2,866 research outputs found
Contributions to the Fourth Solar Wind Conference
Recent results in interplanetary physics are examined. These include observations of shock waves and post-shock magnetic fields made by Voyager 1, 2; observations of the electron temperature as a function of distance between 1.36 AU and 2.25 AU; and observations of the structure of sector boundaries observed by Helios 1. A theory of electron energy transport in the collisionless solar wind is presented, and compared with observations. Alfven waves and Alvenic fluctuations in the solar wind are also discussed
Making Sense of a New Transport System: An Ethnographic Study of the Cambridgeshire Guided Busway
An increase in public transport use has the potential to contribute to improving population health, and there is growing interest in innovative public transport systems. Yet how new public transport infrastructure is experienced and integrated (or not) into daily practice is little understood. We investigated how the Cambridgeshire Guided Busway, UK, was used and experienced in the weeks following its opening, using the method of participant observation (travelling on the busway and observing and talking to passengers) and drawing on Normalization Process Theory to interpret our data. Using excerpts of field notes to support our interpretations, we describe how the ease with which the new transport system could be integrated into existing daily routines was important in determining whether individuals would continue to use it. It emerged that there were two groups of passengers with different experiences and attitudes. Passengers who had previously travelled frequently on regular bus services did not perceive the new system to be an improvement; consequently, they were frustrated that it was differentiated from and not coherent with the regular system. In contrast, passengers who had previously travelled almost exclusively by car appraised the busway positively and perceived it to be a novel and superior form of travel. Our rich qualitative account highlights the varied and creative ways in which people learn to use new public transport and integrate it into their everyday lives. This has consequences for the introduction and promotion of future transport innovations. It is important to emphasise the novelty of new public transport, but also the ways in which its use can become ordinary and routine. Addressing these issues could help to promote uptake of other public transport interventions, which may contribute to increasing physical activity and improving population health. Š 2013 Jones et al
On the tilting of protostellar disks by resonant tidal effects
We consider the dynamics of a protostellar disk surrounding a star in a
circular-orbit binary system. Our aim is to determine whether, if the disk is
initially tilted with respect to the plane of the binary orbit, the inclination
of the system will increase or decrease with time. The problem is formulated in
the binary frame in which the tidal potential of the companion star is static.
We consider a steady, flat disk that is aligned with the binary plane and
investigate its linear stability with respect to tilting or warping
perturbations. The dynamics is controlled by the competing effects of the m=0
and m=2 azimuthal Fourier components of the tidal potential. In the presence of
dissipation, the m=0 component causes alignment of the system, while the m=2
component has the opposite tendency. We find that disks that are sufficiently
large, in particular those that extend to their tidal truncation radii, are
generally stable and will therefore tend to alignment with the binary plane on
a time-scale comparable to that found in previous studies. However, the effect
of the m=2 component is enhanced in the vicinity of resonances where the outer
radius of the disk is such that the natural frequency of a global bending mode
of the disk is equal to twice the binary orbital frequency. Under such
circumstances, the disk can be unstable to tilting and acquire a warped shape,
even in the absence of dissipation. The outer radius corresponding to the
primary resonance is always smaller than the tidal truncation radius. For disks
smaller than the primary resonance, the m=2 component may be able to cause a
very slow growth of inclination through the effect of a near resonance that
occurs close to the disk center. We discuss these results in the light of
recent observations of protostellar disks in binary systems.Comment: 21 pages, 7 figures, to be published in the Astrophysical Journa
Interaction of the solar wind with Venus
Two topics related to the interaction of the solar wind with Venus are considered. First, a short review of the experimental evidence with particular attention to plasma measurements carried out on Mariner-5 and Mariner-10 is given. Secondly, the results of some recent theoretical work on the interaction of the solar wind with the ionosphere of Venus are summarized
Negatively Charged Strangelet Search using the E864 Spectrometer at the AGS
We provide a status report on the progress of searching for negatively
charged strangelets using the E864 spectrometer at the AGS. About 200 million
recorded events representing approximately 14 billion 10% central interactions
of Au + Pt at 11.5 GeV/c taken during the 1996-1997 run of the experiment are
used in the analysis. No strangelet candidates are seen for charges Z=-1 and
Z=-2, corresponding to a 90% confidence level for upper limits of strangelet
production of ~1 x 10^{-8} and ~4 x 10^{-9} per central collision respectively.
The limits are nearly uniform over a wide range of masses and are valid only
for strangelets which are stable or have lifetimes greater than ~50 ns.Comment: 6 pages, 4 figures; Talk at SQM'98, Padova, Italy (July 20-24, 1998
Kelvin-Helmholtz instability in coronal magnetic flux tubes due to azimuthal shear flows
Transverse oscillations of coronal loops are often observed and have been
theoretically interpreted as kink magnetohydrodynamic (MHD) modes. Numerical
simulations by Terradas et al. (2008, ApJ 687, L115) suggest that shear flows
generated at the loop boundary during kink oscillations could give rise to a
Kelvin-Helmholtz instability (KHI). Here, we investigate the linear stage of
the KHI in a cylindrical magnetic flux tube in the presence of azimuthal shear
motions. We consider the basic, linearized MHD equations in the beta = 0
approximation, and apply them to a straight and homogeneous cylindrical flux
tube model embedded in a coronal environment. Azimuthal shear flows with a
sharp jump of the velocity at the cylinder boundary are included in the model.
We obtain an analytical expression for the dispersion relation of the unstable
MHD modes supported by the configuration, and compute analytical approximations
of the critical velocity shear and the KHI growth rate in the thin tube limit.
A parametric study of the KHI growth rates is performed by numerically solving
the full dispersion relation. We find that fluting-like modes can develop a KHI
in time-scales comparable to the period of kink oscillations of the flux tube.
The KHI growth rates increase with the value of the azimuthal wavenumber and
decrease with the longitudinal wavenumber. However, the presence of a small
azimuthal component of the magnetic field can suppress the KHI. Azimuthal
motions related to kink oscillations of untwisted coronal loops may trigger a
KHI, but this phenomenon has not been observed to date. We propose that the
azimuthal component of the magnetic field is responsible for suppressing the
KHI in a stable coronal loop. The required twist is small enough to prevent the
development of the pinch instability.Comment: Submitted in Ap
A First Principles Estimate of Finite Size Effects in Quark-Gluon Plasma Formation
Using lattice simulations of quenched QCD we estimate the finite size effects
present when a gluon plasma equilibrates in a slab geometry, i.e., finite width
but large transverse dimensions. Significant differences are observed in the
free energy density for the slab when compared with bulk behavior. A small
shift in the critical temperature is also seen. The free energy required to
liberate heavy quarks relative to bulk is measured using Polyakov loops; the
additional free energy required is on the order of 30-40 MeV at 2-3 T_c.Comment: 10 pages, 5 figures, RevTeX; revised version includes comparison with
the Bjorken model and various small improvement
Warp propagation in astrophysical discs
Astrophysical discs are often warped, that is, their orbital planes change
with radius. This occurs whenever there is a non-axisymmetric force acting on
the disc, for example the Lense-Thirring precession induced by a misaligned
spinning black hole, or the gravitational pull of a misaligned companion. Such
misalignments appear to be generic in astrophysics. The wide range of systems
that can harbour warped discs - protostars, X-ray binaries, tidal disruption
events, quasars and others - allows for a rich variety in the disc's response.
Here we review the basic physics of warped discs and its implications.Comment: To be published in Astrophysical Black Holes by Haardt et al.,
Lecture Notes in Physics, Springer 2015. 19 pages, 2 figure
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