General comments:

The most significant issue is that some information how the reported O/N2 values (and the brightness of the O and N2 emissions used in deriving the O/N2) compare with other observations, as was done form the NmF2 observations, is needed. The lack of such information is a major shortcoming of the current paper. Such information is needed for others to understand the value of the observations and their limitations.

A minor issue is that substantial editing for clarity is also needed throughout the paper. Some specific suggestions are included below.

The observations described are potentially very valuable for understanding the effects of geomagnetic activity on the composition of the thermosphere and worthy of publication if their relative differences from and consistency with other O/N2 observations can be quantified.

Specific comments and technical corrections:

Line 19: delete “properly”.

Line 21: change “designed requirement” to “design requirements”

Line 26: “can present” doesn’t work well, perhaps “represents the” or “is representative of the”?

Line 27: add space between “N2” and “LBH” (also throughout the paper) and “can be” to “is”. Wording of the sentences could be more concise (“FUV radiation” is repeated).

Line 28: delete “characteristics of”

Line 29: Delete the sentence that begins on this line or reword to combine with previous sentence. It’s redundant with the previous sentence.

Line 31-33: move “based on satellites” to after “ionosphere” and reword slightly (“from satellites” perhaps). Also, delete comma after “as” and “2003)”

Line 35: If the photometer is being used for the thermosphere also, why is it an “ionospheric photometer”? change “equipment” to “instrument”.

Line 36: Photometer suitable for observations of the nighttime ionosphere were made and flown decades before NRL’s instrument (e.g., https://doi.org/10.1029/JA085iA05p02201, and there are probably earlier examples).

Line 46: “leak” to “leaks”

Line 52: suggest changing “and designed” to “and is designed”

Line 57: delete “parameters”

Line 59: use plural “photometers”

Line 81: “added in” to “added in the”

Line 82: Rewording is needed. Airglow at longer wavelengths is the problem, the wording “airglow below 180 nm” indicates that it is the shorter wavelengths. May be clearer to say whether it is longer or shorter wavelengths.

Lines 91-93: delete “times” and use plural “observations” for cases with >1 observation.

Line 96: first “ground” to “the” and delete “in the ground laboratory”.

Line 101-102: The deuterium lamp is completely with a vacuum environment, and the monochromator? That’s what the wording indicates. Seems unlikely and at least unnecessary. A more accurate description may be needed. Typical facility would have vacuum only within the monochromator.

Line 106: delete “following” and “from a” to “from the” (unless multiple lamps are attached at the same time).

Lines 107, 108, 109, 110, 111: “wavelength-selected light” to “wavelength selected”?

Line 109: “counts”, plural, or “signal”; possible “for” rather than “of”.      ????

line 126: “red-leak in daytime” to “red-leak contributions in the daytime”?

line 127: “red-leak in daytime” to “red-leak contributions”?

Line 137: < 10 per second, or what time interval?

Line 138: delete “a peak” and “high”. 2000 counts for what time interval? Might also substitute “in” for “over” since the satellite is flying through region containing energetic particles.

Line 145: by “without red-leak” you mean “with the red-leak signal subtracted”?

Line 146: delete ”which deducted the count of red-leak”? (phrase seems redundant)

Line 149: “deduct”?    ???

Line 150: “The” to “An”

Line 153: “condition kept quiet relatively” to “conditions were relatively quiet”

Line 154: “The example” to “An example”, unless these are the only data collected.

Line 155: “oneither” needs a space between words.

Line 156: “EIA has” to “The EIA has”?

Line 163: “have been” to “have also been”?

Line 166: “further” rather than “furtherly”

Line 187: “2014).”? Something seems to be missing.

Line 181-190: is this agreement you give with NmF2 and TEC typical when observations are compared?

Lines 194-195: wording in the phrase “that the…F2-layer” doesn’t make complete sense, it almost does but something seems off.

Line 196-198: wording, “at day the contribution…of the ionosphere.” Needs some rewording to be clear (unclear what “less than one of the ionosphere” means) and somewhat odd use of prepositions (“at day the contribution” to “on the dayside, contributions” perhaps).

Line 201: “Auroral emission can be derived from the 135.6…”? But 135.6 nm is an auroral emission, nothing to derive, just observe it.

