216 research outputs found
Precision mass determinations with the electric mass filter
Mass determinations of medium weight elements with electric mass filter, and mass ratios of xenon isotope
Diurnal and annual variations of meteor rates at the arctic circle
Meteors are an important source for (a) the metal atoms of the upper atmosphere metal layers and (b) for condensation nuclei, the existence of which are a prerequisite for the formation of noctilucent cloud particles in the polar mesopause region. For a better understanding of these phenomena, it would be helpful to know accurately the annual and diurnal variations of meteor rates. So far, these rates have been little studied at polar latitudes. Therefore we have used the 33 MHz meteor radar of the ALOMAR observatory at 69° N to measure the meteor rates at this location for two full annual cycles. This site, being within 3° of the Arctic circle, offers in addition an interesting capability: The axis of its antenna field points (almost) towards the North ecliptic pole once each day of the year. In this particular viewing direction, the radar monitors the meteoroid influx from (almost) the entire ecliptic Northern hemisphere. <P style='line-height: 20px;'> We report on the observed diurnal variations (averaged over one month) of meteor rates and their significant alterations throughout the year. The ratio of maximum over minimum meteor rates throughout one diurnal cycle is in January and February about 5, from April through December 2.3±0.3. If compared with similar measurements at mid-latitudes, our expectation, that the amplitude of the diurnal variation is to decrease towards the North pole, is not really borne out. <P style='line-height: 20px;'> Observations with the antenna axis pointing towards the North ecliptic pole showed that the rate of deposition of meteoric dust is substantially larger during the Arctic NLC season than the annual mean deposition rate. The daylight meteor showers of the Arietids, Zeta Perseids, and Beta Taurids supposedly contribute considerably to the June maximum of meteor rates. We note, though, that with the radar antenna pointing as described above, all three meteor radiants are close to the local horizon but all three radiants were detected
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Noctilucent clouds and the mesospheric water vapour: The past decade
The topic of this paper is the sensitivity of the brightness of noctilucent clouds (NLC) on the ambient water vapour mixing ratio f(H2O). Firstly, we use state-of-the-art models of NLC layer formation to predict NLC brightness changes in response to changes in the 80km mixing ratio f(H2O) for the two cases of ground-based 532nm lidar observations at 69° N and for hemispheric satellite SBUV observations at 252nm wavelength. In this study, we include a re-evaluation of the sensitivity of NLC brightness to changes in solar Lyman α flux. Secondly, we review observations of episodic changes in f(H2O) and those in NLC brightness, the former being available since 1992, the latter since 1979. To this review, we add a new series of observations of f(H2O), performed in the Arctic summer at the ALOMAR observatory. The episodic change exhibited by the Arctic summer means of f(H2O) turns out to be quite different from all those derived from annual means of f(H2O). The latter indicate that since 1996 a significant reduction of annually averaged upper mesospheric water vapour has occurred at low, mid, and high latitudes. These decreases of f(H2O) have been observed over the same time period in which a slow increase of SBUV NLC albedo has occurred. From this scenario and additional arguments we conclude that the cause for the observed long-term increase in NLC albedo remains to be identified. We close with comments on the very different character of decadal variations in NLC brightness and occurrence rate
Space Vehicle Terrestrial Environment Design Requirements Guidelines
The terrestrial environment is an important driver of space vehicle structural, control, and thermal system design. NASA is currently in the process of producing an update to an earlier Terrestrial Environment Guidelines for Aerospace Vehicle Design and Development Handbook. This paper addresses the contents of this updated handbook, with special emphasis on new material being included in the areas of atmospheric thermodynamic models, wind dynamics, atmospheric composition, atmospheric electricity, cloud phenomena, atmospheric extremes, and sea state. In addition, the respective engineering design elements are discussed relative to terrestrial environment inputs that require consideration. Specific lessons learned that have contributed to the advancements made in the application and awareness of terrestrial environment inputs for aerospace engineering applications are presented
Models of the SL9 Impacts II. Radiative-hydrodynamic Modeling of the Plume Splashback
We model the plume "splashback" phase of the SL9 collisions with Jupiter
using the ZEUS-3D hydrodynamic code. We modified the Zeus code to include gray
radiative transport, and we present validation tests. We couple the infalling
mass and momentum fluxes of SL9 plume material (from paper I) to a jovian
atmospheric model. A strong and complex shock structure results. The modeled
shock temperatures agree well with observations, and the structure and
evolution of the modeled shocks account for the appearance of high excitation
molecular line emission after the peak of the continuum light curve. The
splashback region cools by radial expansion as well as by radiation. The
morphology of our synthetic continuum light curves agree with observations over
a broad wavelength range (0.9 to 12 microns). A feature of our ballistic plume
is a shell of mass at the highest velocities, which we term the "vanguard".
