1,620 research outputs found
Solar total irradiance in cycle 23
The apparently unusual behaviour of the TSI during the most recent minimum of
solar activity has been interpreted as evidence against solar surface magnetism
as the main driver of the secular change in the TSI. We test claims that the
evolution of the solar surface magnetic field does not reproduce the observed
TSI in cycle 23. We use sensitive, 60-minute averaged MDI magnetograms and
quasi-simultaneous continuum images as an input to our SATIRE-S model and
calculate the TSI variation over cycle 23, sampled roughly twice-monthly. The
computed TSI is then compared to the PMOD composite of TSI measurements and to
the data from two individual instruments, SORCE/TIM and UARS/ACRIM II, that
monitored the TSI during the declining phase of cycle 23 and over the previous
minimum in 1996, respectively. Excellent agreement is found between the trends
shown by the model and almost all sets of measurements. The only exception is
the early, i.e. 1996 to 1998, PMOD data. Whereas the agreement between the
model and the PMOD composite over the period 1999-2009 is almost perfect, the
modelled TSI shows a steeper increase between 1996 and 1999 than implied by the
PMOD composite. On the other hand, the steeper trend in the model agrees
remarkably well with the ACRIM II data. A closer look at the VIRGO data, that
make the basis of the PMOD composite after 1996, reveals that only one of the
two VIRGO instruments, the PMO6V, shows the shallower trend present in the
composite, whereas the DIARAD measurements indicate a steeper trend. We
conclude that (1) the sensitivity changes of the PMO6V radiometers within VIRGO
during the first two years have very likely not been correctly evaluated, and
that (2) the TSI variations over cycle 23 and the change in the TSI levels
between the minima in 1996 and 2008 are consistent with the solar surface
magnetism mechanism
ACRIM-gap and total solar irradiance revisited: Is there a secular trend between 1986 and 1996?
A gap in the total solar irradiance (TSI) measurements between ACRIM-1 and
ACRIM-2 led to the ongoing debate on the presence or not of a secular trend
between the minima preceding cycles 22 (in 1986) and 23 (1996). It was recently
proposed to use the SATIRE model of solar irradiance variations to bridge this
gap. When doing this, it is important to use the appropriate SATIRE-based
reconstruction, which we do here, employing a reconstruction based on
magnetograms. The accuracy of this model on months to years timescales is
significantly higher than that of a model developed for long-term
reconstructions used by the ACRIM team for such an analysis. The constructed
`mixed' ACRIM - SATIRE composite shows no increase in the TSI from 1986 to
1996, in contrast to the ACRIM TSI composite.Comment: 4 figure
Folding and unfolding phylogenetic trees and networks
Phylogenetic networks are rooted, labelled directed acyclic graphs which are commonly used to represent reticulate evolution. There is a close relationship between phylogenetic networks and multi-labelled trees (MUL-trees). Indeed, any phylogenetic network can be "unfolded" to obtain a MUL-tree and, conversely, a MUL-tree can in certain circumstances be "folded" to obtain a phylogenetic network that exhibits . In this paper, we study properties of the operations and in more detail. In particular, we introduce the class of stable networks, phylogenetic networks for which is isomorphic to , characterise such networks, and show that they are related to the well-known class of tree-sibling networks.We also explore how the concept of displaying a tree in a network can be related to displaying the tree in the MUL-tree . To do this, we develop a phylogenetic analogue of graph fibrations. This allows us to view as the analogue of the universal cover of a digraph, and to establish a close connection between displaying trees in and reconcilingphylogenetic trees with networks
Reconstructed and measured total solar irradiance: Is there a secular trend between 1978 and 2003?
