Estimating the VLF modal interference distance using the South America VLF Network (SAVNET)

Abstract

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.

VLF modal interference distance and nighttime D region VLF reflection height for west‐east and east‐west propagation paths to Fiji

“…Here we use the notation D SSki = V T [SS k À SS i ] in order to distinguish between each distance obtained from two successive minima, for example, D SS21 = V T [SS 2 À SS 1 ], D SS32 = V T [SS 3 À SS 2 ] and so on. We note from [Samanes et al, 2015] where V N1 and V N2 have been taken from those tabulated by Wait and Spies [1964]. The mean experimental value of D MS estimated for NLK-Suva path is about 16% [((2507-2103)/2507) × 100 = 16%] higher than that for NWC-Suva path.…”

“…The mean D MS calculated using the TT method for NWC-Suva and NLK-Suva VLF propagation paths is 1979 ± 251 km and 2722 ± 266 km, respectively. We have used the TT method in a similar manner as described by Samanes et al [2015]. The estimate of the D MS for Figure 2 data using TT method was Radio Science 10.1002/2016RS006221 computed as follows: (1) we determined the sunrise and sunset amplitude minima times; (2) both the transmitter-receiver paths (NWC-Suva path and NLK-Suva path) were divided into 1300 parts;…”

“…This is because the NLK signal has a longer propagation path and the larger range of local time zones traveled to reach our station Suva. Recently, Samanes et al [2015] analyzed a long-term database of almost 5 years from three different VLF propagation paths, mainly oriented along the west-east direction (NPM-ATI, NPM-PLO, and NPM-ICA) from the South America VLF Network, and estimated the mean value of D MS using the methodology as described in this paper. The authors estimated the mean value of D MS for NPM-ATI, NPM-PLO, and NPM-ICA path as 2190 ± 60 km, 2160 ± 60 km, and 2170 ± 50 km, respectively.…”

“…Kumar [2009] determined the waveguide parameters at 19.8 kHz signal from NWC transmitter recorded at Suva during December 2006 and estimated that the experimental values of the waveguide parameters were consistent by 25-30% with the theoretical values calculated using the mode theory of VLF wave propagation in the waveguide. Recently, Samanes et al [2015] developed a better methodology called terminator time (TT) method to estimate the D MS from the occurrence time of the pronounced VLF amplitude minima and reported that their results show good agreement with other methods. These minima are produced as a result of processes of modal interference in which significant mode conversion is expected to occur at the sunrise or sunset line [Crombie, 1964;Walker, 1965].…”

VLF Monitoring System for Characterizing the Lower Region Ionospheric Layer

“…The decreasing reading during sunrise and sunset is called terminator time (TT). It is a time when the terminator line crosses given locations along the VLF propagation path creating amplitude minima at the receiver [10]. It is also defined as the time of amplitude minimum that coincides very well with the time of maximum rate of phase change [11].…”