Understanding the global metallic ions transport in the thermosphere is important for the field. There is no doubt that WACCM-X is one of the excellent global-scale models to study metallic ions’ transport. Overall, the work on this subject is worthy for publication. However, there are a few points that are not clear enough in the manuscript. I would like to suggest to the authors do a minor revision to clarify all the comments. Here are the detailed comments,

Major Comments:

First and foremost, the ambipolar diffusion velocity in equation 2 of the ion velocity equation adopts the equations (5.54) and (5.70) in Schunk and Nagy (2000), which include the effects of the neutral collision, the number density gradient, and temperature gradient, the gravity force, and the viscous stress along with the magnetic field line. Since the equation 5.54 is derived from the momentum equations 5.51 and 5.52, the neutral-ion momentum transform has been already taken into account. If the authors directly use equation 5.54 in Schunk and Nagy (2000), that could lead to double counting neutral wind effects in equation 2 in the manuscript. The ambipolar diffusion equation shows up as different formulas in various literature. It has to be careful to use them directly, and a strict mathematical derivation is required.

In section 3.1, the peak altitude of Mg+ is ~10 km higher in the summer hemisphere, and the authors suggest that it is caused by the summer to winter neutral wind that transports the metallic ions along with the magnetic field line. Although the vertical drift velocity is provided, the evidence is not enough to support this inference. Firstly, the authors need to clarify the contributions of electric field and neutral wind to the upward drift velocity of ~5 m/s in line 98. Does it mean the ~5 m/s upward drift velocity is only driven by neutral wind? Secondly, since the magnetic field line is roughly symmetric on both sides of the dip equator, we would expect the downward drift velocity on the other side. Some features in Figure 1 may indicate the downward drift, however, more clear evidence should be provided. Thirdly, due to lack of collision, the weak ion-neutral collision may lead to less effect of neutral wind in higher altitudes, but the upward drift velocity somehow increases with respect to altitude in Figure 1. If it is caused by the increasing winds, related evidence should be provided.        

  Compared with Figure 1, Figure 2 shows the Mg+ is barely above 100 km at 10 LT for all months, and much lower than those in Figure 1. It is skeptical because the wind and electric field show no upward effects at all, especially, the diurnal variations of Mg+ in section 3.2 show the Mg+ ions are easily transported to higher altitudes by the fountain effect around the local noon. Is there any explanation, and further investigation on this feature?  

In section 3.3, sentence “…, the ions at subtropical latitude (orange line) are transported upward to a small extent at midday, but transported downward to a lower height (∼1 cm−3 at ∼100 km) at March …, which is in reasonable agreement with the downward drift of the fountain effect along the magnetic field lines at midnight.” is questionable. The dynamo effect originates from the strong ion-neutral coupling in the lower E region, and the electric field produced by the dynamo effect could have substantial effects on the vertical transport of ions in higher altitudes. In the mid and low latitude region, the electric field may not be that important below 120 km, compared with neutral winds. In other words, neutral winds could be the dominant factor that determines in the vertical motion below 120 km in the mid and low latitude region. I would like to suggest the author examine the wind and electric field at March Equinox and June Solstice, and calculate the vertical drift velocities driven by neutral wind and electric field, respectively. In addition, it is hard to judge what factors determine the Fe+ profile shape for the orange and green lines. It’s better to extend the discussion on the role of electric field and neutral winds.

In section 3.4, what are the initial global number densities and distributions of Fe+, Mg+, and Na+? The studies in Huba et al (2019) show that there is not much difference between the transports Fe+ and Mg+, please refer to Figures 1 and 2 in Hube et al (2019). To clarify this issue, it’s better to compare the vertical drift velocities of Fe+ and Mg+.          

Minor Comments:

Line 35, “thermospheric metal atom” -> “the thermospheric metal atom”

Line 43, “affects of ion” -> “effects of ion”

Line 80, “a major sources” -> “a major source”

Line 86, “Na⁺, Fe⁺ and Mg⁺”->” Na⁺, Fe⁺ , and Mg⁺”                                                             

Line 102, “which shows a minimum” -> “which show a minimum”

Line 115, “The modelled” -> “The modeled”

Line 120, “Diurnal variation of” -> “Diurnal variations of”

Caption Figure 3. “… The white dashed lines indicates the position of the the dip equator.” -> “… The white dashed lines indicate the position of the dip equator.”

In Figure 4, the caption says the time is UT, but the titles of all subplots are LT. It is confusing. Please make a consistent statement.

