부산대학교 도서관

Night-time equatorial ionosphere has been known to be characterised by irregularities which manifest as spread F in ionograms and as L-band scintillations in navigation signals. Spread F which is categorised into three types has also been suggested to be influenced by geomagnetic storms. In this study, ionograms obtained at Ilorin (Geomagnetic Lat. 1.87° S, long. 76.73° E, dip angle, –7.9°) during April and May 2010 were used to investigate the impact of moderate geomagnetic storms on the development of spread F. The result shows that geomagnetic storm produced enhancements in both the maximum electron density, NmF2 (up to 250%) and the height of maximum ionisation, hmF2 of the F2 layer (ranging from +2 to +68%) during post-sunset and post-midnight periods, with consequential enhancements in the occurrence percentage of equatorial spread F (ESF). The main phase of geomagnetic storms caused the suppression of ESF while the recovery phase provided atmosphere conducive for the enhancement of ESF occurrences. Although, geomagnetic storms enhance the occurrences of the three types of ESF, its influence is highest during the recovery phase. Percentage occurrence in mixed spread F (MESF) ranged between 8–86% while that of range spread F (RESF) ranged from 10–67%. The implication of this is that geomagnetic storms provide conditions which are conducive for the mechanisms responsible for the occurrence of MESF and RESF. These results suggest that the mechanisms for the formation of the three types of ESF differ, and are enhanced differently by different levels of depression in the magnetic field. [ABSTRACT FROM AUTHOR]