Early Phase of Equatorial Spread-F Seen by HF Ionosonde & VHF Backscatter Radar

Abstract

Comparing the VHF backscatter radar echo power maps at Jicamarca and the vertical incidence ionograms at Huancayo, it has been shown that the post-sunset spread echoes start at the base of the F-Iayer, if no blanketing, sporadic or uniform layer is left in the E-region after sunset. In cases, when a blanketing sporadic-E or any uniform intermediate layer in this E-region persists after sunset, then the spread echoes appear to form first at this E-Iayer and later at the base of the F-Iayer, the two irregularity regions remaining unconnected. It is suggested that the first seeding of the spread irregularities at the equatorial ionosphere in the evening hours occurs due to gradient drift instability at any height between the E and F-Iayers wherever a large vertical plasma density gradient exists. Later the irregularities may extend upwards throughout the F-region due to the buoyancy effects through Rayleigh-Taylor instability mechanism provided proper conditions are present for the bubbles to attain reasonably upward velocities.