4,421 research outputs found
Interactions measurement payload for Shuttle
The Interactions Measurement Payload for Shuttle (IMPS) consisted of engineering experiments to determine the effects of the space environment on projected Air Force space systems. Measurements by IMPS on a polar-orbit Shuttle flight will lead to detailed knowledge of the interaction of the low-altitude polar-auroral environment on materials, equipment and technologies to be used in future large, high-power space systems. The results from the IMPS measurements will provide direct input to MIL-STD design guidelines and test standards that properly account for space-environment effects
Proceedings of the Spacecraft Charging Technology Conference: Executive Summary
Aerospace environments are reviewed in reference to spacecraft charging. Modelling, a theoretical scheme which can be used to describe the structure of the sheath around the spacecraft and to calculate the charging currents within, is discussed. Materials characterization is considered for experimental determination of the behavior of typical spacecraft materials when exposed to simulated geomagnetic substorm conditions. Materials development is also examined for controlling and minimizing spacecraft charging or at least for distributing the charge in an equipotential manner, using electrical conductive surfaces for materials exposed to space environment
Bandit problems with fidelity rewards
The fidelity bandits problem is a variant of the K-armed bandit problem in which the reward of each arm is augmented by a fidelity reward that provides the player with an additional payoff depending on how ‘loyal’ the player has been to that arm in the past. We propose two models for fidelity. In the loyalty-points model the amount of extra reward depends on the number of times the arm has previously been played. In the subscription model the additional reward depends on the current number of consecutive draws of the arm. We consider both stochastic and adversarial problems. Since single-arm strategies are not always optimal in stochastic problems, the notion of regret in the adversarial setting needs careful adjustment. We introduce three possible notions of regret and investigate which can be bounded sublinearly. We study in detail the special cases of increasing, decreasing and coupon (where the player gets an additional reward after every m plays of an arm) fidelity rewards. For the models which do not necessarily enjoy sublinear regret, we provide a worst case lower bound. For those models which exhibit sublinear regret, we provide algorithms and bound their regret
Hysteresis loops of Co-Pt perpendicular magnetic multilayers
We develop a phenomenological model to study magnetic hysteresis in two
samples designed as possible perpendicular recording media. A stochastic
cellular automata model captures cooperative behavior in the nucleation of
magnetic domains. We show how this simple model turns broad hysteresis loops
into loops with sharp drops like those observed in these samples, and explains
their unusual features. We also present, and experimentally verify, predictions
of this model, and suggest how insights from this model may apply more
generally.Comment: 4.5 pages, 5 figure
Expected seismicity and the seismic noise environment of Europa
Seismic data will be a vital geophysical constraint on internal structure of
Europa if we land instruments on the surface. Quantifying expected seismic
activity on Europa both in terms of large, recognizable signals and ambient
background noise is important for understanding dynamics of the moon, as well
as interpretation of potential future data. Seismic energy sources will likely
include cracking in the ice shell and turbulent motion in the oceans. We define
a range of models of seismic activity in Europa's ice shell by assuming each
model follows a Gutenberg-Richter relationship with varying parameters. A range
of cumulative seismic moment release between and Nm/yr is
defined by scaling tidal dissipation energy to tectonic events on the Earth's
moon. Random catalogs are generated and used to create synthetic continuous
noise records through numerical wave propagation in thermodynamically
self-consistent models of the interior structure of Europa. Spectral
characteristics of the noise are calculated by determining probabilistic power
spectral densities of the synthetic records. While the range of seismicity
models predicts noise levels that vary by 80 dB, we show that most noise
estimates are below the self-noise floor of high-frequency geophones, but may
be recorded by more sensitive instruments. The largest expected signals exceed
background noise by 50 dB. Noise records may allow for constraints on
interior structure through autocorrelation. Models of seismic noise generated
by pressure variations at the base of the ice shell due to turbulent motions in
the subsurface ocean may also generate observable seismic noise.Comment: 24 pages, 11 figures, Added in supplementary information from
revision submission, including 3 audio files with sonification of Europa
noise records. To view attachments, please download and extract the gzipped
tar source file listed under "Other formats
Further studies of space-time clustering of Burkitt's lymphoma in Uganda.
All hospital-treated cases of Burkitt's lymphoma (BL), with onset of symptoms in the period 1963-68 and resident in the Lango and Acholi districts of Uganda, were identified. The average annual incidence of BL in the 6-year period was 1-87 X 10(-5), similar to that in the adjacent West Nile district. Contrary to findings in other areas of Uganda, there was no evidence of seasonal variation in the onset of cases, nor of space-time clustering, nor of a decline in the incidence of BL in the study period. An inverse relationship was noted between the median age at onset of BL and the incidence of the disease in different areas of Uganda, a finding consistent with intense malarial infection being a precipitating factor for BL. The variable observations with respect to space-time clustering of BL and seasonal variation in incidence in different areas remains unexplained, but it is suggested that a closer study of the patterns of malarial infection in these areas may help to account for the findings
Frequency-dependent spin susceptibility in the two-dimensional Hubbard model
A Quantum Monte Carlo calculation of dynamical spin susceptibility in the
half-filled 2D Hubbard model is presented for temperature and an
intermediate on-site repulsion . Using the singular value decomposition
technique we succeed in analytically continuing the Matsubara Green's function
into the real frequency domain and in deriving the spectral representation for
the longitudinal and transverse spin susceptibility. The simulation results,
while contradicting the random-phase approximation prediction of
antiferromagnetic long-range order at this temperature, are in agreement with
an extension of a self-consistent renormalization approach of Moriya. The
static susceptibility calculated using this technique is qualitatively
consistent with the simulation results.Comment: 4 pages, Revtex, encoded figs.uu file with 3 figures enclose
Gauge factor of thick film resistors: outcomes of the variable range hopping model
Despite a large amount of data and numerous theoretical proposals, the
microscopic mechanism of transport in thick film resistors remains unclear.
However, recent low temperature measurements point toward a possible variable
range hopping mechanism of transport. Here we examine how such a mechanism
affects the gauge factor of thick film resistors. We find that at sufficiently
low temperatures , for which the resistivity follows the Mott's law
, the gauge factor GF is proportional to
. Moreover, the inclusion of Coulomb gap effects leads to at lower temperatures. In addition, we study a simple
model which generalizes the variable range hopping mechanism by taking into
account the finite mean inter-grain spacing. Our results suggest a possible
experimental verification of the validity of the variable range hopping in
thick film resistors.Comment: 7 pages, 3 eps figures, submitted to Journal of Applied Physic
Kaposi's Sarcoma in Uganda: Geographic and Ethnic Distribution
Over the quinquennium 1964-68 the crude annual incidence of Kaposi's sarcoma in Uganda per million of the population was 7·9 overall, 14·6 for males and 1·1 for females. Statistical analysis indicates that the disease is most prevalent in highland areas to the west and among the indigenous Bantu tribes. There was no correlation with the distribution of squamous cell carcinoma of the lower leg, and Kaposi's sarcoma was not seen in an Indian or European during the period under review
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