Ising Dipoles on the Triangular Lattice

A cluster Monte Carlo method for systems of classical spins with purely dipolar couplings is presented. It is tested and applied for finite arrays of perpendicular Ising dipoles on the triangular lattice. This model is a modification with long-range interactions of the geometrically frustrated Ising antiferromagnet. From measurements of integrated autocorrelation times for energy, magnetization and staggered magnetizations, a high efficiency of the cluster Monte Carlo (MC) algorithm compared to a single-spin-flip algorithm is found. For the investigated model, a finite temperature transition is found which is characterized by a peak in the specific heat and in the staggered susceptibilities.Comment: 4 pages, 3 figures. Presented at the 8th Joint MMM-Intermag Conference, San Antonio 2001. To appear in J.Appl.Physic

Generation of a North/South Magnetic Field Component from Variations in the Photospheric Magnetic Field

We address the problem of calculating the transverse magnetic field in the solar wind outside of the hypothetical sphere called the source surface where the solar wind originates. This calculation must overcome a widely used fundamental assumption about the source surface -- the field is normally required to purely radial at the source surface. Our model rests on the fact that a change in the radial field strength at the source surface is a change in the field line density. Surrounding field lines must move laterally in order to accommodate this field line density change. As the outward wind velocity drags field lines past the source surface this lateral component of motion produces a tilt implying there is a transverse component to the field. An analytic method of calculating the lateral translation speed of the field lines is developed. We apply the technique to an interval of approximately two Carrington rotations at the beginning of 2011 using 2-h averages of data from the Helioseismic Magnetic Imager instrument on the Solar Dynamics Observatory spacecraft. We find that the value of the transverse magnetic field is dominated on a global scale by the effects of high latitude concentrations of field lines being buffetted by supergranular motions.Comment: 23 pages with 8 figures. Accepted by Solar Physics (LaTeX processing with aastex6.cls instead of solarphysics.cls due to compatibility issues

Physical Origin of Differences among various Measures of Solar Meridional Circulation

We show that systematic differences between surface Doppler and magnetic element tracking measures of solar meridional flow can be explained by the effects of surface turbulent magnetic diffusion. Feature-tracking speeds are lower than plasma speeds in low and mid-latitudes, because magnetic diffusion opposes poleward plasma flow in low-latitudes whereas it adds to plasma flow at high latitudes. Flux transport dynamo models must input plasma flow; the model-outputs yield estimates of the surface magnetic feature tracking speed. We demonstrate that the differences between plasma speed and magnetic pattern speed in a flux-transport dynamo are consistent with the observed difference between these speeds.Comment: To appear in Ap

The Problem of Legitimacy in the European Polity. Is Democratization the Answer?

The authors discuss potential sources of legitimacy of the EU, i. e. of the normative bindingness of its decisions. After rejecting the views that such legitimacy is either not needed, not feasible, or provided for already, they focus upon the corrosive impact of the EU upon democratic legitimacy within member states. Brussels-based 'governance' is essentially uncontested and can hardly provide for the legitimacy that results from the interplay between government and opposition within nation states. The problem boils down to achieving legitimacy in the absence of the political community of a 'demos'. The paper outlines a solution to this problem that relies on the apparently oxymoronic model of a 'republican empire' - a political community, that is, which is held together not by the bonds of some presumed sameness, but, to the contrary, by the shared contractual recognition of the dissimilarity of its constituent parts from which legitimacy can flow.democracy; legitimacy; diversity/homogeneity; governance; democratization

Long-Term Measurements of Sunspot Magnetic Tilt Angles

Tilt angles of close to 30,600 sunspots are determined using Mount Wilson daily averaged magnetograms taken from 1974 to 2012, and MDI/SoHO magnetograms taken from 1996 to 2010. Within a cycle, more than 90% of sunspots have a normal polarity alignment along the east-west direction following Hale's law. The median tilts increase with increasing latitude (Joy's law) at a rate of ~0.5 degree per degree of latitude. Tilt angles of spots appear largely invariant with respect to time at a given latitude, but they decrease by ~0.9degree per year on average, a trend which largely reflects Joy's law following the butterfly diagram. We find an asymmetry between the hemispheres in the mean tilt angles. On average, the tilts are greater in the southern than in the northern hemisphere for all latitude zones, and the differences increase with increasing latitude.Comment: 29 pages, 13 figures, to appear on ApJ, October 20, 2012 website: http://www2.ess.ucla.edu/~jingli/ApJ201210