Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks

Plasma processes close to SNR shocks result in the amplification of magnetic fields and in the acceleration of electrons, injecting them into the diffusive acceleration mechanism. The acceleration of electrons and the B field amplification by the collision of two plasma clouds, each consisting of electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of 25 keV are considered. A quasi-planar shock forms at the front of the dense plasma cloud. It is mediated by a circularly left-hand polarized electromagnetic wave with an electric field component along the guiding magnetic field. Its propagation direction is close to that of the guiding field and orthogonal to the collision boundary. It has a low frequency and a wavelength that equals several times the ion inertial length, which would be indicative of a dispersive Alfven wave close to the ion cyclotron resonance frequency of the left-handed mode (ion whistler), provided that the frequency is appropriate. However, it moves with the super-alfvenic plasma collision speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the magnetic amplitude of this wave to values well in excess of those of the quasi-parallel guiding field and of the filamentation modes results in a quasi-perpendicular shock. We present evidence for the instability of this mode to a four wave interaction. The waves developing upstream of the dense cloud give rise to electron acceleration ahead of the collision boundary. Energy equipartition between the ions and the electrons is established at the shock and the electrons are accelerated to relativistic speeds.Comment: 16 pages, 18 figures, Accepted for publication by Astron & Astrophy

Longitudinal EEG Studies in a Kindred with Lafora Disease

We reviewed 18 EEG studies in four members of a family with the Lafora form of progressive myoclonic epilepsy. Each patient was the product of a consan-guinous marriage and presented as a teenager with progressive seizures, myoclonus, dementia, and ataxia, and had biopsy proven disease. The EEG early in Lafora disease has spike-wave activity resembling that seen in a primary generalized epilepsy; the background slowing is more typical of a secondary generalized epilepsy. With disease progression, there is increased epileptiform activity, and a striking change in the spike-wave complexes, with a marked increase in frequency up to 6–12 Hz, and many more short duration poly spike components. Unlike some other forms of secondarily generalized epilepsy, the EEG in Lafora disease is distinguished by an increased frequency of the spike-wave complexes with disease progression. RESUME Les auteurs ont revu 18 enregistrements EEG pratiquÉs chez 4 membres d'une famille prÉsentant une maladie de Lafora. Chaque patient Était issu d'un manage consanguin et a prÉsentÉ dans adolescence association progressive de crises, d'un myoclonus, d'une dÉmence et d'une ataxie; le diagnostic de la maladie a ÉtÉ confirmÉ par une biopsie. EEG prÉsente, À la phase initiale de la maladie de Lafora, un aspect comparable À celui de Épilepsie gÉnÉralisÉe idiopathique, mais le ralentissement de activitÉ de fond Évoque davantage EEG des Épilepsies gÉnÉi-alisÉes symptomatiques (EGS). Pendant la progression de la maladie, les auteurs ont constatÉ une augmentation de activitÉÉp-ileptique EEG, avec modification importante de la morphologie des PO, qui ont augmentÉ de frÉquence, jusqu'À 6–12 c/s, avec composantes accrues en Polypointes rapides. Contrairement À d'autres formes d'EGS, EEG de la maladie de Lafora se distingue par une augmentation de frÉquence des PO au cours de Évolution. RESUMEN Se revisan 18 estudios de EEG en 4 miembros de una familia con la forma de Lafora de una epilepsÍa mioclÓnica progresiva. Cada paciente fue producto de un matrimonio consanguÍneo y en su adolescencia presentaron ataques progresivos, mioclonÍas, demencia y ataxia, y se les practiceÓ una biopsia que confirmÓ la enfermedad. Los EEGs practicados precozmente en la enfer-medad de Lafora tienen una actividad de punta-onda que semeja la que se observa en epilepsÍa generalizada primaria y la lentifi-caciÓn de la actividad de fondo es mÁs tÍpica de epilepsÍa generalizada sencundaria. A medida que la enfermedad progresa se observÓ un incremento de la actividad epileptiforme y un cambio sorprendente en los complejos punta-onda con marcado incremento de la frecuencia hasta 6–12 Hz. y muchos mÁs compo-nentes de polipuntas de breve duraciÓn. Contrariamente a otras formas de epilepsÍa generalizada secundaria el EEG en la enfermedad de Lafora se caracteriza por un incremento de la frecuencia de los complejos punta-onda a medida que la enfermedad progresa. ZUSAMMENFASSUNG Wir beurteilten 18 EEG-Untersuchungen bei 4 Mitgliedern einer Familie mit der Lafora-Form der progressiven Myoklonus-Epilepsie. Jeder Patient entstammte einer konsanguinen Ehe und wurde als Teenager mit progressiven AnfÄllen, Myoklonus, De-menz und Ataxie auffÄllig; die Diagnose wurde durch Biopsie bestÄtigt. Das EEG im frÜhen Verlauf einer Lafora-Erkrankung zeigt Spike Wave AktivitÄt, die der bei primÄr generalisierter Epilepsie Ähnelt, wÄhrend die HintergrundaktivitÄt mehr fÜr eine sekundÄr generaliserte Epilepsie typisch ist. Mit fortschreitender Erkrankung tritt vermehrt epileptische AktivitÄt und ein deutli-cher Wechsel der Spike Wave Komplexe mit starkem Ansteigen der Frequenz auf 6–12 Hz und vermehrten kurzdauernden Polyspike-Komponenten auf. Im Gegensatz zu anderen Formen sekundÄr generalisierter Epilepsien zeigt das EEG bei der Lafora-Erkrankung mit Erkrankungsfortgang eine zunehmende Frequenz der Spike Wave Komplexe.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65436/1/j.1528-1157.1991.tb05548.x.pd

