The combined effects of electrojet strength and the geomagnetic activity (IKsubp/sub/I-index) on the post sunset height rise of the F-layer and its role in the generation of ESF during high and low solar activity periods

摘要

Several investigations have been carried out to identify the factors thatare responsible for the day-to-day variability in the occurrence ofequatorial spread-F (ESF). But the precise forecasting of ESF on aday-to-day basis is still far from reality. The nonlinear development andthe sustenance of ESF/plasma bubbles is decided by the backgroundionospheric conditions, such as the base height of the F-layer (h'F), theelectron density gradient (dN/dz), maximum ionization density (N_max),geomagnetic activity and the neutral dynamics. There is increasingevidence in the literature during the recent past that shows a welldeveloped Equatorial Ionization Anomaly (EIA) during the afternoon hourscontributes significantly to the initiation of ESF during the post-sunset hours.Also, there exists a good correlation between the Equatorial IonizationAnomaly (EIA) and the Integrated Equatorial ElectroJet (IEEJ) strength, asthe driving force for both is the same, namely, the zonal electric field atthe equator.In this paper, we present a linear relationship that exists between thedaytime integrated equatorial electrojet (IEEJ) strength and the maximumelevated height of the F-layer during post-sunset hours (denoted as peakh'F). An inverse relationship that exists between the 6-h averageK_p-index prior to the local sunset and the peak h'F of the F-layer is alsopresented. A systematic study on the combined effects of the IEEJ and theaverage K_p-index on the post-sunset, peak height of the F-layer (peak h'F), whichcontrols the development of ESF/plasma bubbles, is carried out using theionosonde data from an equatorial station, Trivandrum (8.47° N,76.91° E, dip.lat. 0.5° N), an off-equatorial station, SHAR(13.6° N, 79.8° E, dip.lat. 10.8° N) and VHF scintillations (244 MHz)observed over a nearby low-latitude station, Waltair (17.7° N,83.3° E, dip.lat. 20° N). From this study, it has been found thatthe threshold base height of the F-layer at the equator for the developmentof plasma bubbles is reduced from 405 km to 317 km as the solar activitydecreases from March 2001 (mean Rz=113.5) to March 2005 (mean Rz=24.5).This decrease in threshold height with the decreasing solar activity isexplained on the basis of changes in the local linear growth rate ofthe collisional Rayleigh-Taylor instability, due to the variability of variousterms such as inverse density gradient scale length (L−1), ion-neutralcollision frequency (ν_in) and recombination rate (R) with thechanges in the solar activity.