Portions of the vanguard ejected on shallow trajectories produce a lateral
shock front, whose initial expansion accounts for the "third precursors" seen
in the 2-micron light curves of the larger impacts, and for hot methane
emission at early times. Continued propagation of this lateral shock
approximately reproduces the radii, propagation speed, and centroid positions
of the large rings observed at 3-4 microns by McGregor et al. The portion of
the vanguard ejected closer to the vertical falls back with high z-component
velocities just after maximum light, producing CO emission and the "flare" seen
at 0.9 microns. The model also produces secondary maxima ("bounces") whose
amplitudes and periods are in agreement with observations.Comment: 13 pages, 9 figures (figs 3 and 4 in color), accepted for Ap.J.
latex, version including full figures at:
http://oobleck.tn.cornell.edu/jh/ast/papers/slplume2-20.ps.g
Perfect Fluid Theory and its Extensions
We review the canonical theory for perfect fluids, in Eulerian and Lagrangian
formulations. The theory is related to a description of extended structures in
higher dimensions. Internal symmetry and supersymmetry degrees of freedom are
incorporated. Additional miscellaneous subjects that are covered include
physical topics concerning quantization, as well as mathematical issues of
volume preserving diffeomorphisms and representations of Chern-Simons terms (=
vortex or magnetic helicity).Comment: 3 figure
Evolution and Nucleosynthesis of Very Massive Stars
In this chapter, after a brief introduction and overview of stellar
evolution, we discuss the evolution and nucleosynthesis of very massive stars
(VMS: M>100 solar masses) in the context of recent stellar evolution model
calculations. This chapter covers the following aspects: general properties,
evolution of surface properties, late central evolution, and nucleosynthesis
including their dependence on metallicity, mass loss and rotation. Since very
massive stars have very large convective cores during the main-sequence phase,
their evolution is not so much affected by rotational mixing, but more by mass
loss through stellar winds. Their evolution is never far from a homogeneous
evolution even without rotational mixing. All VMS at metallicities close to
solar end their life as WC(-WO) type Wolf-Rayet stars. Due to very important
mass loss through stellar winds, these stars may have luminosities during the
advanced phases of their evolution similar to stars with initial masses between
60 and 120 solar masses. A distinctive feature which may be used to disentangle
Wolf-Rayet stars originating from VMS from those originating from lower initial
masses is the enhanced abundances of neon and magnesium at the surface of WC
stars. At solar metallicity, mass loss is so strong that even if a star is born
with several hundred solar masses, it will end its life with less than 50 solar
masses (using current mass loss prescriptions). At the metallicity of the LMC
and lower, on the other hand, mass loss is weaker and might enable star to
undergo pair-instability supernovae.Comment: 42 pages, 20 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
The Structure of Jupiter, Saturn, and Exoplanets: Key Questions for High-Pressure Experiments
We give an overview of our current understanding of the structure of gas
giant planets, from Jupiter and Saturn to extrasolar giant planets. We focus on
addressing what high-pressure laboratory experiments on hydrogen and helium can
help to elucidate about the structure of these planets.Comment: Invited contribution to proceedings of High Energy Density Laboratory
Astrophysics, 6. Accepted to Astrophysics & Space Science. 12 page
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