Study of variations of the nighttime D region ionosphere during the high solar activity year from multimode tweek atmospherics observed at a lowlatitude station, Tay Nguyen university, Vietnam

English Abstract:

Tweek atmospherics with a maximum up to the eighth harmonics observed during the high solar activity year, 2013, at Tay Nguyen University, Vietnam (Geog. 12.65^o N, 108.02^o E), have been used to study the morphology of the nighttime lower ionosphere. The analysis of 33690 tweeks on five international quiet days during two months each season: summer (May, August), winter (February, November), and equinox (February, September) shows that tweek occurrence of about 51 % during summer season and about 22 % and 27 % occurrences during winter and equinox seasons. The boundary of the D-region ionosphere has the good conductivity for the harmonics m = 5 – 6 in the altitude range of 85 – 86 km. The environment of the D-region during winter and equinox seasons is more inhomogeneous than that during summer season. The mean electron density varies from 30 - 222 cm^-3 corresponding to the harmonics m = 1 - 8 at the mean reflection height from 81 - 87 km. Overall, the reflection height for higher harmonics in the postmidnight is higher during winter and equinox seasons as compared to that during summer season. The result reveals that the lower reference height in our work as compared to other works is due to the higher level of solar activity. The values of the electron density estimated using tweek method during summer, equinox and winter seasons are lower by 19 - 48 %, 34 - 57 %, and 26 - 66 % than those obtained using the International Reference Ionosphere (IRI-2012) model, respectively. The seasonal variation of the electron density profile shows less significant during solar maximum period than that during solar minimum period.