Remote sensing strategies for lithological mapping of pan African assemblages in arid environments: a case study in Eritrea, NE Africa

Over a 30 year period, when major advances were made in understanding the evolution of the Arabian-Nubian Shield, military conflicts prevented the geological mapping and tectonic interpretation of Eritrea, NE Africa. The indication of small occurrences of ultramafic rocks through ancient work in the NW extremity of the country prompted many workers on regional tectonics to propose the line of the Barka river as a Neoproterozoic (Pan African) suture. Several attempts based on remote sensing data have been considered to resolve about the existence of such structure in this region. These previous approaches, based on Landsat TM band ratios, fail comprehensively in Eritrea both to unequivocally map ultramafic occurrences and to serve as a good basis for lithological mapping, producing inconclusive and divergent overall results. In this paper we attempt to verify and amplify the hypothesis of a suture zone in N W Eritrea, employing a novel remote sensing strategy and detailed field work. Using statistically-selected Landsat TM enhanced false colour composites together with renditions from residual information of favored Principal Components (pseudo-ratios), we demonstrate that many important components of the Pan African Terranes of Eritrea - i.e. ophiolitic assemblages - can be mapped with a high degree of confidence. This remote sensing strategy transcends the problems encountered in previous approaches in Eritrea and can potentially serve as a powerful tool for geological mapping of similar arid terrains elsewhere.Durante um período de cerca de 30 anos, enquanto grandes avanços eram alcançados no entendimento da evolução do Escudo Arábico-Nubiano, conflitos militares impediram que qualquer tipo de mapeamento geológico e interpretação tectónica fossem realizados na Eritrea, região do NE da Africa. A indicação, a partir de trabalhos da década de 50, da ocorrência de rochas ultramáficas na extremidade noroeste do país, levou diversos autores a propor a existência de uma zona de sutura Neoproterozóica (Pan Africana) ao longo do rio Barka. Diversas tentativas no sentido de demonstrar a existência desta zona de sutura na região vêm sendo feitas desde então, todas baseadas exclusivamente em dados de sensoriamento remoto. Entre estes trabalhos prévios, destacam-se aqueles que utilizam-se de variações da técnica de razões de bandas do satélite Landsat TM. Essas técnicas, entretanto, são incapazes de mapear inequivocamente a ocorrência de rochas ultramáficas e de gerar produtos que possam servir como uma boa base para o mapeamento litológico. A comparação entre os resultados obtidos nesses trabalhos mostram incertezas e divergências marcantes. Neste artigo, pretende-se verificar e expandir a hipótese da existência de uma zona de sutura no NW da Eritrea, utilizando uma estratégia de sensoriamento remoto inédita e dados de trabalhos de campo de detalhe. Esta estratégia emprega composições coloridas de bandas do sensor Landsat TM, selecionadas por métodos estatísticos e realçadas espectralmente, em conjunto com composições coloridas geradas a partir da informação residual de Componentes Principais (pseudo-razões), para o mapeamento das principais feições dos terrenos Pan Africanos da Eritrea (i.e. sequências ofiolíticas). A estratégia proposta transcende os problemas encontrados em outras tentativas de mapeamento destas feições e potencialmente pode ser adotada como uma poderosa ferramenta na caracterização de áreas geologicamente similares, em regiões de clima árido

