General comment

Present work is a case study of a possible interaction of atmospheric gravity waves (GWs) and equatorial plasma bubbles (EPBs). The authors observed South to North propagation of GWs, when an EPB was drifting eastward. After the interaction, the EPB (DP1) revived, extending its latitudinal extension and the 6300 depletion became much stronger. The authors tried to explain the observational evidence as an interaction of E-field generated by GWs and EPB. They concluded that “fossil depletion” DP1 revived and became an “active depletion” under the passage of GWs.” The interpretation and argument are interesting and worth to pointing and to discuss, since such observational reports are still limited in the literature.

However, I feel a serious concern in the airglow images and the image data analysis. Regarding the signature of thermospheric gravity waves (f1, f2), the authors mentioned that the wave crests can be seen clearly in Figure 1 and Figure 3(c,d). To my seeing, it is very difficult to recognize the wave fronts f1 and f2 from Figure 1 and 3(c, d). Since the wave fronts in question are located close to the image horizon, it is further difficult to resolve horizontal structure. The authors should explain how they transferred the images in geographic coordinates and calculate the wave characteristics. If it is not the case, how the linear coordinates are decided. Besides it, the optical filter for 6300 imaging normally includes OH emissions, of which intensity is much stronger near the horizon, say 75 to 80 degree of zenith angle. How the authors could eliminate the OH contamination. The authors can explain those mattes in the section of Instrumentation and data analysis procedure.

In conclusion, the present manuscript would be necessary to improve data analysis method and to convince readers to see the clear signature of wave structure of GWs in the 6300 emission layer.

Individual comments are below:

Line 133 (filter characteristics of 6300 imaging): please include the filter characteristics (HPBW, for example).

Line 144 (keogram): please mention how to produce the keogram and how to calculate the wave characteristics from the keogram. What is the latitudinal extension (in km) of the keogram Figure 3 (a, b) ?

Line 161 (keogram): Are these keograms made by using original images or geographically coordinated (unwarped) images ? Please make indication by arrow the GW signature in the Figures 3 (a) and (b).

Lines 166-167 (Being faint in nature, GWs signatures in ASAI images were getting lost in the geographic unwarping process): As the authors mentioned, it is difficult to estimate wave characteristics from the wave crests located at a large zenith angle. It means that there is a large uncertainty to get the wave characteristics from the wave fronts located in the corner of an all sky image. If the wave fronts (f1 and f2) are located at around 75 to 80 degrees of zenith angle, for example, one degree of distance corresponds to longer than 40-50 km at 250 km of altitude. The authors should keep in mind such uncertainty in their calculation of wave characteristics.

Line 169 (intensity profiling technique): Please explain the technique,

Line 170 (wave characteristics and Figure 3): The authors used keogramas (Figs 3(a,b) to calculate the wave characteristics. Please show explicitly the wave traces used in the figure. The authors pointed the wave signatures in the south edge of the image, However, the keograms show GW trace in the northern part. No propagating signature in the southern part. Please explain N-S scale of the keogram and put arrow signs to show the GW traces.

Line 176 (G27): Where is G27 ? Please point it.

Line 180 (wave characteristics): please explain how the authors obtained this characteristic. Please remember that an IPP trajectory has two variables, time and space. Figure 3(f), therefore, shows TEC variation as a function of time and coordinates.  

Lines 198-204 and Figure 1: The authors tried to show the interaction of DP1 and GW f1 and f2, using Figure 1. However, it is hard to see such spatial and temporal variations of DP1, f1 and f2, those pointed by the authors. If the authors believe that the process was really happening, they must show airglow images with much clear way, perhaps increasing image contrast by subtracting one image from the other as shown in Figure 3(c,d).

Lines 241-243 (GWs deform ,,,, act as a seed to GRT instability): please put references on it.

END of Review

The manuscript "Simultaneous OI 630 nm imaging observations of thermospheric gravity waves and associated revival of fossil depletions around midnight near the EIA crest" by N. Parihar et al presents observations of airglow depletions from the low-to-mid-latitude station, Ranchi, India. The study focuses on the fossil depletions and their evolution in the presence of gravity waves. The main conclusion of the work is that the gravity waves can revive the observed fossil depletions. The presented observations and focus of the study are interesting and deserve publication. I am listing a few doubts which I encounter while reviewing the manuscript:

(1) The airglow images are linearized or still suffer from the curvature effects?

