19,050 research outputs found
Study of storm time fluxes of heavy ions
Ion composition data sets from Lockheed instruments on a variety of spacecraft were used in combination with each other and with data from other instruments to address a variety of problems regarding plasma sources, energization and transport within the magnetosphere. The availability of data from several differing orbits has given a highly flexible approach to attacking the continually evolving questions of magnetospheric physics. This approach is very successful and should be continued in the future
Astrophysical Effects of Scalar Dark Matter Miniclusters
We model the formation, evolution and astrophysical effects of dark compact
Scalar Miniclusters (``ScaMs''). These objects arise when a scalar field, with
an axion-like or Higgs-like potential, undergoes a second order phase
transition below the QCD scale. Such a scalar field may couple too weakly to
the standard model to be detectable directly through particle interactions, but
may still be detectable by gravitational effects, such as lensing and baryon
accretion by large, gravitationally bound miniclusters. The masses of these
objects are shown to be constrained by the Ly power spectrum to be less
than , but they may be as light as classical axion
miniclusters, of the order of . We simulate the formation and
nonlinear gravitational collapse of these objects around matter-radiation
equality using an N-body code, estimate their gravitational lensing properties,
and assess the feasibility of studying them using current and future lensing
experiments. Future MACHO-type variability surveys of many background sources
can reveal either high-amplification, strong lensing events, or measure density
profiles directly via weak-lensing variability, depending on ScaM parameters
and survey depth. However, ScaMs, due to their low internal densities, are
unlikely to be responsible for apparent MACHO events already detected in the
Galactic halo. A simple estimate is made of parameters that would give rise to
early structure formation; in principle, early stellar collapse could be
triggered by ScaMs as early as recombination, and significantly affect cosmic
reionization.Comment: 13 pages, 12 figures. Replaced to reflect published versio
Identification and control of structures in space
The derivation of the equations of motion for the Spacecraft Control Laboratory Experiment (SCOLE) is reported and the equations of motion of a similar structure orbiting the earth are also derived. The structure is assumed to undergo large rigid-body maneuvers and small elastic deformations. A perturbation approach is proposed whereby the quantities defining the rigid-body maneuver are assumed to be relatively large, with the elastic deformations and deviations from the rigid-body maneuver being relatively small. The perturbation equations have the form of linear equations with time-dependent coefficients. An active control technique can then be formulated to permit maneuvering of the spacecraft and simultaneously suppressing the elastic vibration
The Invincible (1758) site: an integrated geophysical assessment
Chirp sub-bottom profiler and repeat sidescan sonar imaging of the Invincible wreck site (1758) in the Solent (U.K.), interpretation, and implications for management of the site
Uncovering CDM halo substructure with tidal streams
Models for the formation and growth of structure in a cold dark matter
dominated universe predict that galaxy halos should contain significant
substructure. Studies of the Milky Way, however, have yet to identify the
expected few hundred sub-halos with masses greater than about 10^6 Msun. Here
we propose a test for the presence of sub-halos in the halos of galaxies. We
show that the structure of the tidal tails of ancient globular clusters is very
sensitive to heating by repeated close encounters with the massive dark
sub-halos. We discuss the detection of such an effect in the context of the
next generation of astrometric missions, and conclude that it should be easily
detectable with the GAIA dataset. The finding of a single extended cold stellar
stream from a globular cluster would support alternative theories, such as
self-interacting dark matter, that give rise to smoother halos.Comment: 7 pages, 7 figures, submitted to MNRA
Asteroids Observed by The Sloan Digital Sky Survey
We announce the first public release of the SDSS Moving Object Catalog, with
SDSS observations for 58,117 asteroids. The catalog lists astrometric and
photometric data for moving objects observed prior to Dec 15, 2001, and also
includes orbital elements for 10,592 previously known objects. We analyze the
correlation between the orbital parameters and optical colors for the known
objects, and confirm that asteroid dynamical families, defined as clusters in
orbital parameter space, also strongly segregate in color space. Their
distinctive optical colors indicate that the variations in chemical composition
within a family are much smaller than the compositional differences between
families, and strongly support earlier suggestions that asteroids belonging to
a particular family have a common origin.Comment: 6 pages, 1 color figure, to be presented at "Astronomical Telescopes
& Instrumentation", SPIE 200
UCL Verbal Feedback Project Report
This report summarises the findings of a research project involving 13 teachers in England as they reduced or removed written feedback and replaced it with in-class verbal feedback. In this way they improved student engagement and radically reduced teacher workload
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