Onset Conditions and Features of Equatorial F Region Irregularities: New Insight From Collocated Digisonde and Radar Observations From Gadanki

Abstract

In this paper, we study the onset conditions and features of equatorial F region irregularities linked with equatorial plasma bubbles (EPBs) using observations made simultaneously by using a DPS‐4D digisonde and the Gadanki Ionospheric Radar Interferometer (GIRI), both collocated at Gadanki. Importantly, we have employed specific analysis tools on DPS‐4D observations, providing range‐time displays of radio frequency and signal‐to‐noise ratio (SNR) of the reflected/backscattered echoes and the angle‐of‐arrival of the echoes, to characterize the echoes and to study the ambient and disturbed states of the ionosphere. Observations clearly show noticeably different background conditions, in terms of the height of the F layer base, electron density gradient and vertical drift, for the freshly generated and drifting EPBs. The zonal wavelengths of the pre‐sunset large‐scale wave structures (LSWS) observed using the DPS‐4D, however, are found to show a close connection with EPB spacings for freshly generated and drifting EPBs, consistent with earlier findings. The satellite traces were observed just prior to the equatorial spread F (ESF) and were found to be associated with the bottomside upwellings. Comparison of the range‐time displays of radio frequency and SNR of the DPS‐4D observations and the height‐time variations of SNR of the GIRI observations demonstrates that the relatively weak echoes in the DPS‐4D observations, which represent the ESF echoes, correlate fairly well temporally with the plume structures observed by the GIRI. The GIRI observations also reproduce the bottomside upwellings observed by the DPS‐4D. We propose that the bottomside upwellings are due to the growth of the LSWS and these bottomside upwellings are instrumental for the growth of the Rayleigh‐Taylor instability generating EPBs. Finally, the new aspects of the digisonde observations are discussed with regard to their usefulness in understanding and forecasting EPBs/ESF.

On the Upwelling of the F Layer Base and Prediction of Equatorial Plasma Bubble

“…ESF echoes were first noticed at 19:10 LT, 19:25 LT, and 20:10 LT on 11 March, 4 April, and 10 February, respectively. The ST, which was characterized by additional trace in the ionogram with distinctly different angle of arrival (Das, Janardana Reddy, et al., 2022), was observed for short periods (∼10 min) prior to the occurrence of ESF. Note that while STs were observed during the PRE period on 11 March and 4 April, ST‐like echoes were observed much later than the PRE period on 10 February.…”

Determining the Day-to-Day Occurrence of Low-Latitude Scintillation in Equinoxes at Sanya during High Solar Activities (2012–2013)

“…Li et al [8] pointed out that neither Large-Scale Wave Structure (LSWS) nor the post-sunset rise of the F-layer is sufficient alone to cause the development of ESF [51]. LSWS plays a significant role in the generation and growth of equatorial F region irregularities producing ionospheric scintillation [29,52,53]. ESF or scintillation may occur during the existence of LSWS without F-layer lift (weak zonal electric field), or not occur in the absence of LSWS, though the upward drift is large.…”

Identifying the Onset Location of Equatorial Plasma Bubbles (EPBs) and Its Relationship With the Background Ionospheric Conditions

“…This requires the estimation of onset location of EPBs and the examination of the background atmospheric and ionospheric conditions over that location. There are some recent attempts to examine the onset conditions of EPBs by distinguishing freshly generated EPBs from drifting-in EPBs, mainly using beam steering radar observations (Ajith et al, 2015(Ajith et al, , 2016Das et al, 2021;Das, Janardana Reddy, et al, 2022;Li et al, 2013;Patra & Das, 2023;Rodrigues et al, 2023). However, due to the finite Field of View (FoV) of radars, these studies were limited to small longitude sectors.…”

“…Specifically, on a given day, the EPB occurrence pattern shows significant spatial variability even over a longitude sector that have same magnetic declination. This means that on any given day over a specific longitude sector, there can be a possibility of single, periodic or no EPB occurrence (Das, Janardana Reddy, et al., 2022; Huang et al., 2013; Li et al., 2013). This spatially localized occurrence pattern of EPBs primarily leads to the enigmatic day‐to‐day variability in the EPB occurrence.…”