54,373 research outputs found
The June 2012 transit of Venus. Framework for interpretation of observations
Ground based observers have on 5/6th June 2012 the last opportunity of the
century to watch the passage of Venus across the solar disk from Earth. Venus
transits have traditionally provided unique insight into the Venus atmosphere
through the refraction halo that appears at the planet outer terminator near
ingress/egress. Much more recently, Venus transits have attracted renewed
interest because the technique of transits is being successfully applied to the
characterization of extrasolar planet atmospheres. The current work
investigates theoretically the interaction of sunlight and the Venus atmosphere
through the full range of transit phases, as observed from Earth and from a
remote distance. Our model predictions quantify the relevant atmospheric
phenomena, thereby assisting the observers of the event in the interpretation
of measurements and the extrapolation to the exoplanet case. Our approach
relies on the numerical integration of the radiative transfer equation, and
includes refraction, multiple scattering, atmospheric extinction and solar limb
darkening, as well as an up to date description of the Venus atmosphere. We
produce synthetic images of the planet terminator during ingress/egress that
demonstrate the evolving shape, brightness and chromaticity of the halo.
Guidelines are offered for the investigation of the planet upper haze from
vertically-unresolved photometric measurements. In this respect, the comparison
with measurements from the 2004 transit appears encouraging. We also show
integrated lightcurves of the Venus/Sun system at various phases during transit
and calculate the respective Venus-Sun integrated transmission spectra. The
comparison of the model predictions to those for a Venus-like planet free of
haze and clouds (and therefore a closer terrestrial analogue) complements the
discussion and sets the conclusions into a broader perspective.Comment: 14 pages; 14 figures; Submitted on 02/06/2012; A&A, accepted for
publication on 30/08/201
A neural mechanism for binaural pitch perception via ghost stochastic resonance
We present a physiologically plausible binaural mechanism for the perception
of the pitch of complex sounds via ghost stochastic resonance. In this scheme,
two neurons are driven by noise and different periodic signal each (with
frequencies f1=kf0 and f2=(k+1)f0, where k>1), and their outputs (plus noise)
are applied synaptically to a third neuron. Our numerical results, using the
Morris-Lecar neuron model with chemical synapses explicity considered, show
that intermediate noise levels enhance the response of the third neuron at
frequencies close to f0, as in the cases previously described of ghost
resonance. For the case of inharmonic combinations of inputs (both frequencies
shifted by the same amount Df) noise is also seen to enhance the response of
the third neuron at a frequency fr with also shift linearly with Df. In
addition, we show that similar resonances can be observed as a function of the
synaptic time constant. The suggested ghost-resonance-based stochastic
mechanism can thus arise either at the peripheral level or at a higher level of
neural processing in the perception of the pitchComment: 7 pages, 5 figure
Derivation of the physical parameters of the jet in S5 0836+710 from stability analysis
A number of extragalactic jets show periodic structures at different scales
that can be associated with growing instabilities. The wavelengths of the
developing instability modes and their ratios depend on the flow parameters, so
the study of those structures can shed light on jet physics at the scales
involved. In this work, we use the fits to the jet ridgeline obtained from
different observations of S5 B0836710 and apply stability analysis of
relativistic, sheared flows to derive an estimate of the physical parameters of
the jet. Based on the assumption that the observed structures are generated by
growing Kelvin-Helmholtz (KH) instability modes, we have run numerical
calculations of stability of a relativistic, sheared jet over a range of
different jet parameters. We have spanned several orders of magnitude in
jet-to-ambient medium density ratio, and jet internal energy, and checked
different values of the Lorentz factor and shear layer width. This represents
an independent method to obtain estimates of the physical parameters of a jet.
By comparing the fastest growing wavelengths of each relevant mode given by the
calculations with the observed wavelengths reported in the literature, we have
derived independent estimates of the jet Lorentz factor, specific internal
energy, jet-to-ambient medium density ratio and Mach number. We obtain a jet
Lorentz factor , specific internal energy of , jet-to-ambient medium density ratio of , and an internal (classical) jet Mach number of . We also find that the wavelength ratios are better recovered by a
transversal structure with a width of of the jet radius. This
method represents a powerful tool to derive the jet parameters in all jets
showing helical patterns with different wavelengths.Comment: Accepted for publication in A&A, 15 pages, 12 figure
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