162 research outputs found
A Search for EUV Emission from Comets with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)
We have obtained EUV spectra between 90 and 255 \AA of the cometsC/2002 T7
(LINEAR), C/2001 Q4 (NEAT), and C/2004 Q2 (Machholz) near their perihelion
passages in 2004 with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS).
We obtained contemporaneous data on Comet NEAT Q4 with the X-ray
Observatory ACIS instrument, marking the first simultaneous EUV and X-ray
spectral observations of a comet. The total CHIPS/EUV observing times were 337
ks for Q4, 234 ks for T7, and 483 ks for Machholz and for both CHIPS and
we calculate we have captured all the comet flux in the instrument
field of view. We set upper limits on solar wind charge exchange emission lines
of O, C, N, Ne and Fe occurring in the spectral bandpass of CHIPS. The spectrum
of Q4 obtained with can be reproduced by modeling emission lines of
C, N O, Mg, Fe, Si, S, and Ne solar wind ions. The measured X-ray emission line
intensities are consistent with our predictions from a solar wind charge
exchange model. The model predictions for the EUV emission line intensities are
determined from the intensity ratios of the cascading X-ray and EUV photons
arising in the charge exchange processes. They are compatible with the measured
limits on the intensities of the EUV lines. For comet Q4, we measured a total
X-ray flux of 3.7 ergs cm s, and derive from
model predictions a total EUV flux of 1.5 erg cm
s. The CHIPS observations occurred predominantly while the satellite was
on the dayside of Earth. For much of the observing time, CHIPS performed
observations at smaller solar angles than it was designed for and EUV emission
from the Sun scattered into the instrument limited the sensitivity of the EUV
measurements.Comment: 28 pages total, 4 tables, 7 figures. Accepted by The Astrophysical
Journa
On the structure and stability of magnetic tower jets
Modern theoretical models of astrophysical jets combine accretion, rotation,
and magnetic fields to launch and collimate supersonic flows from a central
source. Near the source, magnetic field strengths must be large enough to
collimate the jet requiring that the Poynting flux exceeds the kinetic-energy
flux. The extent to which the Poynting flux dominates kinetic energy flux at
large distances from the engine distinguishes two classes of models. In
magneto-centrifugal launch (MCL) models, magnetic fields dominate only at
scales engine radii, after which the jets become
hydrodynamically dominated (HD). By contrast, in Poynting flux dominated (PFD)
magnetic tower models, the field dominates even out to much larger scales. To
compare the large distance propagation differences of these two paradigms, we
perform 3-D ideal MHD AMR simulations of both HD and PFD stellar jets formed
via the same energy flux. We also compare how thermal energy losses and
rotation of the jet base affects the stability in these jets. For the
conditions described, we show that PFD and HD exhibit observationally
distinguishable features: PFD jets are lighter, slower, and less stable than HD
jets. Unlike HD jets, PFD jets develop current-driven instabilities that are
exacerbated as cooling and rotation increase, resulting in jets that are
clumpier than those in the HD limit. Our PFD jet simulations also resemble the
magnetic towers that have been recently created in laboratory astrophysical jet
experiments.Comment: 16 pages, 11 figures, published in ApJ: ApJ, 757, 6
Applying machine learning to improve simulations of a chaotic dynamical system using empirical error correction
Dynamical weather and climate prediction models underpin many studies of the
Earth system and hold the promise of being able to make robust projections of
future climate change based on physical laws. However, simulations from these
models still show many differences compared with observations. Machine learning
has been applied to solve certain prediction problems with great success, and
recently it's been proposed that this could replace the role of
physically-derived dynamical weather and climate models to give better quality
simulations. Here, instead, a framework using machine learning together with
physically-derived models is tested, in which it is learnt how to correct the
errors of the latter from timestep to timestep. This maintains the physical
understanding built into the models, whilst allowing performance improvements,
and also requires much simpler algorithms and less training data. This is
tested in the context of simulating the chaotic Lorenz '96 system, and it is
shown that the approach yields models that are stable and that give both
improved skill in initialised predictions and better long-term climate
statistics. Improvements in long-term statistics are smaller than for single
time-step tendencies, however, indicating that it would be valuable to develop
methods that target improvements on longer time scales. Future strategies for
the development of this approach and possible applications to making progress
on important scientific problems are discussed.Comment: 26p, 7 figures To be published in Journal of Advances in Modeling
Earth System
The Atmospheric Chemistry Suite (ACS) of Three Spectrometers for the ExoMars 2016 Trace Gas Orbiter
The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (>10,000) and large spectral coverage (0.7 to 17 μm—the visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7–1.6 μm spectral range with a resolving power of ∼20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2–4.4 μm range. MIR achieves a resolving power of >50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7–17 μm with apodized resolution varying from 0.2 to 1.3 cm−1. TIRVIM is primarily dedicated to profiling temperature from the surface up to ∼60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described
Moscow-type NN-potentials and three-nucleon bound states
A detailed description of Moscow-type (M-type) potential models for the NN
interaction is given. The microscopic foundation of these models, which appear
as a consequence of the composite quark structure of nucleons, is discussed.