Line 170, “is thought to be related” -> “are thought to be related”

Line 173, “Figure 7 and 8 compares” -> “Figure 7 and 8 compare”

Line 184, “charge transfer of” -> “the charge transfer of”

The axes of all figures are not friendly. Please add minor ticks in y-axis. Please add ticks from the low limit to the up limit of every axis.

The x-axis of Figure 4 and Figure 8 better shows from 0 to 24 with 2 or 4 hours step.

General comments:

This is an interesting study on model simulations on Mg, Na and Fe (and their ions) in the mesosphere/thermosphere using an extended version of the WACCM-X model. In my opinion the study is suitable for ACP after some revisions have been made. Two more general aspects that will appear again in the specific comments below are:

(1) the differences between the Mg+ results presented here and the ones based on WACCM-Mg in Langowski et al. should be explicitly discussed and perhaps explained – if possible – by differences in the model set-up.

(2) I agree that there are similarities between the simulations presented here and the SCIAMACHY observations in Langowski et al. (which is very good!), but there are also quite significant differences (e.g., in [Mg+] peak altitude or absolute number densities) which are not mentioned at all. Please discuss the similarities and differences in an objective manner and also include the differences.

Specific comments:

Line 4: “The model with full ion transport significantly improves the simulation of global distribution 5 and seasonal variations of Mg+.”

I don’t think this is entirely correct. The WACCM-Mg simulations shown in Langowski et al. were in better agreement with SCIAMACHY observations in terms of number density than the WACCM-X simulations presented here.

Line 15: “and Friedman et al. (2013) investigated a descending thermospheric K layer up to 155

km at Arecibo“

This statement is a little vague. Did the layer descend from altitudes above 155 km downwards to 155 km? The combination of “descending” and “up to” makes the sentence difficult to understand.

Line 33: “However, no previous studies appear to have examined the full transport of metal ions in a self-consistent global chemical-dynamical model.”

I’m not sure what “self-consistent” really means here. “Self-consistent” is used several times throughout the manuscript and at least in some cases with a different meaning. Can you explain, what it means here?

Line 43: “affects” -> “effects” ?

Line 47: “WACCM-X is developed in the present study”

I suggest replacing “developed” by “extended”, because WACCM-X did already exist before.

Same sentence: “combined with interactive chemistry”

Please mention briefly in what sense the chemistry is interactive. Often the chemistry is not “fully” interactive.

Line 53: “and winds is treated in the same way as most active chemical species”

Which species are treated in a different way? Why?

Perhaps “as most” -> “as for most” ?

Line 56: “including a self-consistent electrodynamics module“

Can you mention briefly, what “self-consistent” means here?

Line 61: “constant F107=124“

I suggest to replace “F107” by “F10.7”. Also: The 10.7 cm flux is not dimensionless. Please add sfu (solar flux units) to the numerical value.

Line 95: “At middle latitudes .. , the peak altitude of Mg+ is 10 km higher in the summer hemisphere,”

Higher compared to which region? The same latitudes in the winter hemisphere? Or the rest of the latitudes? Please specify.

Line 104: “To address this, we also present the simulation results at the same local time (10:00 LT) in Figure 2, and they are in better agreement with SCIAMACHY observations.”

Yes, in some respect these results are in better agreement with the SCIAMACHY observations, but the number densities are even lower and the discrepancy to the number densities observed by SCIAMCHY is even bigger. In addition, the Mg+ peak altitude is still about 10 km lower than in the SCIAMCHY data set. These differences should also be explicitly discussed in my opinion, not only the aspects that fit well.

Line 107: “which is absent in the previous models.”

It is unclear – at least to me - what “previous models” refers to. Previous studies by other groups, WACCM-MG?

Line 110: “the Mg+ column density exhibits relatively high distributions at”

I suggest replacing “high distributions” by “high values”. “High distributions” doesn’t really make sense here.

Section 3.1: Please explain the differences between the extended WACCM-X used in this study and WACCM-Mg used Langowski et al.. The WACMM-Mg results agreed much better with SCIA in terms of [Mg+]. Any idea why? In my opinion these differences are an important aspect that should be addressed in the paper.

Also section 3.1: There are of course similarities between your Mg+ results and the SCIAMACHY observations, but there are also significant differences. The Mg+ peak altitude in your simulations is about 10 km lower than in the SCIAMACHY data. Also, the modelled number densities are about a factor of 2 lower than in the SCIAMACHY data set. These differences should also be explicitly mentioned in the paper and possible reasons discussed. I’m not asking for new simulations, just an objective description of the agreement between model and measurement results.

Fig. 2: Here, the low bias compared to SCIAMACHY is even more pronounced than in Fig. 1. Any idea why?

Fig. 3: Units below colour bar is wrong/incorrect. Please correct.

