4,378 research outputs found
Prominence plasma diagnostics through EUV absorption
In this paper we introduce a new diagnostic technique that uses prominence
EUV and UV absorption to determine the prominence plasma electron temperature
and column emission measure, as well as He/H relative abundance; if a realistic
assumption on the geometry of the absorbing plasma can be made, this technique
can also yield the absorbing plasma electron density. This technique
capitalizes on the absorption properties of Hydrogen and Helium at different
wavelength ranges and temperature regimes. Several cases where this technique
can be successfully applied are described. This technique works best when
prominence plasmas are hotter than 15,000 K and thus it is ideally suited for
rapidly heating erupting prominences observed during the initial phases of
coronal mass ejections. An example is made using simulated intensities of 4
channels of the SDO/AIA instrument. This technique can be easily applied to
existing observations from almost all space missions devoted to the study of
the solar atmosphere, which we list.Comment: 17 pages, 4 figures, submitted to Ap
String Geometry and the Noncommutative Torus
We construct a new gauge theory on a pair of d-dimensional noncommutative
tori. The latter comes from an intimate relationship between the noncommutative
geometry associated with a lattice vertex operator algebra A and the
noncommutative torus. We show that the (truncated) tachyon subalgebra of A is
naturally isomorphic to a class of twisted modules representing quantum
deformations of the algebra of functions on the torus. We construct the
corresponding even real spectral triples and determine their Morita equivalence
classes using string duality arguments. These constructions yield simple proofs
of the O(d,d;Z) Morita equivalences between -dimensional noncommutative tori
and give a natural physical interpretation of them in terms of the target space
duality group of toroidally compactified string theory. We classify the
automorphisms of the twisted modules and construct the most general gauge
theory which is invariant under the automorphism group. We compute bosonic and
fermionic actions associated with these gauge theories and show that they are
explicitly duality-symmetric. The duality-invariant gauge theory is manifestly
covariant but contains highly non-local interactions. We show that it also
admits a new sort of particle-antiparticle duality which enables the
construction of instanton field configurations in any dimension. The duality
non-symmetric on-shell projection of the field theory is shown to coincide with
the standard non-abelian Yang-Mills gauge theory minimally coupled to massive
Dirac fermion fields.Comment: 37 pages, LaTeX. Major revisions in section 3. Other minor revisions
in the rest of the paper, references adde
Bright hot impacts by erupted fragments falling back on the Sun: UV redshifts in stellar accretion
A solar eruption after a flare on 7 Jun 2011 produced EUV-bright impacts of
fallbacks far from the eruption site, observed with the Solar Dynamics
Observatory. These impacts can be taken as a template for the impact of stellar
accretion flows. Broad red-shifted UV lines have been commonly observed in
young accreting stars. Here we study the emission from the impacts in the
Atmospheric Imaging Assembly's UV channels and compare the inferred velocity
distribution to stellar observations. We model the impacts with 2D hydrodynamic
simulations. We find that the localised UV 1600A emission and its timing with
respect to the EUV emission can be explained by the impact of a cloud of
fragments. The first impacts produce strong initial upflows. The following
fragments are hit and shocked by these upflows. The UV emission comes mostly
from the shocked front shell of the fragments while they are still falling, and
is therefore redshifted when observed from above. The EUV emission instead
continues from the hot surface layer that is fed by the impacts. Fragmented
accretion can therefore explain broad redshifted UV lines (e.g. C IV 1550A) to
speeds around 400 km/s observed in accreting young stellar objects.Comment: 12 pages, 4 figures (movies available upon request), accepted for
publicatio
Acceleration of weakly collisional solar-type winds
One of the basic properties of the solar wind, that is the high speed of the
fast wind, is still not satisfactorily explained. This is mainly due to the
theoretical difficulty of treating weakly collisional plasmas. The fluid
approach implies that the medium is collision dominated and that the particle
velocity distributions are close to Maxwellians. However the electron velocity
distributions observed in the solar wind depart significantly from Maxwellians.
