Experimental Electron Density Profiles Research Articles

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.

Effects of gradients of the electron density on Earth-space communications

This paper is a review of the main results achieved in the framework of COST 271 Action Working Group 4, under the activities of the Work Package 4.4. The first topic treated deals with the influence of ionospheric space and time gradients in the slant to vertical and vertical to slant ionospheric delay conversion when the thin shell approximation of the ionosphere is assumed and with the effects of geomagnetic activity on the errors that this conversion introduces. The second topic is related to the comparison of ionospheric topside models with experimental electron density profiles to check the ability of the models to reproduce the observed topside shape and characteristics that determine the electron density gradients. The analysis that has been done allows pointing out the changes needed to improve the models. Finally a third topic covers a model simulation study of the total electron content that can be encountered in GPS-to-geostationary satellite ray paths. It takes into account that the propagation paths for such satellite-to-satellite links are very long and they have the potential to intersect regions of the ionised atmosphere where the electron density is high when the geometry is close to eclipse.

A new force field for simulating phosphatidylcholine bilayers

A new force field for the simulation of dipalmitoylphosphatidylcholine (DPPC) in the liquid-crystalline, fluid phase at zero surface tension is presented. The structure of the bilayer with the area per lipid (0.629 nm(2); experiment 0.629-0.64 nm(2)), the volume per lipid (1.226 nm(3); experiment 1.229-1.232 nm(3)), and the ordering of the palmitoyl chains (order parameters) are all in very good agreement with experiment. Experimental electron density profiles are well reproduced in particular with regard to the penetration of water into the bilayer. The force field was further validated by simulating the spontaneous assembly of DPPC into a bilayer in water. Notably, the timescale on which membrane sealing was observed using this model appears closer to the timescales for membrane resealing suggested by electroporation experiments than previous simulations using existing models.

Use of raytracing in models to investigate ionospheric channel performance

The main parameters that characterize an ionospheric channel, frequency, elevation angle, group path, ground range, and apogee have been obtained by means of 2D raytracing program called abcray03. The analysis has considered different hourly ionospheric conditions for four quiet days (25–28 March 2000) and five geographic locations distributed in the two hemispheres. Electron density profiles define the selected ionospheric conditions. Raytracing is run considering experimental electron density profiles from manually scaled measurements obtained with ionosondes. The resulting radio propagation parameters are compared with those obtained using the IRI and NeQuick models driven by measured F2 peak values.

Structure and Phase Behavior of Self-Assembled DPPC−DNA−Metal Cation Complexes

Multilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) in solution with DNA and bivalent metal cations (Ca2+, Mn2+, Mg2+) self-assemble into a ternary DPPC-DNA-Me2+ complex. The supramolecular structure of the complex consists of an ordered multilamellar assembly where hydrated DNA helices are sandwiched between the lipid bilayers and the metal cations bind the phosphate groups of DNA to the lipid polar heads. In the range of explored incubation times, the complex coexists with the uncomplexed DPPC over the whole temperature range investigated (20-55 degrees C). Accordingly, two distinct coexisting lamellar phases are observed, one corresponding to the ternary complex and the other to the uncomplexed lipid. The structure and thermotropic phase behavior of both of these have been investigated by means of synchrotron X-ray diffraction, and the relevant structural data are deduced from experimental electron density profiles. While the uncomplexed lipid exhibits the same phase behavior as pure DPPC, that is, L beta'-P beta'-L alpha, the thermotropic behavior of the bound lipid in the complex is partially altered. This is manifested as an increase in the main transition temperature and the disappearance of the ripple phase leading to the single -phase transition. The role of the different metal cations in promoting and stabilizing the DNA condensation into the ternary complex is also discussed.

Day-to-day changes in experimental electron density profiles and their implications to IRI model

The electron density variability at fixed heights is studied for use in the International Reference Ionosphere IRI model. Monthly median, upper and lower quartile values were obtained for ƒ oF2, hmF2, B0 and B1 as deduced from ionograms. The IRI electron density profiles established with these values were then compared with the median and quartiles at fixed heights. Results are shown for the three ionospheric stations El Arenosillo (37.1 N, 353.3 E), Tucuman (26.9 S, 294.6 E) and San Juan (31.5 S, 290.4 E) as a function of solar activity, season and local time. As found by other authors the height of maximum variability, hvmax, is located below the peak electron density height ( hmF2) inall the cases. Values of differences between hmF2 and hvmax are analized. Variability defined as the interquartile difference and hvmax results are calculated from experimental electron density profiles measured at the three stations.