The Density Spike in Cosmic-Ray-Modified Shocks: Formation, Evolution, and Instability

We examine the formation and evolution of the density enhancement (density spike) that appears downstream of strong, cosmic-ray-modified shocks. This feature results from temporary overcompression of the flow by the combined cosmic-ray shock precursor/gas subshock. Formation of the density spike is expected whenever shock modification by cosmic-ray pressure increases strongly. That occurence may be anticipated for newly generated strong shocks or for cosmic-ray-modified shocks encountering a region of higher external density, for example. The predicted mass density within the spike increases with the shock Mach number and with shocks more dominated by cosmic-ray pressure. We find this spike to be linearly unstable under a modified Rayleigh-Taylor instability criterion at the early stage of its formation. We confirm this instability numerically using two independent codes based on the two-fluid model for cosmic-ray transport. These two-dimensional simulations show that the instability grows impulsively at early stages and then slows down as the gradients of total pressure and gas density decrease. Observational discovery of this unstable density spike behind shocks, possibly through radio emission enhanced by the amplified magnetic fields would provide evidence for the existence of strongly cosmic-ray modified shock structures.Comment: 26 pages in Latex and 6 figures. Accepted to Ap

Inverse Compton Emission from Galactic Supernova Remnants: Effect of the Interstellar Radiation Field

The evidence for particle acceleration in supernova shells comes from electrons whose synchrotron emission is observed in radio and X-rays. Recent observations by the HESS instrument reveal that supernova remnants also emit TeV gamma-rays; long awaited experimental evidence that supernova remnants can accelerate cosmic rays up to the ``knee'' energies. Still, uncertainty exists whether these gamma-rays are produced by electrons via inverse Compton scattering or by protons via neutral pion decay. The multi-wavelength spectra of supernova remnants can be fitted with both mechanisms, although a preference is often given to neutral pion decay due to the spectral shape at very high energies. A recent study of the interstellar radiation field indicates that its energy density, especially in the inner Galaxy, is higher than previously thought. In this paper we evaluate the effect of the interstellar radiation field on the inverse Compton emission of electrons accelerated in a supernova remnant located at different distances from the Galactic Centre. We show that contribution of optical and infra-red photons to the inverse Compton emission may exceed the contribution of cosmic microwave background and in some cases broaden the resulted gamma-ray spectrum. Additionally, we show that if a supernova remnant is located close to the Galactic Centre its gamma-ray spectrum will exhibit a ``universal'' cutoff at very high energies due to the Klein-Nishina effect and not due to the cut-off of the electron spectrum. As an example, we apply our calculations to the supernova remnants RX J1713.7-3946 and G0.9+0.1 recently observed by HESS.Comment: 4 pages, 4 figures. Uses emulateapj.cls. Accepted by ApJ

On the mechanism for breaks in the cosmic ray spectrum

The proof of cosmic ray (CR) origin in supernova remnants (SNR) must hinge on full consistency of the CR acceleration theory with the observations; direct proof is impossible because of the orbit stochasticity of CR particles. Recent observations of a number of galactic SNR strongly support the SNR-CR connection in general and the Fermi mechanism of CR acceleration, in particular. However, many SNR expand into weakly ionized dense gases, and so a significant revision of the mechanism is required to fit the data. We argue that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by \emph{exactly one power}. The spectral break is caused by a partial evanescence of Alfven waves that confine particles to the accelerator. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is also calculated. Using the recent Fermi spacecraft observation of the SNR W44 as an example, we demonstrate that the parent proton spectrum is a classical test particle power law $\propto E^{-2}$, steepening to $E^{-3}$ at $E_{br}\approx7GeV$.Comment: APS talk to appear in PoP, 4 figure

Nonlinear Diffusive Shock Acceleration with Magnetic Field Amplification

We introduce a Monte Carlo model of nonlinear diffusive shock acceleration allowing for the generation of large-amplitude magnetic turbulence. The model is the first to include strong wave generation, efficient particle acceleration to relativistic energies in nonrelativistic shocks, and thermal particle injection in an internally self-consistent manner. We find that the upstream magnetic field can be amplified by large factors and show that this amplification depends strongly on the ambient Alfven Mach number. We also show that in the nonlinear model large increases in the magnetic field do not necessarily translate into a large increase in the maximum particle momentum a particular shock can produce, a consequence of high momentum particles diffusing in the shock precursor where the large amplified field converges to the low ambient value. To deal with the field growth rate in the regime of strong fluctuations, we extend to strong turbulence a parameterization that is consistent with the resonant quasi-linear growth rate in the weak turbulence limit. We believe our parameterization spans the maximum and minimum range of the fluctuation growth and, within these limits, we show that the nonlinear shock structure, acceleration efficiency, and thermal particle injection rates depend strongly on the yet to be determined details of wave growth in strongly turbulent fields. The most direct application of our results will be to estimate magnetic fields amplified by strong cosmic-ray modified shocks in supernova remnants.Comment: Accepted in ApJ July 2006, typos corrected in this versio

