Role of Bottom-Side Density Gradient in the Development of Equatorial Plasma Bubble/Spread F Irregularities: Solar Minimum and Maximum Conditions

摘要

From the analysis of Digisonde data over Brazilian equatorial and low-latitude sites, we investigate the relative importance of the different parameters driving the generation of rising bubble-type and bottom-type spread F (SF) irregularities. Data for the complete month of October 2001, a solar maximum epoch (F-10.7 = 210), and that of October 2008, an extended solar minimum period (F-10.7 = 70), are analyzed to examine the SF intensity and occurrence rate as a function of the evening prereversal vertical drift velocity and the corresponding F layer heights and the bottom-side density gradient. While the SF at the equatorial site is indicative of both the bottom-side irregularities and rising bubbles, the SF at the low latitude represents exclusively the latter. Comparison of the results, from the two epochs, reveals a large decrease in the intensity and occurrence rate of plasma bubbles, with a decrease in solar flux. But a notable increase in these characteristics is observed in the case of bottom-side SF. It is found that a larger (steeper) density gradient of the F layer bottom side that exists in the low solar flux condition is responsible for an enhanced Raleigh-Taylor instability growth, counterbalancing a reduction in this rate that may arise from a smaller prereversal vertical drift and lower layer height that also characterize the low solar flux condition. Thus, the role of the bottom-side density gradient in the ESF instability growth has been identified for the first time in terms of its ability to explain the contrasting irregularity features as observed during solar flux maximum and minimum years.