80,214 research outputs found
The color of sea level: importance of spatial variations in spectral shape for assessing the significance of trends
We investigate spatial variations in the shape of the spectrum of sea level variability, based on a homogeneously-sampled 12-year gridded altimeter dataset. We present a method of plotting spectral information as color, focusing on periods between 2 and 24 weeks, which shows that significant spatial variations in the spectral shape exist,
and contain useful dynamical information. Using the Bayesian Information Criterion, we determine that, typically, a 5th order autoregressive model is needed to capture the structure in the spectrum. Using this model, we show that statistical errors in fitted local trends range between less than 1 and more than 5 times what would be calculated assuming “white” noise, and the time needed to detect a 1 mm/yr trend ranges between about 5 years and many decades. For global-mean sea level, the statistical error reduces to 0.1 mm/yr over 12 years, with only 2 years needed to detect a 1 mm/yr trend. We find significant regional differences in trend from the global mean. The
patterns of these regional differences are indicative of a sea level trend dominated by dynamical ocean processes, over this perio
The excitation of O2 in auroras
Newly measured electron impact cross sections for excitation of the a 1 Delta g and b 1 Sigma g+ electronic states of O2 were employed to predict the absolute volume emission rates from these states under auroral conditions. A secondary electron electron flux typical of an IBC II nighttime aurora was used and the most important quenching processes were included in the calculations. The new excitation cross sections for the a 1 Delta g and b 1 Sigma g+ states are more than an order of magnitude larger than previous estimates, and lead to correspondingly greater intensities in the atmospheric and IR-atmospheric band systems. The calculated intensity ratios of the volume emission rates of 7621 A and 1.27 microns to that for 3914 A are smaller than obtained from aircraft observations and recent rocket experiments
Space telescope observatory management system preliminary test and verification plan
The preliminary plan for the Space Telescope Observatory Management System Test and Verification (TAV) is provided. Methodology, test scenarios, test plans and procedure formats, schedules, and the TAV organization are included. Supporting information is provided
Molecules, ices and astronomy
Molecules in interstellar gas and in interstellar ices play a fundamental role in astronomy. However, the formation of the simplest molecule, molecular hydrogen, is still not fully understood. Similarly, although interstellar ice analogues have received much attention in the laboratory, the evolution of ices in the interstellar medium still requires further study. At UCL we have developed two separate experiments to address these issues and explore the following questions: How is H formed on dust-grain surfaces? What is the budget between internal, kinetic and surface energies in the formation process? What are the astronomical consequences of these results? For ices, we ask: How do molecules desorb from pure and from mixed ices in regions warmed by newly formed stars? What can molecules released from ices tell us about the star-formation process? We put our results in the context of other laboratory work and we describe their application to current problems in astronomy
Cancellation of the collisional frequency shift in caesium fountain clocks
We have observed that the collisional frequency shift in primary caesium
fountain clocks varies with the clock state population composition and, in
particular, is zero for a given fraction of the |F = 4, mF = 0> atoms,
depending on the initial cloud parameters. We present a theoretical model
explaining our observations. The possibility of the collisional shift
cancellation implies an improvement in the performance of caesium fountain
standards and a simplification in their operation. Our results also have
implications for test operation of fountains at multiple pi/2 pulse areas
Sampling Time Effects for Persistence and Survival in Step Structural Fluctuations
The effects of sampling rate and total measurement time have been determined
for single-point measurements of step fluctuations within the context of
first-passage properties. Time dependent STM has been used to evaluate step
fluctuations on Ag(111) films grown on mica as a function of temperature
(300-410 K), on screw dislocations on the facets of Pb crystallites at 320K,
and on Al-terminated Si(111) over the temperature range 770K - 970K. Although
the fundamental time constant for step fluctuations on Ag and Al/Si varies by
orders of magnitude over the temperature ranges of measurement, no dependence
of the persistence amplitude on temperature is observed. Instead, the
persistence probability is found to scale directly with t/Dt where Dt is the
time interval used for sampling. Survival probabilities show a more complex
scaling dependence which includes both the sampling interval and the total
measurement time tm. Scaling with t/Dt occurs only when Dt/tm is a constant. We
show that this observation is equivalent to theoretical predictions that the
survival probability will scale as Dt/L^z, where L is the effective length of a
step. This implies that the survival probability for large systems, when
measured with fixed values of tm or Dt should also show little or no
temperature dependence.Comment: 27 pages, 10 figure
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