31,711 research outputs found
Jupiter as an exoplanet: UV to NIR transmission spectrum reveals hazes, a Na layer and possibly stratospheric H2O-ice clouds
Currently, the analysis of transmission spectra is the most successful
technique to probe the chemical composition of exoplanet atmospheres. But the
accuracy of these measurements is constrained by observational limitations and
the diversity of possible atmospheric compositions. Here we show the UV-VIS-IR
transmission spectrum of Jupiter, as if it were a transiting exoplanet,
obtained by observing one of its satellites, Ganymede, while passing through
Jupiter's shadow i.e., during a solar eclipse from Ganymede. The spectrum shows
strong extinction due to the presence of clouds (aerosols) and haze in the
atmosphere, and strong absorption features from CH4. More interestingly, the
comparison with radiative transfer models reveals a spectral signature, which
we attribute here to a Jupiter stratospheric layer of crystalline H2O ice. The
atomic transitions of Na are also present. These results are relevant for the
modeling and interpretation of giant transiting exoplanets. They also open a
new technique to explore the atmospheric composition of the upper layers of
Jupiter's atmosphere.Comment: Accepted for publication in ApJ Letter
A figure of merit measuring picture resolution
Figure of merit measuring picture resolutio
The centre-to-limb variations of solar Fraunhofer lines imprinted upon lunar eclipse spectra - Implications for exoplanet transit observations
The atmospheres of exoplanets are commonly studied by observing the transit
of the planet passing in front of its parent star. The obscuration of part of
the stellar disk during a transit will reveal aspects of its surface structure
resulting from general centre-to-limb variations (CLVs). These become apparent
when forming the ratio between the stellar light in and out of transit. These
phenomena can be seen particularly clearly during the progress of a penumbral
lunar eclipse, where the Earth transits the solar disk and masks different
regions of the solar disk as the eclipse progresses. When inferring the
properties of the planetary atmosphere, it is essential that this effect
originating at the star is properly accounted for. Using the data observed from
the 2014-April-15 lunar eclipse with the ESPaDOnS spectrograph mounted on the
Canada France Hawaii Telescope (CFHT), we have obtained for the first time a
time sequence of the penumbral spectra. These penumbral spectra enable us to
study the centre-to-limb variations of solar Fraunhofer lines when the Earth is
transiting Sun. The Na i and Ca ii absorption features reported from previous
lunar eclipse observations are demonstrated to be CLV features, which dominate
the corresponding line profiles and mask possible planetary signal. Detecting
atmospheric species in exoplanets via transit spectroscopy must account for the
CLV effect.Comment: 9 pages, 11 figures, accepted, A&
Toward a unified PNT, Part 1: Complexity and context: Key challenges of multisensor positioning
The next generation of navigation and positioning systems must provide greater accuracy and reliability in a range of challenging environments to meet the needs of a variety of mission-critical applications. No single navigation technology is robust enough to meet these requirements on its own, so a multisensor solution is required. Known environmental features, such as signs, buildings, terrain height variation, and magnetic anomalies, may or may not be available for positioning. The system could be stationary, carried by a pedestrian, or on any type of land, sea, or air vehicle. Furthermore, for many applications, the environment and host behavior are subject to change. A multi-sensor solution is thus required. The expert knowledge problem is compounded by the fact that different modules in an integrated navigation system are often supplied by different organizations, who may be reluctant to share necessary design information if this is considered to be intellectual property that must be protected
Cosmic-ray propagation around the Sun: investigating the influence of the solar magnetic field on the cosmic-ray Sun shadow
The cosmic-ray Sun shadow, which is caused by high-energy charged cosmic rays
being blocked and deflected by the Sun and its magnetic field, has been
observed by various experiments, such as Argo-YBJ, HAWC, Tibet, and IceCube.
Most notably, the shadow's size and depth was recently shown to correlate with
the 11-year solar cycle. The interpretation of such measurements, which help to
bridge the gap between solar physics and high-energy particle astrophysics,
requires a solid theoretical understanding of cosmic-ray propagation in the
coronal magnetic field. It is the aim of this paper to establish theoretical
predictions for the cosmic-ray Sun shadow in order to identify observables that
can be used to study this link in more detail. To determine the cosmic-ray Sun
shadow, we numerically compute trajectories of charged cosmic rays in the
energy range of 5-316 TeV for five different mass numbers. We present and
analyze the resulting shadow images for protons and iron, as well as for
typically measured cosmic-ray compositions. We confirm the observationally
established correlation between the magnitude of the shadowing effect and both
the mean sunspot number and the polarity of the magnetic field during the solar
cycle. We also show that during low solar activity, the Sun's shadow behaves
similarly to that of a dipole, for which we find a non-monotonous dependence on
energy. In particular, the shadow can become significantly more pronounced than
the geometrical disk expected for a totally unmagnetized Sun. For times of high
solar activity, we instead predict the shadow to depend monotonously on energy,
and to be generally weaker than the geometrical shadow for all tested energies.
These effects should become visible in energy-resolved measurements of the Sun
shadow, and may in the future become an independent measure for the level of
disorder in the solar magnetic field.Comment: 18 pages, 88 figure
- …