Review of “Revisiting Mirror Modes in the Plasma Environment of Comet 67P/Churyumov-Gerasimenko” by Fallau, Goetz, Simon Wedlund, Volwerk and Moeslinger

This is an interesting paper but it is unconvincing that it is are correctly identifying solitary mirror mode waves. If one looks at Figure 1 and the identification of the mirror modes, the readership will ask what are all the other waves which have not been identified as mirror modes? They look pretty much the same as the events that you have identified.  Is it just by chance that sometimes the angular changes fit their conditions that the authors have set for mirror modes and sometimes they do not? I realize that the authors are claiming that these other waves are evolutionary structures of mirror modes, but there has not been any past observations/evidence of this. This is just pure speculation.  Maybe this is an entirely different wave mode, or possible this is a coupling of mirror modes with another wave mode? Or could there be two instabilities occurring, say a mirror mode instability and an ion cycloton wave instability?  The authors should discuss this in some detail. Give balance to the paper so that the readership will not be misled.  In light of this I suggest that the authors put in the word “possible” in the title before “Mirror Modes” and in line 3 of the abstract.  Future work on this by other people might be able to resolve what is creating these perplexing waves.

In reading through the paper, it appears as if this is a reporting of the authors’ efforts in research and not a streamlined scientific paper.   In the abstract, sentence line 8 -9, you mention a magnetic field only method of detecting mirror modes and later  in the body of the text, you discard it for a magnetic field and plasma density method.  It would be best if you delete this sentence in the abstract and in the body of the text and get to the main point of the results.  In the sentence on lines 9-10, I think you should correctly state that the 565 events were detected by the standard technique of identifying mirror mode events (anticorrelated magnetic fields and plasma densities)?  This is not a new idea and should be the starting point of your discussions. 

Abstract, sentence lines 12-13, you mention that you have detected only one mirror mode train.  This is interesting and should be expanded upon in the body of the text.  It should be noted that for mirror modes in planetary magnetosheaths, the waves are always in trains, not single events separated by other types of waves.   It should also be noted that previously observed mirror modes at comets were trains, but of very short duration.  Some discussion of this need to be added.  References to mirror modes at comets are: GRL, 14, 644, 1987 doi:10.1016/S0237-1177(97)004730; JGR 98, 20955, 1993; NPG, 6, 229, 1999 doi:10.5194/npg-6-229-1999. These references should be added to the paper. Previous reports of mirror modes in planetary sheaths and at comets (and elsewhere) have all reported trains, not single events as shown here.  This should be discussed in the body of the text.

Detailed Comments   

Introduction line 27. I suggest the deletion of the Ip reference and the replacement by Wu and Davidson.  Ip was just a copy of the original.  They did nothing new.

Lines 27-28, references should be given for the different wave modes mentioned. For magnetosonic waves, the original discovery was GRL, 13, 3, 259, 1986. The most recent work on this is the Ostaszewski et al article that you quote.  Both should be mentioned here.

The term “Alfven wave” should be deleted here.  It is realized that theoretically Alfven waves are the low frequency analog to ion cyclotron waves, but detailed studies of Alfven waves indicate that they are not left hand polarized, but are arc polarized spherical waves.  Please see JGRSP 123, https://doi.org/10.1002/2017JA024203 for details.  Nothing like these waves were detected at comets, thus the suggested deletion.  The readership will be confused.   

For ion cyclotron waves, a good reference is JGR, 98, 20921, 1993. https://doi.org/10.1029/93JA02583.

You might wish to add other wave modes that have been detected at comets (and references).

Line 32.  I suggest the deletion of the term “magnetic holes”.  There are many people that do not believe that magnetic holes are mirror modes.  Please note the references in Tsurutani et al., 2011 which indicates many references to mechanisms for magnetic hole generation which are not temperature or pressure anisotropy generated. If you discuss magnetic holes here or elsewhere, these references should be quoted as alternative explanations.

There has been a problem with the analysis of “linear magnetic holes” as well.  It is the same problem as discussed with your waves.  Singular events were selected out of a string of events that did not fit linear holes.  What are the rest of the events?  Are they due to some other mechanism? The idea that they are the evolution of mirror modes is not convincing.  I don’t believe anyone has shown a case where mirror modes have only partially evolved. Again this is pure conjecture that these are due to the evolution of mirror mode waves.  Others have suggested mechanisms for magnetic hole formation.  These should be quoted and discussed.

Line 33.  The original theoretical paper and observational paper on mirror modes and proton anisotropies should be quoted here instead of what you have. They are Phys Fluids, 12, 2642, 1969 and JGR, 87, A8, 6060, 1982.

Line 49.  Remove “Alfven wave” here.  See previous comment.

Line 51. Remove “Alfven” in front of ion cyclotron waves.  See above comment.  A couple of references on ion cyclotron waves should be added here:  JGR, 118, 785, 2013 doi:10.1002/jga.50091 and ApJ, 793:6, 2014 doi:10.1088/0004-637X/793/1/6.  They are follow- ons to the Gary 1992 paper and give further information for the instability and the waves.

Line 51, sentence beginning with “Moreover…”.  This sentence should be reworded.  This was a result from theory but have never been verified experimentally.  And a “minute” amount of heavy ions would not have much of an effect. Almost nothing depending on the “minuteness”.

Line 52. Remove “Alfven”

Line 60.  A reference to field-line draping should be added here.  I recommend the original paper: JGR 68, 5111, 1963.