(2) Authors should enlarge the ROI frame for clarity and to distinguish the gravity waves and depletions. 

(3) Should one refer to any dark patch at the corner of the airglow image as depletion?      

(4) Both gravity waves and depletions characteristics are drawn from airglow observations. Does the author use any criteria to distinguish them?

(5) Obviously, if one draws their characteristics from the same image and the same region of interest, one would expect a kind of relationship that authors have found. To what extent this kind of approach is consistent and reasonable?

(6) Are the revivals of these fossil depletions are in-situ or the reflections of the equatorial energetics?

(7) If the revival is in-situ then why do authors discuss the apex height characteristics?

(8) If the revival is a reflection of the equatorial energetics then is it possible to have gravity waves reaching at 600 km altitude at the equator? 

With these comments, I recommend the manuscript for publication with a minor revision.

 

The manuscript titled "Simultaneous OI 630 nm imaging observations of thermospheric gravity waves and associated revival of fossil depletions around midnight near the EIA crest" presents a possible interaction between thermospheric gravity waves (GWs) and fossil equatorial plasma bubbles (EPBs) over Ranchi, India, on 16 April 2012. The authors argue that after the interaction, the EPBs return to the growth stage.

However,  the present manuscript contains some unclear data analysis and incomplete reasoning, which necessitates significant revision and clarification before it can be considered acceptable for publication. Only a few detailed comments are listed below.

1. Regarding the new findings, the author claims to be the first to report the interaction between Gravity Waves (GW) and Equatorial Plasma Bubbles (EPBs) leading to latitudinal growth. However, Wrasse et al. (2021) (http://www.eppcgs.org//article/doi/10.26464/epp2021045?pageType=en ) presented observational evidence of an interaction between EPBs and wave-like perturbations known as Medium-Scale Traveling Ionospheric Disturbances (MSTID) at low latitudes over the Brazilian sector. Wrasse et al. (2021) argued that electric fields associated with MSTID can intensify the growth of EPBs, leading to latitudinal and height expansion. Therefore, the authors should conduct an extensive bibliography review to properly address the novel findings presented in the study.
2. Regarding Figures 1 and 2, no clear signature of thermospheric gravity waves (GWs) can be observed in the OI 630 nm images. Additionally, there is no evident northward development visible in the equatorial plasma bubble (EPBs) structures. It is suggested that the authors include an OI 630 nm movie for the complete night of observation as "Supporting Information." This would enable a thorough assessment of the presence or absence of GWs, the evolution of EPBs, and the nature of their interaction throughout the observation period.  Furthermore, the authors should consider employing alternative techniques to emphasize the interaction between GWs and EPBs, such as the detrended unwarped image technique demonstrated by Wrasse et al. (2021).
3. The GWs signature in the TEC GPS IPP tracks are associated to a fluctuation of about 1-5% of the TEC level (e.g., Otsuka et al., 2013; Figueiredo et al., 2018; Takahashi et al., 2021/ https://angeo.copernicus.org/articles/31/163/2013/; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JA025021; http://www.eppcgs.org/en/article/doi/10.26464/epp2021047). Figure 3e and 3f present a TEC oscillation of about 10 TECU (see GPS PRN 28). This kind of TEC fluctuations are usually associated to EPBs signature (Barrros et al., 2018/https://angeo.copernicus.org/articles/36/91/2018/ ). Same interpretation can be done for Figure 3a and 3b, the north-south keograms clearly show EPBs signatures with their bifurcation.  Therefore, the author should consider employing multiple GNSS receivers positioned near the event, or utilize various GNSS constellations (including GPS, GLONASS, Galileo, BeiDou), to accurately determine satellite IPP tracks corresponding to the same location as the OI 630 nm event. To achieve this, generating unwrapped images of OI 630 nm emissions is essential.
4. The author should make an effort to present new analyses to thoroughly discuss the physical mechanisms underlying a possible revival of the EPBs. For instance, analysis of any enhancement of the polarization electric field inside the EPBs could be beneficial, with the assistance of ionosonde data collected near the event.