Abstract
Abstract. Several investigations have been carried out to identify the factors that are responsible for the day-to-day variability in the occurrence of equatorial spread-F (ESF). But the precise forecasting of ESF on a day-to-day basis is still far from reality. The nonlinear development and the sustenance of ESF/plasma bubbles is decided by the background ionospheric conditions, such as the base height of the F-layer (h'F), the electron density gradient (dN/dz), maximum ionization density (Nmax), geomagnetic activity and the neutral dynamics. There is increasing evidence in the literature during the recent past that shows a well developed Equatorial Ionization Anomaly (EIA) during the afternoon hours contributes significantly to the initiation of ESF during the post-sunset hours. Also, there exists a good correlation between the Equatorial Ionization Anomaly (EIA) and the Integrated Equatorial ElectroJet (IEEJ) strength, as the driving force for both is the same, namely, the zonal electric field at the equator. In this paper, we present a linear relationship that exists between the daytime integrated equatorial electrojet (IEEJ) strength and the maximum elevated height of the F-layer during post-sunset hours (denoted as peak h'F). An inverse relationship that exists between the 6-h average Kp-index prior to the local sunset and the peak h'F of the F-layer is also presented. A systematic study on the combined effects of the IEEJ and the average Kp-index on the post-sunset, peak height of the F-layer (peak h'F), which controls the development of ESF/plasma bubbles, is carried out using the ionosonde 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 that the threshold base height of the F-layer at the equator for the development of plasma bubbles is reduced from 405 km to 317 km as the solar activity decreases from March 2001 (mean Rz=113.5) to March 2005 (mean Rz=24.5). This decrease in threshold height with the decreasing solar activity is explained on the basis of changes in the local linear growth rate of the collisional Rayleigh-Taylor instability, due to the variability of various terms such as inverse density gradient scale length (L−1), ion-neutral collision frequency (νin) and recombination rate (R) with the changes in the solar activity.
Highlights
The equatorial and low-latitude ionosphere is the region where the most important post-sunset electrodynamics play a great role towards the generation of a wide spectrum of plasma density irregularities, generally referred to as equatorial spread-F (ESF)
We present a linear relationship that exists between the daytime integrated equatorial electrojet (IEEJ) strength and the maximum elevated height of the F-layer during post-sunset hours
These results suggest that the daytime EEJ strength contributes positively for the post-sunset height rise of the equatorial F-layer thereby creating conditions favorable for the development of ESF, while the average Kp-index acts as a suppressant for the post-sunset height rise of the equatorial F-layer during equinoxes and winter solstices
Summary
The equatorial and low-latitude ionosphere is the region where the most important post-sunset electrodynamics play a great role towards the generation of a wide spectrum of plasma density irregularities, generally referred to as equatorial spread-F (ESF). BeckerGuedes et al (2004), from their case studies on the ESF occurrence/suppression during geomagnetic storms over the Brazilian sector, have shown that the enhanced geomagnetic activity acts as an inhibitor during the high spread-F season and acts as an initiator during low spread-F season, possibly due to the corresponding changes in the quiet and disturbed drift patterns during the different seasons These results suggest that the daytime EEJ strength contributes positively for the post-sunset height rise of the equatorial F-layer thereby creating conditions favorable for the development of ESF, while the average Kp-index acts as a suppressant for the post-sunset height rise of the equatorial F-layer during equinoxes and winter solstices. In this paper, this result is explained, for the first time, on the basis of changes in the local linear growth rate of collisional Rayleigh-Taylor instability due to the variability of various terms, such as the inverse density gradient scale length (L−1), ion-neutral collision frequency (νin) and recombination rate (R) with the changes in the solar activity
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