22,022 research outputs found
Remote sensing of tropical tropopause layer radiation balance using A-train measurements
Determining the level of zero net radiative heating (LZH) is critical to understanding parcel trajectory in the Tropical Tropopause Layer (TTL) and associated stratospheric hydration processes. Previous studies of the TTL radiative balance have focused on using radiosonde data, but remote sensing measurements from polar-orbiting satellites may provide the relevant horizontal and vertical information for assessing TTL solar heating and infrared cooling rates, especially across the Pacific Ocean. CloudSat provides a considerable amount of vertical information about the distribution of cloud properties relevant to heating rate analysis. The ability of CloudSat measurements and ancillary information to constrain LZH is explored. We employ formal error propagation analysis for derived heating rate uncertainty given the CloudSat cloud property retrieval algorithms. Estimation of the LZH to within approximately 0.5 to 1 km is achievable with CloudSat, but it has a low-altitude bias because the radar is unable to detect thin cirrus. This can be remedied with the proper utilization of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar backscatter information. By utilizing an orbital simulation with the GISS data set, we explore the representativeness of non-cross-track scanning active sounders in terms of describing the LZH distribution. In order to supplement CloudSat, we explore the ability of Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Scanning Radiometer-EOS (AMSR-E) to constrain LZH and find that these passive sounders are useful where the cloud top height does not exceed 7 km. The spatiotemporal distributions of LZH derived from CloudSat and CALIPSO measurements are presented which suggest that thin cirrus have a limited effect on LZH mean values but affect LZH variability
SDSS Standard Star Catalog for Stripe 82: the Dawn of Industrial 1% Optical Photometry
We describe a standard star catalog constructed using multiple SDSS
photometric observations (at least four per band, with a median of ten) in the
system. The catalog includes 1.01 million non-variable unresolved
objects from the equatorial stripe 82 ( 1.266) in
the RA range 20h 34m to 4h 00m, and with the corresponding band
(approximately Johnson V band) magnitudes in the range 14--22. The
distributions of measurements for individual sources demonstrate that the
photometric pipeline correctly estimates random photometric errors, which are
below 0.01 mag for stars brighter than (19.5, 20.5, 20.5, 20, 18.5) in ,
respectively (about twice as good as for individual SDSS runs). Several
independent tests of the internal consistency suggest that the spatial
variation of photometric zeropoints is not larger than 0.01 mag (rms). In
addition to being the largest available dataset with optical photometry
internally consistent at the 1% level, this catalog provides practical
definition of the SDSS photometric system. Using this catalog, we show that
photometric zeropoints for SDSS observing runs can be calibrated within nominal
uncertainty of 2% even for data obtained through 1 mag thick clouds, and
demonstrate the existence of He and H white dwarf sequences using photometric
data alone. Based on the properties of this catalog, we conclude that upcoming
large-scale optical surveys such as the Large Synoptic Survey Telescope will be
capable of delivering robust 1% photometry for billions of sources.Comment: 63 pages, 24 figures, submitted to AJ, version with correct figures
and catalog available from
http://www.astro.washington.edu/ivezic/sdss/catalogs/stripe82.htm
Analysis of Neptune's 2017 Bright Equatorial Storm
We report the discovery of a large (8500 km diameter) infrared-bright
storm at Neptune's equator in June 2017. We tracked the storm over a period of
7 months with high-cadence infrared snapshot imaging, carried out on 14 nights
at the 10 meter Keck II telescope and 17 nights at the Shane 120 inch reflector
at Lick Observatory. The cloud feature was larger and more persistent than any
equatorial clouds seen before on Neptune, remaining intermittently active from
at least 10 June to 31 December 2017. Our Keck and Lick observations were
augmented by very high-cadence images from the amateur community, which
permitted the determination of accurate drift rates for the cloud feature. Its
zonal drift speed was variable from 10 June to at least 25 July, but remained a
constant m s from 30 September until at least 15
November. The pressure of the cloud top was determined from radiative transfer