M-type models are shown to arise naturally in a coupled channel approach when
compound or bag-like six-quark states, strongly coupled to the NN channel, are
eliminated from the complete multiquark wave function. The role of the
deep-lying bound states that appear in these models is elucidated. By
introducing additional conditions of orthogonality to these compound six-quark
states, a continuous series of almost on-shell equivalent nonlocal interaction
models, characterized by a strong reduction or full absence of a local
repulsive core (M-type models), is generated. The predictions of these
interaction models for 3N systems are analyzed in detail. It is shown that
M-type models give, under certain conditions, a stronger binding of the 3N
system than the original phase-equivalent model with nodeless wave functions.
An analysis of the 3N system with the new versions of the Moscow NN potential
describing also the higher even partial waves is presented. Large deviations
from conventional NN force models are found for the momentum distribution in
the high momentum region. In particular, the Coulomb displacement energy for
nuclei ^3He - ^3H displays a promising agreement with experiment when the ^3H
binding energy is extrapolated to the experimental value.Comment: 23 pages Latex, 9 figures, to appear in Phys.Rev.
Evidence for Dust Related X-ray Emission from Comet C/1995 O1 (Hale-Bopp)
We report the discovery of X-ray emission from comet C/1995 O1 (Hale-Bopp) by
the LECS instrument on-board BeppoSAX on 1996 September 10--11. The 0.1--2.0
keV luminosity decayed by a factor of 2 on a timescale of ~10 hr with a mean
value of 5.10E16 erg s-1. The spectrum is well fit by a thermal bremsstrahlung
model with a temperature of 0.29 +/- 0.06 keV, or a power-law with a photon
index of 3.1 +{0.6} -{0.2}. The lack of detected C and O line emission places
severe constraints on many models for cometary X-ray emission, especially those
which involve X-ray production in cometary gas. The luminosity is a factor of
at least 3.4 greater than measured by Extreme Ultraviolet Explorer (EUVE) 4
days later. This difference may be related to the emergence from the nucleus on
1996 September 9 of a dust-rich cloud. Over the next few days the cloud
continued to expand becoming increasingly tenuous, until it had reached an
extent of ~3.10E5 km (or ~2 arcmin) by the start of EUVE observation. We
speculate that the observed reduction in X-ray intensity is evidence for dust
fragmentation. These results support the view that cometary X-ray emission
arises from the interaction between solar X-rays and cometary dust.Comment: 17 pages. 4 postscript figs (2 in color). Accepted for publication in
ApJ (Letters
Comparing Suzaku and XMM-Newton Observations of the Soft X-ray Background: Evidence for Solar Wind Charge Exchange Emission
We present an analysis of a pair of Suzaku spectra of the soft X-ray
background (SXRB), obtained from pointings on and off a nearby shadowing
filament in the southern Galactic hemisphere. Because of the different Galactic
column densities in the two pointing directions, the observed emission from the
Galactic halo has a different shape in the two spectra. We make use of this
difference when modeling the spectra to separate the absorbed halo emission
from the unabsorbed foreground emission from the Local Bubble (LB). The
temperatures and emission measures we obtain are significantly different from
those determined from an earlier analysis of XMM-Newton spectra from the same
pointing directions. We attribute this difference to the presence of previously
unrecognized solar wind charge exchange (SWCX) contamination in the XMM-Newton
spectra, possibly due to a localized enhancement in the solar wind moving
across the line of sight. Contemporaneous solar wind data from ACE show nothing
unusual during the course of the XMM-Newton observations. Our results therefore
suggest that simply examining contemporaneous solar wind data might be
