Coronal mass ejections as expanding force-free structures

We mode Solar coronal mass ejections (CMEs) as expanding force-fee magnetic structures and find the self-similar dynamics of configurations with spatially constant \alpha, where {\bf J} =\alpha {\bf B}, in spherical and cylindrical geometries, expanding spheromaks and expanding Lundquist fields correspondingly. The field structures remain force-free, under the conventional non-relativistic assumption that the dynamical effects of the inductive electric fields can be neglected. While keeping the internal magnetic field structure of the stationary solutions, expansion leads to complicated internal velocities and rotation, induced by inductive electric field. The structures depends only on overall radius R(t) and rate of expansion \dot{R}(t) measured at a given moment, and thus are applicable to arbitrary expansion laws. In case of cylindrical Lundquist fields, the flux conservation requires that both axial and radial expansion proceed with equal rates. In accordance with observations, the model predicts that the maximum magnetic field is reached before the spacecraft reaches the geometric center of a CME.Comment: 19 pages, 9 Figures, accepted by Solar Physic

Effects of momentum conservation on the analysis of anisotropic flow

We present a general method for taking into account correlations due to momentum conservation in the analysis of anisotropic flow, either by using the two-particle correlation method or the standard flow vector method. In the latter, the correlation between the particle and the flow vector is either corrected through a redefinition (shift) of the flow vector, or subtracted explicitly from the observed flow coefficient. In addition, momentum conservation contributes to the reaction plane resolution. Momentum conservation mostly affects the first harmonic in azimuthal distributions, i.e., directed flow. It also modifies higher harmonics, for instance elliptic flow, when they are measured with respect to a first harmonic event plane such as one determined with the standard transverse momentum method. Our method is illustrated by application to NA49 data on pion directed flow.Comment: RevTeX 4, 10 pages, 1 eps figure. Version accepted for publication in Phys Rev

Magnetic Field Generation in Stars

Enormous progress has been made on observing stellar magnetism in stars from the main sequence through to compact objects. Recent data have thrown into sharper relief the vexed question of the origin of stellar magnetic fields, which remains one of the main unanswered questions in astrophysics. In this chapter we review recent work in this area of research. In particular, we look at the fossil field hypothesis which links magnetism in compact stars to magnetism in main sequence and pre-main sequence stars and we consider why its feasibility has now been questioned particularly in the context of highly magnetic white dwarfs. We also review the fossil versus dynamo debate in the context of neutron stars and the roles played by key physical processes such as buoyancy, helicity, and superfluid turbulence,in the generation and stability of neutron star fields. Independent information on the internal magnetic field of neutron stars will come from future gravitational wave detections. Thus we maybe at the dawn of a new era of exciting discoveries in compact star magnetism driven by the opening of a new, non-electromagnetic observational window. We also review recent advances in the theory and computation of magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo theory. These advances offer insight into the action of stellar dynamos as well as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field generation in stars to appear in Space Science Reviews, Springe

Tullock contests of weakly heterogeneous players

I explore asymmetric equilibria in a Tullock contest of heterogeneous players in the case when the players’ heterogeneity is weak, and the effort optimization problem can be analyzed in the linear approximation. With increasing discriminatory power of the contest, players sequentially drop out. The corresponding threshold values of the discriminatory power are related to those identified earlier for a Tullock contest of identical players. Weak heterogeneity, however, is sufficient to make the players’ behavior strongly asymmetric and qualitatively alter the structure of the equilibria as compared to the homogeneous case. Copyright Springer Science+Business Media, LLC 2007Contests, Rent-seeking, Heterogeneous players,

Ex Post Problems in Buyer–Supplier Transactions: Effects of Transaction Characteristics, Social Embeddedness, and Contractual Governance

This paper focuses on ex post governance of inter-firm transactions. We develop and test hypotheses on the occurrence of ex post problems like delivery delays, inferior quality, and insufficient service in buyer–supplier transactions. Our hypotheses address effects of transaction characteristics, of social embeddedness, and of contractual governance on the occurrence of problems. Other than earlier research on embeddedness effects in this field, we consider not only effects of dyadic embeddedness but also effects of network embeddedness. We test hypotheses using rich survey data on more than 1200 purchases of information technology (IT) products: hardware and software, both standard and complex. We find evidence for effects of transaction characteristics on the occurrence of problems, while our data do not support hypotheses on effects of contractual governance. Our data provide rather consistent support for hypotheses on the effects of embeddedness. Specifically, we find evidence that network embeddedness reduces problems. Copyright Springer Science+Business Media, Inc. 2006buyer–supplier relations, embeddedness, ex post problems, inter-firm networks, governance, transaction cost theory,