5,730 research outputs found
Retrieval analysis of 38 WFC3 transmission spectra and resolution of the normalisation degeneracy
A comprehensive analysis of 38 previously published Wide Field Camera 3
(WFC3) transmission spectra is performed using a hierarchy of nested-sampling
retrievals: with versus without clouds, grey versus non-grey clouds, isothermal
versus non-isothermal transit chords and with water, hydrogen cyanide and/or
ammonia. We revisit the "normalisation degeneracy": the relative abundances of
molecules are degenerate at the order-of-magnitude level with the absolute
normalisation of the transmission spectrum. Using a suite of mock retrievals,
we demonstrate that the normalisation degeneracy may be partially broken using
WFC3 data alone, even in the absence of optical/visible data and without
appealing to the presence of patchy clouds, although lower limits to the mixing
ratios may be prior-dominated depending on the measurement uncertainties. With
James Webb Space Telescope-like spectral resolutions, the normalisation
degeneracy may be completely broken from infrared spectra alone. We find no
trend in the retrieved water abundances across nearly two orders of magnitude
in exoplanet mass and a factor of 5 in retrieved temperature (about 500 to 2500
K). We further show that there is a general lack of strong Bayesian evidence to
support interpretations of non-grey over grey clouds (only for WASP-69b and
WASP-76b) and non-isothermal over isothermal atmospheres (no objects). 35 out
of 38 WFC3 transmission spectra are well-fitted by an isothermal transit chord
with grey clouds and water only, while 8 are adequately explained by flat
lines. Generally, the cloud composition is unconstrained.Comment: Accepted by MNRAS. 33 pages, 29 figures, 3 table
The role of word frequency and morpho-orthography in agreement processing
Agreement attraction in comprehension (when an ungrammatical verb is read quickly if preceded by a feature-matching local noun) is well described by a cue-based retrieval framework. This suggests a role for lexical retrieval in attraction. To examine this, we manipulated two probabilistic factors known to affect lexical retrieval: local noun word frequency and morpho-orthography (agreement morphology realised with or without –s endings) in a self-paced reading study. Noun number and word frequency affected noun and verb region reading times, with higher-frequency words not eliciting attraction. Morpho-orthography impacted verb processing but not attraction: atypical plurals led to slower verb reading times regardless of verb number. Exploratory individual difference analyses further underscore the importance of lexical retrieval dynamics in sentence processing. This provides evidence that agreement operates via a cue-based retrieval mechanism over lexical representations that vary in their strength and association to number features
Negative emotions boost users activity at BBC Forum
We present an empirical study of user activity in online BBC discussion
forums, measured by the number of posts written by individual debaters and the
average sentiment of these posts. Nearly 2.5 million posts from over 18
thousand users were investigated. Scale free distributions were observed for
activity in individual discussion threads as well as for overall activity. The
number of unique users in a thread normalized by the thread length decays with
thread length, suggesting that thread life is sustained by mutual discussions
rather than by independent comments. Automatic sentiment analysis shows that
most posts contain negative emotions and the most active users in individual
threads express predominantly negative sentiments. It follows that the average
emotion of longer threads is more negative and that threads can be sustained by
negative comments. An agent based computer simulation model has been used to
reproduce several essential characteristics of the analyzed system. The model
stresses the role of discussions between users, especially emotionally laden
quarrels between supporters of opposite opinions, and represents many observed
statistics of the forum.Comment: 29 pages, 6 figure
Power-law Distributions in Information Science - Making the Case for Logarithmic Binning
We suggest partial logarithmic binning as the method of choice for uncovering
the nature of many distributions encountered in information science (IS).
Logarithmic binning retrieves information and trends "not visible" in noisy
power-law tails. We also argue that obtaining the exponent from logarithmically
binned data using a simple least square method is in some cases warranted in
addition to methods such as the maximum likelihood. We also show why often used
cumulative distributions can make it difficult to distinguish noise from
genuine features, and make it difficult to obtain an accurate power-law
exponent of the underlying distribution. The treatment is non-technical, aimed
at IS researchers with little or no background in mathematics.Comment: Accepted for publication in JASIS
Retrieving Neptune's aerosol properties from Keck OSIRIS observations. I. Dark regions
We present and analyze three-dimensional data cubes of Neptune from the
OSIRIS integral-field spectrograph on the 10-m Keck telescope, from July 2009.
These data have a spatial resolution of 0.035"/pixel and spectral resolution of
R~3800 in the H and K broad bands. We focus our analysis on regions of
Neptune's atmosphere that are near-infrared dark- that is, free of discrete
bright cloud features. We use a forward model coupled to a Markov chain Monte
Carlo algorithm to retrieve properties of Neptune's aerosol structure and
methane profile above ~4 bar in these near-infrared dark regions.