Also, the MgII VCDs shown here are significantly lower than the ones determined from the SCIAMACHY observations (compare to Fig. 16 (right) in Langowski et al.).

Line 173: “Figure 7 and 8 compares“ -> “ Figures 7 and 8 compare” ?

Line 186: “In contrast, the reduced densities of the molecular ions (and electrons) at night means that

their increased lifetimes become comparable to transport lifetimes.“

Is the logic behind this sentence entirely correct? Please check.

Fig. 6: The Mg+/Na+ ratios exhibit an interesting interhemispheric difference for solstice conditions. Please comment on it and perhaps provide a qualitative explanation, if possible.  

Line 190: “the full life cycle of multiple meteoric”

I’m not sure what this really means: “the full life cycle”. Is the formation of meteoric smoke particles included as well?

Line 195: “(1) A clear seasonal cycle is found in the monthly averaged global distributions of Mg+, in good agreement with the SCIAMACHY measurements”

I agree the seasonal cycle is in good overall agreement with the SCIAMACHY measurements. However, several other aspects show significant differences (e.g. peak altitude, absolute values etc.). The differences should also be mentioned.

This work is an implementation of transports of metallic ions (Mg+, Fe+, and Na+) in WACCM-X for the purpose of understanding the effects of atmospheric motions on their distributions.  The work lacks sufficient description of the implementation as well as scientific insights.

The implication of the transport effect, mainly described by equation (2), is questionable.  There are terms missing as compared with Chu and Yu (2017), 2nd and 6th term in their eq(6).  This is not explained.  The effect of neutral wind (6th term) is especially important in the E region.

“Self-consistency” appears in numerous places but there is no explanation of what “self-consistency” really means, and what was not consistent in any previous model works.  The difference of this work from that by Langowski et al. (2015) is not clear, except that the model used is WACCM-X instead of WACCM.  

Almost all conclusions were drawn based on comparisons of the distributions from model simulations with observations or other model runs.  This is a poor way to gain much insight into any physical/chemical processes.  Most of the conclusions are speculative, without any quantitative assessments.  The benefit of modeling is to enable the examination of individual effects associated with different processes.  If only the final results are examined, the value of modeling is not utilized much.  Several examples are listed below:

1. The authors argue that the “upward transport of Fe+ does not significantly contribute to changes of Fe,” (line 152).  To prove this, the contributions of Fe+ transport to the time tendency of Fe need to be calculated separately and compared with other effects.  

2. The authors argue that “uplift of metal ions” is “driven by “meridional wind,” (line 196-197).  This could be supported if the authors can demonstrate that the magnitude of uplift due to the meridional wind is indeed much larger than other wind components.

3. The authors believe that this uplift can “explain the summer maximum occurrence of thermospheric sodium layers.” If that were the case, then the contribution of the uplift term to the production of neutral sodium would show being much larger than others.  Furthermore, the authors did not even show that the model actually produced a summer maximum of Na in the thermosphere.  In addition, it is not true that thermospheric Na appears mostly in summer.   As Cai et al. (2017) stated, the thermospheric sodium appeared at this Southern Hemisphere site mostly in spring and fall and rarely in summer and winter.

Most of the thermospheric Na observed showed a downward phase progression similar to that of diurnal or semidiurnal tides.  Since tides are well resolved in WACCM-X, this feature should be reproducible if the model results are to be believed.  On the other hand, the thermospheric Fe at high latitude showed variations on much shorter timer scales (Chu and Yu 2017).  Comparison with Fe observations with a model that cannot resolve the short-time scale dynamics (gravity waves) is not meaningful.

Other minor points

According to Liu et al. (2018), WACCM-X "neglects the influence of ion-neutral collisions on ion motion perpendicular to B,” which is significant “in the E-region where the O+ lifetime is short and transport is unimportant.”  Since E-region is an area of focus in this study, how does this neglected effect influence the simulation result?

169-170: TIFe and TINa are not defined.  In fact, there is no point in using such acronyms as they are mentioned only once and they are not widely accepted.

Figure 3e needs to adopt a different color range to show more structure.

I suggest that the authors cite the following paper, which is the first report of thermospheric Na in the southern hemisphere.  Furthermore, the temperature and wind reported in this paper is a rare dataset that can be used to validate model simulations trying to reproduce the thermospheric Na.

Liu, A. Z., Guo, Y., Vargas, F., and Swenson, G. R. (2016), First measurement of horizontal wind and temperature in the lower thermosphere (105–140 km) with a Na Lidar at Andes Lidar Observatory, Geophys. Res. Lett., 43, 2374– 2380, doi:10.1002/2016GL068461.