The cure kinetics of an unsaturated polyester resin were studied by differential scanning calorimetry (DSC), and different dynamic and isothermal procedures were compared. It was established that the isothermal kinetic analysis through the isoconversional adjustment lnt = A + E/RT is the method that offers the most accurate results for unsaturated polyester resin cure kinetics. From this comparative study it was noted that the activation energy not only varies according to the degree of conversion but also according to the method used to evaluate the kinetic parameters. Furthermore, it was shown that the activation energy cannot be separated from the other adjustment parameters, so the different kinetic procedures used are not generally comparable. Different methods of evaluating the degrees of conversion α and the reaction rates dα/dt according to the experimental reaction heat were also studied. It was found that the method used has a strong influence on the values of α and dα/dt, but only a slight one on the kinetic parameters.

Comparative study of the cure kinetics of an unsaturated polyester resin using different procedures

Quasi-stationary coronal magnetic field and electron density from Faraday rotation experiment, using theoretical model

The quasi-stationary coronal magnetic field and electron density as determined from a Faraday rotation experiment

Pioneer VI was launched into a circumsolar orbit on December 16, 1965, and was occulted by the sun in the latter half of November, 1968. During the occultation period, the 2292-MHz S-band telemetry carrier underwent Faraday rotation due to the interaction of this signal with the plasma and magnetic field in the solar corona. The NASA/JPL 210-ft diameter antenna of the Deep Space Net- work near Barstow, California, was used for the measurement. The antenna feed was modified for automatic polarization tracking for this experiment. The measurement results are interpreted with a theoretical model of the solar corona. This model consists of a modified Allen-Baumbach electron density and a coronal magnetic field calculated both from Mount Wilson magnetograph observations using a source surface model and field extrapolations from the Explorer 33 satellite magnetometer. The observations and the calculated rotation show gene- ral agreement with respect to magnitude, sense, and timing, suggesting the source-surface model and field extrapolations from 1 AU are a valid technique to obtain the magnetic field in the corona from 4 to 12 solar radii. Variations present can easily be ascribed to density enhancements known to be present in the corona. Longitudinal variations of the density in the corona cannot be obtained from coronagraph observations, and thus a purely radial variation was assumed. An improved fit to the Faraday rotation data is obtained with an equatorial electron density

Coronal magnetic field and electron density as determined from a faraday rotation experiment

The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere–lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3–30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth–ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique.

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On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

Effect of the 16 February 1980 solid eclipse on VLF propagation

VLF Propagation Effects produced by the Eclipse

The occurrence of sudden ionospheric disturbances is studied in relation to changes in the conductivity parameter and the conductivity gradient in the lower ionosphere. The analysis is based on phase and amplitude data from a great number of such events measured at several very low frequencies. The change of the conductivity parameter was found to be more important as the SID affects deeper regions. The results also suggest that during these anomalies the ionospheric conductivity gradient is reduced, generally by small amounts. Such disturbed gradients may be determined if initial conditions for daytime normal gradient, and for collision frequencies above the D region, are known. A first estimate shows that, for the majority of the events selected, the new gradient changes from the normal daytime value of 0.30 to 0.28 km−1. The existence of special kinds of SID is discussed; they produce an important modification in the gradient and a relatively small height variation for the earth-ionosphere waveguide upper boundary.

Relative changes on lower ionosphere conductivity gradients during SID events

There are microwave bursts not observable in the total flux from the Sun, but only in circular polarization. The production of these events in the Sun requires equivalent changes of the magnetic field imposed on the emission regions during the bursts.

Polarization bursts in the sun observed at microwave frequencies.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JA073i019p06404

Correction to paper by Pierre Kaufmann and Aracy M. Mendes, ‘Relative changes on lower ionosphere conductivity gradients during SID events’

Five months of very-low-frequency phase measurements along the propagation paths Balboa (Panama)-Sao Paulo and Criggion (GB)-Sao Paulo, were studied in order to find apparent and equivalent diurnal height variation for the upper boundary of the earth-ionosphere waveguide. The results suggest that an explanation of the apparent height variation dependence on the wavelength and propagation path may be found if the second order mode of propagation is considered during nighttime and the diurnal change of the ionospheric conductivity gradient is taken into account. The results fit well with an “average ionosphere” model presenting nighttime conductivity gradient of 0.5 km ?1 and daytime gradient of 0.3 km ?1 . As a consequence, the equivalent diurnal height variation found is situated near 10 km, the assumption of a single waveguide configuration for both propagation paths being valid. DOI: 10.1111/j.2153-3490.1968.tb00411.x

Aracy M. Mendes

Papers

VLF Propagation Effects produced by the Eclipse

Relative changes on lower ionosphere conductivity gradients during SID events

Polarization bursts in the sun observed at microwave frequencies.

Correction to paper by Pierre Kaufmann and Aracy M. Mendes, ‘Relative changes on lower ionosphere conductivity gradients during SID events’

A note on lower ionosphere equivalent height diurnal variation