Theory of "Jitter" Radiation from Small-Scale Random Magnetic Fields and Prompt Emission from Gamma-Ray Burst Shocks

Abridged.-- We demonstrate that the radiation emitted by ultrarelativistic electrons in highly nonuniform, small-scale magnetic fields is different from synchrotron radiation if the electron's transverse deflections in these fields are much smaller than the beaming angle. A quantitative analytical theory of this radiation, which we refer to as jitter radiation, is developed. It is shown that the emergent spectrum is determined by statistical properties of the magnetic field. As an example,we then use the model of a magnetic field in internal shocks of GRBs. The spectral power distribution of radiation produced by the power-law electrons is well described by a sharply broken power-law with indices 1 and -(p-1)/2 and the jitter break frequency is independent of the field strength but depends on the electron density in the ejecta. Since large-scale fields may also be present in the ejecta, we construct a two-component, jitter+synchrotron spectral model of the prompt $\gamma$-ray emission. Quite surprisingly, this model seems to be readily capable of explaining several properties of time-resolved spectra of some GRBs, such as (i) the violation of the constraint on the low-energy spectral index called the synchrotron ``line of death'', (ii) the sharp spectral break at the peak frequency, inconsistent with the broad synchrotron bump, (iii) the evidence for two spectral sub-components, and (iv) possible existence of emission features called ``GRB lines''. We believe these facts strongly support both the existence of small-scale magnetic fields and the proposed radiation mechanism from GRB shocks. As an example, we use the composite model to analyze GRB 910503 which has two spectral peaks.Comment: 12 pages (emulateapj), 11 figures (EPS), ApJ, accepted. For related work, see http://cfa-www.harvard.edu/~mmedved

Exotic Halophila stipulacea is an introduced carbon sink for the Eastern Mediterranean Sea

Carbon and nitrogen storage in exotic Halophila stipulacea were compared to that in native Posidonia oceanica and Cymodocea nodosa meadows and adjacent unvegetated sediments of the Eastern Mediterranean Sea and to that in native H. stipulacea of the Red Sea at sites with different biogeochemical conditions and level of human pressure. Exotic H. stipulacea possessed considerable storing capacity, with 2-fold higher Corg stock (0.71 ± 0.05 kg m−2 in the top 20 cm of sediment) and burial (14.78 gCorg m−2 y−1) than unvegetated areas and C. nodosa meadows and, surprisingly, comparable to P. oceanica. N (0.07 ± 0.01 kg m−2) and Cinorg (14.06 ± 8.02 kg m−2) stocks were similar between H. stipulacea and C. nodosa or unvegetated sediments, but different to P. oceanica. Corg and N stocks were higher in exotic than native H. stipulacea populations. Based on isotopic mixing model, organic material trapped in H. stipulacea sediments was mostly allochthonous (seagrass detritus 17% vs seston 67%). Corg stock was similar between monospecific and invaded C. nodosa meadows by H. stipulacea. Higher stocks were measured in the higher human pressure site. H. stipulacea introduction may contribute in the increase of carbon sequestration in the Eastern Mediterranean

Signs of low frequency dispersions in disordered binary dielectric mixtures (50-50)

Dielectric relaxation in disordered dielectric mixtures are presented by emphasizing the interfacial polarization. The obtained results coincide with and cause confusion with those of the low frequency dispersion behavior. The considered systems are composed of two phases on two-dimensional square and triangular topological networks. We use the finite element method to calculate the effective dielectric permittivities of randomly generated structures. The dielectric relaxation phenomena together with the dielectric permittivity values at constant frequencies are investigated, and significant differences of the square and triangular topologies are observed. The frequency dependent properties of some of the generated structures are examined. We conclude that the topological disorder may lead to the normal or anomalous low frequency dispersion if the electrical properties of the phases are chosen properly, such that for ``slightly'' {\em reciprocal mixture}--when $\sigma_1\gg\sigma_2$, and $\epsilon_1<\epsilon_2$--normal, and while for ``extreme'' {\em reciprocal mixture}--when $\sigma_1\gg\sigma_2$, and $\epsilon_1\ll\epsilon_2$--anomalous low frequency dispersions are obtained. Finally, comparison with experimental data indicates that one can obtain valuable information from simulations when the material properties of the constituents are not available and of importance.Comment: 13 pages, 7 figure

PROBABILISTIC EVALUATION OF LIQUEFACTION-INDUCED GROUND FAILURES TRIGGERED BY SEISMIC LOADING IN URBAN ENVIRONMENT; CASE STUDIES FROM GREECE

