Formation of Ionization Ledges in the Equatorial Topside Ionosphere

Topside ionograms obtained in the low-latitude region around the magnetic equator reveal cusp formation, indicating the existence of a ledge due to an enhancement of ionization in the vertical electron density profiles. The present work discusses two examples of ledge observations, one at noontime and the other at a pre-midnight hour. The noontime observations indicate that the ledge is formed in the region above the F2 layer maximum where O(+) ions are predominant. A mechanism for ledge formation is discussed, in which a field-aligned anomaly in neutral density and temperature plays a significant role. Such an anomaly inhibits plasma flow along the field line and causes an enhancement of ionization along a particular field line over the magnetic equator.