6,841 research outputs found
Drift induced perpendicular transport of solar energetic particles
Drifts are known to play a role in galactic cosmic ray transport within the heliosphere and are a standard component of cosmic ray propagation models. However, the current paradigm of solar energetic particle (SEP) propagation holds the effects of drifts to be negligible, and they are not accounted for in most current SEP modeling efforts. We present full-orbit test particle simulations of SEP propagation in a Parker spiral interplanetary magnetic field (IMF), which demonstrate that high-energy particle drifts cause significant asymmetric propagation perpendicular to the IMF. Thus in many cases the assumption of field-aligned propagation of SEPs may not be valid. We show that SEP drifts have dependencies on energy, heliographic latitude, and charge-to-mass ratio that are capable of transporting energetic particles perpendicular to the field over significant distances within interplanetary space, e.g., protons of initial energy 100 MeV propagate distances across the field on the order of 1 AU, over timescales typical of a gradual SEP event. Our results demonstrate the need for current models of SEP events to include the effects of particle drift. We show that the drift is considerably stronger for heavy ion SEPs due to their larger mass-to-charge ratio. This paradigm shift has important consequences for the modeling of SEP events and is crucial to the understanding and interpretation of in situ observations. © 2013. The American Astronomical Society. All rights reserved.
Pair-production of charged Dirac particles on charged Nariai and ultracold black hole manifolds
Spontaneous loss of charge by charged black holes by means of pair-creation
of charged Dirac particles is considered. We provide three examples of exact
calculations for the spontaneous discharge process for 4D charged black holes
by considering the process on three special non-rotating de Sitter black hole
backgrounds, which allow to bring back the problem to a Kaluza-Klein reduction.
Both the zeta-function approach and the transmission coefficient approach are
taken into account. A comparison between the two methods is also provided, as
well as a comparison with WKB results. In the case of non-zero temperature of
the geometric background, we also discuss thermal effects on the discharge
process.Comment: 27 page
Sequence Expression of Supernumerary B Chromosomes: Function or Fluff?
B chromosomes are enigmatic heritable elements found in the genomes of numerous plant and animal species. Contrary to their broad distribution, most B chromosomes are non-essential. For this reason, they are regarded as genome parasites. In order to be stably transmitted through generations, many B chromosomes exhibit the ability to "drive", i.e., they transmit themselves at super-Mendelian frequencies to progeny through directed interactions with the cell division apparatus. To date, very little is understood mechanistically about how B chromosomes drive, although a likely scenario is that expression of B chromosome sequences plays a role. Here, we highlight a handful of previously identified B chromosome sequences, many of which are repetitive and non-coding in nature, that have been shown to be expressed at the transcriptional level. We speculate on how each type of expressed sequence could participate in B chromosome drive based on known functions of RNA in general chromatin- and chromosome-related processes. We also raise some challenges to functionally testing these possible roles, a goal that will be required to more fully understand whether and how B chromosomes interact with components of the cell for drive and transmission
The First Billion Years project: gamma-ray bursts at z>5
Long gamma-ray burst's (LGRB's) association to the death of massive stars
suggest they could be used to probe the cosmic star formation history (CSFH)
with high accuracy, due to their high luminosities. We utilise cosmological
simulations from the First Billion Years project to investigate the biases
between the CSFH and the LGRB rate at z>5, assuming various different models
and constraints on the progenitors of LGRBs. We populate LGRBs using a
selection based on environmental properties and demonstrate that the LGRB rate
should trace the CSFH to high redshifts. The measured LGRB rate suggests that
LGRBs have opening angles of theta_jet=0.1 deg, although the degeneracy with
the progenitor model cannot rule out an underlying bias. We demonstrate that
proxies that relate the LGRB rate with global LGRB host properties do not
reflect the underlying LGRB environment, and are in fact a result of the host
galaxy's spatial properties, such that LGRBs can exist in galaxies of solar
metallicity. However, we find a class of host galaxies that have low stellar
mass and are metal-rich, and that their metallicity dispersions would not allow
low-metallicity environments. Detection of hosts with this set of properties
would directly reflect the progenitor's environment. We predict that 10% of
LGRBs per year are associated with this set of galaxies that would have
forbidden line emission that could be detected by instruments on the James Webb
Space Telescope. Such a discovery would place strong constraints on the
collapsar model and suggest other avenues to be investigated.Comment: 13 pages, 8 figures, 1 table, accepted for publication in MNRA
Exact quantisation of the relativistic Hopfield model
We investigate the quantisation in the Heisenberg representation of a
relativistically covariant version of the Hopfield model for dielectric media,
which entails the interaction of the quantum electromagnetic field with the
matter dipole fields. The matter fields are represented by a mesoscopic
polarization field. A full quantisation of the model is provided in a covariant
gauge, with the aim of maintaining explicit relativistic covariance. Breaking
of the Lorentz invariance due to the intrinsic presence in the model of a
preferred reference frame is also taken into account. Relativistic covariance
forces us to deal with the unphysical (scalar and longitudinal) components of
the fields, furthermore it introduces, in a more tricky form, the well-known
dipole ghost of standard QED in a covariant gauge. In order to correctly
dispose of this contribution, we implement a generalized Lautrup trick.
Furthermore, causality and the relation of the model with the Wightman axioms
are also discussed.Comment: 24 page
Path integral quantization of the relativistic Hopfield model
The path integral quantization method is applied to a relativistically
covariant version of the Hopfield model, which represents a very interesting
mesoscopic framework for the description of the interaction between quantum
light and dielectric quantum matter, with particular reference to the context
of analogue gravity. In order to take into account the constraints occurring in
the model, we adopt the Faddeev-Jackiw approach to constrained quantization in
the path integral formalism. In particular we demonstrate that the propagator
obtained with the Faddeev-Jackiw approach is equivalent to the one which, in
the framework of Dirac canonical quantization for constrained systems, can be
directly computed as the vacuum expectation value of the time ordered product
of the fields. Our analysis also provides an explicit example of quantization
of the electromagnetic field in a covariant gauge and coupled with the
polarization field, which is a novel contribution to the literature on the
Faddeev-Jackiw procedure.Comment: 16 page
Experimental quantum cosmology in time-dependent optical media
It is possible to construct artificial spacetime geometries for light by
using intense laser pulses that modify the spatiotemporal properties of an
optical medium. Here we theoretically investigate experimental possibilities
for studying spacetime metrics of the form
. By tailoring the laser
pulse shape and medium properties, it is possible to create a refractive index
variation that can be identified with . Starting from a
perturbative solution to a generalised Hopfield model for the medium described
by an we provide estimates for the number of photons generated by the
time-dependent spacetime. The simplest example is that of a uniformly varying
that therefore describes the Robertson-Walker metric, i.e. a
cosmological expansion. The number of photon pairs generated in experimentally
feasible conditions appears to be extremely small. However, large photon
production can be obtained by periodically modulating the medium and thus
resorting to a resonant enhancement similar to that observed in the dynamical
Casimir effect. Curiously, the spacetime metric in this case closely resembles
that of a gravitational wave. Motivated by this analogy we show that a periodic
gravitational wave can indeed act as an amplifier for photons. The emission for
an actual gravitational wave will be very weak but should be readily observable
in the laboratory analogue.Comment: Version accepted fro publication in New Journal of Physic
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