"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 $(d_e/L)^2$ 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

Bidimensional Tandem Mass Spectrometry for Selective Identification of Nitration Sites in Proteins.

Nitration of protein tyrosine residues is very often regarded as a molecular signal of peroxynitrite formation during development, oxidative stress, and aging. However, protein nitration might also have biological functions comparable to protein phosphorylation, mainly in redox signaling and in signal transduction. The major challenge in the proteomic analysis of nitroproteins is the need to discriminate modified proteins, usually occurring at substoichiometric levels from the large amount of nonmodified proteins. Moreover, precise localization of the nitration site is often required to fully describe the biological process. Existing methodologies essentially rely on immunochemical techniques either using 2D-PAGE fractionation in combination with western blot analyses or exploiting immunoaffinity procedures to selectively capture nitrated proteins. Here we report a totally new approach involving dansyl chloride labeling of the nitration sites that rely on the enormous potential of MSn analysis. The tryptic digest from the entire protein mixture is directly analyzed by MS on a linear ion trap mass spectrometer. Discrimination between nitro- and unmodified peptide is based on two selectivity criteria obtained by combining a precursor ion scan and an MS3 analysis. This new procedure was successfully applied to the identification of 3-nitrotyrosine residues in complex protein mixtures

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

STUDIO DELLA BIRIFRANGENZA DELLE ONDE DI TAGLIO NELL'AREA GEOTERMICA DI LARDERELLO-TRAVALE

RIASSUNTO Negli ultimi decenni, numerosi studi sulla birifrangenza delle onde sismiche di taglio (onde S) hanno contribuito a fornire informazioni utili sul campo di sforzo attivo e sulla deformazione crostale. L’onda di taglio, quando attraversa un mezzo anisotropo, si separa in due componenti a polarizzazione ortogonale, che si propagano con velocità differenti (fast e slow). In particolare, nel caso in cui l'anisotropia sia imputabile alla fratturazione del mezzo di propagazione, la componente più veloce avrà una direzione di polarizzazione parallela a quella delle fratture e quindi dello sforzo massimo orizzontale σh. In generale, l'anisotropia elastica si manifesta nelle registrazioni sismiche con uno sdoppiamento dell'impulso della fase S che, tuttavia, rappresenta un effetto del secondo ordine non sempre facilmente rilevabile. In letteratura sono presenti vari metodi per l’analisi della birifrangenza sismica (Crampin & Gao, 2006). Quello da me utilizzato si basa sulla procedura descritta da Bowman and Ando (1987) ed implementata nel codice di calcolo “Anisomat+” (Piccinini et al., 2013). La procedura utilizza in ingresso le due componenti orizzontali del sismogramma, che vengono ruotate sul piano orizzontale nell'intervallo 0-180°. Per ogni angolo di rotazione, il calcolo della funzione di cross-correlazione permette di stimare la similitudine delle due componenti del moto ed il rispettivo ritardo temporale. Il ritardo fast-slow e la direzione di polarizzazione sono scelti in corrispondenza di quell'angolo di rotazione per il quale è massimizzata la similarità tra le due componenti del moto. Tale metodologia è stata applicata a registrazioni di terremoti localizzati nell'area geotermica di Larderello-Travale. I dati utilizzati provengono da una rete sismica temporanea di tredici stazioni simiche a larga banda, e sono relativi ad un periodo di circa dieci mesi, da Maggio 2012 a Febbraio 2013. Le misure di anisotropia così ottenute sono state messe a confronto con le informazioni geologico-strutturali riportate in letteratura. Per alcune delle stazioni analizzate, la polarizzazione dell'onda fast mostra una direzione prevalente NW-SE, consistente con la direzione media delle fratture a scala regionale, come indicato dai rilievi strutturali disponibili. Ciò nonostante, i parametri dell'anisotropia mostrano una marcata variabilità sia spaziale che temporale, legata alle eterogeneità del campo di sforzo, ed alla presenza di volumi crostali soggetti a rapide variazioni nella pressione dei fluidi. Infine, per identificare i livelli crostali a comportamento maggiormente anisotropo, si è implementata una inversione dei tempi di ritardo tra le onde fast e slow durante il loro tragitto sorgente-ricevitore. Sebbene l’inversione risenta di una non omogenea copertura di raggi alle varie profondità investigate, per molte stazioni si nota che il picco di anisotropia è ben correlabile con la profondità dell'orizzonte K, un riflettore sismico caratteristico dell'area, il cui andamento ricalca approssimativamente quello dell'isoterma di 450°C

Reconnection of Quasi-singular Current Sheets: The "Ideal" Tearing Mode

A strong indication that fast reconnection regimes exist within resistive magnetohydrodynamics was given by the proof that the Sweet-Parker current sheet, maintained by a flow field with an appropriate inflow-outflow structure, could be unstable to a reconnecting instability which grows without bound as the Lundquist number, S, tends to infinity. The requirement of a minimum value for S in order for the plasmoid instability to kick in does little to resolve the paradoxical nature of the result. Here we argue against the realizability of Sweet-Parker current sheets in astrophysical plasmas with very large S by showing that an ''ideal'' tearing mode takes over before current sheets reach such a thickness. While the Sweet-Parker current sheet thickness scales as ∼S {sup –1/2}, the tearing mode becomes effectively ideal when a current sheet collapses to a thickness of the order of ∼S {sup –1/3}, up to 100 times thicker than S {sup –1/2}, when (as happens in many astrophysical environments) S is as large as 10{sup 12}. Such a sheet, while still diffusing over a very long time, is unstable to a tearing mode with multiple x-points: here we detail the characteristics of the instability and discuss how it may help solve themore » flare trigger problem and effectively initiate the turbulent disruption of the sheet.« les