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51.
Vladimir Truhlik Ludmila Triskova Dieter Bilitza Katerina Podolska 《Journal of Atmospheric and Solar》2009,71(17-18):2055-2063
A database of the electron temperature (Te) comprising of most of the available LEO satellite measurements is used for studying the solar activity variations of Te. The Te data are grouped for two levels of solar activity (low LSA and high HSA), five altitude ranges between 350 and 2000 km, and day and night. By fitting a theoretical expression to the Te values we obtain variation of Te along magnetic field lines and heat flux for LSA and HSA. We have found that Te increases with increase in solar activity at low and mid-latitudes during nighttime at all altitudes studied. During daytime the Te response to solar activity depends on latitude, altitude, and season. This analysis shows existence of anti-correlation between Te and solar activity at mid-latitudes below 700 km during the equinox and winter day hours. Heat fluxes show small latitudinal dependence for daytime but substantial for nighttime. 相似文献
52.
A method is presented for simulating arrays of spatially varying ground motions, incorporating the effects of incoherence, wave passage, and differential site response. Non‐stationarity is accounted for by considering the motions as consisting of stationary segments. Two approaches are developed. In the first, simulated motions are consistent with the power spectral densities of a segmented recorded motion and are characterized by uniform variability at all locations. Uniform variability in the array of ground motions is essential when synthetic motions are used for statistical analysis of the response of multiply‐supported structures. In the second approach, simulated motions are conditioned on the segmented record itself and exhibit increasing variance with distance from the site of the observation. For both approaches, example simulated motions are presented for an existing bridge model employing two alternatives for modeling the local soil response: i) idealizing each soil‐column as a single‐degree‐of‐freedom oscillator, and ii) employing the theory of vertical wave propagation in a single soil layer over bedrock. The selection of parameters in the simulation procedure and their effects on the characteristics of the generated motions are discussed. The method is validated by comparing statistical characteristics of the synthetic motions with target theoretical models. Response spectra of the simulated motions at each support are also examined. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献