Behavior of the bottomside electron density profile over Pruhonice

Ionosonde data obtained at Pruhonice (50.0°N; 15.0°E) during different times of the day, seasons and periods of the solar cycle are used to study the behavior of the F2 thickness and shape parameters, B 0 and B 1, respectively. Experimental values of both parameters are compared with the predictions of the latest version of the International Ionosphere Reference, IRI-2000. A preliminary analysis of the variability of the ionospheric parameters ƒ oF2, M(3000)F2, ƒ o F1 and ƒ oE is also done using the variability index Cup-Clo = interquartile range/median. The study shows that: (i) the IRI Gulyaeva B O option reproduces better the experimental electron density profiles than the B 0-Table option, (ii) the new values introduced for B 1 in the IRI-2000 model are more realistic than the constant value of three adopted by the IRI-90 and IRI-95 versions, (iii) the variability in the E and Fl regions is less than that observed in the F2 region, (iv) the M(3000)F2 factor exhibits low variability. Additional studies will be done in order to contribute to the development of a model for the variability indices used in this study.

Orientation and Conformation of a Lipase at an Interface Studied by Molecular Dynamics Simulations

Electron density profiles calculated from molecular dynamics trajectories are used to deduce the orientation and conformation of Thermomyces lanuginosa lipase and a mutant adsorbed at an air-water interface. It is demonstrated that the profiles display distinct fine structures, which uniquely characterize enzyme orientation and conformation. The density profiles are, on the nanosecond timescale, determined by the average enzyme conformation. We outline a computational scheme that from a single molecular dynamics trajectory allows for extraction of electron density profiles referring to different orientations of the lipase relative to an implicit interface. Profiles calculated for the inactive and active conformations of the lipase are compared with experimental electron density profiles measured by x-ray reflectivity for the lipase adsorbed at an air-water interface. The experimental profiles contain less fine structural information than the calculated profiles because the resolution of the experiment is limited by the intrinsic surface roughness of water. Least squares fits of the calculated profiles to the experimental profiles provide areas per adsorbed enzyme and suggest that Thermomyces lanuginosa lipase adsorbs to the air-water interface in a semiopen conformation with the lid oriented away from the interface.

Model computations of the influence of carbon impurities on the ionization relaxation in krypton shock waves

Chemical reactions in shock waves can be strongly affected by minute impurity concentrations. Thus it is not adequate to take into account the additional impurity electron production in relaxation studies simply by global adjustment of the atom—atom excitation cross-section constant to the measured electron density.A definite improvement, however, can only be achieved if the ionization relaxation model is extended to include all relevant impurity atom reactions. Consequently we treated the real test gas as a mixture of krypton and impurity carbon atoms. For the carbon model it is important to take the lower real excitation levels into consideration. Carrying out a sensitivity analysis we were able to reduce the number of reactions substantially. A comparison with experimental electron density profiles yielded 3.0 × 10−6 m2/J for the Kr—Kr excitation cross-section constant as well as values for the C—Kr constants.For a temperature of about 8000 K and an impurity concentration of about 40 p.p.m. it is shown that the impurity reactions dominate the electron production in the initial relaxation zone. This effect causes a pronounced decrease of the relaxation time with increasing concentration.By comparing computational results of the Kr–C model with those of the simplistic pure Kr model it is possible to explain the dependence of the Kr–Kr excitation cross-section constant on the impurity concentration and the plasma temperature.

Azelaic acid in model membranes: A thermodynamical and structural study

We report a thermodynamical and structural investigation on the effect of a dicarboxylic acid, the azelaic acid, on a synthetic phosophatidylcholine in excess water condition. The results indicate that the drug interacts with the lipid headgroup region, near to the hydrocarbon chains: in particular, at higher concentrations investigated, an interdigitated «gel phase», with rigid chains, stiff and perpendicular to the lamellar planes, is induced. The experimental electron density profiles, obtained for samples containing low and high quantity of azelaic acid, support the proposed interaction. Moreover, step electron density models, used to confirm the selected sign combination of the observed structure factors, show the best agreement with the experimental data when the chain terminal methyl groups are frozen in fixed position.

Model starting heights for N( h) analyses of ionograms

Using experimental electron density profiles, expressions have been obtained for the real height corresponding to a plasma frequency of 0.5 MHz and for the electron density at real heights of 90 and 150 km. These expressions may be used to define a starting point for the N( h) analysis of an ionogram containing only an ordinary-mode reflection trace.

The initial ionization of hydrogen in a strong shock wave

A theoretical and experimental investigation has been made of the initial ionization processes in a strong shock wave in hydrogen. The relaxation length for ionization, which is principally determined by the rate of excitation, was measured and from a comparison with the theory an estimate was obtained for the cross-section for atom-atom excitation collisions.Detailed theoretical calculations showed that the electron temperature approaches to within 1 % of the atomic temperature in a distance that is small compared with the total relaxation length for ionization. This enabled considerable simplification, for it indicated that a single-temperature model could be used in calculating the theoretical relaxation profile over the experimental range of operating conditions. An electromagnetic shock tube, with a Philippov pinch to create the driver plasma, was employed to produce shock waves of the required velocity. The ionization behind the shock front was studied by means of a double-frequency Mach-Zehnder interferometer, with a ruby laser and a K.D.P. crystal as the light source. A close agreement between the theoretical and experimental electron density profiles, behind the shock front, was obtained for small relaxation lengths, when the cross-section for the atom-atom excitation collisions was assumed to be about 7 × 10−2 times that of the corresponding cross-section for electron-atom excitation collisions.