The article describes possible effects of enhanced X-ray fluxes on the Earth’s stratospheric ozone, and subsequently on biosphere, caused by a hypothetical supernova explosion. The authors extrapolate the effects of X-ray fluxes from documented solar flares and assume that such X-ray fluxes could persist for multiple days. The article is innovative and addresses important issues of the planetary habitability and possible space weather dangers. I have some questions regarding the numerical procedures used (in particular, the choice of nudging for the TIME-GCM simulations), as well as regarding the underlying physical assumptions. Also, I think the referencing of earlier works can be improved (see specific comments below). 

Lines 93-94 and subsequently: The assumption that the effects of flares could be extrapolated in time needs extra justification. Is it assumed that the effects of increased X-ray flux would linearly accumulate in time? Should one also expect some qualitative, possibly non-linear, changes in NOx/ozone? One possibility I could see is due to a tidal feedback. Since atmospheric thermal tides are at least partly generated due to stratospheric ozone, the reduction in ozone would presumably change the tides and reduce the tidal mixing and NOx transport from mesosphere to stratosphere. Since tides have large amplitudes in middle atmosphere, their effects on transport/mixing could be substantial. In the current study, diurnal and semidiurnal tides in TIME-GCM are assumed to be given by GSWM (lines 202-203). Thus, the tides are presumably unaffected by ozone changes due to the flare. Does this mean that possible tidal feedback is disabled? Please clarify.

Lines 214-225: The authors chose to constraint the TIME-GCM simulations (winds and temperature fields) by MERRA-2 reanalysis instead of, e.g., NAVGEM-HA fields. Can they comment on the validity of MERRA-2 fields in the region of interest (55 and 75 km altitude)? Also, in their previous works the authors noted a great performance of NAVGEM-HA with respect to radar observations of middle atmosphere dynamics (e.g., McCormack et al., 2017). Thus, I am confused of why they chose to nudge the simulations to MERRA-2 dynamical fields instead of NAVGEM-HA. 

Line 384-385: Can the authors clarify the claim about the only qualitative analysis? I thought that few other studies addressed the impact of solar flares on nitric oxide, including studies with ground instruments (e.g., Enell et al., 2008 and further references there).

As a minor note, there are some inconsistencies in spelling. For example, it alternates between “X-ray”, “X ray” and “Xray”. 

References

Enell et al., 2008, https://doi.org/10.5194/angeo-26-2311-2008

McCormack et al., 2017, https://doi.org/10.1016/j.jastp.2016.12.007

Review of the paper by Siskind et al.

The paper examines the impact of X-ray emission of nearby young supernova remnants (SNR). Using a realistic model of the middle atmosphere and lower thermosphere including photochemistry, the authors test for planet Earth the hypothesis brought up by Brunton et al. who found a possible threat to biospheres in the wider cosmic neighbourhood of such SNRs by the prolonged strong X-ray emission phase. The paper shows that the rough estimation of Brunton et al. for the lethal distance between a SNR and the impacted biosphere does not hold and that in the case of Earth its atmosphere effectively shields also against the threat by extended X-ray emission from young SNRs.

The paper is absolutely in time and is a very valuable contribution to the field of harsh environments for early life and existence of biospheres in stellar system with habitable planets. The methods applied are essentially sound, even when the authors deduce their esimation of the NOx input and related ozone loss from some kind of extrapolation.

I only have a few general comments:

1. The authors use the TIME-GCM for their study. The authors state themselves that the model in this configuration is not able to simulate elevated stratosphere events, which are key for strong NOx intrusions in the NH, and probably underestimates downward transport inside the polar winter vortex in the lower mesophere in general. The authors comment that comparisons with MIPAS data show good agreement for midlatitudes instead, but these airmasses see the Sun also during winter and NOx here has a limited lifetime. Somehow related, NOx transported below the lower boundary is lost in their model. But during summer with the change of the circulation this NOx is brought to the middle stratosphere again where it could contribute to ozone loss. Can the authors give an estimation of this contribution which is lost in the model?

In addition, the second, strong case does not run through and the authors must rely on some reasonable extrapolation. Despite the authors state this clearly, perhaps other models like WACCM are obviously better suited to study such events if the shortwave photolysis part developed by Siskind would have been implemented there.

2. Despite the rather large energy input, the modelled impact on the ozone layer is small. Typical particle events connected to magnetic storms show hemispheric power values of a few hundred GigaWatts for several hours which corresponds to energy input around 50 J/m2. Obviously, the energy spectrum of the ionizing radiation or particles is most important for the impact in the stratosphere. Brunton et al. speculate (section 3.1) that the spectrum of young SNRs may be significantly harder than assumed in this paper. This would result in deeper penetration into the middle atmosphere and possibly could strongly enhance longlived NOx in the stratosphere as then NOx could bridge the gap of photolysis loss descending NOx faces in spring. The authors should point to that critical uncertainty. Of course, observations in the extended spectral range are needed to further reduce this uncertainty, but also some sensitivity model study would be helpful. In this context, a

3. The authors should estimate the total amount of NOx brought into the atmosphere, not just for the MLT. For comparison, particle events like the Halloween storm show inputs of about 2 Gmol of NOx, and a recent study by Reddmann et al. https://doi.org/10.35097/1104, modelling an extreme SPE/storm event with an input of 30 Gmol also show limited impact on the ozone layer, in line with this study.

4. The position of the source in ecliptic plane may be not the position with maximum impact. Could the authors try to have a source at the celestial poles? This would put the NOx enhancement deeper into the atmosphere due to the vertical infall and could bring NOx to also the stratosphere.

5. The results of this paper is at strong odds with the result of Brunton et al.. For further studies it would be helpful, if the authors could explain why the results of their study and that of Brunton et al. differ so much. A possible reason for the differences could be the hard X-ray spectrum part. Can you compare your spectrum with Fig. 1 of Ejzak et al?

6. The labeling of most figures does not confirm with standard. Often the y-axis shows the unit but not the quantitity or vs versa; how to show units (with brackets or without) is not consistent.

Minor comments and typos:

Title: "Supernova effects" is rather unspecific. Perhaps "No threat to the ozone layer by X-ray luminous SNs" ?

L14 "planetary": The paper only deals with the Earth, so perhaps "ozone layers of Earth-like planets"

L24 "most global": "strongest global"?

L35 "these": delete

L88 "Thus our": delete "our"

L117 "For our purposes ...": This is too general

L156: "our spectrum" replace "our"

L158ff:  "well covered with modern spectra" "suggests that .... agrees" : please reformulate to be more concise

L232: Kp = 3 (L235) still shows some particle ionization.

L236: 30°

L276: Michelson

L289: "descent" is purely dynamic. You mean the amount which descends.

L348, Fig. 5: Why not showing diffs?

L361, "1.0": unit?