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.

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Total Electron content is measured using a dual frequency GPS receiver in Lamgung (LMJG) Station located at 84.57° longitude and 28.17° latitude of the year 2015 as it is considered as geo-magnetically active year. In this study, diurnal variation of VTEC has been studied separately for quiet and disturbed days and, for the effective study of the case the PRN wise data of VTEC have been also used. The maximum VTEC is seen from 0700 LT to 1100 LT (LT=UT+5.45). PRN wise VTEC is studied taking the lower bound (LB) and upper bound (UB). The correlation of VTEC with Dst index, Kp index and Solar flux have been studied. Positive correlation has been found in disturbed days with Kp index and solar flux but negative correlation with Dst index. Dst index shows positive correlation in quiet days but Kp index shows negative correlation. BIBECHANA 17(2020) 123-132

The US Navy VLF/LF Transmitter's NSY signal (45.9 kHz) transmitted from Niscemi, Sicily, Italy, and received at the Kiel Long Wave Monitor, Germany, was analyzed for the period of two months, May and June (EQ-month) of 2013. There were 12 earthquakes of magnitude greater than 4 that hit Italy in these two months, of which the earthquake of 21 st June having magnitude of 5.2 and a shallow focal depth of 5 km was the major one. We studied the earthquake of 21 st of June 2013, which struck Tuscany, Central Italy, (44.1713˚N and 10.2082˚E) at 10:33 UT, and also analyzed the effects of this earthquake on the sub-ionospheric VLF/LF signals. In addition, we also studied another earthquake, of magnitude 4.9, which hit the same place at 14:40 UT on 30 th of June and had shallow focal depth of 10 km. We assessed the data using terminator time (TT) method and night time fluctuation method and found unusual changes in VLF/LF amplitudes/phases. Analysis of trend, night time dispersion, and night time fluctuation was also carried and several anomalies were detected. Most ionospheric perturbations in these parameters were found in the month of June, from few days to few weeks prior to the earthquakes. Moreover, we filtered the possible effects due to geomagnetic storms, auroras, and solar activities using parameters like Dst index, AE index, and Kp index for analyzing the geomagnetic effects, and Bz (sigma) index, sunspot numbers, and solar index F10.7 for analyzing the solar activities for the confirmation of anomalies as precursors.

This study presents an attempt to derive Zenith Total Delay (ZTD) and Precipitable Water Vapour (PWV) from Global Positioning System (GPS) and observed meteorological data over three topological regions of Nepal: Terai, Hilly and Himalayan regions. Four months of the year 2015, each from four different seasons was taken as the study period and the obtained data were plotted and the trends were compared. The result showed that the average PWV is higher in the Terai region in comparison to the Hilly and Himalayan regions. This in turn explained the rainfall trends in Himalayan region of Nepal. The diurnal behaviour of PWV shows that it was highest during midday of the local time and was lowest during night.