Some characteristics of the nighttime subionopheric propagation of VLF waves

Daily amplitude of Very Low Frequency (VLF) waves shows deep amplitude minima. They are attributed to modal interference of waves propagating in the nighttime Earth - Ionosphere Waveguide (EIW). The time of these minima are related with the localization of the Terminator line along the propagation path, and, therefore, is denominated of Terminator Time or TT. In this work we use the monitor of TTs to discuss some aspects and parameters of the nighttime subionospheric wave propagation, such as distance of modal interference D and temporal variations of the nighttime low ionosphere height on different scales. We also show the possibility to search for seismo-electromagnetic effects using this method. For this study we are using amplitude data, obtained since 2006, by the South America VLF Network (SAVNET) on East-West and North-South oriented VLF propagation paths.