263 research outputs found
D-region ion chemistry
D-region ion chemistry, nitrogen oxides and oxygen allotropes in mesosphere, and ionization source
From the elasticity theory to cosmology and vice versa
The paper shows how a generalization of the elasticity theory to four
dimensions and to space-time allows for a consistent description of the
homogeneous and isotropic universe, including the accelerated expansion. The
analogy is manifested by the inclusion in the traditional Lagrangian of general
relativity of an additional term accounting for the strain induced in the
manifold (i.e. in space-time) by the curvature, be it induced by the presence
of a texture defect or by a matter/energy distribution. The additional term is
sufficient to account for various observed features of the universe and to give
a simple interpretation for the so called dark energy. Then, we show how the
same approach can be adopted back in three dimensions to obtain the equilibrium
configuration of a given solid subject to strain induced by defects or applied
forces. Finally, it is shown how concepts coming from the familiar elasticity
theory can inspire new approaches to cosmology and in return how methods
appropriated to General Relativity can be applied back to classical problems of
elastic deformations in three dimensions.Comment: 11 pages, 3 figure
Comparison of ionospheric scintillation models with experimental data for satellite navigation applications
A comparison between two of the most used scintillation models and experimental data is presented. The experimental
data have been derived from a GPS scintillation monitor developed at Cornell University and placed in Tucuman
(Argentina), under the peak of the anomaly. The models used (GISM and WBMOD) have been run for the
geophysical conditions corresponding to the measurements. The comparison is done by subdividing the information
on the basis of an ionospheric grid of 5°×5° surface square boxes. The comparison has been performed for
several local times, from 18 LT until 04 LT. Here, only a few cases of particular interest are shown. The goal is to
understand if the models are able to forecast actual scintillation morphology (from the satellite navigation systems
point of view) and if they could be used to yield an estimate of scintillation effects on satellite navigation systems
Space plasma effects on Earth-spaceand satellite-to-satellite communications:Working Group 4 overview
This paper summarizes the the activities carried out by Working Group 4 of COST 271 Action. The structure of this Working Group included four Working packages that were dealing with different aspects of the same overall problem related to space plasma variability and irregularities effects
on advanced satellite systems. General comments about the most relevant achievements and possible future lines of research are given
Ionospheric topside models compared with experimental electron density profiles
Recently an increasing number of topside electron density profiles has been made available to the scientific community
on the Internet. These data are important for ionospheric modeling purposes, since the experimental information
on the electron density above the ionosphere maximum of ionization is very scarce. The present work
compares NeQuick and IRI models with the topside electron density profiles available in the databases of the
ISIS2, IK19 and Cosmos 1809 satellites. Experimental electron content from the F2 peak up to satellite height
and electron densities at fixed heights above the peak have been compared under a wide range of different conditions.
The analysis performed points out the behavior of the models and the improvements needed to be assessed
to have a better reproduction of the experimental results. NeQuick topside is a modified Epstein layer,
with thickness parameter determined by an empirical relation. It appears that its performance is strongly affected
by this parameter, indicating the need for improvements of its formulation. IRI topside is based on Booker's
approach to consider two parts with constant height gradients. It appears that this formulation leads to an overestimation
of the electron density in the upper part of the profiles, and overestimation of TEC
Vector field theories in cosmology
Recently proposed theories based on the cosmic presence of a vectorial field
are compared and contrasted. In particular the so called Einstein aether theory
is discussed in parallel with a recent proposal of a strained space-time theory
(Cosmic Defect theory). We show that the latter fits reasonably well the cosmic
observed data with only one, or at most two, adjustable parameters, whilst
other vector theories use much more. The Newtonian limits are also compared.
Finally we show that the CD theory may be considered as a special case of the
aether theories, corresponding to a more compact and consistent paradigm.Comment: 19 pages, 1 figure, to appear on Phys. Rev.
A different approach to the analysis of GPS scintillation data
Amplitude scintillation data from GPS were analyzed. The objective is to estimate the impact of ionospheric scintillations at Satellite Based Augmentation Systems Ranging and Integrity Monitoring Station (SBAS RIMS) level and at GPS user level. For this purpose, a new approach to the problem was considered. Data were studied from the point of view of the impact of scintillations on the calculation of VTEC at pierce points and ionospheric grid points. An ionospheric grid of 5° 5° surface squares was assumed. From geometrical considerations and
taking into account the basic principle to compute VTEC at grid points, with the data analyzed it is shown that scintillations very seldom affect the calculation of a grid point VTEC. Data from all the RIMS and for the entire GPS satellites network must be analyzed simultaneously to describe a realistic scenario for the impact of scintillations on SBAS. Finally, GPS scintillation data were analyzed at user level: service availability problems were encountered
An improved bottomside for the ionospheric electron density model NeQuick
The ionospheric electron density model NeQuick is a «profiler» which uses the peaks of the E-layer, the F1-layer
and the F2-layer as anchor points. In the version prepared for and submitted to the International Telecommunication
Union (ITU) the model uses the ITU-R (CCIR) maps for foF2 and M(3000)F2 and adapted maps similar
to the ITU-R ones for foE and foF1. Since users found problematic behaviour of NeQuick under conditions
of strong differences of foE and foF2 map structures, the profiling was adapted by changing the properties of the
Epstein layers used for this purpose. The new formulation avoids both strange horizontal structures of the geographic
distribution of electron density in fixed heights and unrealistic peculiarities of the height profile which
occasionally occurred with the old version of the model. Since the Epstein layer approach allows for 8 parameters
only (3 layer amplitudes and 5 semi-thicknesses) the adaptation was no minor task but needed careful planning
of suitable strategies
Comparison of analytical functions used to describe topside electron density profiles with satellite data
Electron density models of the ionosphere use different analytical formulations for the electron density vertical
profile in the topside. The present paper compares some single-layer topside analytical descriptions (Chapman,
Epstein, modified Epstein used in the NeQuick model) with experimental topside profiles obtained from measurements
of IK19 and ISIS2 satellites. The limits of height range and shape for each formulation are described
and analyzed and suggestions for the use of multiple layers solution to reproduce experimental results are given
Ionogram inversion F1-layer treatment effect in raytracing
This paper shows the importance of the F1-layer shape in the electron density profiles obtained from ionograms
with different inversion techniques when the profiles are used in ray tracing. This layer often controls the propagation
on the path with ranges less than about 2000 km, particularly for spring and summer periods. Ionograms
from two different stations, Hainan (19.4N, 109E) and El Arenosillo (37.1N, -6.7E), obtained during the month
of July 2002 (average sunspot number: 99.6) during geomagnetic quiet conditions (Ap-index between 9 and 15)
are analyzed. The profiles obtained with two different inversion techniques with different options are used together
with the ray tracing program of the Proplab-Pro software. This program calculates the features of the received signal
as angle of arrival, path length, height of reflection and range for each given profile assumed to define a spherically
symmetric ionosphere in the region along the path. For each ionospheric condition (location, day, hour) the
difference between range values obtained with Proplab-Pro program using profiles from the two techniques and
the different options (POLAN no valley, POLAN valley, POLAN1-layer and NHPC) are considered
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