Implications of solar flare hard X-ray "knee" spectra observed by RHESSI

We analyse the RHESSI photon spectra of four flares that exhibit significant deviations from power laws - i.e. changes in the "local" Hard X-ray spectral index. These spectra are characterised by two regions of constant power law index connected by a region of changing spectral index - the "knee". We develop theoretical and numerical methods of describing such knees in terms of variable photon spectral indices and we study the results of their inversions for source mean thin target and collisional thick target injection electron spectra. We show that a particularly sharp knee can produce unphysical negative values in the electron spectra, and we derive inequalities that can be used to test for this without the need for an inversion to be performed. Such unphysical features would indicate that source model assumptions were being violated, particularly strongly for the collisional thick target model which assumes a specific form for electron energy loss. For all four flares considered here we find that the knees do not correspond to unphysical electron spectra. In the three flares that have downward knees we conclude that the knee can be explained in terms of transport effects through a region of non-uniform ionisation. In the other flare, which has an upward knee, we conclude that it is most likely a feature of the accelerated spectrum

Energy saving in typical architecture: The flow energy in traditional solutions in a sustainable perspective

Quick urbanization increment causes a few difficult problems, such as social assessment, more energy demands, and pollution increase. A positive factor for a city is the concentration of energy requests. On the other hand, urbanization trend is going to fragmentation of settlements, with a consequent expansion of the energy distribution networks but also an increase of the energy wasted. Each building needs some of the total energy distributed in the city. Energy needs can be grouped into four essential parameters: construction, heating, electricity, and water. How were these four parameters satisfied in the XIX century, before the industrial revolution? In those days' energy requirement wasn't so high and could be supplied with basic energy production technology. The knowledge of appropriate building technology (project), the use of (energy saving) materials, with a low and punctual heating system (fireplace, stove) or passive refreshment all intervened in this process. These requirements were achieved in each country using building plans studied to face different weather conditions with different architectural typology and with human effort and time (work-energy). This paper analyses an energy balance in a single typical building in the center of Italy, describing the energy flow that will show the logical and technical solution for "energy-saving". Old buildings (before the introduction of "building-plant") were always designed and built with an energy-saving concern. This article wants to propose the study of a typical building in which are used only low-energy systems to meet all the comfort requirements, to demonstrate that it is not necessary to use high-energy technology. This is how architecture urbanization studies and implementation can be used to reduce high-energy production needs. © 2019 Author(s)

Feature augmentation for the inversion of the Fourier transform with limited data

We investigate an interpolation/extrapolation method that, given scattered observations of the Fourier transform, approximates its inverse. The interpolation algorithm takes advantage of modeling the available data via a shape-driven interpolation based on variably scaled Kernels (VSKs), whose implementation is here tailored for inverse problems. The so-constructed interpolants are used as inputs for a standard iterative inversion scheme. After providing theoretical results concerning the spectrum of the VSK collocation matrix, we test the method on astrophysical imaging benchmarks

Electron-Electron Bremsstrahlung Emission and the Inference of Electron Flux Spectra in Solar Flares

Although both electron-ion and electron-electron bremsstrahlung contribute to the hard X-ray emission from solar flares, the latter is normally ignored. Such an omission is not justified at electron (and photon) energies above $\sim 300$ keV, and inclusion of the additional electron-electron bremsstrahlung in general makes the electron spectrum required to produce a given hard X-ray spectrum steeper at high energies. Unlike electron-ion bremsstrahlung, electron-electron bremsstrahlung cannot produce photons of all energies up to the maximum electron energy involved. The maximum possible photon energy depends on the angle between the direction of the emitting electron and the emitted photon, and this suggests a diagnostic for an upper cutoff energy and/or for the degree of beaming of the accelerated electrons. We analyze the large event of January 17, 2005 observed by RHESSI and show that the upward break around 400 keV in the observed hard X-ray spectrum is naturally accounted for by the inclusion of electron-electron bremsstrahlung. Indeed, the mean source electron spectrum recovered through a regularized inversion of the hard X-ray spectrum, using a cross-section that includes both electron-ion and electron-electron terms, has a relatively constant spectral index $\delta$ over the range from electron kinetic energy $E = 200$ keV to $E = 1$ MeV. However, the level of detail discernible in the recovered electron spectrum is not sufficient to determine whether or not any upper cutoff energy exists.Comment: 7 pages, 5 figures, submitted to Astrophysical Journa

Principles at the basis of the denorms round robin test on the low frequency sound absorption measurements in reverberation rooms and impedance tube

In recent years low frequency measurements in building and room acoustics fields gained attention. Moreover the low frequency sound absorption coefficient (below 100 Hz) has always been difficult to determine and the results coming from different laboratories cannot always be compared. This paper describes the principles at the basis of the round robin test carried out in the framework of the DENORMS cost action (Designs for Noise Reducing Materials and Structures). The same samples have been measured in reverberation rooms and in impedance tube by the different laboratories participating to the Round Robin Test.status: accepte

