165 research outputs found
The Public Service Publisher - an Obituary
No full textIn 2005, the British communications regulator Ofcom released its review of public service broadcasting (PSB), the remit governing the majority of broadcasters throughout British broadcasting history.
While supporting a continuing role for PSB in a digital era, Ofcom found that the arrangement whereby commercial broadcasters provided public service programming in return for access to the airwaves was breaking down because of the declining value of analogue spectrum in the run-up to the switchover to digital television. In this situation, Ofcom, as the body with statutory responsibility for ensuring broadcast diversity and with specific oversight over commercial channels, concluded that the BBC was likely to end up as the near monopoly provider of PSB. It therefore recommended the creation of a new competitive supplier of PSB content, a Public Service Publisher (PSP), whose role would be to facilitate the creation of innovative and publicly engaged material
On magnetic reconnection and flux rope topology in solar flux emergence
No full textWe present an analysis of the formation of atmospheric flux ropes in a magnetohydrodynamic solar flux emergence simulation. The simulation domain ranges from the top of the solar interior to the low corona. A twisted magnetic flux tube emerges from the solar interior and into the atmosphere where it interacts with the ambient magnetic field. By studying the connectivity of the evolving magnetic field, we are able to better understand the process of flux rope formation in the solar atmosphere. In the simulation, two flux ropes are produced as a result of flux emergence. Each has a different evolution resulting in different topological structures. These are determined by plasma flows and magnetic reconnection. As the flux rope is the basic structure of the coronal mass ejection, we discuss the implications of our findings for solar eruptions.Peer reviewe
Pine Ridge Reservation Trip Email
No full textPine Ridge Reservation Trip Email
From: Douglas L. MacTaggart
To: Kevin EERC Dalsted; Mary Jo Benton EERC Lee; Dan Swets; Donald Ohlen; Hank Lehre
Magnetohydrostatic modelling of stellar coronae
No full textWe introduce to the stellar physics community a method of modelling stellar coronae that can be considered to be an extension of the potential field. In this approach, the magnetic field is coupled to the background atmosphere. The model is magnetohydrostatic and is a balance between the Lorentz force, the pressure gradient and gravity. Analytical solutions are possible and we consider a particular class of equilibria in this paper. The model contains two free parameters and the effects of these on both the geometry and topology of the coronal magnetic field are investigated. A demonstration of the approach is given using a magnetogram derived from Zeeman–Doppler imaging of the 0.75 M⊙ M-dwarf star GJ 182.Peer reviewe
Non-symmetric magnetohydrostatic equilibria: a multigrid approach
No full textAims: Linear magnetohydrostatic (MHS) models of solar magnetic fields balance plasma pressure gradients, gravity and Lorentz forces where the current density is composed of a linear force-free component and a cross-field component that depends on gravitational stratification. In this paper, we investigate an efficient numerical procedure for calculating such equilibria.
Methods: The MHS equations are reduced to two scalar elliptic equations – one on the lower boundary and the other within the interior of the computational domain. The normal component of the magnetic field is prescribed on the lower boundary and a multigrid method is applied on both this boundary and within the domain to find the poloidal scalar potential. Once solved to a desired accuracy, the magnetic field, plasma pressure and density are found using a finite difference method.
Results: We investigate the effects of the cross-field currents on the linear MHS equilibria. Force-free and non-force-free examples are given to demonstrate the numerical scheme and an analysis of speed-up due to parallelization on a graphics processing unit (GPU) is presented. It is shown that speed-ups of ×30 are readily achievable
Flux emergence within mature solar active regions
No full text<i>Aims.</i> Recent insterest in flux emergence within mature active regions has led to several observational studies. Our aim is to model such a scenario and investigate the evolution of the system.<p></p>
<i>Methods.</i> We solve the 3D MHD equations numerically with a Lagrangian-remap scheme. The mature active region is modelled, in the initial condition, with a potential field. The smaller emerging region is a twisted flux tube and is placed between the two polarities of the mature region. The polarities of the new flux are aligned the same way as those of the mature region. The new flux emerges closer to the main negative polarity than the main positive polarity. To investigate the effects of reconnection, the distribution of the parallel electric field is calculated throughout the simulation. The topology of the magnetic field is then studied in regions of interest indicated by the electric field distribution.<p></p>
<i>Results.</i> The expansion of the new negative polarity is restricted due to the curvature of the overlying field and also because it is of the same sign. Reconnection is found to be strongest at low heights (below the corona) and along the outer side of the new positive polarity and its magnetic tongue. The effect of reconnection, in combination with the pressure between the two flux systems, is to resist the expansion of the new flux. The system then relaxes. Large-scale eruptions, such as CMEs, are not expected from the setup considered. At the new negative polarity, the high magnetic pressure can generate strong parallel electric fields which may lead to localized reconnection. The results of the model are in qualitative agreement with observations.<p></p>
Finite deformation in ideal magnetohydrodynamics
No full textAims. In this paper we investigate the finite deformation of magnetic fields that can enable one to find complex analytical magnetohydrostatic (MHS) equilibria. These can be used as input to non-linear simulations.
