23,472 research outputs found
Charging of DMSP/F6 spacecraft in aurora on 10 January 1983
Spacecraft charging has been widely observed in geosynchronous orbit on the ATS-5 and ATS-6 pair and on the SCATHA spacecraft. An adequate theory for explaining the observations exist. Neither the data or theory can be exported to low polar orbit and its drastically different environment. Evidence of charging on the DMSP F6 spacecraft is presented. A simple model is set up explaining the observations. Two independent instruments on the spacecraft showed charging to a moderate (44 volts) negative potential. The selection spectrometer showed a flux of 2 billion electrons per sq. cm. sec. ster. peaked at 9.5 keV. This was marginally sufficient to overcome the flux of cold ambient ions. Charging calculations are presented showing where simplications are justified and where serious uncertainties exist. More serious charging is predicted for the Shuttle in polar orbit
Shock-Wave Heating Model for Chondrule Formation: Prevention of Isotopic Fractionation
Chondrules are considered to have much information on dust particles and
processes in the solar nebula. It is naturally expected that protoplanetary
disks observed in present star forming regions have similar dust particles and
processes, so study of chondrule formation may provide us great information on
the formation of the planetary systems.
Evaporation during chondrule melting may have resulted in depletion of
volatile elements in chondrules. However, no evidence for a large degree of
heavy-isotope enrichment has been reported in chondrules. In order to meet this
observed constraint, the rapid heating rate at temperatures below the silicate
solidus is required to suppress the isotopic fractionation.
We have developed a new shock-wave heating model taking into account the
radiative transfer of the dust thermal continuum emission and the line emission
of gas molecules and calculated the thermal history of chondrules. We have
found that optically-thin shock waves for the thermal continuum emission from
dust particles can meet the rapid heating constraint, because the dust thermal
emission does not keep the dust particles high temperature for a long time in
the pre-shock region and dust particles are abruptly heated by the gas drag
heating in the post-shock region. We have also derived the upper limit of
optical depth of the pre-shock region using the radiative diffusion
approximation, above which the rapid heating constraint is not satisfied. It is
about 1 - 10.Comment: 58 pages, including 5 tables and 15 figures, accepted for publication
in The Astrophysical Journa
Greenhouse Gas Impacts of Ethanol from Iowa Corn: Life Cycle Analysis versus System-wide Accounting
Life cycle analysis (LCA) is the standard approach used to evaluate the greenhouse gas (GHG) benefits of biofuels. However, it is increasingly recognized that LCA results do not account for some impacts including land use changes that have important implications on GHGs. Thus, an alternative accounting system that goes beyond LCA is needed. In this paper, we contribute to the literature by laying out the basics of a system-wide accounting (SWA) method that takes into account all potential changes in GHGs resulting from biofuel expansion. We applied both LCA and SWA to assess the GHG impacts of ethanol based on Iowa corn. Growing corn in rotation with soybeans generated 35% less GHG emissions than growing corn after corn. Based on average corn production, ethanol's GHG benefits were lower in 2007 than in 2006 because of an increase in continuous corn in 2007. When only additional corn was considered, ethanol emitted about 22% less GHGs than gasoline. Results from SWA varied with the choice of baseline and the definition of geographical boundaries. Using 2006 as a baseline and 2007 as a scenario, corn ethanol's benefits were about 20% of the emissions of gasoline. If we expand geographical limits beyond Iowa, but assume the same emission rates for soybean production and land use changes as those in Iowa, then corn ethanol generated more GHG emissions than gasoline. These results highlight the importance of boundary definition for both LCA and SWAbiofuels, corn ethanol, greenhouse gas, life cycle analysis, system-wide accounting, Research Methods/ Statistical Methods, Resource /Energy Economics and Policy,
Stationary Points of Scalar Fields Coupled to Gravity
We investigate the dynamics of gravity coupled to a scalar field using a
non-canonical form of the kinetic term. It is shown that its singular point
represents an attractor for classical solutions and the stationary value of the
field may occur distant from the minimum of the potential. In this paper
properties of universes with such stationary states are considered. We reveal
that such state can be responsible for modern dark energy density.Comment: H. Kroger, invited talk, FFP6, Udine (2004), revised version with
corrected author lis
Polar orbit electrostatic charging of objects in shuttle wake
A survey of DMSP data has uncovered several cases where precipitating auroral electron fluxes are both sufficiently intense and energetic to charge spacecraft materials such as teflon to very large potentials in the absence of ambient ion currents. Analytical bounds are provided which show that these measured environments can cause surface potentials in excess of several hundred volts to develop on objects in the orbiter wake for particular vehicle orientations
Evidence for Cosmic Acceleration is Robust to Observed Correlations Between Type Ia Supernova Luminosity and Stellar Age
Type Ia Supernovae (SNe Ia) are powerful standardizable candles for
constraining cosmological models and provided the first evidence of the
accelerated expansion of the universe. Their precision derives from empirical
correlations, now measured from SNe Ia, between their luminosities,
light-curve shapes, colors and most recently with the stellar mass of their
host galaxy. As mass correlates with other galaxy properties, alternative
parameters have been investigated to improve SN Ia standardization though none
have been shown to significantly alter the determination of cosmological
parameters. We re-examine a recent claim, based on 34 SN Ia in nearby passive
host galaxies, of a 0.05 mag/Gyr dependence of standardized SN Ia luminosity on
host age which if extrapolated to higher redshifts, would be a bias up to 0.25
mag, challenging the inference of dark energy. We reanalyze this sample of
hosts using both the original method and a Bayesian hierarchical model and find
after a fuller accounting of the uncertainties the significance of a dependence
on age to be and after the removal of a single
poorly-sampled SN Ia. To test the claim that a trend seen in old stellar
populations can be applied to younger ages, we extend our analysis to a larger
sample which includes young hosts. We find the residual dependence of host age
(after all standardization typically employed for cosmological measurements) to
be consistent with zero for 254 SNe Ia from the Pantheon sample, ruling out the
large but low significance trend seen in passive hosts.Comment: 9 pages, 3 figures, 3 tables. Accepted for publication in ApJ
Self-stabilization of extra dimensions
We show that the problem of stabilization of extra dimensions in Kaluza-Klein
type cosmology may be solved in a theory of gravity involving high-order
curvature invariants. The method suggested (employing a slow-change
approximation) can work with rather a general form of the gravitational action.
As examples, we consider pure gravity with Lagrangians quadratic and cubic in
the scalar curvature and some more complex ones in a simple Kaluza-Klein
framework. After a transition to the 4D Einstein conformal frame, this results
in effective scalar field theories with certain effective potentials, which in
many cases possess positive minima providing stable small-size extra
dimensions. Estimates made in the original (Jordan) conformal frame show that
the problem of a small value of the cosmological constant in the present
Universe is softened in this framework but is not solved completely.}Comment: 10 pages, 4 figures, revtex4. Version with additions and corrections,
accepted at Phys. Rev.
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