An issue that should be taken into account by urban planners in Greece, for the extension of urban areas, is the likelihood of earthquake-induced ground deformations. In this paper, information is provided regarding the methodology that is applied for the prediction of liquefaction manifestations in urban environment. In particular, a review of our research concerning the evaluation of the probability of liquefaction-induced ground disruption in three towns, Larissa, Edessa and Lefkada, is presented. The evaluation of probability was achieved using the Liquefaction Potential Index methodology and the relatively classification, introduced in late 80’s and modified by Papathanassiou (2008)

Amplitude measurements of Faraday waves

A light reflection technique is used to measure quantitatively the surface elevation of Faraday waves. The performed measurements cover a wide parameter range of driving frequencies and sample viscosities. In the capillary wave regime the bifurcation diagrams exhibit a frequency independent scaling proportional to the wavelength. We also provide numerical simulations of the full Navier-Stokes equations, which are in quantitative agreement up to supercritical drive amplitudes of 20%. The validity of an existing perturbation analysis is found to be limited to 2.5% overcriticaly.Comment: 7 figure

Applying the INQUA scale to the Sofades 1954, Central Greece, earthquake

Οι μακροσκοπικές κλίμακες έντασης ενός σεισμού έχουν χρησιμοποιηθεί για την εκτίμηση του μεγέθους του, βασιζόμενες στην καταγραφή της επίδρασης την εδαφικής κίνησης στην συμπεριφορά του ανθρώπου και τις μεταβολές που προκαλούνται στο φυσικό και τεχνητό περιβάλλον. Η προτεινόμενη, από την επιτροπή της Παλαιοσεισμολογίας, κλίμακα INQUA λαμβάνει υπόψη της, κατά την εκτίμηση της σεισμικής έντασης, μονάχα τις εδαφικές παραμορφώσεις και μεταβολές τις οποίες προκάλεσε η σεισμική δόνηση. Fia τον έλεγχο της ακρίβειας και της αξιοπιστίας της INQUA, αποφασίστηκε η εφαρμογή της κλίμακας στον σεισμό των Σοφάδων το 1954. Από τη σύγκριση των τιμών της έντασης INQUA με τις αντίστοιχες τιμές της κλίμακας MM, καταλήξαμε στο συμπέρασμα ότι στην περίπτωση σεισμών οι οποίοι προκαλούν έντονα δευτερογενή φαινόμενα, οι τιμές των εντάσεων είναι περίπου ίσες. Παρόλα αυτά, προτείνεται η χρησιμοποίηση της κλίμακας INQUA, σε συνδυασμό με τις υπάρχουσες, καθώς δίνει έμφαση στις γεωλογικές παραμορφώσεις, αποφεύγοντας να χρησιμοποιήσει δεδομένα τα οποία σχετίζονται με την μεταβαλλόμενη από θέση σε θέση τρωτότητα των κατασκευώνMacroseismic intensity scales are used in order to measure the size of an earthquake using the impact of the ground shaking on humans, man made environment and nature. The INQUA scale is a new scale based solely on the earthquake-induced ground deformations, proposed by the INQUA Subcommission on Paleoseismicity. This scale is applied to the Sofades 1954 earthquake in order to test its accuracy and reliability. From the comparison among the evaluated intensities based on MM scale with the degrees of INQUA intensity at several locations, we conclude that, in case of earthquakes which triggered remarkable geological effects, these intensity values are about the same. Nonetheless, the use of INQUA scale is suggested in combination with the existing ones, as an assessing tool of the intensity based only on geological effect

Using collective intelligence to identify barriers to teaching 12–19 year olds about the ocean in Europe

Since the degradation of the marine environment is strongly linked to human activities, having citizens who appreciate the ocean's influence on them and their influence on the ocean is important. Research has shown that citizens have a limited understanding of the ocean and it is this lack of ocean literacy that needs to change. This study maps the European landscape of barriers to teaching 12–19 year olds about the ocean, through the application of Collective Intelligence, a facilitation and problem solving methodology. The paper presents a meta-analysis of the 657 barriers to teaching about the ocean, highlighting how these barriers are interconnected and influence one another in a European Influence Map. The influence map shows 8 themes: Awareness and Perceived knowledge; Policies and Strategies; Engagement, formal education sector; the Ocean itself; Collaboration; Connections between humans and the ocean and the Blue Economy, having the greatest influence and impact on marine education. “Awareness and Perceived knowledge” in Stage 1, exerts the highest level of overall influence in teaching 12–19 year olds about the ocean. This map and study serves as a roadmap for policy makers to implement mobilisation actions that could mitigate the barriers to teaching about the ocean. Examples of such actions include free marine education learning resources such as e-books, virtual laboratories or hands-on experiments. Thus, supporting educators in taking on the challenge of helping our youth realise that the ocean supports life on Earth is essential for education, the marine and human well-being