224 research outputs found
"Ideal" tearing and the transition to fast reconnection in the weakly collisional MHD and EMHD regimes
This paper discusses the transition to fast growth of the tearing instability
in thin current sheets in the collisionless limit where electron inertia drives
the reconnection process. It has been previously suggested that in resistive
MHD there is a natural maximum aspect ratio (ratio of sheet length and breadth
to thickness) which may be reached for current sheets with a macroscopic length
L, the limit being provided by the fact that the tearing mode growth time
becomes of the same order as the Alfv\`en time calculated on the macroscopic
scale (Pucci and Velli (2014)). For current sheets with a smaller aspect ratio
than critical the normalized growth rate tends to zero with increasing
Lundquist number S, while for current sheets with an aspect ratio greater than
critical the growth rate diverges with S. Here we carry out a similar analysis
but with electron inertia as the term violating magnetic flux conservation:
previously found scalings of critical current sheet aspect ratios with the
Lundquist number are generalized to include the dependence on the ratio
where de is the electron skin depth, and it is shown that there are
limiting scalings which, as in the resistive case, result in reconnecting modes
growing on ideal time scales. Finite Larmor Radius effects are then included
and the rescaling argument at the basis of "ideal" reconnection is proposed to
explain secondary fast reconnection regimes naturally appearing in numerical
simulations of current sheet evolution.Comment: 15 pages, 3 Figures, 1 Tabl
Dynamic evolution of current sheets, ideal tearing, plasmoid formation and generalized fractal reconnection scaling relations
Magnetic reconnection may be the fundamental process allowing energy stored
in magnetic fields to be released abruptly, solar flares and coronal mass
ejection (CME) being archetypal natural plasma examples. Magnetic reconnection
is much too slow a process to be efficient on the large scales, but accelerates
once small enough scales are formed in the system. For this reason, the fractal
reconnection scenario was introduced (Shibata and Tanuma 2001) to explain
explosive events in the solar atmosphere: it was based on the recursive
triggering and collapse via tearing instability of a current sheet originally
thinned during the rise of a filament in the solar corona. Here we compare the
different fractal reconnection scenarios that have been proposed, and derive
generalized scaling relations for the recursive triggering of fast, `ideal' -
i.e. Lundquist number independent - tearing in collapsing current sheet
configurations with arbitrary current profile shapes. An important result is
that the Sweet-Parker scaling with Lundquist number, if interpreted as the
aspect ratio of the singular layer in an ideally unstable sheet, is universal
and does not depend on the details of the current profile in the sheet. Such a
scaling however must not be interpreted in terms of stationary reconnection,
rather it defines a step in the accelerating sequence of events of the ideal
tearing mediated fractal cascade. We calculate scalings for the expected number
of plasmoids for such generic profiles and realistic Lundquist numbers.Comment: 11 pages, 2 figure
Identification of proteinaceous binders in paintings: A targeted proteomic approach for cultural heritage
Abstract Identification of proteins in paintings and polychrome objects is a challenge, which requires the development of tailored analytical approaches. In the present study, a targeted proteomics approach was developed for discriminating among the three most common proteinaceous materials used as paint binders, i.e. milk, egg, and animal glue. In this study a specific database of peptides was created based on tandem MS analyses of tryptic digests of several paint samples collected from a variety of art objects of different ages and conservation conditions. Specific peptide markers of each protein were then selected and monitored by LC-MSMS in Multiple Reaction Monitoring (MRM) ion mode, together with their specific precursor ion-product ion transitions, as defined by their unique amino acid sequence. The developed method enabled a sensitive and reliable detection of the target peptides in a selection of case studies, leading to the unambiguous identification of the proteins used as paint binders. The method showed greatly increased sensitivity compared to currently available strategies
La mobilità pendolare in Lombardia
Analisi dei flussi pendolari in Lombardi
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