Recent kinetic collisionless models (called exospheric) using velocity
distributions with a suprathermal tail have been able to reproduce the high
speeds of the fast solar wind. In this letter we present new developments of
these models by generalizing them over a large range of corona conditions. We
also present new results obtained by numerical simulations that include
collisions. Both approaches calculate the heat flux self-consistently without
any assumption on the energy transport. We show that both approaches - the
exospheric and the collisional one - yield a similar variation of the wind
speed with the basic parameters of the problem; both produce a fast wind speed
if the coronal electron distribution has a suprathermal tail. This suggests
that exospheric models contain the necessary ingredients for the powering of a
transonic stellar wind, including the fast solar one.Comment: Accepted for publication in The Astrophysical Journal Letters
(accepted: 13 May 2005
Activation of MHD reconnection on ideal timescales
Magnetic reconnection in laboratory, space and astrophysical plasmas is often
invoked to explain explosive energy release and particle acceleration. However,
the timescales involved in classical models within the macroscopic MHD regime
are far too slow to match the observations. Here we revisit the tearing
instability by performing visco-resistive two-dimensional numerical simulations
of the evolution of thin current sheets, for a variety of initial
configurations and of values of the Lunquist number , up to . Results
confirm that when the critical aspect ratio of is reached in the
reconnecting current sheets, the instability proceeds on ideal (Alfv\'enic)
macroscopic timescales, as required to explain observations. Moreover, the same
scaling is seen to apply also to the local, secondary reconnection events
triggered during the nonlinear phase of the tearing instability, thus
accelerating the cascading process to increasingly smaller spatial and temporal
scales. The process appears to be robust, as the predicted scaling is measured
both in inviscid simulations and when using a Prandtl number in the
viscous regime.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
Differential and Twistor Geometry of the Quantum Hopf Fibration
We study a quantum version of the SU(2) Hopf fibration and its
associated twistor geometry. Our quantum sphere arises as the unit
sphere inside a q-deformed quaternion space . The resulting
four-sphere is a quantum analogue of the quaternionic projective space
. The quantum fibration is endowed with compatible non-universal
differential calculi. By investigating the quantum symmetries of the fibration,
we obtain the geometry of the corresponding twistor space and
use it to study a system of anti-self-duality equations on , for which
we find an `instanton' solution coming from the natural projection defining the
tautological bundle over .Comment: v2: 38 pages; completely rewritten. The crucial difference with
respect to the first version is that in the present one the quantum
four-sphere, the base space of the fibration, is NOT a quantum homogeneous
space. This has important consequences and led to very drastic changes to the
paper. To appear in CM
The social diversification of fashion
We propose a model to investigate the dynamics of fashion traits purely driven by social interactions. We assume that people adapt their style to maximize social success, and we describe the interaction as a repeated group game in which the payoffs reflect the social norms dictated by fashion. On one hand, the tendency to imitate the trendy stereotypes opposed to the tendency to diverge from them to proclaim identity; on the other hand, the exploitation of sex appeal for dating success opposed to the moral principles of the society. These opposing forces promote diversity in fashion traits, as predicted by the modeling framework of adaptive dynamics. Our results link the so-called horizontal dynamics—the primary driver of fashion evolution, compared with the vertical dynamics accounting for interclass and economic drivers—to style variety
Dia t\=on gramm\=on: Euclidean geometry to solve trigonometric problems
Ancient astronomers faced the problem of dealing with arcs and angles in
their observations and predictions without the help of modern trigonometry. The
usual method to deal with such problems was the Menelaus Theorem, explicitly
discussed and extensively used by Ptolemy. It is generally assumed that
Hipparchus, who in his Exegesis explicitly states that he developed a method to
calculate Simultaneous Risings and Settings, used either the Menelaus' Theorem,
or stereographic projections, even if both methods are only attested for later
periods and their availability to Hipparchus is debated. In this paper we argue
that simultaneous risings and settings could be calculated using less advanced
mathematical tools unquestionably available to Hipparchus: the theorem of
Pythagoras, the theory of proportions, and the table of chords. Without
claiming that this was indeed the method that Hipparchus used, we argue that
there is no need to postulate the availability of more sophisticated tools to
carry out the calculations which Hipparchus mentions in the Exegesis
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