In this paper, we present low‐latitude ionospheric response over Indian longitude to the recent super geomagnetic storm of 17 March 2015, using the Sami2 is Another Model of the Ionosphere (SAMI2) model which incorporates ionosonde‐derived vertical drift impacted by prompt penetration eastward electric field occurring during the evening prereversal enhancement (PRE) in the vertical drift. The importance of this storm is that (1) Dst reaches as low as −228 nT and (2) prompt penetration of eastward electric field coincided with evening hours PRE. The daytime vertical E × B drifts in the SAMI2 model are, however, considered based on Scherliess‐Fejer model. The simulations indicate a significant enhancement in F layer height and equatorial ionization anomaly (EIA) in the post sunset hours on 17 March 2015 vis‐a‐vis quiet day. The model simulations during recovery phase, considering disturbance dynamo vertical E × B drift along with equatorward disturbance wind, indicate suppression of the daytime EIA. SAMI2 simulations considering the disturbance wind during the recovery phase suggest that equatorward wind enhances the ionospheric density in the low latitude; however, its role in the formation of the EIA depends on the polarity of the zonal electric field. Comparison of model derived total electron content (TEC) with the TEC from ground GPS receivers indicates that model does reproduce enhancement of the EIA during the main phase and suppression of the EIA during the recovery phase of the superstorm. However, peculiarities pertaining to the ionospheric response to prompt penetration electric field in the Indian sector vis‐a‐vis earlier reports from American sector have been discussed.

Equatorial spread F observations have been categorized into three categories based on ionograms recorded over Sriharikota. First category comprised cases where the onset of equatorial spread F (ESF) was concurrent with the peak h′F time. Second and third categories comprised cases where the onset of ESF happened with a delay of 30 min and more than 30 min, respectively, with reference to the peak h′F time. Average peak h′F in the first category was more than 35 km higher than that in the second and third categories. Also, the peak vertical (upward) plasma drift was higher in the first category. Assuming the genesis of F region irregularity to have happened at or before the time of F layer attaining the peak height, late onset of ESF indicates the genesis of irregularities to have happened westward of Sriharikota. The fact that the peak h′F values were remarkably different in the three categories indicates a zonal variation of eastward electric field and postsunset height rise of F layer. The relative magnitude of the F layer height rise in the three different categories over Sriharikota has also been found to be significantly different than that over Thumba, an equatorial (magnetic) station located ~360 km westward of Sriharikota longitude. This scenario points toward the existence of a large-scale zonal standing wave in the F layer and its important role in F region instability process. Results presented in the manuscript have been discussed in the light of current understanding on the large-scale wave structure.

A comprehensive investigation of spread F irregularities over the Indian sector has been carried out using VHF radar and ionosonde observations. Two different categories of spread F observations, one where the onset of the range spread F (RSF) was concurrent with the peak h′F (category 1) and another where the RSF onset happened ~90 min after the peak h′F time (category 2), are presented. RSF in category 2 was preceded by the presence of oblique echoes in ionograms, indicating the irregularity genesis westward of Sriharikota. The average peak h′F in category 1 was ~30 km higher than that in category 2 indicating the presence of standing large‐scale wave structure (LSWS). Occurrence of the blanketing E_s during 19:30 to 20:30 Indian Standard Time in category 1 (category 2) was 0% (>50%). Model computation is also carried out to further substantiate the observational results. Model computation indicates that zonal variation of low‐latitude E_s can generate zonal modulation in the F layer height rise. It is found that the modulation of the F layer height, linked with the low‐latitude E_s, assists the equatorial spread F onset by modifying both the growth rate of the collisional Rayleigh‐Taylor (R‐T) instability and also its efficiency. A predominant presence of low‐latitude E_s has been observed, but the increase in the F layer height and the R‐T instability growth in the evening hours will maximize with complete absence of low‐latitude E_s. A new mechanism for the generation of LSWS and its implications on R‐T instability is discussed.

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.