Active (Evolving) Phase Of Equatorial Plasma Bubbles Observed Over Tirunelveli

Usual post-sunset generation of equatorial spread F (ESF) irregularities on quiet days (Q-days) and midnight or post-midnight generation of ESF on magnetically disturbed days (D-days) is widely known. However, the duration of evolving or active phase of these freshly generated ESF (FESF) irregularities is not clearly understood. Here, the active phase duration refers to the time period during which the perturbation electric field associated with equatorial plasma bubbles drifting over Tirunelveli is alive. Recently, Gurram et al. (2018) quantified the active phase duration of FESF for both Q-days and D-days. It was the first exercise to obtain the average active phase duration of FESF from observations. In view of this, we carried out detailed analysis to examine their dependence on solar flux and geomagnetic activity using long-term amplitude scintillation observations on 251-MHz signal. It is found that percentage of days with longer active phase (t(d) >= 50 min) is higher on D-days, and it is consistent for all levels of solar flux. Average duration of the active phase of FESF,< t(d)>, is 57.9 +/- 52.3 min on D-days and 33.8 +/- 23.4 min on Q-days. The active phase of FESF has tendency to sustain for longer time during low solar flux, which is attributed to presence of weaker ambient ionospheric electric field during that period. It is noted that FESF generated on days with stronger geomagnetic activity tend to have longer active phase. Also, days with FESF cause more moderate-strong scintillations as compared to days with drifted-in ESF.