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Longitudinal Variations in Equatorial Ionospheric TEC from GPS, Global Ionosphere Map and International Reference Ionosphere-2016 during the Descending and Minimum Phases of Solar Cycle 24
(2022-11-01)
Ogwala, Aghogho
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Oyedokun, Oluwole Johnson
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Ogunmodimu, Olugbenga
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Akala, Andrew Ovie
;
Ali, Masood Ashraf
Research on longitudinal discrepancies in local ionospheric variability, especially in equatorial and low-latitude regions, is a focal point of interest for the space weather modeling community. The ionosphere over these regions is influenced by complex electrodynamics, wind, and temperature dynamics that can seriously impact dynamic technological systems such as satellite tracking and positioning, satellite radio communication, and navigation control systems. Here, we researched the longitudinal variability in the ionospheric total electron content (TEC) by analyzing observed global positioning system (GPS)-derived TEC values along with those extracted from the most reliable global ionospheric maps (GIMs) and the International Reference Ionosphere (IRI-2016) model at selected stations in the vicinity of the magnetic equator along the American, African, and Asian longitude sectors. The period of study covered the descending (2016–2017) and deep solar minimum (2018–2019) years in the 24th solar cycle. Apart from the decreasing trend of the TEC from the descending to deep solar minimum period irrespective of season and longitude sector, the results showed a relatively higher magnitude of TEC in the African longitude than the other two longitude sectors. Despite evident overestimation and underestimations of TEC in both models, GIM predictions generally looked better in terms of observed variation patterns, especially in the African longitude. The study also highlights the seasonal and semiannual effects of longitudinal variations in TEC, manifesting in local time offsets and some peculiar anomalies, which seemed to be different from previously reported results, especially during the solar minimum years at the three longitude sectors. The insignificant effects of longitudinal variations on the equinoctial asymmetry are attributed to the diverse electron density distribution and ionospheric morphology at the three longitude sectors that will prompt further investigations in the future. The outcomes from this study may augment the past efforts of scientists to understand the seasonal effects of the longitudinal variations in TEC, thereby complementing the improvements of ionospheric representations in global ionosphere models and maps.
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Individual performance of multi-GNSS signals in the determination of STEC over Thailand with the applicability of Klobuchar model
(2022-02-01)
Seok, Hong Woo
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Ansari, Kutubuddin
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Panachai, Chaiwat
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Jamjareegulgarn, Punyawi
Thailand is situated in the southern low latitude region in the Asian longitude sector having much importance as major portion of southern hemisphere is covered by ocean resulting in a sparse density of ground-based monitoring systems. With the establishment of GNSS monitoring stations across the Thailand and neighboring region, it accentuates the ionospheric variability study in the southern hemisphere in the Asian longitude. Therefore, in the current study, we selected four GNSS station located at different part of the country (i.e., CHMA, DPT9, NKRM, and SRTN) and studied the variations of ionospheric slant total electron content (STEC). Here, the STEC observations are estimated by Klobuchar model (namely, Klobuchar-modeled STEC values) and compared with the Global Ionospheric Map (GIM) STEC values for its validation. As an initial study, the Klobuchar-modeled STEC values obtained from five multi-constellation GNSSs over Thailand region (i.e., GPS, GLONASS, Galileo, BeiDou and QZSS) are computed and compared with the GIM STEC values during the intense geomagnetic storm on May 12, 2021 (DOY 132) and during June 2020 to May 2021 for monthly variations. Moreover, to show the relationships between the proposed Klobuchar-modeled STEC values and the GIM STEC values, the correlation coefficients and the root mean square errors between them are computed. The results showed that among the five multi-constellations of GNSSs, the GIM STEC values frequently overestimate the Klobuchar-modeled STEC values, except the QZSS system with the least differences ranging from −10 TECU to 20 TECU. Also, the correlation coefficient between the proposed Klobuchar-modeled and GIM STEC variations span between 0.87 and 0.89, and their RMSEs range from 10 TECU to 11 TECU, excluding QZSS system with less than 10 TECU. The correlation coefficients of higher than 0.85 can be considered as a good indicator for the applicability of Klobuchar model in practice with multi constellation systems.
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Singular spectrum analysis of GPS derived ionospheric TEC variations over Nepal during the low solar activity period
(2020-04-01)
Ansari, Kutubuddin
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Panda, Sampad Kumar
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Jamjareegulgarn, Punyawi
Accurate modeling of ionospheric total electron content (TEC) is an important aspect for mitigating the threats of trans-ionospheric delay error in satellite communication, earth observation, space-based navigation, timing applications as well as space weather forecasting services. In recent years, singular spectrum analysis (SSA) has been proved to be a powerful technique giving a relatively accurate estimate in time-series analysis comparable to the contemporary methods. In the current study, the SSA has been implemented on the GPS-derived TEC during the low solar activity year of 2017 over Nepal region which locates itself almost in the vicinity of low-latitudes being sandwiched between India and Tibet, China. The country foresees an explicit investigation and modeling of ionospheric TEC variations and corresponding delay error to precisely accomplish the space-based trans-ionospheric applications. The semi-annual variability of TEC with higher magnitudes during equinoctial seasons and lower values during solstice seasons is clearly noticed in the diurnal plots which are further substantiated by the trajectory matrix of time-series. The decomposed modes in the principal component analysis (PCA) signifies diurnal (first), semidiurnal (second), semiannual (third), monthly (fourth) with higher orders representing associated noise errors in the signals. Correlation coefficients (CC) between the reconstructed and observed time-series demonstrates the SSA method could be a successful tool for forecasting the TEC series over the region. The results are compared with empirical global ionospheric maps (GIMs) and IRI-Plas 2017 models during different seasons, emphasizing the suitability of SSA technique for relatively better precise TEC forecasting over the region.

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