inadequate for determining if a spectrum of the SXRB is contaminated by SWCX
emission. If our Suzaku spectra are not badly contaminated by SWCX emission,
our best-fitting LB model gives a temperature of log T = 5.98 +0.03/-0.04 and a
pressure of p/k = 13,100-16,100 cm^-3 K. These values are lower than those
obtained from other recent observations of the LB, suggesting the LB may not be
isothermal and may not be in pressure equilibrium. Our halo modeling,
meanwhile, suggests that neon may be enhanced relative to oxygen and iron,
possibly because oxygen and iron are partly in dust.Comment: 17 pages, 8 figures. Accepted for publication in the Astrophysical
Journa
Vertical structure and size distributions of Martian aerosols from solar occultation measurements
Solar occultations performed with a spectrometer on board the Soviet spacecraft Phobos 2 (Blamont et al. 1991) provided data on the vertical structure of the Martian aerosols in the equatorial region (0[deg]-20[deg] N latitude) near the northern spring equinox (LS = 0[deg]-20[deg]). All measurements were made close to the evening terminator. Five clouds were detected above 45 km altitude and their vertical structure recorded at six wavelengths between 0.28 and 3.7 [mu]m. They have a small vertical extent (3-6 km) and a vertical optical depth less than 0.03. The thermal structure, as derived from saturated profiles of water vapor observed by our instrument in the infrared, does not allow the CO2 frost point to be reached at cloud altitude, strongly suggesting that cloud particles are formed of H2O ice. Under the assumption of spherical particles, a precise determination of their effective radius, which varies from cloud to cloud and with altitude, is obtained and ranges from 0.15 to 0.85 [mu]m; an estimate of the effective variance of the particle size distribution is ~ 0.2. The number density of cloud particles at the peak extinction level is ~1 cm-3. Dust was also observed and monitored at two wavelengths, 1.9 and 3.7 [mu]m, on nine different occasions. The top of the dust opaque layer, defined as the level above which the atmosphere becomes nearly transparent at the wavelengths of observation, is located near 25 km altitude, with variations smaller than +/-3 km from place to place. The scale height of dust at this altitude is 3-4 km. The effective radius of dust particles near the top of the opaque layer is 0.95 +/- 0.25 [mu]m and increases below with a vertical gradient of ~0.05 [mu]m km-1. Assuming that particles are levitated by eddy mixing, the eddy diffusion coefficient, K, is found to be ~106 cm2 sec-1 at 25 km and 105-106 cm2 sec-1 at 50 km using, respectively, dust and cloud observations. An effective variance of 0.25 (+/-50%) for the dust size distribution is obtained on the basis of a simple theoretical model for the observed vertical gradient of the effective radius of dust particles. Three clouds observed by Viking at midlatitude during the northern summer are reanalyzed. The analysis gives K [approximate] 106 cm2 sec-1 below 50 km altitude and at least 107 cm2 sec-1 above. Since the clouds seen from Phobos 2 are observed at twilight, which coincides with the diurnal maximum of the ambient temperature, they can be assumed to be in a steady state. If their thermodynamic state were to vary quickly during the day, our optical thickness at twilight would correspond to unrealistic values in earlier hours when the temperature is lower. Clouds are well fitted by theoretical profiles obtained assuming the steady state. An atmospheric temperature of 165-170 K at ~50 km is inferred. The negative temperature gradient above the cloud is large (1.5-2 K km-1). A parallel is established between these thin clouds and the polar mesospheric clouds observed on Earth. It is shown that upwelling in equatorial regions at equinox could be a significant factor in levitating cloud particles.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30061/1/0000431.pd
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