Nighttime Ionospheric Scintillations & E- & F-region Irregularities at the Magnetic Equator

Abstract

Associations of the amplitude scintillations of ATS-6 beacons during nighttime hours at Huancayo with the ionospheric irregularities indicated in the corresponding vertical incidence ionograms are discussed. The occurrence of blanketing type of sporadic-E layer during nighttime causes scintillations of about 1-2 dB on VHF and of about 5 dB on HF bands. The occurrences of range-type equatorial spread-F showing different layers of strong reflections on the ionograms are associated With very fast and almost saturated scintillations on all frequencies and these events occur predominantly in the post-sunset .ours (2100-2200 hrs LT) and also less frequently in the early morning hours (0300-0400 hrs LT). The range spread showing uniform amorphous spreading of echoes cause only moderate scintillations on VHF band less than 5 dB in magnitude. The frequency spread with no range spreading at lower frequencies either causes no scintillations on VHF bands or sometimes produces weak ones and also fluctuations ofthe signals of about 1-2 dB magnitude only. It is concluded that strong equatorial ionospheric scintillations during the nighttime hours are the result of multiple scattering between the regions of large plasma density gradient produced at fully developed stage of range spread. A thick F-region irregularity, indicated by uniform range spread, is not very efficient for scintillations of satellite radio signals.