Methods. In order to find analytical equilibria, one normally has to consider simplifications or exploit a particular symmetry. Even with these measures, however, the desired equilibrium is often out of analytical reach. Here we describe a method that can work when traditional methods fail. It is based on the smooth deformation of simple magnetic fields into complex ones.
Results. Examples are given, to demonstrate the method, that are of practical importance in coronal physics. This technique will prove useful in setting up the initial conditions of non-linear magnetohydrodynamic simulations
On signatures of twisted magnetic flux tube emergence
No full textRecent studies of NOAA active region 10953, by Okamoto et al. (Astrophys. J. Lett. 673, 215, 2008; Astrophys. J. 697, 913, 2009), have interpreted photospheric observations of changing widths of the polarities and reversal of the horizontal magnetic field component as signatures of the emergence of a twisted flux tube within the active region and along its internal polarity inversion line (PIL). A filament is observed along the PIL and the active region is assumed to have an arcade structure. To investigate this scenario, MacTaggart and Hood (Astrophys. J. Lett. 716, 219, 2010) constructed a dynamic flux emergence model of a twisted cylinder emerging into an overlying arcade. The photospheric signatures observed by Okamoto et al. (2008, 2009) are present in the model although their underlying physical mechanisms differ. The model also produces two additional signatures that can be verified by the observations. The first is an increase in the unsigned magnetic flux in the photosphere at either side of the PIL. The second is the behaviour of characteristic photospheric flow profiles associated with twisted flux tube emergence. We look for these two signatures in AR 10953 and find negative results for the emergence of a twisted flux tube along the PIL. Instead, we interpret the photospheric behaviour along the PIL to be indicative of photospheric magnetic cancellation driven by flows from the dominant sunspot. Although we argue against flux emergence within this particular region, the work demonstrates the important relationship between theory and observations for the successful discovery and interpretation of signatures of flux emergence
PRESSURE NARROWING OF THE ROTATIONAL LINES IN THE FUNDAMENTAL INDUCED INFRARED ABSORPTION BAND OF H. IN , AND GAS MIXTURES
No full textJ. De Remigis, J. W. Mactaggart and H. L. Welsh, Can. J. Phys. 49, 381 (1971). H. R. Zaidi and J. Van Kranendonk, Can J. Phys. 49, 385 (1971).""Author Institution: Department of Physics, University of TorontoIn a recent the observation of a density-dependent narrowing of the quadrupole-induced rotational lines in the pressure-induced fundamental infrared band of hydrogen in gas and liquid mixtures was reported. It was found that for Ar densities greater than 300 amagat the half-width of the S (1) line of hydrogen varies inversely as the Ar density. Zaidi and Van have interpreted this effect as a form of diffusional narrowing and have developed an expression for the half-width, , in terms of the mutual diffusion coefficient of hydrogen in argon. In this paper the results of similar experiments on para-, , , and pure are presented. In particular, the (O) line in exhibits the same density dependence as previously found for the (1) line. For and for the (1) line of varies inversely as the rare gas density for Kr and Xe densities greater than 380 and 246 amagat, respectively. By combining these results with the theory of Zaidi and Van Kranendonk the diffusion coefficients of in high density Ar, Kr and Xe gas are computed and found to compare favourably with the kinetic theory values
Non-linear tearing of 3D null point current sheets
No full textThe manner in which the rate of magnetic reconnection scales with the Lundquist number in realistic three-dimensional (3D) geometries is still an unsolved problem. It has been demonstrated that in 2D rapid non-linear tearing allows the reconnection rate to become almost independent of the Lundquist number (the “plasmoid instability”). Here, we present the first study of an analogous instability in a fully 3D geometry, defined by a magnetic null point. The 3D null current layer is found to be susceptible to an analogous instability but is marginally more stable than an equivalent 2D Sweet-Parker-like layer. Tearing of the sheet creates a thin boundary layer around the separatrix surface, contained within a flux envelope with a hyperbolic structure that mimics a spine-fan topology. Efficient mixing of flux between the two topological domains occurs as the flux rope structures created during the tearing process evolve within this envelope. This leads to a substantial increase in the rate of reconnection between the two domains.Copyright (2014) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics
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