Statistical analysis and modelling of equatorial spread F parameters obtained from VHF radar at three longitudinal sectors

Abstract

Equatorial Spread F (ESF) is a manifestation of ionospheric interchange instabilities in the nighttime equatorial F region. These instabilities generate plasma density irregularities with scale sizes ranging from kilometers to thousands of kilometers. In the present study, statistics of various aspects of spread F occurrence are presented from HF/VHF radar located at three equatorial stations: Christmas Island (2oN, 202.6oE, 2.9oN dip latitude, VHF radar), São Luís (2.59oS, 315.8oE, 0.5oS dip latitude, HF radar) and Jicamarca (12oS, 283.1oE, 0.6oN dip latitude). The spread F parameters presented here are the onset altitude and onset time (onset refers to first appearance of signal in the radar field of view) of the bottom-type and plume, and the peak altitude of the plume which are known to be associated with the spread F occurrence characteristics. The study reveals novel features namely, seasonal and solar flux dependence of spread F occurrence over Christmas island /São Luís, and longitudinal dependence of spread F occurrence characteristics from these three stations based on the chosen parameters. The importance of this work lies in the spread F parameter empirical model developed combining statistical analysis of three equatorial and longitudinally separated stations, which is important to study evolution of irregularities in different longitudinal sectors for space weather forecasting and nowcasting programs, and improving scintillation warning models. These parameters show generally linear correlation with solar flux index (F10.7 cm) and variation with season and magnetic declination angle. The fit correlation with F10.7cm is shown as useful information to implement one spread-F development empirical model based on small scale irregularities detected by VHF radars.