Lines 203-205: Unclear why the sentence that begins on line 203 is included. This is the only mention of the WAI instrument in the paper. If it is included, more relevance to the current paper should be added.

Lines 210-213: The two sentences, immediately following the auroral discussion, may give an incorrect impression. Column O/N2 derivation would be from dayglow observations, but not from auroral.

Line 219-220: Needs some minor rewording for clarity, unless the 135.6 nm brightness can be derived from either channel. Maybe just say the 135.6 nm brightness can be derived “using” the dayside 135.6 nm and N2 LBH channels?

Line 221: “cloud top” to “cloud tops”

Line 223-224: “result of column O and N2 ratio”, simplify to “column O/N2 ratio”?

Line 230: suggest spelling out the name of the satellite (FY3D) the first time it’s mentioned in the conclusions, then using the abbreviation (as on line 231) if desired.

Line 235: How do the changes seen in O/N2 compare with other observations? There are probably TIMED/GUVI observations available to compare against. Later storms could possibly be compared with O/N2 observations from the GOLD mission, in addition to GUVI. Some comparison of the reported O/N2 values with other observations, as was done form the NmF2 observations, is needed in the paper.

Review Report

The manuscript describes the data and products from an ionospheric photometer (IPM). The IPM measures FUV emissions, such as O 135.6 nm and N2 LBH bands. It is a sensitive instrument. The data products include TEC, NmF2 and O/N2 column density ratio. They  are useful data products. However, it lacks of details on the methods for estimating these products. The O/N2 product is not validated or compared with existing O/N2 data from other missions. Some of the O/N2 features, likely artifacts, are not discussed.

Specific comments

• Are the IPM data open to public for an independent evaluation since that data in published papers are usually required to be accessible by public?
• IPM calibration was done on ground. Was IPM calibrated in orbit?
• Fig 2 (nightside channel). The non-zero responsivity around and below (likely) 130 nm suggests it is possible to pick up bright Lyman α emission around 121.6 nm. A discussion on this will be helpful.
• Fig 3. Dayside. Both bright 130.4 and 121.6 nm emissions could contribute the 135.6 channel.
• Figure 4. The LBH band includes emission of N-1493, NO ε band, etc. Does the algorithm ignore the impact of none N2 LBH emissions?
• Fig 5. It is necessary to show the data from the red leak channel. Furthermore, what is the responsivity of the red leak channel?
• Fig 6b. Change the color bar to show the equatorial arcs. Is the SAA contamination removed in the same way as the red leak? If yes, why can it be done this way? If not, describe the method.
• The plots show that data cover the entire Earth. Does the photometer scan in a cross track direction?
• Line 175. How can one estimate NmF2 based on the ratio between NmF2 and 135.6 nm intensity? It needs more details on the method.
• Figure 8. The derived NmF2 should be plotted in the same format of Figure 6 to show the ionosphere morphology. It also needs a map of errors in the derived NmF2. What is the altitude of the Feng Yun 3-D? If the altitude is around NmF2 or above, the method wouldn’t work.
• Line 165. What is the local time of the observations in other studies?
• Line 180. How is the TEC estimated using the 135.6 nm radiance?
• Line 215. The net 135.6 nm and LBH radiances are estimated already. What is the reason to use a Butterworth filter to estimate the red leak due to cloud?
• This reviewer found a reference Wang and Wang 2016. It has the same title “ Airglow simulation based on the Atmospheric Ultraviolet Radiance Integrated Code of 2012” and author names. Is this the same to the reference (Wang and Wang, 2015)?

If they are the same paper, this reviewer couldn’t find a AURIC based lookup table for the IPM O/N2 calculation.

• Line 220. A plot of the O/N2 look up table should be added in the manuscript. Since both 1356 and LBH channel include LBH, N-1493, NO ε bands. Are the contributions removed?

• Figure 9. How is the O/N2 data product validated? O/N2 depletion was seen on Aug 26, 2018 (storm-time). What are the sporadic enhanced O/N2 (vertical bars in red) over one or two orbits? Are they artifacts? If not, what cause the enhancements? What is the reference N2 column density for the O/N2 ratio?