Effect of local stiffeners and warping constraints on the buckling of symmetric open thin-walled beams with high warping stiffness

Local stiffeners affect the behaviour of thin-walled beams (TWBs). An in-house code based on a one-dimensional model proved effective in several instances of compressive buckling of TWBs but gave counterintuitive results for locally stiffened TWBs. To clarify the matter, we investigated TWBs with multi-symmetric double I cross-section, widely used in practical applications where high bending stiffness is required. Several samples were manufactured and stiffened on purpose, closing them over a small portion of the axis at different places. The samples were tested with end constraints accounting for various warping conditions. The experimental and numerical outputs from a commercial FEM code gave a key to overcome the unexpected results by the in-house code, paving the way for further studies

Impedance tube measurements on the denorms round robin test material samples

The DENORMS Round Robin Test aims to study and improve the techniques used to determine the sound absorption coefficient of materials. Within this framework, samples taken from the same batch have been tested in both reverberation room and impedance tube. The latter technique presents some challenges in that the results may strongly depend on the specimen preparation and set-up. The purpose of this paper is to present the results obtained in custom-made impedance tube for the RRT set of materials. The tests have been carried out using a four-microphone impedance tube, according to the method outlined in the ASTM E2611-09 standard. The results obtained for the different parameters provided by the test, in particular the sound absorption coefficient and the sound transmission loss, are taken into account and discussed

The DENORMS Round Robin test: Measurement procedure and post-processing of time data

The DENORMS Round Robin Test (RRT) is intended to study and improve the techniques used for the determination of the sound absorption coefficient of materials, with particular focus on the low-frequency range and on measurements in reverberation rooms. It is well known that the main reason why it is difficult to extend the frequency range of interest below 100 Hz is the low modal density. The modal behavior of a room is dependent on its geometry and this is one of the reasons why a discrepancy of the results coming from different laboratories can be found even if the same material is tested. This paper describes the measurement procedure developed to allow uniform acquisition and post-processing of acoustic response data of laboratories participating in the RRT, with and without absorbing materials inside. The tests and the post-processing operations performed on the measured data are also discussed in the paper

Slip distribution inversion by trans-dimensional Monte Carlo sampling: application to the 2009 L’Aquila Earthquake (Central Italy)

Non-uniform slip distribution on a fault plane from geodetic data is usually estimated in two steps. First, the geometric fault parameters are inferred by non -linear inversion assuming a uniform slip on a rectangular fault. A second analysis, based on linear inversion techniques, infers the slip distribution on an arbitrary subdivision of the fault plane into patches. Two main concerns arise. First, the fault geometry determined under the assumption of a uniform slip i s not guaranteed to properly represent the fault geometry for a spatially variable slip distribution. Moreover, an arbitrary fault subdivision into patches u nrelated to the observed data could bias the model resolution, introducing spurious features. In recent years, the availability of large coverage data, such as DInSAR images, improved mapping the coseismic displacements. The large amount of geodetic da ta from the area surrounding earthquake faults allows for improving the slip models and refining the knowledge of earthquake dynamics. Less attention has been given to the development of new inversion algorithms that can resolve the main concerns above. In particular, the question is whether the data themselves ca n constrain the slip model complexity, i.e., the unknown number and distribution of the fault patches needed to fit the observations. The reversible jump Mar kov chain Monte Carlo (RJMCMC) algorithm has been recently introduced in the geosciences to solve a variety of non linear inverse problems. RJMCMC combines a classical Markov chain Monte Carlo method with the ability to shift between models with a different number of unknowns. A posterior probability distribution of the num ber of unknowns is obtained at the end of the Markov chain, so that the model resolution is determined by the observed data. In this study, we apply a RJMCMC method to the Mw 6.3 L’Aquila earthquake that occurred on April 6th 2009 in Central Italy. Three DInSAR images, mapping the c oseismic displacement, are inverted to constrain not only the slip distribution but also the number of unknowns (i.e., the number of fault patches) and the ge ometry of non-rectangular patches

Sound radiation and sound insulation performances of maritime bulkheads

The research of materials matching low weight and high resistance has always been a key factor in the shipbuilding industry to increase performances and loading capacity. Nowadays, other issues add up to economical convenience, and building quiet ships is important not only for passengers and cabin crew, but also to make harbor areas more comfortable and to respect the aquatic environment. In this context, using sandwich or composite materials must be carefully evaluated and the sound insulation performances must be considered throughout all stages of the design process. This work presents some evaluations about the sound insulation performances of a ribbed fiberglass bulkhead and of a balsa-core sandwich bulkhead. In particular, the bending stiffness and the sound transmission loss obtained by sound transmission suites and mobility measurements are provided. From such measurements it has also been possible to determine the radiation efficiency of the structures, whose optimization is particularly important when a reduction of the noise pollution is required