1,978 research outputs found
Fast reconnection in relativistic plasmas: the magnetohydrodynamics tearing instability revisited
Fast reconnection operating in magnetically dominated plasmas is often
invoked in models for magnetar giant flares, for magnetic dissipation in pulsar
winds, or to explain the gamma-ray flares observed in the Crab nebula, hence
its investigation is of paramount importance in high-energy astrophysics. Here
we study, by means of two dimensional numerical simulations, the linear phase
and the subsequent nonlinear evolution of the tearing instability within the
framework of relativistic resistive magnetohydrodynamics, as appropriate in
situations where the Alfven velocity approaches the speed of light. It is found
that the linear phase of the instability closely matches the analysis in
classical MHD, where the growth rate scales with the Lundquist number S as
S^-1/2, with the only exception of an enhanced inertial term due to the thermal
and magnetic energy contributions. In addition, when thin current sheets of
inverse aspect ratio scaling as S^-1/3 are considered, the so-called "ideal"
tearing regime is retrieved, with modes growing independently on S and
extremely fast, on only a few light crossing times of the sheet length. The
overall growth of fluctuations is seen to solely depend on the value of the
background Alfven velocity. In the fully nonlinear stage we observe an inverse
cascade towards the fundamental mode, with Petschek-type supersonic jets
propagating at the external Alfven speed from the X-point, and a fast
reconnection rate at the predicted value R~(ln S)^-1.Comment: 14 pages, 9 figures, accepted for publication (MNRAS
Activation of MHD reconnection on ideal timescales
Magnetic reconnection in laboratory, space and astrophysical plasmas is often
invoked to explain explosive energy release and particle acceleration. However,
the timescales involved in classical models within the macroscopic MHD regime
are far too slow to match the observations. Here we revisit the tearing
instability by performing visco-resistive two-dimensional numerical simulations
of the evolution of thin current sheets, for a variety of initial
configurations and of values of the Lunquist number , up to . Results
confirm that when the critical aspect ratio of is reached in the
reconnecting current sheets, the instability proceeds on ideal (Alfv\'enic)
macroscopic timescales, as required to explain observations. Moreover, the same
scaling is seen to apply also to the local, secondary reconnection events
triggered during the nonlinear phase of the tearing instability, thus
accelerating the cascading process to increasingly smaller spatial and temporal
scales. The process appears to be robust, as the predicted scaling is measured
both in inviscid simulations and when using a Prandtl number in the
viscous regime.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
Can Gravity Distinguish Between Dirac and Majorana Neutrinos?
We show that spin-gravity interaction can distinguish between Dirac and
Majorana neutrino wave packets propagating in a Lense-Thirring background.
Using time-independent perturbation theory and gravitational phase to generate
a perturbation Hamiltonian with spin-gravity coupling, we show that the
associated matrix element for the Majorana neutrino differs significantly from
its Dirac counterpart. This difference can be demonstrated through significant
gravitational corrections to the neutrino oscillation length for a two-flavour
system, as shown explicitly for SN1987A.Comment: 4 pages, 2 figures; minor changes of text; typo corrected; accepted
in Physical Review Letter
BANK EROSION AND INSTABILITY MONITORING WITH A LOW COST TERRESTRIAL LASER SCANNER
ABSTRACT: Among the dominant processes taking place in a river basin, especially mountain ones, sediments
creation and transport play a key role in morphological processes. Studies usually focus on big mass movements,
such as landslides and debris flows, or on wide spread slope erosion due to rainfalls, while bank erosion is
neglected or not considered essential for sediment budget at basin scale. Nevertheless, authors consider bank
erosion a process that deserve more careful studies; not only the sediment share from bank erosion is not
negligible in steep mountain rivers, but also the process can threat structures on river sides due the possibility to
have limited, but still significant, mass collapse of bank sections during intense events. The paper present an
attempt to monitor bank erosion in a section of a river in Northern Italy Alps and to put it in relation with
weather and water discharge. Survey campaign was set up at regular time intervals, or after particularly intense
rainfalls, and uses a Terrestrial Laser Scanner (TLS) to acquire the bank surface. The tool was developed
internally, at Politecnico di Milano, to meet requirements about low cost level and good accuracy. Successive
acquisitions of point clouds were elaborated, via an ad-hoc MatLab code, to determine erosion, or deposition,
volumes of sediments. These volumetric results have been evaluated in relation with rainfalls and freeze-thaw
cycles looking for a relationship between environmental conditions and bank failures. Some interesting results
are shown, such as a relation between erosion rates and temperature or water flow in the river. The path to a
complete process understanding and modelling is long, however the results reported can be considered a first
step towards objective
Orographic Precipitation Extremes: An Application of LUME (Linear Upslope Model Extension) over the Alps and Apennines in Italy
Critical hydrometeorological events are generally triggered by heavy precipitation. In complex terrain, precipitation may be perturbed by the upslope raising of the incoming humid airflow, causing in some cases extreme rainfall. In this work, the application of LUME-Linear Upslope Model Extension-to a group of extreme events that occurred across mountainous areas of the Central Alps and Apennines in Italy is presented. Based on the previous version, the model has been "extended" in some aspects, proposing a methodology for physically estimating the time-delay coefficients as a function of precipitation efficiency. The outcomes of LUME are encouraging for the cases studied, revealing the intensification of precipitation due to the orographic effect. A comparison between the reference rain gauge data and the results of the simulations showed good agreement. Since extreme precipitation is expected to increase due to climate change, especially across the Mediterranean region, LUME represents an effective tool to investigate more closely how these extreme phenomena originate and evolve in mountainous areas that are subject to potential hydrometeorological risks
Pseudohermitian Hamiltonians, time-reversal invariance and Kramers degeneracy
A necessary and sufficient condition in order that a (diagonalizable)
pseudohermitian operator admits an antilinear symmetry T such that T^{2}=-1 is
proven. This result can be used as a quick test on the T-invariance properties
of pseudohermitian Hamiltonians, and such test is indeed applied, as an
example, to the Mashhoon-Papini Hamiltonian.Comment: 6 page
IMAGE-BASED RECONSTRUCTION AND ANALYSIS OF DYNAMIC SCENES IN A LANDSLIDE SIMULATION FACILITY
The application of image processing and photogrammetric techniques to dynamic reconstruction of landslide simulations in a scaled-down facility is described. Simulations are also used here for active-learning purpose: students are helped understand how physical processes happen and which kinds of observations may be obtained from a sensor network. In particular, the use of digital images to obtain multi-temporal information is presented. On one side, using a multi-view sensor set up based on four synchronized GoPro 4 Black® cameras, a 4D (3D spatial position and time) reconstruction of the dynamic scene is obtained through the composition of several 3D models obtained from dense image matching. The final textured 4D model allows one to revisit in dynamic and interactive mode a completed experiment at any time. On the other side, a digital image correlation (DIC) technique has been used to track surface point displacements from the image sequence obtained from the camera in front of the simulation facility. While the 4D model may provide a qualitative description and documentation of the experiment running, DIC analysis output quantitative information such as local point displacements and velocities, to be related to physical processes and to other observations. All the hardware and software equipment adopted for the photogrammetric reconstruction has been based on low-cost and open-source solutions
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