Modeling of long‐path propagation characteristics of VLF radio waves as observed from Indian Antarctic station Maitri

Abstract

Propagation of very low frequency (VLF) radio signal through the Earth‐ionosphere waveguide depends strongly on the plasma properties of the ionospheric D layer. Solar extreme ultraviolet radiation plays the central role in controlling physical and chemical properties of the lower ionospheric layers and hence determining the propagation characteristics of a VLF signal. The nature of interference among different propagating modes varies widely with the length of the propagation path. For a very long path, exposure of solar radiation and thus the degree of ionization vary by a large amount along the path. This influences the VLF signal profile by modulating the sky wave propagation. To understand the propagation characteristics over such a long path, we need a thorough investigation of the chemical reactions of the lower ionosphere which is lacking in the literature. Study of radio signal characteristics in the Antarctic region during summer period in the Southern Hemisphere gives us a unique opportunity to explore such a possibility. In addition, there is an extra feature in this path—the presence of solar radiation and hence the D region for the whole day during summer in at least some sections of the path. In this paper, we present long‐distance propagation characteristics of VLF signals transmitted from VTX (18.2 kHz) and NWC (19.8 kHz) transmitters recorded at the Indian permanent station Maitri (latitude 70° 45′S, longitude 114° 40′E) in 2007–2008. A very stable diurnal variation of the signal has been obtained with no signature of nighttime fluctuation due the presence of 24 h of sunlight. Using ion production and recombination profiles by solar irradiance and incorporating D region ion chemistry processes, we calculate the electron density profile at different heights. Using this profile in the Long Wavelength Propagation Capability code, we are able to reproduce the amplitude of VLF signal.

Modeling of temporal variation of very low frequency radio waves over long paths as observed from Indian Antarctic stations

“…Second, some portion of the signal propagation path lies over both continental ice (for Maitri station) and southern sea ice (for both Maitri and Bharati stations), which is responsible for high attenuation in signal amplitude. It has been found from previous existing literatures [ Field and Greifinger , ; Sasmal et al , ] that there is a significant loss of VLF signal amplitude for propagation over ice mass. It has also been pointed out that for a fixed upper boundary of the Earth‐ionosphere waveguide (say 60–70 km), the attenuation is higher (around 8 times) for propagation over ice mass than sea surface.…”

“…Smaller segment could generate more realistic ionospheric conditions thereby increasing the chance of agreements between the observed and simulation results. In our previous work [ Sasmal et al , ] we extensively used the D region ion chemistry model for our simulation. In that work, we manually segmented the entire path to calculate the simulated electron density profile.…”

“…Earlier, in Sasmal et al [2014Sasmal et al [ , 2015, the signal amplitude variation with distance was successfully reproduced using both numerical simulation and theoretical modeling. In Sasmal et al [2014], the well-known long wavelength propagation capability (LWPC) code was used to compare the signal characteristics corresponding to long propagation paths (∼10000 km) with observations.…”

“…It was also seen that all the modes suffered maximum attenuation near the equatorial region and at the Antarctic ice mass [ Sasmal et al , ]. In Sasmal et al [], the signal amplitude was reproduced using the well‐known D region ion chemistry model and LWPC code. The whole path was divided into 20 segments, and electron density was calculated from the ion chemistry model for each segment incorporating the solar zenith angle information into the model.…”

First Results of the Wave Measurements by the WHU VLF Wave Detection System at the Chinese Great Wall Station in Antarctica

“…Due to its unique geographical location and electromagnetically quiet environment, Antarctica is ideal for radio wave studies, as well as the underlying physical processes (Clilverd et al, 2010;He et al, 1987;Sasmal et al, 2015). The energetic particles scattered by plasma waves can precipitate into the atmosphere along the geomagnetic field lines and interact with the high-latitude ionosphere (e.g., Ni et al, 2017Ni et al, , 2019.…”