Comments on Solar wind driving of auroral outflow….. by Zhao et al.

W.K. Peterson

This is an expanded re-analysis of an event analyzed by Strangeway now including corrected mass resolved ion outflow observations and an analysis of the nightside region. The analysis is almost completely qualitative, not quantitative. The exception is shown in Figure 5 where orbit average correlation coefficients between in situ Poynting Flux and soft electron precipitation vs solar wind inputs parameters are given and discussed without presenting the uncertainties of the correlations. The conclusion of the paper is that the data presented are consistent with what has been deduced from prior observations and modeling.

In spite of minimal quantitative analysis, the data presented and discussed here will be useful as qualitative checks for large scale magnetospheric models attempting to account for the many processes occurring in the coupled solar wind, magnetosphere, ionosphere system. 

I recommend publication after the authors have considered the general and specific comments below.

General  Comments:

The paper presents no quantitative evaluation of the role of solar wind drivers on ion outflows. The authors should consider changing the title to: A reexamination of the drivers of auroral outflow during the September 23-26, 1998 storm.

No direct correlations were presented of ion outflow rates vs solar wind inputs.  Orbit average cusp and auroral O+ outflow rates over the latitude range  could have been compared to the solar wind drivers shown in Figure 5.  If such an analysis was attempted, the authors should discuss what prevented them from presenting the analysis. If the analysis had been done, comparisons could then have been made to total O+ escape rates as a function of solar wind pressure and other solar wind inputs as presented by Ramstad and Barabash, 2021; Schillings et al, 2019; Lennartsson et al. 2004; and Peterson et al. 2024.  

O+ outflowing fluxes are a strong function of solar activity. Please state somewhere the  F10.7 index range for the orbits analyzed.

Specific Comments.

Line 149: The paper should explicitly state that the TEAMS data have been corrected for spacecraft potential.

Line 192: Significant fluxes of photoelectrons are observed below 60 eV as reported by Peterson 2021, Front. Astron. Space Sci. and others.

Lines: 197-200: Why are averages of some quantities compared to maxima of others?  The median of all quantities would be consistent as well as a good filter of extreme values.  If different methods of selecting values of quantities presented in the figures are used, this fact should be noted in the figure caption.

Line 207: This reads like the H+ number flux on orbit 8284 wasn’t what we expected so we ignored it. The readers deserve a better explanation. In the reviewer’s opinion, presenting the median H+  number flux for all intervals, even if orbit 8284 it is low would be better and would reflect the variability of the night side outflows as discussed in the paper.

Line 304:  The O+ and total fluxes track very well indicating what?

Line 321: are fen in the text here and in other places and Fen on Figure 4 the same parameter?  If so choose one for consistency.

Figure 5: The reader can and should be better informed about the quality of the correlation coefficients shown in Figure 5. This could be done by moving the coefficients to a separate table and reporting their uncertainties.  There would then be room to put lines showing the best fits for the cusp intervals which show the best, but still poor, correlations of ~0.8

Review of "Solar wind driving of auroral outflow during a CME storm" by Zhao et al

In this study the authors revisit FAST measurements during the CME storm that occurred September 23–26, 1998, including relatively recently recalibrated mass spectrometer measurements made by the TEAMS instrument. This storm and the corresponding FAST measurements have been the subject of a number of studies now spanning two decades. I am a little confused about the purpose of this study, because the territory covered by the authors has already been covered elsewhere, including in their own studies (Zhao et al, 2020, 2022; Nowrouzi et al, 2023). There are also plenty of studies describing qualitative correlations between so-called solar wind drivers and various measures of local energy input, so I struggle to see what's new here.

Beyond not really seeing what's new, I have a hard time with the statement in the abstract that the energy inputs are "strongly correlated with the solar wind - magnetosphere coupling functions". The correlation is generally low quite low, with correlation coefficients of 0.5 or less in Figure 5. (The highest value is 0.75 for the correlation between dayside DC Poynting flux and the Newell coupling function.) I also find the results in Figure 5 somewhat misleading, because as far as I can tell, the authors do not calculate the general correlation between the quantities in Figure 5; they specifically calculate statistics of these quantities during observations of ion outflows. Given that ion outflows are the final result of multi-stage processes on day- and nightside, and that the geometry of the magnetic field is such that energy inputs are not necessarily collocated with the ion outflows they induce (e.g., Sánchez and Strømme, 2014, doi: 10.1002/2013JA019096; Gorney et al, 1985, doi: 10.1029/ja090ia05p04205), the connection is tenuous at best. Sánchez and Strømme (2014) have a good discussion of the problem with trying to correlate nightside outflows with auroral drivers, if we may call them that.  

In other words, I have a problem with the methodology. It doesn't make sense to me to identify short periods of ion outflow at altitudes of a few thousand kilometers, and to then calculate (for example) the average Bz during these short periods as though there should be a relationship between Bz at the bow shock and intense ion outflow that takes at least a few minutes to make it out of the ionosphere. This problem is even worse on the nightside, where there is really no reason to expect that observations of outflows should have anything to do with what is happening at the bow shock at that moment. 

Given these difficulties with the methodology, I unfortunately don't think the authors succeed in answering the second question they pose in the last paragraph of the introduction: "(2) Which solar wind drivers are responsible for the enhanced energy input?" The authors are looking for causation, and the dataset they use just doesn't put them in a position to do this.

On a related note, the first question the authors pose ("How do the energy input and the outflow flux change with time during the storm?") has been addressed by a number of studies, the first that come to mind being the work of Yau et al (1985, doi: 10.1029/JA090iA09p08417) Audrey Schillings (https://ltu.diva-portal.org/smash/record.jsf?pid=diva2%3A1203208&dswid=3966), and my own work (Hatch et al, 2016, doi:10.1002/2016JA022805), and there are many others that I think several coauthors of this study must be aware of.

Conclusion

In summary, I unfortunately cannot recommend this study for publication. The first question that the authors pose has been answered in many previous studies (just do a forward citation search on the several hundreds of citations of Yau et al, 1985), and I don't think the dataset or methodology are appropriate for answering the second question. 

Minor comments

205–210: I, like the other reviewer, was confused by the statement on these lines that H+ fluxes were too low during orbit 8285. Does this mean that the measurements were bad in some way? If the measurements were not bad, it seems to me data from that orbit should be included. As Jesper Gjerloev likes to say (in paraphrase), the measurement that doesn't match your expectations is often where the discovery is waiting to be made.

Section 3.4: "To find the driving factors …" Again, I don't think the dataset or the methodology puts the authors in a position to positively identify driving factors (i.e., causation) of variation in energy inputs. What they present are correlations, and as we hear so often, correlation just isn't causation.