Ionospheric TEC response to severe geomagnetic storm and annular solar eclipse through GNSS based TEC observations and assessment of IRI-2016 model and global ionosphere maps over Sri Lankan equatorial and low latitude region

The recent establishment of permanent Global Navigation Satellite System (GNSS) monitoring stations across the Sri Lankan region prompts investigating the challenging equatorial ionospheric dynamics and modeling advancements across the Indian longitudes. In this paper, we tried to investigate the ionospheric response to the solar-terrestrial events over Sri Lanka and assess the performance of the latest International Reference Ionosphere (IRI-2016) and Global Ionospheric Map (GIM) over the region. The total electron content (TEC) observations from two GNSS stations at Jaffna (9.66(0) N, 80.02(0) E) and Kalutara (6.58(0) N, 79.96(0) E) are considered for investigating a couple of major solar-terrestrial events with peculiar electrodynamics in the declining phase of 24th solar cycle: namely, (i) geomagnetic storm event during September 2017 and (ii) annular solar eclipse of 26th of December 2019. During the storm time, IRI-2016 model followed the variation pattern of GNSS-TEC, indicating a significant underestimation during noontime (similar to 18 TECU) and nighttime (similar to 7 TECU) which attains a maximum deviation of 86% during the early morning. Concerning the solar eclipse day, the model did not witness any transient response, rather remained undervalued throughout the period. On the other hand, the GIM-TEC shows a relatively better agreement with GNSS-TEC following its variation pattern during both events. A daytime overestimation of 10-15%, a higher nighttime overestimation of around 40% with a significant stormtime deviation of 84% is noticed in the GIM data. Moreover, both the models do not exhibit a clear storm-time or eclipse-time ionospheric fluctuations as clearly seen from the GNSS-TEC. This could be due to the underlying sparse experimental data coverage across the region. From the analysis of both the events and supporting literature, we emphasize further improvements in the IRI-2016 model with due consideration of the topside and plasmasphere variations. Further, the improvement in time resolution of the GIM would improve the level of understanding of the low latitude ionosphere over the Sri Lankan region.