On the coexistence of kilometer‐ and meter‐scale irregularities in the nighttime equatorial <i>F</i> region

Abstract

Nighttime multifrequency scintillation and 50‐MHz radar backscatter observations simultaneously performed over a nearly common ionospheric volume at the dip equator in Peru during March 1977 were used to study the relationship between the large‐scale irregularities (∼0.1–1 km) giving rise to scintillations and small‐scale irregularities (3 m) causing 50‐MHz backscatter. It is shown that during the generation phase of equatorial irregularities in the evening hours, the kilometer‐ and meter‐scale irregularities coexist, whereas in the later phase, approximately an hour after the onset, the meter‐scale irregularities decay but the large‐scale ones continue to retain their high spectral intensities. Further, multistation scintillation observations from a host of geostationary satellites as well as from the Wideband satellite indicate that eastward‐drifting irregularity structures detected around midnight cause significant scintillations at UHF and L band but generally fail to give rise to appreciable backscatter. Thus, contrary to expectations, it is possible to have even L band scintillations without any plume structure on backscatter maps. This indicates that at later local time a cutoff of the spectral intensity probably occurs at some scale length between 100 and 3 m. These observational results are discussed in the context of current theories of plasma instability in the equatorial ionosphere.