Annual and semi-annual variations of TEC over Nepal during the period of 2007-2017 and possible drivers

The total electron content (TEC) variation of the ionosopheric layer is mostly controlled by Geomagnetic and solar activity. The TEC of the ionosphere can be estimated using the satellite signal delay recorded at GPS sites. In this study, the TEC data from three nearby GPS stations (CHLM, BMCL, and LMJG) from Nepal are extracted for about 11 years period (2007-2017). For the computation of the TEC data, wavelet transform, global wavelet power spectrum, cross wavelet transform, and wavelet coherence techniques are used. Utilizing such long-term GPS TEC data, Annual Oscillation (AO) and Semi-annual Oscillation (SAO) are identified in the daytime and nighttime TEC over Nepal. The SAO is found to be dominating periodicity in the daytime TEC, whereas the AO is found to dominant at night. In addition, possible connections with the indicators of geomagnetic and solar activity were studied. The geomagnetic indices AE and AU are exhibit a change in phase and are most consistent with both daytime and nighttime AO, implying that these indices could be the likely drivers of TEC's AO and SAO periodicities. The Dst index, on the other hand, is recognized as the most prominent driver of SAO in both daytime and nighttime TEC.

Automated summary

This study utilizes TEC data from three GPS stations in Nepal to investigate the periodic oscillations and the relationship with geomagnetic and solar activity indicators. It is found that daytime TEC is mostly driven by SAO, while nighttime TEC is dominated by AO. The geomagnetic indices AE and AU show consistency with AO, while the Dst index is strongly correlated with SAO.