Seasonal and local-time patterns of VTEC and dTEC RMS over Hainan: temporal and horizontal characteristics of winter and summer enhancements

Abstract. Background vertical total electron content (VTEC) and short-timescale TEC variability describe different aspects of the low-latitude ionosphere. Previous East Asian studies established winter daytime and warm-season nighttime disturbances, but it remained unclear whether the winter and summer enhancements have the same temporal and horizontal character. We analyzed daily-balanced GPS and BeiDou observations from 1,793 dates during 2015-2020 over Hainan. VTEC followed daytime plasma buildup and was strongest in autumn, with a later afternoon maximum in midsummer. Detrended TEC root-mean-square amplitude (dTEC RMS) instead peaked in winter afternoons and shifted toward late evening and midnight during spring and summer. We then compared BeiDou paths that remained near one ionospheric location with paths that crossed the surrounding region. In winter, both types showed the afternoon enhancement, including within comparable viewing directions. The winter structures therefore evolve over or pass through Hainan; a stationary horizontal gradient alone cannot explain the climatological maximum. In summer, the near-midnight enhancement was also present at nearly fixed locations, confirming a temporal increase, but its strength changed markedly among viewing directions and sampled regions. The summer structures are therefore more likely concentrated in particular regions or directions than spread uniformly across the Hainan sector. Spring and autumn show the orderly transition between these winter-afternoon and warm-season near-midnight environments. Together, the results show that winter and summer dTEC enhancements share temporal development but differ in their horizontal distribution.