Evolution of Freshly Generated Equatorial Spread F (F‐ESF) Irregularities on Quiet and Disturbed Days

Abstract

We examined the seasonal and solar flux dependence of the occurrence of freshly generated intermediate scale (100 m to few km) equatorial spread F (ESF) irregularities during magnetically quiet (Q) and disturbed (D) periods. We utilized long‐term (1992–2006 and 2013–2015) amplitude scintillation data on a 251 MHz signal recorded at Tirunelveli (dip lat. 1.5°N ). Also, ionosonde data (1990–2003) recorded at Trivandrum (dip lat. 0.5°N) are used. The presence of fresh ESF (F‐ESF) is identified using the maximum cross‐correlation between intensity variations recorded by two spaced receivers on a magnetic east‐west baseline. We find distinct differences in the seasonal and solar flux dependence of the usual postsunset (<22 LT) generation of F‐ESF on both Q‐ and D‐days. Interesting feature is that F‐ESF linked moderate–strong scintillations are more prevalent on D‐days as compared to Q‐days in both early (18–22 LT) and later (>22 LT) phase of evolution of the irregularities. It directly hints toward the difference in the spatial structuring (spatial scales) of F‐ESF on D‐days as compared to Q‐days. On D‐days, the occurrence of F‐ESF is more likely around midnight and early‐morning hours in all seasons. Whereas on Q‐days, the postmidnight F‐ESF is found to occur mainly during solstices of low solar activity. The possible sources for the generation of F‐ESF around midnight on Q‐days of solstices during low solar activity are examined. We also find that perturbation electric field linked with F‐ESF on D‐days sustains for longer time, which results in longer durations of the active phase of equatorial plasma bubbles.

Earthquake/Tsunami‐Linked Imprints in the Equatorial F Region Zonal Plasma Drifts and Spatial Structures of Plasma Bubbles

“…Day‐to‐day variation in the occurrence of ESF irregularities and their characteristics and ambient F region parameters like electron density, electric field, conductivities, and height are well reported in many earlier studies (Burke et al, ; Gurram et al, ; Kakad, Tiwari, & Pant, ; Li et al, ; Nishioka et al, ; Yamazaki et al, ). Apart from day‐to‐day variations, these parameters are also affected by the modulated neutral winds resulting from the enhanced Joule energies at high latitudes and prompt penetration electric fields during periods of geomagnetic storms (Bhattacharyya et al, ; Kakad et al, ; ; Kikuchi et al, ).…”

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

“…We noticed that days with FESF produce more moderate-strong scintillations during the night as compared to days with drifted-in ESF. Recently, Gurram et al (2018) have reported active phase durations of the FESF observed at dip equatorial station Tirunelveli. However, in their study, they did not investigate the solar flux and geomagnetic activity dependence of active phase of FESF, which we have examined here.…”

“…We do not have much information about the duration of the perturbation electric field of EPBs. Recently, Gurram et al (2018) for the first time estimated the average duration of active phase of EPBs drifting over Tirunelveli using long-term VHF scintillation observations. In their study, efforts have been made to quantify the active phase duration, and it is reported that the perturbation electric field linked with the FESF survives for more time on magnetically disturbed days (D-days) as compared to that on Q-days.…”

“…The occurrence of L-band scintillation in the Journal of Geophysical Research: Space Physics 10.1029/2019JA027359 anomaly region (where the ambient plasma density is higher) is mainly controlled by the presence/absence of ESF irregularities with shallower power spectrum (smaller spatial scales) in that region (Bhattacharyya et al, 2017). In recent study, occurrence of ESF irregularities generated as a result of magnetic activity has been studied statistically, and it is reported that during the active phase of evolution, these FESF irregularities cause more moderate-strong scintillations on very high frequency (VHF) radio signal as compared to the quiet time (Gurram et al, 2018).…”

“…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.…”