1,191 research outputs found
An altitude-dependent spacecraft charging model
A model for the altitude dependence of the hot plasma parameters responsible for the electrostatic charging of spacecraft was developed. Based upon plasma orbit theory, the directed velocity is a function of the ambient magnetic field flux density. A consequence of this approach is that while the thermal velocity distributions (assumed to be Maxwellian) of the plasma particles are independent of the magnetic field strength (and hence altitude), the particle densities increase with magnetic field strength. Thus, according to this model, while the equilibrium voltage is independent of altitude, the charging current density increases with decreasing altitude. However, the probability of such spacecraft charging decreases with decreasing altitude
Environmental effects on spacecraft materials
The effects on the natural space environments on materials are presented, which may be used for SDI applications. The current state-of-the-art knowledge of those effects was studied, and a literature search, a questionnaire mailing, and some visits to NASA and Air Force research facilities were performed. Phase 2 will be a study of what materials may be used for SDI applications and to what natural space environments they may be vulnerable. Deficiencies in knowledge of the effects of the natural space environments on these materials are to be identified and recommendations are to be made to eliminate these knowledge deficiencies
Statistical evaluation of proton radiation from solar flares Final technical report
Statistical evaluation of proton radiation from solar flare
A flight experiment to determine GPS photochemical contamination accumulation rates
It was recently suggested that photochemically deposited contamination, originating from volatiles outgassed by a spacecraft, may be responsible for the anomalous degradation in power seen on the GPS Block 1 vehicles. In an attempt to confirm, or deny, the photochemical deposition rates predicted, a study was undertaken to design a flight experiment to be incorporated on the GPS vehicles currently in production. The objective was to develop an inexpensive, light weight instrument package that would give information on the contamination levels within a few months of launch. Three types of apparatus were studied, Quartz Crystal Microbalances, (QCM's), modified solar cells, and calorimeters. A calorimeter was selected due primarily to its impact on the production schedule of the GPS vehicles. An analysis of the sensitivity of the final design is compared to the predicted contamination accumulation rates in order to determine how long after launch it will take the experiment to show the effects of photochemical contamination
WHAM Observations of H-Alpha, [S II], and [N II] toward the Orion and Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium
A large portion of the Galaxy (l = 123 deg to 164 deg, b = -6 deg to -35
deg), which samples regions of the Local (Orion) spiral arm and the more
distant Perseus arm, has been mapped with the Wisconsin H-Alpha Mapper (WHAM)
in the H-Alpha, [S II] 6716, and [N II] 6583 lines. Several trends noticed in
emission-line investigations of diffuse gas in other galaxies are confirmed in
the Milky Way and extended to much fainter emission. We find that the [S
II]/H-Alpha and [N II]/H-Alpha ratios increase as absolute H-Alpha intensities
decrease. For the more distant Perseus arm emission, the increase in these
ratios is a strong function of Galactic latitude and thus, of height above the
Galactic plane. The [S II]/[N II] ratio is relatively independent of H-Alpha
intensity. Scatter in this ratio appears to be physically significant, and maps
of it suggest regions with similar ratios are spatially correlated. The Perseus
arm [S II]/[N II] ratio is systematically lower than Local emission by 10%-20%.
With [S II]/[N II] fairly constant over a large range of H-Alpha intensities,
the increase of [S II]/H-Alpha and [N II]/H-Alpha with |z| seems to reflect an
increase in temperature. Such an interpretation allows us to estimate the
temperature and ionization conditions in our large sample of observations. We
find that WIM temperatures range from 6,000 K to 9,000 K with temperature
increasing from bright to faint H-Alpha emission (low to high [S II]/H-Alpha
and [N II]/H-Alpha) respectively. Changes in [S II]/[N II] appear to reflect
changes in the local ionization conditions (e.g. the S+/S++ ratio). We also
measure the electron scale height in the Perseus arm to be 1.0+/-0.1 kpc,
confirming earlier, less accurate determinations.Comment: 28 pages, 10 figures. Figures 2 and 3 are full color--GIFs provided
here, original PS figures at link below. Accepted for publication in ApJ.
