Magnetic-Island Contraction and Particle Acceleration in Simulated Eruptive Solar Flares

The mechanism that accelerates particles to the energies required to produce the observed high-energy impulsive emission in solar flares is not well understood. Drake et al. (2006) proposed a mechanism for accelerating electrons in contracting magnetic islands formed by kinetic reconnection in multi-layered current sheets. We apply these ideas to sunward-moving flux ropes (2.5D magnetic islands) formed during fast reconnection in a simulated eruptive flare. A simple analytic model is used to calculate the energy gain of particles orbiting the field lines of the contracting magnetic islands in our ultrahigh-resolution 2.5D numerical simulation. We find that the estimated energy gains in a single island range up to a factor of five. This is higher than that found by Drake et al. for islands in the terrestrial magnetosphere and at the heliopause, due to strong plasma compression that occurs at the flare current sheet. In order to increase their energy by two orders of magnitude and plausibly account for the observed high-energy flare emission, the electrons must visit multiple contracting islands. This mechanism should produce sporadic emission because island formation is intermittent. Moreover, a large number of particles could be accelerated in each magnetohydrodynamic-scale island, which may explain the inferred rates of energetic-electron production in flares. We conclude that island contraction in the flare current sheet is a promising candidate for electron acceleration in solar eruptions.Comment: Accepted for publication in The Astrophysical Journal (2016

Plant Diversity in an Intensively Cultivated Vineyard Agroecosystem (Langhe, North-West Italy)

In areas of intensive agriculture, wild plant species are confined to field margins, thus they play a role inprotecting biodiversity. The aim of the present study was to assess plant diversity in an area of intensiveviticulture and to evaluate, for the first time, the impact of field margins on vineyard flora biodiversity. Thestudy was conducted in North-West Italy, were five categories of floristic lists in vineyard-margin pairs weresampled and compared. Five margins were identified: grass-covered (A) and bare (B) headlands, small (C)and wide (D) woodlands, and shrub and herbaceous (E) areas. Two hundred and fifty-two taxa were found,although only 19 were widespread. Differences among categories emerged, highlighting the high floristiccomplexity of the sites surrounded by wide wooded areas (D). The findings suggest an influence of marginsize, in addition to margin type, on the floristic richness of the vineyard. Moreover, an inverse relationshipbetween species richness and both the presence of Poaceae and the degree of soil grass coverage emerged.Enhancing biodiversity, at landscape and field level, by the appropriate management of cover crops andecological infrastructures, within and around vineyards, could be a strategy in sustainable viticulture.The increase in plant species richness is not an end in itself, but it might help to promote biodiversity atdifferent trophic levels

Topologically decoherence-protected qubits with trapped ions

We show that trapped ions can be used to simulate a highly symmetrical Hamiltonian with eingenstates naturally protected against local sources of decoherence. This Hamiltonian involves long range coupling between particles and provides a more efficient protection than nearest neighbor models discussed in previous works. Our results open the perspective of experimentally realizing in controlled atomic systems, complex entangled states with decoherence times up to nine orders of magnitude longer than isolated quantum systems.Comment: 4 page

Gas-dynamic shock heating of post-flare loops due to retraction following localized, impulsive reconnection

We present a novel model in which shortening of a magnetic flux tube following localized, three-dimensional reconnection generates strong gas-dynamic shocks around its apex. The shortening releases magnetic energy by progressing away from the reconnection site at the Alfven speed. This launches inward flows along the field lines whose collision creates a pair of gas-dynamic shocks. The shocks raise both the mass density and temperature inside the newly shortened flux tube. Reconnecting field lines whose initial directions differ by more that 100 degrees can produce a concentrated knot of plasma hotter that 20 MK, consistent with observations. In spite of these high temperatures, the shocks convert less than 10% of the liberated magnetic energy into heat - the rest remains as kinetic energy of bulk motion. These gas-dynamic shocks arise only when the reconnection is impulsive and localized in all three dimensions; they are distinct from the slow magnetosonic shocks of the Petschek steady-state reconnection model

Double-lambda microscopic model for entangled light generation by four-wave-mixing

