TITLE:
A Study on the VLF/LF Long Term Amplitude Oscillations Associated with Frequencies 37.5 kHz and 45.9 kHz Received at Keil Longwave Monitor, Keil, Germany
AUTHORS:
Govinda Sharma, Keshav Prasad Kandel, Balaram Khadka, Peter Wilhelm Schnoor, Karan Bhatta, Basu Dev Ghimire, Aditya Singh Thapa
KEYWORDS:
VLF/LF, D-Layer, SAO, AO, TRGCP, NSY, Grindavik
JOURNAL NAME:
International Journal of Geosciences,
Vol.8 No.9,
September
18,
2017
ABSTRACT: The middle and upper atmosphere of Earth displays many large-scale oscillations
in several parameters. Of these oscillations, Atmospheric Oscillation (AO)
and Semi-annual Oscillation (SAO) are prominent ones. In this work, we
have analyzed the Very Low Frequency/Low Frequency (VLF/LF) data from
two of the transmitters of frequency 45.9 kHz at NSY, Sicily, Italy and 37.5
kHz at Grindavik, Iceland. The selected Trasmitter-Receiver Great Circle Path
(TRGCP) is middle latitude which is marine in case of Grindavik TRGCP and
terrestrial in case of NSY TRGCP. The VLF/LF signals are bounced back and
forth from D-layer of ionosphere (altitude at ~65 km during day and ～95 km
during night) in Earth-ionospheric waveguide. This infers the presence of atmospheric
oscillations as a consequence of change in ionization and recombination
rates. Many works related to AO and SAO are mostly done only for equatorial
region of the ionosphere and authors have reported the elegant dominancy
of AO and SAO in the VLF/LF amplitude pattern over years. To our surprise,
in our work it is seen that not only AO and SAO but also other oscillations are
necessary to model the oscillation pattern of middle latitude ionosphere.