Shear band formation in granular media as a variational problem

Strain in sheared dense granular material is often localized in a narrow region called shear band. Recent experiments in a modified Couette cell provided localized shear flow in the bulk away from the confining walls. The non-trivial shape of the shear band was measured as the function of the cell geometry. First we present a geometric argument for narrow shear bands which connects the function of their surface position with the shape in the bulk. Assuming a simple dissipation mechanism we show that the principle of minimum dissipation of energy provides a good description of the shape function. Furthermore, we discuss the possibility and behavior of shear bands which are detached from the free surface and are entirely covered in the bulk.Comment: 4 pages, 5 figures; minor changes, typos and journal-ref adde

The origin of net electric currents in solar active regions

There is a recurring question in solar physics about whether or not electric currents are neutralized in active regions (ARs). This question was recently revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetic flux emergence into the solar atmosphere. Such simulations showed that flux emergence can generate a substantial net current in ARs. Another source of AR currents are photospheric horizontal flows. Our aim is to determine the conditions for the occurrence of net vs. neutralized currents with this second mechanism. Using 3D MHD simulations, we systematically impose line-tied, quasi-static, photospheric twisting and shearing motions to a bipolar potential magnetic field. We find that such flows: (1) produce both {\it direct} and {\it return} currents, (2) induce very weak compression currents - not observed in 2.5D - in the ambient field present in the close vicinity of the current-carrying field, and (3) can generate force-free magnetic fields with a net current. We demonstrate that neutralized currents are in general produced only in the absence of magnetic shear at the photospheric polarity inversion line - a special condition rarely observed. We conclude that, as magnetic flux emergence, photospheric flows can build up net currents in the solar atmosphere, in agreement with recent observations. These results thus provide support for eruption models based on pre-eruption magnetic fields possessing a net coronal current.Comment: 14 pages and 11 figures (Accepted in The Astrophysical Journal

Anti-Saccharomyces cerevisiae antibodies in patients with inflammatory bowel disease and their first-degree relatives: Potential clinical value

Anti-Saccharomyces cerevisiae antibodies (ASCA) have been described as specific markers in Crohn's disease and their healthy first-degree relatives. 171 patients with Crohn's disease, their 105 first-degree relatives, 145 patients with ulcerative colitis and 101 first-degree relatives of patients with ulcerative colitis, 50 patients with infectious enterocolitis and 100 healthy controls were tested for ASCA employing the ELISA technique. When compared with the healthy controls (p < 0.0001) and patients with infectious enterocolitis (p < 0.0001) the prevalence of ASCA was significantly increased in patients with Crohn's disease and their first-degree relatives (p < 0.01). Further significant differences concerning the frequency of ASCA within the different groups of our study population were not observed. In particular, ASCA were not found in increased prevalence in infectious enterocolitis. These observations are compatible with a role of ASCA as a marker of genetic predisposition to Crohn's disease. Copyright (C) 2002 S. Karger AG, Basel

Static QˉQ\bar{Q}Q pair free energy and screening masses from correlators of Polyakov loops: continuum extrapolated lattice results at the QCD physical point

We study the correlators of Polyakov loops, and the corresponding gauge invariant free energy of a static quark-antiquark pair in 2+1 flavor QCD at finite temperature. Our simulations were carried out on $N_t$ = 6, 8, 10, 12, 16 lattices using Symanzik improved gauge action and a stout improved staggered action with physical quark masses. The free energies calculated from the Polyakov loop correlators are extrapolated to the continuum limit. For the free energies we use a two step renormalization procedure that only uses data at finite temperature. We also measure correlators with definite Euclidean time reversal and charge conjugation symmetry to extract two different screening masses, one in the magnetic, and one in the electric sector, to distinguish two different correlation lengths in the full Polyakov loop correlator