11:00 〜 13:00
[PEM13-P01] Diurnal and seasonal variations in the D-region ionosphere using LF transmitter signals
The ionospheric D-region is maintained by solar radiation. The D-region electron density varies depending on the solar zenith angle, which is a function of local time (LT) and season (month). In addition to the regular variation, D-region variations associated with various factors have been reported, for example, by solar flares, geomagnetic storms, atmospheric gravity waves, planetary waves, and sudden stratosphere warming (SSW) (e.g., Lastovicka, 2006; Correia et al., 2011; Pal et al., 2017). The detailed investigation of these factors is required to fully understand the D-region characteristics. In this study, we investigated the seasonal and LT dependences of the D-region ionosphere using Low Frequency (LF) transmitter signals. The transmitter and receiver were JJY (60kHz, 33.47N, 130.18E) and RKB (Rikubetsu, Hokkaido, 43.45N, 143.77E), respectively. For removing effects of geomagnetic storms, five geomagnetically quiet days (q1-q5) in each month in 2017 were selected for present analysis. We calculated amplitude of the variations in the intensity (ΔA) and phase (ΔP) by subtracting the average value from the raw data. In daytime, both ΔA and ΔP around 09:00-15:00 LT are larger than those in other LTs. ΔA was large in summer and winter, while the ΔP was large in spring and autumn. In nighttime, the ΔP was large during sunrise and sunset, although the ΔA did not show clear trend. The observed results were compared with the calculated results using the wave-hop method. In daytime, calculated ΔA was larger during sunrise and sunset than those around 09:00~15:00 LT. This is opposite to the observation. The change in ΔP between observed and calculated results was opposite during sunrise and sunset, although it was similar with observation from 09:00 LT to 15:00 LT. The calculation results showed that the ΔP was large in summer. In the nighttime, both ΔA and ΔP were constant in the calculation, while there were several variations in the observation. In observation, both ΔA and ΔP in January, 2017, were larger than the calculated values. The increases in the ΔA and ΔP in January, 2017 suggest that observed LF variations could be caused by neutral atmospheric waves, because a SSW occurred in the analyzed period. In this presentation, we will discuss the differences between the observed and calculated ΔA and ΔP in detail.