842 research outputs found
Distinction between clouds and ice/snow covered surfaces in the identification of cloud-free observations using SCIAMACHY PMDs
SCIAMACHY on ENVISAT allows measurement of different trace gases including those most abundant in the troposphere (e.g. CO<sub>2</sub>, NO<sub>2</sub>, CH<sub>4</sub>, BrO, SO<sub>2</sub>). However, clouds in the observed scenes can severely hinder the observation of tropospheric gases. Several cloud detection algorithms have been developed for GOME on ERS-2 which can be applied to SCIAMACHY. The GOME cloud algorithms, however, suffer from the inadequacy of not being able to distinguish between clouds and ice/snow covered surfaces because GOME only covers the UV, VIS and part of the NIR wavelength range (240-790 nm). As a result these areas are always flagged as clouded, and therefore often not used. Here a method is presented which uses the SCIAMACHY measurements in the wavelength range between 450 nm and 1.6 µm to make a distinction between clouds and ice/snow covered surfaces. The algorithm is developed using collocated MODIS observations. The algorithm presented here is specifically developed to identify cloud-free SCIAMACHY observations. The SCIAMACHY Polarisation Measurement Devices (PMDs) are used for this purpose because they provide higher spatial resolution compared to the main spectrometer measurements
On graphs with a large chromatic number containing no small odd cycles
In this paper, we present the lower bounds for the number of vertices in a
graph with a large chromatic number containing no small odd cycles
Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon
Aims: We demonstrate the universal character of the quiet-Sun chromosphere
among inactive stars (solar-type and giants). By assessing the main physical
processes, we shed new light on some common observational phenomena. Methods:
We discuss measurements of the solar Mt. Wilson S-index, obtained by the
Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the
established chromospheric basal Ca II K line flux to the Mt. Wilson S-index
data of inactive ("flat activity") stars, including giants. Results: During the
unusually deep and extended activity minimum of 2009, the Sun reached S-index
values considerably lower than in any of its previously observed minima. In
several brief periods, the Sun coincided exactly with the S-indices of inactive
("flat", presumed Maunder Minimum-type) solar analogues of the Mt. Wilson
sample; at the same time, the solar visible surface was also free of any plages
or remaining weak activity regions. The corresponding minimum Ca II K flux of
the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson
sample are found to be identical to the corresponding Ca II K chromospheric
basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a
universal phenomenon among inactive stars. Its mixed-polarity magnetic field,
generated by a local, "fast" turbulent dynamo finally provides a natural
explanation for the minimal soft X-ray emission observed for inactive stars.
Given such a local dynamo also works for giant chromospheres, albeit on larger
length scales, i.e., l ~ R/g, with R and g as stellar radius and surface
gravity, respectively, the existence of giant spicular phenomena and the
guidance of mechanical energy toward the acceleration zone of cool stellar
winds along flux-tubes have now become traceable.Comment: 6 pages, 4 figures; Astronomy & Astrophysics (Research Note), in
pres
Lifting Bell inequalities
A Bell inequality defined for a specific experimental configuration can always be extended to a situation involving more observers, measurement settings, or measurement outcomes. In this article, such "liftings" of Bell inequalities are studied. It is shown that if the original inequality defines a facet of the polytope of local joint outcome probabilities then the lifted one also defines a facet of the more complex polytope
Newly Discovered Global Temperature Structures in the Quiet Sun at Solar Minimum
Magnetic loops are building blocks of the closed-field corona. While active
region loops are readily seen in images taken at EUV and X-ray wavelengths,
quiet Sun loops are seldom identifiable and therefore difficult to study on an
individual basis. The first analysis of solar minimum (Carrington Rotation
2077) quiet Sun (QS) coronal loops utilizing a novel technique called the
Michigan Loop Diagnostic Technique (MLDT) is presented. This technique combines
Differential Emission Measure Tomography (DEMT) and a potential field source
surface (PFSS) model, and consists of tracing PFSS field lines through the
tomographic grid on which the Local Differential Emission Measure (LDEM) is
determined. As a result, the electron temperature Te and density Ne at each
point along each individual field line can be obtained. Using data from
STEREO/EUVI and SOHO/MDI, the MLDT identifies two types of QS loops in the
corona: so-called "up" loops in which the temperature increases with height,
and so-called "down" loops in which the temperature decreases with height. Up
loops are expected, however, down loops are a surprise, and furthermore, they
are ubiquitous in the low-latitude corona. Up loops dominate the QS at higher
latitudes. The MLDT allows independent determination of the empirical pressure
and density scale heights, and the differences between the two remain to be
explained. The down loops appear to be a newly discovered property of the solar
minimum corona that may shed light on the physics of coronal heating. The
results are shown to be robust to the calibration uncertainties of the EUVI
instrument.Comment: Accepted for publication in The Astrophysical Journal, waiting for
the full biblio inf
The effect of magnetic activity saturation in chromospheric flux-flux relationships
We present a homogeneous study of chromospheric and coronal flux-flux
relationships using a sample of 298 late-type dwarf active stars with spectral
types F to M. The chromospheric lines were observed simultaneously in each star
to avoid spread due to long term variability. Unlike other works, we subtract
the basal chromospheric contribution in all the spectral lines studied. For the
first time, we quantify the departure of dMe stars from the general relations.
We show that dK and dKe stars also deviate from the general trend. Studying the
flux-colour diagrams we demonstrate that the stars deviating from the general
relations are those with saturated X-ray emission and that those stars also
present saturation in the H line. Using several age spectral
indicators, we show that they are younger stars than those following the
general relationships. The non-universality of flux-flux relationships found in
this work should be taken into account when converting between fluxes in
different chromospheric activity indicators.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Societ
Can surface flux transport account for the weak polar field in cycle 23?
To reproduce the weak magnetic field on the polar caps of the Sun observed
during the declining phase of cycle 23 poses a challenge to surface flux
transport models since this cycle has not been particularly weak. We use a
well-calibrated model to evaluate the parameter changes required to obtain
simulated polar fields and open flux that are consistent with the observations.
We find that the low polar field of cycle 23 could be reproduced by an increase
of the meridional flow by 55% in the last cycle. Alternatively, a decrease of
the mean tilt angle of sunspot groups by 28% would also lead to a similarly low
polar field, but cause a delay of the polar field reversals by 1.5 years in
comparison to the observations.Comment: 9 pages, 8 figures, Space Science Reviews, accepte
Microbial community management in aquaculture
Microbial community management in aquaculture creates benefits at the nutritional as well as at health level for cultured species. In addition, in case of biofloc application, it allows to link species at different trophic levels, making bioflocs the potential link in integrated multispecies aquaculture
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