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.
Highlights
The Earth’s middle and upper atmosphere mainly implies stratosphere, mesosphere and thermosphere
A trend can be seen in time series, positive in all-time series except in the midday of Grindavik Trasmitter-Receiver Great Circle Path (TRGCP), where it was negatively correlated with its coefficient −0.00082
We examined five years of VLF narrow band (NB) data received in North-East hemisphere (NSY TRGCP) and North-west hemisphere (Grindavik TRGCP), during midday and midnight hours
Summary
The Earth’s middle and upper atmosphere (above 20 km from the surface of the earth) mainly implies stratosphere, mesosphere and thermosphere. This region reveals several dominant large-scale oscillations in many measured parameters. These dynamic oscillations are the main features of these regions These oscillations can be found at all latitudes, from the Equator to high latitudes. The important oscillation is SAO occurring mainly at equator in the regions of stratosphere and mesosphere [7]. AO is found to be prominent mainly in high latitudes The amplitudes of those oscillations are higher in mesosphere than in stratosphere. As not many works are done for AO and SAO oscillations of VLF/LF, this work of ours is of great importance
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