Nonthermal Emission from a Supernova Remnant in a Molecular Cloud

In evolved supernova remnants (SNRs) interacting with molecular clouds, such as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a forward shock of moderate Mach number, a cooling layer, a dense radiative shell and an interior region filled with hot tenuous plasma is expected. We present a kinetic model of nonthermal electron injection, acceleration and propagation in that environment and find that these SNRs are efficient electron accelerators and sources of hard X- and gamma-ray emission. The energy spectrum of the nonthermal electrons is shaped by the joint action of first and second order Fermi acceleration in a turbulent plasma with substantial Coulomb losses. Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal electrons produce multiwavelength photon spectra in quantitative agreement with the radio and the hard emission observed by ASCA and EGRET from IC 443. We distinguish interclump shock wave emission from molecular clump shock wave emission accounting for a complex structure of molecular cloud. Spatially resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and 3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST would distinguish the contribution of the energetic lepton component to the gamma-rays observed by EGRET.Comment: 14 pages, 4 figure, Astrophysical Journal, v.538, 2000 (in press

Analytic solution for nonlinear shock acceleration in the Bohm limit

The selfconsistent steady state solution for a strong shock, significantly modified by accelerated particles is obtained on the level of a kinetic description, assuming Bohm-type diffusion. The original problem that is commonly formulated in terms of the diffusion-convection equation for the distribution function of energetic particles, coupled with the thermal plasma through the momentum flux continuity equation, is reduced to a nonlinear integral equation in one variable. Its solution provides selfconsistently both the particle spectrum and the structure of the hydrodynamic flow. A critical system parameter governing the acceleration process is found to be $\Lambda = M^{-3/4}\Lambda_1$, where $\Lambda_1 =\eta p_1/mc$, with a suitably normalized injection rate $\eta$, the Mach number M >> 1, and the cut-off momentum $p_1$. We particularly focus on an efficient solution, in which almost all the energy of the flow is converted into a few energetic particles. It was found that (i) for this efficient solution (or, equivalently, for multiple solutions) to exist, the parameter $\zeta =\eta\sqrt{p_0 p_1}/mc$ must exceed a critical value $\zeta_{cr} \sim 1$ ($p_0$ is the injection momentum), (ii) the total shock compression ratio r increases with M and saturates at a level that scales as $ r \propto \Lambda_1 (iii) the downstream power-law spectrum has the universal index q=3.5 over a broad momentum range. (iv) completely smooth shock transitions do not appear in the steady state kinetic description.Comment: 39 pages, 3 PostScript figures, uses aasms4.sty, to appear in Aug. 20, 1997 issue ApJ, vol. 48

Cooperative and competitive behaviour among passengers during the Costa Concordia disaster

Despite the increase in regulation and codes, there is a relatively small body of scientific literature on maritime disasters management, especially in terms of human factors that affect the success of the evacuation and safety procedures. This paper provides an analysis of passengers’ behaviour during the Costa Concordia disaster of 2012, in which 32 people died. We use 49 passengers’ witness statement made available by the court of Grosseto to understand how the evacuation occurred. We examine whether the main factor in reducing the effectiveness of the evacuation procedure was the lack of effective management or the behaviours among evacuees, or a combination of the two. Results of the analysis suggest that passengers reacted with solidarity, helped each other and that such spontaneous and pro-social behaviour possibly contributed to reduce the number of casualties. By contrast, competitive behaviours happened only in relation to specific environmental constraints and were limited to the proximity of safety boats. The deficiencies in command in the Costa Concordia evacuation highlights the need to increase the skills of personnel called to manage an emergency at sea and the need to create ad hoc training programs that consider also unexpected scenarios. Understanding how people (both staff and public) deal with an emergency and the factors that affect their decision is pivotal to help planners to review their strategy, anticipate similar events, and consider all the factors in future plans and regulations. While human error is always a big factor in maritime disaster, its impact can be considered and mitigated with specific procedures and adaptable plans