More information about the WHAM project can be found at
http://www.astro.wisc.edu/wham/ . REVISION: Figure 6, bottom panel now
contains the proper points. No other changes have been mad
Early Results from the Wisconsin H-Alpha Mapper Southern Sky Survey
After a successful eleven-year campaign at Kitt Peak, we moved the Wisconsin
H-Alpha Mapper (WHAM) to Cerro Tololo in early 2009. Here we present some of
the early data after a few months under southern skies. These maps begin to
complete the first all-sky, kinematic survey of the diffuse H-alpha emission
from the Milky Way. Much of this emission arises from the Warm Ionized Medium
(WIM), a significant component of the ISM that extends a few kiloparsecs above
the Galactic disk. While this first look at the data focuses on the H-alpha
survey, WHAM is also capable of observing many other optical emission lines,
revealing fascinating trends in the temperature and ionization state of the
WIM. Our ongoing studies of the physical conditions of diffuse ionized gas will
continue from the southern hemisphere following the H-alpha survey. In
addition, future observations will cover the full velocity range of the
Magellanic Stream, Bridge, and Clouds to trace the ionized gas associated with
these neighboring systems.Comment: 4 pages, 2 figures. To appear in "The Dynamic ISM: A celebration of
the Canadian Galactic Plane Survey," ASP Conference Serie
Persistent organic pollutants in Blanchard\u27s cricket frogs (Acris crepitants blanchardi) from Ohio
Analysis of Spatial Structure of the SPica H II Region
Far ultraviolet (FUV) spectral images of the Spica H II region are first
presented here for the Si II* 1533.4A and Al II 1670.8A lines and then compared
with the optical Halpha image. The H alpha and Si II* images show enhanced
emissions in the southern part of the H II region where H I density increases
outwards. This high density region, which we identify as part of the
"interaction ring" of the Loop I superbubble and the Local Bubble, seems to
bound the southern H II region. On the other hand, the observed profile of Al
II shows a broad central peak, without much difference between the northern and
southern parts, which we suspect results from multiple resonant scattering. The
extended tails seen in the radial profiles of the FUV intensities suggest that
the nebula may be embedded in a warm ionized gas. Simulation with a spectral
synthesis code yields the values of the Lyman continuum luminosity and the
effective temperature of the central star similar to previous estimates with
10^46.2 photons s^-1 and 26,000 K, respectively, but the density of the
northern H II region, 0.22 cm^-3, is much smaller than previous estimates for
the H alpha brightest region.Comment: 15 pages, 5 figures, accepted for Ap
miRNAs are essential for survival and differentiation of newborn neurons but not for expansion of neural progenitors during early neurogenesis in the mouse embryonic neocortex
Neurogenesis during the development of the mammalian cerebral cortex involves a switch of neural stem and progenitor cells from proliferation to differentiation. To explore the possible role of microRNAs (miRNAs) in this process, we conditionally ablated Dicer in the developing mouse neocortex using Emx1-Cre, which is specifically expressed in the dorsal telencephalon as early as embryonic day (E) 9.5. Dicer ablation in neuroepithelial cells, which are the primary neural stem and progenitor cells, and in the neurons derived from them, was evident from E10.5 onwards, as ascertained by the depletion of the normally abundant miRNAs miR-9 and miR-124. Dicer ablation resulted in massive hypotrophy of the postnatal cortex and death of the mice shortly after weaning. Analysis of the cytoarchitecture of the Dicer-ablated cortex revealed a marked reduction in radial thickness starting at E13.5, and defective cortical layering postnatally. Whereas the former was due to neuronal apoptosis starting at E12.5, which was the earliest detectable phenotype, the latter reflected dramatic impairment of neuronal differentiation. Remarkably, the primary target cells of Dicer ablation, the neuroepithelial cells, and the neurogenic progenitors derived from them, were unaffected by miRNA depletion with regard to cell cycle progression, cell division, differentiation and viability during the early stage of neurogenesis, and only underwent apoptosis starting at E14.5. Our results support the emerging concept that progenitors are less dependent on miRNAs than their differentiated progeny, and raise interesting perspectives as to the expansion of somatic stem cells
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