Total solar irradiance reconstructed between 1978 and 2003 using solar
surface magnetic field distributions is compared with three composites of total
solar irradiance measurements. A good correspondence is found with the total
solar irradiance composite from PMOD/WRC, with no bias between the three
cycles. The agreement with the other composites (the ACRIM composite, mainly
based on the Active Cavity Radiometer Irradiance Monitors I, II & III, and the
IRMB composite from the Institut Royal Meteorologique Belgique) is
significantly poorer. In particular, a secular increase in the irradiance
exhibited by these composites is not present in the reconstructions. Hence any
secular trend in total solar irradiance between 1978 and 2003 is not due to
magnetic fields at the solar surface
Resonance as the Mechanism of Daytime Periodic Breathing in Patients with Heart Failure
Rationale: In patients with chronic heart failure, daytime oscillatory breathing at rest is associated with a high risk of mortality. Experimental evidence, including exaggerated ventilatory responses to CO2 and prolonged circulation time, implicates the ventilatory control system and suggests feedback instability (loop gainâ>â1) is responsible. However, daytime oscillatory patterns often appear remarkably irregular versus classic instability (Cheyne-Stokes respiration), suggesting our mechanistic understanding is limited.
Objectives: We propose that daytime ventilatory oscillations generally result from a chemoreflex resonance, in which spontaneous biological variations in ventilatory drive repeatedly induce temporary and irregular ringing effects. Importantly, the ease with which spontaneous biological variations induce irregular oscillations (resonance âstrengthâ) rises profoundly as loop gain rises toward 1. We tested this hypothesis through a comparison of mathematical predictions against actual measurements in patients with heart failure and healthy control subjects.
Methods: In 25 patients with chronic heart failure and 25 control subjects, we examined spontaneous oscillations in ventilation and separately quantified loop gain using dynamic inspired CO2 stimulation.
Measurements and Main Results: Resonance was detected in 24 of 25 patients with heart failure and 18 of 25 control subjects. With increased loop gainâconsequent to increased chemosensitivity and delayâthe strength of spontaneous oscillations increased precipitously as predicted (râ=â0.88), yielding larger (râ=â0.78) and more regular (interpeak interval SD, râ=ââ0.68) oscillations (Pâ<â0.001 for all, both groups combined).
Conclusions: Our study elucidates the mechanism underlying daytime ventilatory oscillations in heart failure and provides a means to measure and interpret these oscillations to reveal the underlying chemoreflex hypersensitivity and reduced stability that foretells mortality in this population
Detection of circumstellar material in a normal Type Ia Supernova
Type Ia supernovae are thought to be thermonuclear explosions of accreting
white dwarfs that reach a critical mass limit. Despite their importance as
cosmological distance indicators, the nature of their progenitors has remained
controversial. Here we report the detection of circumstellar material in a
normal Type Ia supernova. The expansion velocities, densities and dimensions of
the circumstellar envelope indicate that this material was ejected from the
progenitor system. The relatively low expansion velocities appear to favor a
progenitor system where a white dwarf accretes material from a companion star
which is in the red-giant phase at the time of explosion.Comment: 25 pages, 7 figures. Accepted for publication in Science. Full
resolution version at
http://www.hq.eso.org/~fpatat/science/sn06X/preprint.pdf . The original paper
can be found at http://www.sciencemag.org/cgi/content/abstract/114300
On the intensity contrast of solar photospheric faculae and network elements
Sunspots, faculae and the magnetic network contribute to solar irradiance
variations. The contribution due to faculae and the network is of basic
importance, but suffers from considerable uncertainty. We determine the
contrasts of active region faculae and the network, both as a function of
heliocentric angle and magnetogram signal. To achieve this, we analyze
near-simultaneous full disk images of photospheric continuum intensity and
line-of-sight magnetic field provided by the Michelson Doppler Interferometer
(MDI) on board the SOHO spacecraft. Starting from the surface distribution of
the solar magnetic field we first construct a mask, which is then used to
determine the brightness of magnetic features, and the relatively field-free
part of the photosphere separately. By sorting the magnetogram signal into
different bins we are able to distinguish between the contrasts of different
concentrations of magnetic field. We find that the contrasts of active region
faculae (large magnetogram signal) and the network (small signal) exhibit a
very different CLV, showing that the populations of magnetic flux tubes are
different. This implies that these elements need to be treated separately when
reconstructing variations of the total solar irradiance with high precision. We
have obtained an analytical expression for the contrast of photospheric
magnetic features as a function of both position on the disk and magnetic field
strength, by performing a 2-dimensional fit to the observations.Comment: 12 pages, 8 figures, uses aa.cl
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