Semiannual Anomaly of TEC in Northern Midlatitude Regions During Low Solar Activity Years
11th International Conference on Recent Advances in Air and Space Technologies, Conference Program, RAST 2026, İstanbul, Türkiye, 13 - 15 Mayıs 2026, (Tam Metin Bildiri)
- Yayın Türü: Bildiri / Tam Metin Bildiri
- Doi Numarası: 10.1109/rast69551.2026.11672308
- Basıldığı Şehir: İstanbul
- Basıldığı Ülke: Türkiye
- Anahtar Kelimeler: Dissimilarity Measure (DM), Low Solar Activity (LSA), Planar Trend Model, Semiannual Anomaly, Total Electron Content (TEC)
- Hacettepe Üniversitesi Adresli: Evet
Özet
Semiannual anomaly refers to the phenomenon where ionospheric ionization is greater during equinoxes compared to solstices. In this study, the semiannual anomaly is investigated using the base-level coefficients of the locally planar trend model over all Northern Midlatitude regions and for all Low Solar Activity (LSA) years between 2000 and 2025. The seasons of the Northern Midlatitude regions - Spring Equinox (SE), Summer Solstice (SS), Autumn Equinox (AE), and Winter Solstice (WS) - are grouped into 60-day-long sets that include base-level coefficients for all days within the seasonal months and for all hours. To compare interseasonal variability, the Dissimilarity Measure (DM), a combined indicator of both metric and entropic distances, is employed. DM heatmaps show that, during LSA years, geomagnetic forcing more strongly shapes ionospheric variability, consistent with geomagnetic field geometry. In particular, the DM patterns closely track the geomagnetic equator, while the lowest dissimilarity is observed between SE and AE. The results further show that the upper Northern Midlatitude regions maintain systematically higher annual mean DM values than the lower regions, implying stronger seasonal contrast in the upper band. It is further observed that, under LSA conditions, although the upper ionospheric layers still retain a measurable contribution, the spatial distribution of the semiannual anomaly over the Northern Midlatitude regions appears to be influenced more strongly by contributions associated with the lower ionospheric layers. Regional departures from the dominant ordering indicate that the observed semiannual behavior is affected not only by latitude but also by longitude-dependent climatological and geomagnetic effects. Regions located within the North Atlantic Oscillation sector, together with the Siberia Yakutsk Anomaly sector and the Pacific Decadal Oscillation sector, exhibit a pronounced separation from the other Northern Midlatitude regions.