文摘
Recording the tweeks with a maximum up to eight harmonics using the receiver installed at Tay Nguyen University (12.65° N, 108.02° E) during 2013–2014, we investigated the morphology of the nighttime D-region ionosphere. Tweeks were recorded on 5 quiet nights per month. The results show that the mean reflection height in 2014 (Rz=79.3) is lower by 3.3 km than that in 2013 (Rz=64.9). The reflection height at low latitudes is higher than that at high latitudes. The mean reference height h′ in 2013 is higher about 0.9 km than that in 2014 and the mean sharpness factor β in 2013 is higher by 0.07 km−1 than that in 2014. The short-term variation of reflection heights for tweeks with harmonics m = 1–3 and sunspot number have the negative correlation coefficients. However, the correlations between them are not clear. On some nights, from 19:00–21:00 LT, the reflection height temporal variability shows a moderate to strong negative correlation with the tweek occurrence. This suggests that the reflection height variation may be caused by QE fields generated by lightning discharges. The variations of tweek reflection heights observed during 2013–2014, at low latitudes could be significantly caused by the ionization effect by Lyman-α and Lyman-β coming from geocorona, variation of neutral density, particle precipitations, and by direct energy coupling between lightning and lower ionosphere.