Motivated by recent experiments, we study four-wave-mixing in an atomic double-{\Lambda} system driven by a far-detuned pump. Using the Heisenberg-Langevin formalism, and based on the microscopic properties of the medium, we calculate the classical and quantum properties of seed and conju- gate beams beyond the linear amplifier approximation. A continuous variable approach gives us access to relative-intensity noise spectra that can be directly compared to experiments. Restricting ourselves to the cold-atom regime, we predict the generation of quantum-correlated beams with a relative-intensity noise spectrum well below the standard quantum limit (down to -6 dB). Moreover entanglement between seed and conjugate beams measured by an inseparability down to 0.25 is expected. This work opens the way to the generation of entangled beams by four-wave mixing in a cold atomic sample.Comment: 11 pages, 6 figures, submitted to PR

Road traffic injuries in the province of Grosseto

Introduction. Road traffic injuries constitute a major public health issue. The Province of Grosseto is one of the territories most affected in the Region of Tuscany. The objective of the study, part of the Road Safety Provincial Council?s project, is to describe the epidemiology of the road accidents in order to contribute to the reduction of the burden of deaths and injuries. Methods. The data relative to road accidents occurring in the Province were drawn from the various sources available: Death Certificates (1991-2005), Police Reports (1991-2003), Hospital Discharge Records (1996-2005), Emergency Room visits (2004-2005). Results. On average, each year road accidents cause 30 deaths, at least 530 hospitalizations, and approximately 3,300 Emergency Room visits. The standardized mortality rate (2003-2005, males: 20.6; females: 6.0), the mortality ratio (2003: 34.6 deaths for every 1,000 accidents), and the severity ratio (2003: 1,432 injured for every 1,000 accidents) are higher than regional figures. Discussion. The greater relative number of fatalities, casualties and crashes can be explained by various physical and social environmental factors such as vast flatland, few greater urban settlements, deprived area. The territory specifically demonstrates an accentuated seasonality in August, a month in which a peak in both the number of accidents and their severity is reported, brought about by the intense volume of commuter and transit traffic, and highlighted by the fact that in that same month approximately half of Emergency Room visits concern non-residents. Conclusion. The complexity of the issue, the number of determinant factors involved, and the disproportionately greater impact on the more disadvantaged and vulnerable segments of society require the development of inter-sectoral strategies and the sharing of responsibility among individuals, groups and communities

Spin liquid ground state in a two dimensional non-frustrated spin model

We consider an exchange model describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on the square lattice. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit, and is known to show a spin-Peierls-like dimerization in one dimension. In two dimensions we calculate energy gaps, susceptibilities, and correlation functions with a Green's Function Monte Carlo. We find a finite spin gap and no evidence of any kind of order. We conclude that the ground state is, most likely, a spin liquid of resonating valence bonds.Comment: 4 pages, 4 figures, Revte

Shocks and Thermal Conduction Fronts in Retracting Reconnected Flux Tubes

We present a model for plasma heating produced by time-dependent, spatially localized reconnection within a flare current sheet separating skewed magnetic fields. The reconnection creates flux tubes of new connectivity which subsequently retract at Alfv\'enic speeds from the reconnection site. Heating occurs in gas-dynamic shocks which develop inside these tubes. Here we present generalized thin flux tube equations for the dynamics of reconnected flux tubes, including pressure-driven parallel dynamics as well as temperature dependent, anisotropic viscosity and thermal conductivity. The evolution of tubes embedded in a uniform, skewed magnetic field, following reconnection in a patch, is studied through numerical solutions of these equations, for solar coronal conditions. Even though viscosity and thermal conductivity are negligible in the quiet solar corona, the strong gas-dynamic shocks generated by compressing plasma inside reconnected flux tubes generate large velocity and temperature gradients along the tube, rendering the diffusive processes dominant. They determine the thickness of the shock that evolves up to a steady-state value, although this condition may not be reached in the short times involved in a flare. For realistic solar coronal parameters, this steady-state shock thickness might be as long as the entire flux tube. For strong shocks at low Prandtl numbers, typical of the solar corona, the gas-dynamic shock consists of an isothermal sub-shock where all the compression and cooling occur, preceded by a thermal front where the temperature increases and most of the heating occurs. We estimate the length of each of these sub-regions and the speed of their propagation.Comment: 39 pages (AASTeX: 29 pages of text, 10 figures), accepted for publication in the Astrophysical Journa