[1] A numerical modeling of Earth's atmosphere is carried out by means of the Transmission Line Matrix (TLM) numerical method with the aim of calculating the Schumann resonance frequencies. The numerical results obtained are very close to the experimental ones and those obtained with the widely accepted two-scale-height ionospheric model, which allows us to affirm that this is a valid numerical tool for predicting the Schumann frequencies in the atmospheres of other planets and moons. The great flexibility of the TLM numerical method also allows the study of slight shifts in the Schumann resonance frequencies due to an increase in electrical conductivity at the Earth's poles, originating from high-energy particle precipitation emitted from the Sun in conjunction with solar flares. A slight increase in the first Schumann resonance frequency is observed during these events, which is associated with a reduction in the dimensions of the electromagnetic cavity.

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[1] More than four years of VLF electric field data recorded by DEMETER have been analyzed, in order to monitor the first cut-off frequency (QTM 1 ) of the Earth-ionosphere waveguide, at around 1.6-1.8 kHz. Since losses in a waveguide are maximized right at the cut-off frequency, DEMETER ($700 km orbit) can detect the minimum of energy of the leaking fields coming from the waveguide. This measurement permits to draw a global map of its value (f 1 ), which is directly related to the effective height of the ionosphere (h) by the relation f 1 = c/2h (c is the speed of light). It enables the remote sensing of the D region, which is one of the less known layers of the ionosphere, because it is too low for satellites to orbit inside it and too high for balloons to reach it. The effective height depends mainly on the electron density (N e ) and neutral density (N n ) profiles, which determine the plasma frequency and the electron mobility. The effective height shifts downward 5-10 km in southern warm season in the South Pacific Ocean. Another effect is observed in the Indian and Atlantic Oceans; the effective height decreases its value twice a year, in the area of roughly AE15 from the geomagnetic equator. The main causes for the changes on the effective reflection height are the solar radiation and the thunderstorm activity. However, the observed shifts are more prominent over the oceans, and a possible explanation for this difference could be attributed to i) less polluted conditions above the oceans (aerosols change the atmospheric conductivity and then the global atmospheric electric circuit), ii) the effect of the current associated to the thunderclouds on the bottom of the ionosphere because thunderstorms are much more numerous above land, or iii) ionization by elves because their occurrence is larger above oceans.Citation: Toledo-Redondo, S., M. Parrot, and A. Salinas (2012), Variation of the first cut-off frequency of the Earth-ionosphere waveguide observed by DEMETER,

Poly(vinyl) butyral (PVB) nanofibers (NFs) and carbon nanotube (CNT) reinforced PVB NF composites were developed by using the ForcespinningV R technology. PVB was dissolved in a mixture of ethanol and methanol (7:3 wt/wt) at various concentrations, and the solutions were spun at rotational speeds varying between 3,000 and 9,000 rpm. The CNT/PVB solutions were prepared using the same solvent ratio with varying the concentration of CNTs. The results show that the diameter of the PVB fibers increased with increasing rotational speed; however the standard deviation of the fiber diameter distribution decreased. The morphology and thermal properties of the developed fiber systems were studied by DSC, TGA, Raman, and FTIR. The effect of CNT on the mechanical properties of the developed fibers was investigated by carrying out tensile tests at different strain rates. Raman and FTIR analyses indicate a noncovalent p-p stacking interactions and hydrogen bonding between CNT and the PVB NFs. Adding CNT to the PVB NF matrix resulted in improved tensile strength by 150%. POLYM. ENG. SCI., 55:81-87,