Pronounced amplitude minima are observed during the subionospheric propagation of VLF waves at times (Terminator Times) when the Terminator Line crosses given locations along the propagation path. The distance between such two successive minima is called the modal interference distance D, which is related to nighttime mode propagation in the Earth-ionosphere waveguide. Therefore, the temporal behavior of the distance D can bring information on the dynamics of the nighttime lower ionosphere and on the presence of external forcing agents, including those associated with seismic activity. In this paper we present a methodology to estimate D based on the measure and analysis of the pronounced VLF amplitude minima. We have used a long-term database of almost 5 years from three different VLF propagation paths from the South America VLF Network. We emphasize that the accuracy of the determination of the distance D achieved by our method is better than those obtained in earlier studies. The reason for that is the use of a long-term continuous database, from different parallel propagation paths mainly oriented along the west-to-east direction. We discuss typical properties of the obtained distance D, as the simultaneous occurrence of amplitude minima for parallel propagation paths, anomalous values of D at locations where the Terminator Line is close to the receiver, and the derivation of the undisturbed nighttime ionospheric height at h N~8 8 km.
[1] Daily profiles of phase measurements as observed on fixed VLF paths generally show a transient phase advance, followed by a phase delay, for about 90 min after sunrise hours. This is indicative of a reflecting ionospheric C region developing along the terminator line at an altitude below the normal D region. The suggested occurrence of a C region is consistent with rocket measurements made in the 1960s, showing a maximum of the electron density between 64 and 68 km, and by radio sounding in the 1980s. In order to correctly describe the properties of the phase effect associated with the presence of a C region, it is important to understand the subionospheric propagation characteristics of the VLF paths. In this paper, we analyze the variations presented by the temporal properties of the VLF narrowband phase effect and determined a parameter associated with the appearance of the C region at sunrise hours observed by receivers from the South America VLF Network. Periodic patterns emerge from the parameter curves. Two distinct temporal behavior regimes can be identified: one exhibiting slow variations between March and October, and another one exhibiting faster variations between October and March. Solar illumination conditions and the geometrical configuration of the VLF paths relative to the sunrise terminator partly explain the slow variation regime. During periods of faster variations, we have observed good association with atmospheric temperature variability found in the measurements of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics and Sounding of the Atmosphere using Broadband Emission Radiometry satellite instrument, which we assume to be related to the winter anomaly atmospheric phenomenon. However, when comparing the parameter time series with temperature curves, no direct one-to-one correspondence was found for transient events.Citation: Bertoni, F. C. P., J.-P. Raulin, H. R. Gavila´n, P. Kaufmann, R. Rodriguez, M. Clilverd, J. S. Cardenas, and G. Fernandez (2013), Lower ionosphere monitoring by the South America VLF Network (SAVNET): C region occurrence and atmospheric temperature variability,
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