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.

中文翻译：

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底端密度梯度在赤道等离子气泡/扩散F不规则发展中的作用：太阳的最小和最大条件

通过对巴西赤道和低纬度地区的Digisonde数据进行分析，我们调查了驱动上升气泡型和底部型展布F（SF）不规则现象的不同参数的相对重要性。分析了2001年10月整个月的太阳最大历时（F _10.7  = 210）和2008年10月整个月的太阳最短时期（F _10.7  = 70）的数据，以检查SF强度和发生率。反转前垂直漂移速度和相应的F的函数层高和底部密度梯度。赤道站点的SF既指示了底部不规则性，又显示了上升的气泡，而低纬度的SF仅表示后者。从两个时期比较结果表明，等离子体气泡的强度和发生率大大降低，而太阳通量降低。但是，在底部SF的情况下，这些特性显着增加。发现F的密度更大（更陡）在低太阳通量条件下存在的低层底面有助于提高Raleigh-Taylor的不稳定性，从而抵消了由较小的逆向垂直漂移和较低层高度（也代表低太阳通量条件）引起的该速率的降低。因此，已经首次确定了底面密度梯度在ESF不稳定增长中的作用，因为它能够解释在太阳通量最大和最小年期间观察到的对比的不规则特征。