49,983 research outputs found
New near-IR observations of mesospheric CO2 and H2O clouds on Mars
Carbon dioxide clouds, which are speculated by models on solar and
extra-solar planets, have been recently observed near the equator of Mars. The
most comprehensive identification of Martian CO2 ice clouds has been obtained
by the near-IR imaging spectrometer OMEGA. CRISM, a similar instrument with a
higher spatial resolution, cannot detect these clouds with the same method due
to its shorter wavelength range. Here we present a new method to detect CO2
clouds using near-IR data based on the comparison of H2O and CO2 ice spectral
properties. The spatial and seasonal distributions of 54 CRISM observations
containing CO2 clouds are reported, in addition to 17 new OMEGA observations.
CRISM CO2 clouds are characterized by grain size in the 0.5-2\mum range and
optical depths lower than 0.3. The distributions of CO2 clouds inferred from
OMEGA and CRISM are consistent with each other and match at first order the
distribution of high altitude (>60km) clouds derived from previous studies. At
second order, discrepancies are observed. We report the identification of H2O
clouds extending up to 80 km altitude, which could explain part of these
discrepancies: both CO2 and H2O clouds can exist at high, mesospheric
altitudes. CRISM observations of afternoon CO2 clouds display morphologies
resembling terrestrial cirrus, which generalizes a previous result to the whole
equatorial clouds season. Finally, we show that morning OMEGA observations have
been previously misinterpreted as evidence for cumuliform, and hence
potentially convective, CO2 clouds.Comment: Vincendon, M., C. Pilorget, B. Gondet, S. Murchie, and J.-P. Bibring
(2011), New near-IR observations of mesospheric CO2 and H2O clouds on Mars,
J. Geophys. Res., 116, E00J0
A high-precision liDAR-based method for surveying and classifying coastal notches
Formation of notches is an important process in the erosion of seaside cliffs. Monitoring of coastal notch erosion rate and processes has become a prime research focus for many coastal geomorphologists. Observation of notch erosion rate considers a number of characteristics, including cliff collapse risk, distinction of historical sea levels, and recognition of ongoing erosional mechanisms. This study presents new approaches for surveying and classifying marine notches based on a high-precision light detection and ranging (LiDAR)-based experiment performed on a small region of a coastal cliff in southern Portugal. A terrestrial LiDAR scanner was used to measure geometrical parameters and surface roughness of selected notches, enabling their classification according to shape and origin. The implemented methodology proved to be a highly effective tool for providing an unbiased analysis of marine morphodynamic processes acting on the seaside cliffs. In the analyzed population of voids carved into Miocene calcarenites in a coastal cliff section, two types of notch morphology were distinguished, namely U-shaped and V-shaped. The method presented here provides valuable data for landscape evaluation, sea-level changes, and any other types of analyses that rely on the accurate interpretation of cliff morphological features.National Science Centre [UMO-2015/17/D/ST10/02191
Radio emission from Supernova Remnants
The explosion of a supernova releases almost instantaneously about 10^51 ergs
of mechanic energy, changing irreversibly the physical and chemical properties
of large regions in the galaxies. The stellar ejecta, the nebula resulting from
the powerful shock waves, and sometimes a compact stellar remnant, constitute a
supernova remnant (SNR). They can radiate their energy across the whole
electromagnetic spectrum, but the great majority are radio sources. Almost 70
years after the first detection of radio emission coming from a SNR, great
progress has been achieved in the comprehension of their physical
characteristics and evolution. We review the present knowledge of different
aspects of radio remnants, focusing on sources of the Milky Way and the
Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief
overview of theoretical background, analyze morphology and polarization
properties, and review and critical discuss different methods applied to
determine the radio spectrum and distances. The consequences of the interaction
between the SNR shocks and the surrounding medium are examined, including the
question of whether SNRs can trigger the formation of new stars. Cases of
multispectral comparison are presented. A section is devoted to reviewing
recent results of radio SNRs in the Magellanic Clouds, with particular emphasis
on the radio properties of SN 1987A, an ideal laboratory to investigate
dynamical evolution of an SNR in near real time. The review concludes with a
summary of issues on radio SNRs that deserve further study, and analyzing the
prospects for future research with the latest generation radio telescopes.Comment: Revised version. 48 pages, 15 figure
Meteorology of Jupiter's Equatorial Hot Spots and Plumes from Cassini
We present an updated analysis of Jupiter's equatorial meteorology from
Cassini observations. For two months preceding the spacecraft's closest
approach, the Imaging Science Subsystem (ISS) onboard regularly imaged the
atmosphere. We created time-lapse movies from this period in order to analyze
the dynamics of equatorial hot spots and their interactions with adjacent
latitudes. Hot spots are quasi-stable, rectangular dark areas on
visible-wavelength images, with defined eastern edges that sharply contrast
with surrounding clouds, but diffuse western edges serving as nebulous
boundaries with adjacent equatorial plumes. Hot spots exhibit significant
variations in size and shape over timescales of days and weeks. Some of these
changes correspond with passing vortex systems from adjacent latitudes
interacting with hot spots. Strong anticyclonic gyres present to the south and
southeast of the dark areas appear to circulate into hot spots. Impressive,
bright white plumes occupy spaces in between hot spots. Compact cirrus-like
'scooter' clouds flow rapidly through the plumes before disappearing within the
dark areas. These clouds travel at 150-200 m/s, much faster than the 100 m/s
hot spot and plume drift speed. This raises the possibility that the scooter
clouds may be more illustrative of the actual jet stream speed at these
latitudes. Most previously published zonal wind profiles represent the drift
speed of the hot spots at their latitude from pattern matching of the entire
longitudinal image strip. If a downward branch of an equatorially-trapped
Rossby waves controls the overall appearance of hot spots, however, the
westward phase velocity of the wave leads to underestimates of the true jet
stream speed.Comment: 33 pages, 11 figures; accepted for publication in Icarus; for
supplementary movies, please contact autho
Gamma-ray signatures of cosmic ray acceleration, propagation, and confinement in the era of CTA
Galactic cosmic rays are commonly believed to be accelerated at supernova
remnants via diffusive shock acceleration. Despite the popularity of this idea,
a conclusive proof for its validity is still missing. Gamma-ray astronomy
provides us with a powerful tool to tackle this problem, because gamma rays are
produced during cosmic ray interactions with the ambient gas. The detection of
gamma rays from several supernova remnants is encouraging, but still does not
constitute a proof of the scenario, the main problem being the difficulty in
disentangling the hadronic and leptonic contributions to the emission. Once
released by their sources, cosmic rays diffuse in the interstellar medium, and
finally escape from the Galaxy. The diffuse gamma-ray emission from the
Galactic disk, as well as the gamma-ray emission detected from a few galaxies
is largely due to the interactions of cosmic rays in the interstellar medium.
On much larger scales, cosmic rays are also expected to permeate the
intracluster medium, since they can be confined and accumulated within clusters
of galaxies for cosmological times. Thus, the detection of gamma rays from
clusters of galaxies, or even upper limits on their emission, will allow us to
constrain the cosmic ray output of the sources they contain, such as normal
galaxies, AGNs, and cosmological shocks. In this paper, we describe the impact
that the Cherenkov Telescope Array, a future ground-based facility for
very-high energy gamma-ray astronomy, is expected to have in this field of
research.Comment: accepted to Astroparticle Physics, special issue on Physics with the
Cherenkov Telescope Arra
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