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

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. Plain Language Summary: Equatorial plasma bubbles (EPBs) and associated irregularities are the important constituents of the near‐earth space weather. They are highly detrimental for satellite‐based communication/navigation applications. In this paper, we investigate the onset conditions and features of equatorial F region irregularities linked with EPBs using observations made simultaneously by a DPS‐4D digisonde and the Gadanki Ionospheric Radar Interferometer (GIRI), both collocated at Gadanki. 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 for the freshly generated and drifting EPBs. The zonal wavelengths of the pre‐sunset large‐scale wave structures (LSWS) observed by 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 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 (RT) instability generating EPBs. The usefulness of the new aspects of the digisonde observations are discussed in understanding and forecasting EPBs/ESF. Key Points: Untapped potentials of a digisonde in studying the ambient and disturbed states of the equatorial ionosphere are demonstratedUse of digisonde observations for predicting overhead equatorial plasma bubbles (EPBs) has been demonstratedResults indicate that bottomside upwellings are due to growth of LSWS and are responsible for the satellite trace and subsequent growth of the Rayleigh‐Taylor instability generating EPBs [ABSTRACT FROM AUTHOR]