22,842 research outputs found
Development of solar wind shock models with tensor plasma pressure for data analysis
The development of solar wind shock models with tensor plasma pressure and the comparison of some of the shock models with the satellite data from Pioneer 6 through Pioneer 9 are reported. Theoretically, difficulties were found in non-turbulent fluid shock models for tensor pressure plasmas. For microscopic shock theories nonlinear growth caused by plasma instabilities was frequently not clearly demonstrated to lead to the formation of a shock. As a result no clear choice for a shock model for the bow shock or interplanetary tensor pressure shocks emerged
A statistical analysis of multiple temperature proxies: Are reconstructions of surface temperatures over the last 1000 years reliable?
Predicting historic temperatures based on tree rings, ice cores, and other
natural proxies is a difficult endeavor. The relationship between proxies and
temperature is weak and the number of proxies is far larger than the number of
target data points. Furthermore, the data contain complex spatial and temporal
dependence structures which are not easily captured with simple models. In this
paper, we assess the reliability of such reconstructions and their statistical
significance against various null models. We find that the proxies do not
predict temperature significantly better than random series generated
independently of temperature. Furthermore, various model specifications that
perform similarly at predicting temperature produce extremely different
historical backcasts. Finally, the proxies seem unable to forecast the high
levels of and sharp run-up in temperature in the 1990s either in-sample or from
contiguous holdout blocks, thus casting doubt on their ability to predict such
phenomena if in fact they occurred several hundred years ago. We propose our
own reconstruction of Northern Hemisphere average annual land temperature over
the last millennium, assess its reliability, and compare it to those from the
climate science literature. Our model provides a similar reconstruction but has
much wider standard errors, reflecting the weak signal and large uncertainty
encountered in this setting.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS398 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
3C273 variability at 7 mm: Evidences of shocks and precession in the jet
We report 4 years of observations of 3C273 at 7 mm obtained with the
Itapetinga Radiotelescope, in Brazil, between 2009 and 2013. We detected a
flare in 2010 March, when the flux density increased by 50% and reached 35 Jy.
After the flare, the flux density started to decrease and reached values lower
than 10 Jy. We suggest that the 7 mm flare is the radio counterpart of the
-ray flare observed by Fermi/LAT in 2009 September, in which the flux
density at high energies reached a factor of fifty of its average value. A
delay of 170 days between the radio and -ray flares was revealed using
the Discrete Correlation Function (DCF) that can be interpreted in the context
of a shock model, in which each flare corresponds to the formation of a compact
superluminal component that expands and becomes optically thin at radio
frequencies at latter epochs. The difference in flare intensity between
frequencies and at a different times, is explained as a consequence of an
increase in the Doppler factor , as predicted by the 16 year precession
model proposed by Abraham & Romero, which has a large effect on boosting at
high frequencies while does not affect too much the observed optically thick
radio emission. We discuss other observable effects of the variation in
, as the increase in the formation rate of superluminal components, the
variations in the time delay between flares and the periodic behaviour of the
radio light curve that we found compatible with changes in the Doppler factor.Comment: 9 pages, 8 figure
UK Centre for Coaching Excellence Scoping Project Report: National and International Best Practice in Level 4 Coach Development
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