32 research outputs found
Heating plasma loops in the solar corona
We find a new heat source term for hot coronal loops and include it in the energy equation. This term requires the loop to be hotter than the ambient corona and depends on the combined effect of electron fluid shear and the temperature gradient. Under certain circumstances, the shear drives the heat up the radial temperature gradient into a cross section of the magnetic flux tube from which it leaves by radiation and by conduction down the axial temperature gradient in the usual manner. The heat source is thus a surface term applied over the whole of the loop rather than a volume-distributed term, and its strength is proportional to the cube of the temperature. We apply it to the usual scaling law and obtain an expression for the radius of the flux tube for thermal equilibrium to hold. The temperature distribution around the plasma loop is determined and compared with recent observations and is found to be in satisfactory agreement with them
On the magnetic structure of the solar transition region
We examine the hypothesis that ``cool loops'' dominate emission from solar
transition region plasma below temperatures of K. We compare
published VAULT images of H L, a lower transition region line, with
near-contemporaneous magnetograms from Kitt Peak, obtained during the second
flight (VAULT-2) on 14 June 2002. The measured surface fields and potential
extrapolations suggest that there are too few short loops, and that L
emission is associated with the base regions of longer, coronal loops. VAULT-2
data of network boundaries have an asymmetry on scales larger than
supergranules, also indicating an association with long loops. We complement
the Kitt Peak data with very sensitive vector polarimetric data from the
Spectro-Polarimeter on board Hinode, to determine the influence of very small
magnetic concentrations on our analysis. From these data two classes of
behavior are found: within the cores of strong magnetic flux concentrations ( Mx) associated with active network and plage, small-scale mixed
fields are absent and any short loops can connect just the peripheries of the
flux to cell interiors. Core fields return to the surface via longer, most
likely coronal, loops. In weaker concentrations, short loops can connect
between concentrations and produce mixed fields within network boundaries as
suggested by Dowdy and colleagues. The VAULT-2 data which we examined are
associated with strong concentrations. We conclude that the cool loop model
applies only to a small fraction of the VAULT-2 emission, but we cannot
discount a significant role for cool loops in quieter regions. We suggest a
physical picture for how network L emission may occur through the
cross-field diffusion of neutral atoms from chromospheric into coronal plasma.Comment: Accepted by ApJ, 9 May 200
Black hole pairs and supergravity domain walls
We examine the pair creation of black holes in the presence of supergravity
domain walls with broken and unbroken supersymmetry. We show that black holes
will be nucleated in the presence of non- extreme, repulsive walls which break
the supersymmetry, but that as one allows the parameter measuring deviation
from extremality to approach zero the rate of creation will be suppressed. In
particular, we show that the probability for creation of black holes in the
presence of an extreme domain wall is identically zero, even though an extreme
vacuum domain wall still has repulsive gravitational energy. This is consistent
with the fact that the supersymmetric, extreme domain wall configurations are
BPS states and should be stable against quantum corrections. We discuss how
these walls arise in string theory, and speculate about what string theory
might tell us about such objects.Comment: 21 pages LaTeX, special style files (psfrag.sty, efsf_psfrag.sty,
a4local.sty, epsf.tex), minor revisions and amended reference
Abelian Higgs hair for extreme black holes and selection rules for snapping strings
It has been argued that a black hole horizon can support the long range
fields of a Nielsen-Olesen string, and that one can think of such a vortex as
black hole ``hair''. We show that the fields inside the vortex are completely
expelled from a charged black hole in the extreme limit (but not in the near
extreme limit). This would seem to imply that a vortex cannot be attached to an
extreme black hole. Furthermore, we provide evidence that it is energetically
unfavourable for a thin vortex to interact with a large extreme black hole.
This dispels the notion that a black hole can support `long' Abelian Higgs hair
in the extreme limit. We discuss the implications for strings that end at black
holes, as in processes where a string snaps by nucleating black holes.Comment: 4 pages REVTeX plus 3 figures. Additional figures and mpeg movies
available at http://www.damtp.cam.ac.uk/user/ats25/strhole.html This paper is
a condensed version of gr-qc/9706004, and is essentially the talk presented
at The Eighth Marcel Grossmann Meeting on General Relativity, 22-27 June
1997, The Hebrew University, Jerusalem, Israe
Can extreme black holes have (long) Abelian Higgs hair?
