work, non-regular streams have been defined corecursively, by representing them with finitary equational systems built on top of various operators, besides the standard constructor. With finitary equational systems based only on the stream constructor, one can use the free theory of regular (a.k.a. rational) trees to get a sound and complete procedure to decide whether two streams are equal. However, this is not the case if one allows other operators in equations, since the underlying equational theory becomes non-trivial, hence equality of regular trees is too strong to guarantee termination of corecursive functions defined even only with the constructor and tail operators. To overcome this problem, we provide a weaker definition of equality between streams denoted by finitary equational systems built on different stream operators, including tail operator and constructor, and prove its soundness

Seismic vulnerability of Santa Maria Novella Basilica in Florence

This paper presents the evaluation of the seismic vulnerability of Santa Maria Novella Basilica in Florence. Santa Maria Novella is one of the most important historical churches in Italy and, for this reason, different studies on the structural behavior of this monument were conducted during the last decades. Particularly, this work is focused on the dynamic behavior of the church. Mechanical properties of masonries were determined through "in situ" and laboratory tests, according to the National Italian Code (Norme Tecniche per le Costruzioni, 2018). An eigenvalue analysis on a finite element model of the Basilica was performed to obtain the fundamental vibration mode shapes.Finally, to evaluate the seismic risk index in terms of ratio between the minimum peak ground acceleration which leads to the first collapse of a structural element and the design peak ground acceleration, a response spectrum analysis was carried out to evaluate the stress fields in both columns and walls

Experimental investigation on a Fe-Ga close yoke vibrational harvester by matching magnetic and mechanical biases

Abstract The output power generated by a vibrational magnetostrictive energy harvester depends on several parameters, some of them linked to the mechanical source, as vibration amplitude and frequency, others related to design quantities, like mechanical preload, magnetic bias, coil turns and load impedance. Complex models have been developed in literature to reproduce the behavior of these devices. However, for output variables such as power and voltage, one moves in a space of many variables and it is not trivial to reconstruct an overall behavior of the device. The aim of this paper is to provide a wide picture concerning the device behavior investigating experimentally the output power and voltage as a function of the mechanical and especially magnetic bias, varying the amplitude and frequency of the driving vibration. A galfenol rod (Fe 81 Ga 19 ) sample inserted in a three-legged magnetizer is utilized to vary the magnetic bias and to provide the flux closure to the sample, while a dynamic test machine provides both the mechanical bias and the driving vibration at different frequencies up to 100 Hz. The paper analysis has highlighted that the output power and voltage depend on the magnetic bias according to an exponentially modified Gaussian distribution. Keeping constant the other parameters and varying the mechanical bias, a family of modified Gaussian distributions is obtained. Moreover, fixing the electric load, the amplitude and frequency of the vibration, the couple of values "magnetic bias – mechanical preload" corresponding to the maximum output power of the device depicts a linear behavior. The results here obtained point out that it is possible to simplify the design of magnetostrictive energy harvesters and to obtain high output power even with permanent magnets providing a relatively small coercive field. The results have been confirmed by using two yokes equipped with permanent magnets on the external columns. The maximum output average power obtained with permanent magnets has been 796 mW equal to 6.5 mW/cm 3 with a sinusoidal vibration amplitude of 40 MPa at 100 Hz

The influence of the geometrical features on the seismic response of historical churches reinforced by different cross lam roof-solutions

Recent Italian earthquakes have shown the seismic vulnerability of many typical historical masonry churches characterized by one nave and wooden roofs. Under transverse earthquake, the nave transverse response of this kind of churches can be influenced by the geometrical and material features. To increase the seismic performance, strengthening interventions aimed to pursue the global box-behavior by the realization of dissipative roof-structure represent a valid strategy, especially to avoid out-of-plane mechanisms. In this way, the roof structure must be able to represent a tool for the damped rocking of the perimeter walls. Cross-laminated timber (CLT) panels with calibrated metal connections have been recently adopted in experimental tests as a valid solution to obtain a roof-diaphragm with ductile behavior, satisfying the conservative restoration criteria at the same time. In this paper, after a description of the numerical approach for the damped rocking mechanism for one nave configuration church, the effectiveness of different CLT based roof-diaphragms in the nave transverse response is investigated for four historical churches. The seismic responses are performed by comparative dynamic nonlinear analyses and the results are shown in terms of displacements and shear actions transferred to the façade. The influence of the geometrical features of the churches on the nave transversal response is deepened by sensitivity analyses with the aim to predict the displacements and shear variations under the same earthquake excitation

The renovation of Santa Maria di Collemaggio Basilica: the intervention on the nave’s columns

The earthquake of L’Aquila in 2009 caused the partial collapse of the S. Maria di Collemaggio Basilica. The XIII century nave’s columns showed serious cracking throughout their height. Therefore, a retrofitting intervention was planned with the purpose of protecting these valuable elements from future additional damages. The intervention designed involved the complete unloading and reloading of the columns. To avoid additional damages, it was necessary to verify that the procedure would not alter significantly the existing stress state and would not lead to the occurrence of tensile stresses greater than the materials’ tensile strength. Therefore, a 3D FE model of the colonnade and the supported wall has been realized and a construction stage non-linear analysis implemented. The geometry was modeled through the acquisition of the results of a laser scanner survey in order to reflect faithfully the Basilica’s irregularities (lack of verticality of the wall, eccentricities, different dimensions from one column to the other). Moreover, elastic-brittle constitutive laws were assigned to the model’s materials to reproduce the intrinsic non-linearity of the masonry. The analysis results in terms of stresses and displacements were attentively analyzed. Furthermore, a comparison between the numerical results and the values measured during the intervention has been executed. An acceptable deviation between the values was observed and, therefore, the reliability of the model was assured

Enhancing Expressivity of Checked Corecursive Streams

We propose a novel approach to stream definition and manipulation. Our solution is based on two key ideas. Regular corecursion, which avoids non termination by detecting cyclic calls, is enhanced, by allowing in equations defining streams other operators besides the stream constructor. In this way, some non-regular streams are definable. Furthermore, execution includes a runtime check to ensure that the stream generated by a function call is well-defined, in the sense that access to an arbitrary index always succeeds. We extend the technique beyond the simple stream operators considered in previous work, notably by adding an interleaving combinator which has a non-trivial recursion scheme