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171.
Interpretation of the global anisotropy in the radio polarizations of cosmologically distant sources
We present a detailed statistical study of the observed anisotropy in radio polarizations from distant extragalactic objects.
This anisotropy was earlier found by Birch (1982) and reconfirmed by Jain and Ralston (1999) in a larger data set. A very
strong signal was seen after imposing the cut
, whereRM is the rotation measure and
its mean value. In this paper, we show that there are several indications that this anisotropy cannot be attributed to bias
in the data. We also find that a generalized statistic shows a very strong signal in the entire data without imposing theRM dependent cut. Finally we argue that an anisotropic background pseudoscalar field can explain the observations. 相似文献
172.
The relationship between the velocity of CMEs and the plasma temperature of the associated X-ray solar flares is investigated.The velocity of CMEs increases with plasma temperature(R=0.82)and photon index below the break energy(R=0.60)of X-ray flares.The heating of the coronal plasma appears to be significant with respect to the kinetics of a CME from the reconnection region where the flare also occurs.We propose that the initiation and velocity of CMEs perhaps depend upon the dominant process of conversion of the magnetic field energy of the active region to heating/accelerating the coronal plasma in the reconnected loops.Results show that a flare and the associated CME are two components of one energy release system,perhaps,magnetic field free energy. 相似文献
173.
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175.
Operational activities in the ocean like planning for structural repairs or fishing expeditions require real time prediction of waves over typical time duration of say a few hours. Such predictions can be made by using a numerical model or a time series model employing continuously recorded waves. This paper presents another option to do so and it is based on a different time series approach in which the input is in the form of preceding wind speed and wind direction observations. This would be useful for those stations where the costly wave buoys are not deployed and instead only meteorological buoys measuring wind are moored. The technique employs alternative artificial intelligence approaches of an artificial neural network (ANN), genetic programming (GP) and model tree (MT) to carry out the time series modeling of wind to obtain waves. Wind observations at four offshore sites along the east coast of India were used. For calibration purpose the wave data was generated using a numerical model. The predicted waves obtained using the proposed time series models when compared with the numerically generated waves showed good resemblance in terms of the selected error criteria. Large differences across the chosen techniques of ANN, GP, MT were not noticed. Wave hindcasting at the same time step and the predictions over shorter lead times were better than the predictions over longer lead times. The proposed method is a cost effective and convenient option when a site-specific information is desired. 相似文献
176.
We present multiwavelength observations of a large-amplitude oscillation of a polar-crown filament on 15 October 2002, which
has been reported by Isobe and Tripathi (Astron. Astrophys.
449, L17, 2006). The oscillation occurred during the slow rise (≈1 km s−1) of the filament. It completed three cycles before sudden acceleration and eruption. The oscillation and following eruption
were clearly seen in observations recorded by the Extreme-Ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). The oscillation was seen only in a part of the filament, and it appears to be a standing oscillation rather than
a propagating wave. The amplitudes of velocity and spatial displacement of the oscillation in the plane of the sky were about
5 km s−1 and 15 000 km, respectively. The period of oscillation was about two hours and did not change significantly during the oscillation.
The oscillation was also observed in Hα by the Flare Monitoring Telescope at the Hida Observatory. We determine the three-dimensional
motion of the oscillation from the Hα wing images. The maximum line-of-sight velocity was estimated to be a few tens of kilometers
per second, although the uncertainty is large owing to the lack of line-profile information. Furthermore, we also identified
the spatial displacement of the oscillation in 17-GHz microwave images from Nobeyama Radio Heliograph (NoRH). The filament
oscillation seems to be triggered by magnetic reconnection between a filament barb and nearby emerging magnetic flux as was
evident from the MDI magnetogram observations. No flare was observed to be associated with the onset of the oscillation. We
also discuss possible implications of the oscillation as a diagnostic tool for the eruption mechanisms. We suggest that in
the early phase of eruption a part of the filament lost its equilibrium first, while the remaining part was still in an equilibrium
and oscillated. 相似文献
177.
Growing interest in the use of artificial neural networks (ANNs) in rainfall‐runoff modelling has suggested certain issues that are still not addressed properly. One such concern is the use of network type, as theoretical studies on a multi‐layer perceptron (MLP) with a sigmoid transfer function enlightens certain limitations for its use. Alternatively, there is a strong belief in the general ANN user community that a radial basis function (RBF) network performs better than an MLP, as the former bases its nonlinearities on the training data set. This argument is not yet substantiated by applications in hydrology. This paper presents a comprehensive evaluation of the performance of MLP‐ and RBF‐type neural network models developed for rainfall‐runoff modelling of two Indian river basins. The performance of both the MLP and RBF network models were comprehensively evaluated in terms of their generalization properties, predicted hydrograph characteristics, and predictive uncertainty. The results of the study indicate that the choice of the network type certainly has an impact on the model prediction accuracy. The study suggests that both the networks have merits and limitations. For instance, the MLP requires a long trial‐and‐error procedure to fix the optimal number of hidden nodes, whereas for an RBF the structure of the network can be fixed using an appropriate training algorithm. However, a judgment on which is superior is not clearly possible from this study. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
178.
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180.
Sanjay Kumar Jain Sharad K. Jain V. Hariprasad Anju Choudhry 《Journal of the Indian Society of Remote Sensing》2011,39(2):259-270
Water balance of a basin involves estimation of input precipitation, runoff, infiltration and evapotranspiration (ET). Although
ET may have large variations over a big basin, it is commonly estimated using a few point measurements and this makes the
estimation error prone. Satellite based remote sensing data provides few parameters for estimation of energy fluxes, at the
land surface and atmosphere interaction in a distributed manner using the meteorological parameters. These parameters through
surface energy balance equation have been used for the estimation of ET in this study. Various spatially distributed variables
required for ET estimation; viz. NDVI, surface albedo, surface temperature etc. have been derived using remote sensing and
ancillary data for Tapi basin located in western India. Beside this field data such as rainfall, air temperature, relative
humidity, sunshine hours etc. have been used. For computation of runoff, Soil Conservation Services (SCS) approach has been
considered. Tapi basin up to Ukai dam has been selected as the study area. Satellite data from National Oceanic and Atmospheric
Administration (NOAA), Polar Orbiting Environmental Satellite, which carries the Advanced Very High Resolution Radiometer
(AVHRR), have been used for preparation of various maps required for runoff and ET analysis. The results of runoff and ET
have been compared with observed data for 2 years, 2002–2003 and the results have been found in good agreement with observed
data. 相似文献