To investigate their role on the occurrence of equatorial spread F (ESF), the low-latitude E region in the evening hours has been characterized in terms of sporadic E (E_s) activity based on ionosonde observations from Sriharikota, a low-latitude station in India. While prior to the sunset, either Chapman type E layer, or E_s, or a combination of the two was observed, after sunset, when observed, the E layer was exclusively E _s. E_s, however, could be either blanketing or nonblanketing type. Observations revealed that while the occurrence of ESF had a broad relationship with the type of E_s, a detailed investigation suggested that (1) ESF occurred (did not occur) irrespective of E_s type if the virtual height of the F layer base (h'F) was greater than ～360 km (less than ～290 km), and (2) ESF occurrence depended strongly on the type of E_s if 290 km < h'F < 360 km. Model computation shows that low-latitude E_s can significantly influence the growth of the Rayleigh-Taylor instability for the cases corresponding to category 2 by increasing the total field-line integrated Pedersen conductivity, consistent with the observations. The results presented in this paper highlight under what circumstances the low-latitude E_s would play a critical role in the occurrence of ESF, which can be used to understand the day-to-day variability of ESF. Key Points Low latitude E region Observations has been categorized into three categories F region response to the three categories of E region observations is presented Role of E region conductivity on RT instability has been examined
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