Line 71.  Does the “two different sizes of mirror mode waves” still hold after you found that the magnetic field only technique produced errors?  If not, then this sentence should be removed.

Lines 80-89.  Are all of your possible mirror modes pressure balance structures or can you not say?  This should be discussed in the paper.  On line 89 besides the Hasegawa (1969) reference, you should add Price et al.  1986 and Tsurutani et al. 1982.

Lines 90-96, linear magnetic holes.  A comment concerning this was mentioned before.  Although this conjecture is possible, for balance some statement indicating that they are possibly not mirror mode structures should be made.  The many possible mechanisms for their generation should be quoted.

Lines 96-99.  This conclusion is too strong.  These structures may not be mirror modes at all and may be generated by processes not involving anisotropic protons.  A statement should be added stating this.

Lines 144-146.  It seems unnecessary to describe your selection process as a two step procedure.  Why don’t you cut to the chase and state your final selection criteria?

Table 1.  Your criteria for detection of your events are clear.  But later in the paper you mention that some of the selected events do not have anticorrelations between B and N.  This seems contradictory.  Can you please clean this up?  Throw out events that don’t have clear anticorrelations?

Lines 171-176.  Is all this detail necessary to tell the readership of AG?  I think it is important to tell the readership that the “magnetic field only technique” does not work for identifying possible mirror mode waves at your specific comet, but I think this could be done in one sentence. When you mention different wave types earlier in the paper, you should add “steepened waves” (plus reference) and “singing comet waves” (and references).

Line 181. Telling the reader that the Volwerk et al. (2016a) results are now known to be incorrect will be useful to the readership, but perhaps state this more succinctly?

Line 185.  Why don’t you start the true data analysis section with the discussion of the Wedlund et al. (2022) method which works?  Why discuss methods which don’t work?  And by the way this is not a new method but was discussed by Hasegawa 1969; Tsurutani et al. 1982 and Price et al. 1986. Plus of course the anisotropic protons and pressure balance, both topics which you have not discussed/shown. This latter point should be mentioned prominently.  Since you were not able to do these types of analyses, your method only partially identifies mirror mode structures.

Line 205.  What does the presence of magnetic holes have to do with the generation of mirror modes?  Please explain or delete.  Also note previous comments that many people do not believe that magnetic holes are the remnants of mirror modes.  So this point should also be made if you wish to revise this sentence.

Figure 1/lines 223-226.  Please shade the intervals that you believe are mirror modes for the readership.  I don’t see where your criteria are being met.

Lines 227-233.  Since these “events” do not meet your criteria, why don’t you delete the vertical lines and delete these sentences?

Lines 277-280. This is a highly biased statement.  Another possibility is that magnetic holes have nothing to do with mirror modes.  As previously mentioned there have been many proposed mechanisms for magnetic holes, which you should include in your revision.  Please revise the discussion so it is more even handed for the readership of AG.

Figure 3.  I think this is perhaps the most interesting figure of the paper.  It will help others who wish to tackle this wave problem to realize that this may have nothing to do with mirror mode generation. 

Lines 328-342 discussion.  The authors should mention that the previous studies of the Saturnian magnetosheath have show no evolution of mirror modes into magnetic holes.  Thus the comment by Plasche et al 2018 is conjecture, not observations. 

This article revisits the presence of mirror modes in the plasma environment of the comet 67P with Rosetta data. If the findings are not completely conclusive, the methods and discussions are worth reading and nicely complement previous studies on mirror modes. I have mostly minor comments to this version.

l52: the peculiarities due to the presence of heavier ions are more general and not restricted "to the solar wind".

l61-62: do the authors mean that the field line draping at the MP acts to increase the anisotropy ? Does this come in competition with the reduction of the anisotropy under the action of mirror mode activity itself (the instability "consumes" the free energy contained in the anisotropy) ? How does this competition take place ?

l95-96: on the theoretical possibility to observe mirror modes (mostly dips) in mirror stable plasmas, the authors could refer to Passot et al., 2006 (https://doi.org/10.1063/1.2356485) together with Génot et al., 2011 (https://doi.org/10.5194/angeo-29-1849-2011) which presents a scenario of mirror mode evolution based on simulation and observations. This also applies to the discussion around l330.

l120: same sentence as l143

l179: is there a way to check file versions on the Rosetta database ? (at ESAC ?)

l204: if no IC waves have been detected at 67P, why putting so stringent constrains on mirror mode detection ? ie, a reduction of 32000 to 565 events. What other mode could this be ?

l380: throughout the paper the term "magnetic-field only method" is used. Why is it well-known ? Also the present paper complements the analysis with a check of the B-N anti-correlation. So I don't understand why insisting on the ""magnetic-field only". This comment agrees with the one of another referee who recommends a better description of the method used in the paper (I think mostly the naming should be adapted).

l385: the down selection from 32000 to 500 could be repeated here.

l387: the very rare observation of mirror mode trains is very puzzling, if not strange. Could this be linked to the method itself ? Otherwise I agree with the conclusion that most of the observed events are linked to mirror activity happening elsewhere and are just remnants of this.

l414-415: to my knowledge, mirror modes in cometary environments have already been studied. What is done for the first time exactly ?

last sentence: sure it would be good to have such measurements. But what exactly remains to be undestood ? And what do we learn, from the mirror activity, on the comet itself and/or cometary processes in general ?