calculations to be 0.3-0.6 bar; this value remained constant over the course of
the observations. Multiple cloud break-up events, in which a bright cloud band
wrapped around Neptune's equator, were observed over the course of our
observations. No "dark spot" vortices were seen near the equator in HST imaging
on 6 and 7 October. The size and pressure of the storm are consistent with
moist convection or a planetary-scale wave as the energy source of convective
upwelling, but more modeling is required to determine the driver of this
equatorial disturbance as well as the triggers for and dynamics of the observed
cloud break-up events.Comment: 42 pages, 14 figures, 6 tables; Accepted to Icaru
The jovian environment
Jovian atmosphere and environment - composition, structure, and photometric properties of atmosphere, period of rotation, magnetic field, and Jovian radiofrequency spectru
Readout of GEM Detectors Using the Medipix2 CMOS Pixel Chip
We have operated a Medipix2 CMOS readout chip, with amplifying, shaping and
charge discriminating front-end electronics integrated on the pixel-level, as a
highly segmented direct charge collecting anode in a three-stage gas electron
multiplier (Triple-GEM) to detect the ionization from Fe X-rays and
electrons from Ru. The device allows to perform moderate energy
spectroscopy measurements (20 % FWHM at 5.9 keV -rays) using only digital
readout and two discriminator thresholds. Being a truly 2D-detector, it allows
to observe individual clusters of minimum ionizing charged particles in
(70:30) and (70:30) mixtures and to achieve excellent
spatial resolution for position reconstruction of primary clusters down to
, based on the binary centroid determination method.Comment: 18 pages, 14 pictures. submitted to Nuclear Instruments and Methods
in Physics Research
The Role of Stellar Feedback and Dark Matter in the Evolution of Dwarf Galaxies
Supernova and multiple supernova events regulate several structural
properties of dwarf galaxies. In particular, they govern the metal enrichment
and the energy budget of the ISM; they might induce partial (blowout) or total
(blowaway) gas removal from the galaxy; the morphology of the galactic gaseous
body. Significant amounts of dark matter may play an equally important role:
the dark matter gravitational potential tends to concentrate baryons towards
the center, thus enhancing both the star formation rate and metal production.
Also, the dynamical properties of the ISM, and the occurrence of a blowout or
blowaway are shown to be determined by the dark matter content. We present
detailed analytical/numerical models describing the evolution of dwarf
Irregular galaxies including the above and other effects. The main results are:
(i) dwarfs with total masses M\simlt 5\times 10^6 M_\odot are blown away;
those with gas masses up to lose mass in an outflow; (ii)
metallicities are found to correlate tightly with dark matter content and are
consistent with a range of dark-to-visible mass ratios with
about 65% of the dwarfs in the sample having ; (iii) we
predict a lower limit to the oxygen abundance in dIs of ; (iv) outflows are not particularly important for the metallicity
evolution of dwarf galaxies and certainly less than star formation for gas
consumption; however, dwarfs with gas masses few are
shown to be the major pollutants of the IGM; (v) the ISM HI velocity dispersion
correlates with metallicity and, indepentently of dark matter, scales as
. (Abridged)Comment: 56 pages, aasms4.sty, LaTeX, 12 figures. MNRAS, submitte
Siting Multiple Observers for Maximum Coverage: An Accurate Approach
The selection of the minimal number of observers that ensures the maximum visual coverage
over an area represented by a digital elevation model (DEM) have great interest in many
elds, e.g., telecommunications, environment planning, among others. However, this problem
is complex and intractable when the number of points of the DEM is relatively high. This
complexity is due to three issues: 1) the di culty in determining the visibility of the terrain
from one point, 2) the need to know the visibility at all points of the terrain and 3) the
combinatorial complexity of the selection of observers.
The recent progress in total-viewshed maps computation not only provides an e cient solu-
tion to the rst two problems, but also opens other ways to new solutions that were unthinkable
previously. This paper presents a new type of cartography, called the masked total viewshed
map, and provides optimal solutions for both sequential and simultaneous observers location.Universidad de Málaga. Campus de Excelencia Internacional AndalucÃa Tech
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