Using a set of high signal-to-noise spectra in a cloud-free band from 2-12N,
we find that Neptune's cloud opacity is dominated by a compact, optically thick
cloud layer with a base near 3 bar and composed of low albedo, forward
scattering particles, with an assumed characteristic size of ~1m. Above
this cloud, we require a vertically extended haze of smaller (~0.1 m)
particles, which reaches from the upper troposphere (~0.6 bar) into the
stratosphere. The particles in this haze are brighter and more isotropically
scattering than those in the deep cloud. When we extend our analysis to 18
cloud-free locations from 20N to 87S, we observe that the optical depth in
aerosols above 0.5 bar decreases by a factor of 2-3 or more at mid- and
high-southern latitudes relative to low latitudes.
We also consider Neptune's methane (CH) profile, and find that our
retrievals indicate a strong preference for a low methane relative humidity at
pressures where methane is expected to condense. Our preferred solution at most
locations is for a methane relative humidity below 10% near the tropopause in
addition to methane depletion down to 2.0-2.5 bar. We tentatively identify a
trend of lower CH columns above 2.5 bar at mid- and high-southern latitudes
over low latitudes.Comment: Published in Icarus: 15 September 201
Some implications of sampling choices on comparisons between satellite and model aerosol optical depth fields
The comparison of satellite and model aerosol optical depth (AOD) fields provides useful information on the strengths and weaknesses of both. However, the sampling of satellite and models is very different and some subjective decisions about data selection and aggregation must be made in order to perform such comparisons. This work examines some implications of these decisions, using GlobAerosol AOD retrievals at 550 nm from Advanced Along-Track Scanning Radiometer (AATSR) measurements, and aerosol fields from the GEOS-Chem chemistry transport model. It is recommended to sample the model only where the satellite flies over on a particular day; neglecting this can cause regional differences in model AOD of up to 0.1 on monthly and annual timescales. The comparison is observed to depend strongly upon thresholds for sparsity of satellite retrievals in the model grid cells. Requiring at least 25% coverage of the model grid cell by satellite data decreases the observed difference between the two by approximately half over land. The impact over ocean is smaller. In both model and satellite datasets, there is an anticorrelation between the proportion <i>p</i> of a model grid cell covered by satellite retrievals and the AOD. This is attributed to small <i>p</i> typically occuring due to high cloud cover and lower AODs being found in large clear-sky regions. Daily median AATSR AODs were found to be closer to GEOS-Chem AODs than daily means (with the root mean squared difference being approximately 0.05 smaller). This is due to the decreased sensitivity of medians to outliers such as cloud-contaminated retrievals, or aerosol point sources not included in the model
Neural differentiation is moderated by age in scene- but not face-selective cortical regions
The aging brain is characterized by neural dedifferentiation, an apparent decrease in the functional selectivity of category-selective cortical regions. Age-related reductions in neural differentiation have been proposed to play a causal role in cognitive aging. Recent findings suggest, however, that age-related dedifferentiation is not equally evident for all stimulus categories and, additionally, that the relationship between neural differentiation and cognitive performance is not moderated by age. In light of these findings, in the present experiment, younger and older human adults (males and females) underwent fMRI as they studied words paired with images of scenes or faces before a subsequent memory task. Neural selectivity was measured in two scene-selective (parahippocampal place area (PPA) and retrosplenial cortex (RSC)] and two face-selective [fusiform face area (FFA) and occipital face area (OFA)] regions using both a univariate differentiation index and multivoxel pattern similarity analysis. Both methods provided highly convergent results, which revealed evidence of age-related reductions in neural dedifferentiation in scene-selective but not face-selective cortical regions. Additionally, neural differentiation in the PPA demonstrated a positive, age-invariant relationship with subsequent source memory performance (recall of the image category paired with each recognized test word). These findings extend prior findings suggesting that age-related neural dedifferentiation is not a ubiquitous phenomenon, and that the specificity of neural responses to scenes is predictive of subsequent memory performance independently of age
Characterization of dust activity on Mars from MY27 to MY32 by PFS-MEX observations
We present spatial and temporal distributions of dust on Mars from Ls = 331
in MY26 until Ls = 80 in MY33 retrieved from the measurements taken by the
Planetary Fourier Spectrometer (PFS) aboard Mars Express. In agreement with
previous observations, large dust opacity is observed mostly in the southern
hemisphere spring/summer and particularly over regions of higher terrain and
large topographic variation. We present a comparison with dust opacities
obtained from Thermal Emission Spectrometer (TES) - Mars Global Surveyor (MGS)
measurements. We found good consistency between observations of two instruments
during overlapping interval (Ls = 331 in MY26 until Ls = 77 in MY27). We found
a different behavior of the dust opacity with latitude in the various Martian
years (inter-annual variations). A global dust storm occurred in MY28. We
observe a different spatial distribution, a later occurrence and dissipation of
the dust maximum activity in MY28 than in other Martian years. A possible
precursor signal to the global dust storm in MY 28 is observed at Ls = 200 -
235 especially over west Hellas. Heavy dust loads alter atmospheric
temperatures. Due to the absorption of solar radiation and emission of infrared
radiation to space by dust vertically non-uniformly distributed, a strong
heating of high atmospheric levels (40 - 50 km) and cooling below around 30 km
are observed.Comment: in press in Icarus. 47 pages, 15 figure
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