It has been argued that a black hole horizon can support the long range
fields of a Nielsen-Olesen string, and that one can think of such a vortex as
black hole ``hair''. In this paper, we examine the properties of an Abelian
Higgs vortex in the presence of a charged black hole as we allow the hole to
approach extremality. Using both analytical and numerical techniques, we show
that the magnetic field lines (as well as the scalar field) of the vortex are
completely expelled from the black hole in the extreme limit. This was to be
expected, since extreme black holes in Einstein-Maxwell theory are known to
exhibit such a ``Meissner effect'' in general. This would seem to imply that a
vortex does not want to be attached to an extreme black hole. We calculate the
total energy of the vortex fields in the presence of an extreme black hole.
When the hole is small relative to the size of the vortex, it is energetically
favoured for the hole to remain inside the vortex region, contrary to the
intuition that the hole should be expelled. However, as we allow the extreme
horizon radius to become very large compared to the radius of the vortex, we do
find evidence of an instability. This proves that it is energetically
unfavourable for a thin vortex to interact with a large extreme black hole.
This would seem to dispel the notion that a black hole can support `long'
abelian Higgs hair in the extreme limit. We show that these considerations do
not go through in the near extreme limit. Finally, we discuss whether this has
implications for strings that end at black holes.Comment: 21 pages REVTeX plus 9 figures. Additional figures and mpeg movies
available at http://www.damtp.cam.ac.uk/user/ats25/strhole.html We have made
several cosmetic changes, and we have revised and extended the discussion of
strings which end on extreme horizon
From O'Shaughnessy to opportunity: Innovating Hepatology Trials in the UK
Developing new treatments that improve outcomes for patients with decompensated cirrhosis remains an unmet area of clinical need. The UK has a rich history of being on the forefront of clinical trials for this patient group. However, there have been challenges in achieving this goal in the past decade, with several negative studies as well as trials struggling to achieve recruitment. This has been further exacerbated by the changed clinical landscape following the COVID-19 pandemic. In response to this, the O'Shaughnessy report was commissioned to identify potential opportunities to improve clinical trial performance in the UK. In this review article, we identify critical areas for the UK hepatology community to collaborate and develop sustainable partnerships for clinical trial delivery which will ensure that outcomes are representative, inclusive and patient-centred
Heating the solar corona by plasma loops
We investigate the heating of the corona via plasma loops.
It is shown that it may be possible to maintain the high corona temperatures
using plasma loops as conduits. Under certain conditions heat can flow across
magnetic fields up temperature gradients, a mechanism that has been previously
applied to the heating of plasma loops. A typical conduit loop is hotter than
the ambient plasma in the upper chromosphere and transition layer, and is
cooler than the ambient plasma in the background corona. Hence, heat enters the
loop at the bottom, is transported by a combination of conduction (if there is
a negative temperature gradient), convection and shock waves up the loop into
the corona. Typical values show that this type of heating is sufficient to
maintain both the quiet and active corona and that it also has the
non-homogeneous temperature distribution observed in the lower corona. The
behaviour of some brightening events seen in TRACE data support the proposed
convective and shock wave mechanisms. The model offers a possible explanation
of a long-standing problem, namely why the corona is so hot.
Pickover biomorphs and non-standard complex numbers
In this study Pickover biomorphs are analysed as being dependent on the chosen complex number system in which iterations of analytic functions are performed. Moran’s spatial autocorrelation function and two forms of entropy, the Shannon entropy and the sample entropy, are chosen in order to find correlations and measure complexity in Pickover biomorphs. These turn out to be strongly correlated and low-entropy objects with a fractal dimension between 1.4 and 2. It is shown that there is a strong maximum in correlation and a strong minimum in entropy for the case of Galilean complex numbers corresponding to the square of the generalised imaginary unit being equal to zero
Numerical simulation of two-dimensional and three-dimensional axisymmetric advection-diffusion systems with complex geometries using finite-volume methods
A finite-volume method has been developed that can deal accurately with complicated, curved boundaries for both two-dimensional and three-dimensional axisymmetric advection-diffusion systems. The motivation behind this is threefold. Firstly, the ability to model the correct geometry of a situation yields more accurate results. Secondly, smooth geometries eliminate corner singularities in the calculation of, for example, mechanical variables and thirdly, different geometries can be tested for experimental applications. An example illustrating each of these is given: fluid carrying a dye and rotating in an annulus, bone fracture healing in mice, and using vessels of different geometry in an ultracentrifuge