16284410
Differential rotation and convection in the Sun
We show that the differential rotation profile of the solar convection zone, apart from inner and outer boundary layers, can be reproduced with great accuracy if the isorotation contours correspond to characteristics of the thermal wind equation. This requires that there be a formal quantitative relationship involving the entropy and the angular velocity. Earlier work has suggested that this could arise from magnetohydrodynamic stability constraints; here, we argue that purely hydrodynamical processes could also lead to such a result. Of special importance to the hydrodynamical solution is the fact that the thermal wind equation is insensitive to radial entropy gradients. This allows a much more general class of solutions to fit the solar isorotation contours, beyond just those in which the entropy itself must be a function of the angular velocity. In particular, for this expanded class, the thermal wind solution of the solar rotation profile remains valid even when large radial entropy gradients are present. A clear and explicit example of this class of solution appears to be present in published numerical simulations of the solar convective zone. Though hydrodynamical in character, the theory is not sensitive to the presence of weak magnetic fields. Thus, the identification of solar isorotation contours with the characteristics of the thermal wind equation appears to be robust, accommodating, but by no means requiring, magnetic field dynamics.
53074129
Magnetic buoyancy instabilities in the presence of magnetic flux pumping at the base of the solar convection zone
We perform idealized numerical simulations of magnetic buoyancy instabilities in three dimensions, solving the equations of compressible magnetohydrodynamics in a model of the solar tachocline. In particular, we study the effects of including a highly simplified model of magnetic flux pumping in an upper layer (‘the convection zone’) on magnetic buoyancy instabilities in a lower layer (‘the upper parts of the radiative interior – including the tachocline’), to study these competing flux transport mechanisms at the base of the convection zone. The results of the inclusion of this effect in numerical simulations of the buoyancy instability of both a preconceived magnetic slab and a shear-generated magnetic layer are presented. In the former, we find that if we are in the regime that the downward pumping velocity is comparable with the Alfven speed of the magnetic layer, magnetic flux pumping is able to hold back the bulk of the magnetic field, with only small pockets of strong field able to rise into the upper layer. 
 
In simulations in which the magnetic layer is generated by shear, we find that the shear velocity is not necessarily required to exceed that of the pumping (therefore the kinetic energy of the shear is not required to exceed that of the overlying convection) for strong localized pockets of magnetic field to be produced which can rise into the upper layer. This is because magnetic flux pumping acts to store the field below the interface, allowing it to be amplified both by the shear and by vortical fluid motions, until pockets of field can achieve sufficient strength to rise into the upper layer. In addition, we find that the interface between the two layers is a natural location for the production of strong vertical gradients in the magnetic field. If these gradients are sufficiently strong to allow the development of magnetic buoyancy instabilities, strong shear is not necessarily required to drive them (cf. previous work by Vasil & Brummell). We find that the addition of magnetic flux pumping appears to be able to assist shear-driven magnetic buoyancy in producing strong flux concentrations that can rise up into the convection zone from the radiative interior.
122311483
Multiple length scales and screening lengths in QCD at high temperatures
122195564
Possible origins of a small, nonzero cosmological constant
Abstract The idea that the cosmological constant Λ may have a small, nonzero value (Λ ∼ (10 −33 eV) 2 ) has been recently revived in attempts to understand several cosmological issues. Despite the fact that such a small, nonzero Λ is aesthetically unappealing the issue of its existence will eventually be decided by experiments. In this paper the implications to particle physics of the existence of such a small, nonzero value of Λ are examined. Models are described in which the ground state of the system is assumed to have a cosmological constant equal to zero but in which we are not, presently in this ground state. The possibility that this deviation from the ground state leads to such a small, effective cosmological constant is studied.
120222832
Anyonic superconductivity: Analysis of Chern-Simons theory at finite density
119520942
Quantum fluctuations of Wilson loops from string models
Abstract We discuss the impact of quadratic quantum fluctuations on the Wilson loop extracted from classical string theory. We show that a large class of models, which includes the near horizon limit of D p branes with 16 supersymmetries, admits a Luscher type correction to the classical potential. For a BPS configuration of a single quark in the AdS 5 × S 5 model, we show that the bosonic and the fermionic determinants cancel, in particular confirming the absence of divergences. We find that for the Wilson loop in that model, however, there is no such cancellation. For string models that correspond to gauge theories in the confining phase, we show that the correction to the potential is of a Luscher type and is attractive.