Review of “Responses of CIPS/AIM Noctilucent Clouds to the Interplanetary Magnetic Field “ by Zhang et al.

General Comments

This paper investigates possible connections between the interplanetary magnetic field (IMF) By component and Noctilucent clouds (NLCs) in Earth’s mesosphere. The paper is mostly written well, although there is a tendency for very long sentences, an there are instances when the ideas are poorly expressed.

The Authors show some reasonably convincing correlations between NLC properties observed by CIPS and measurements of By. Still, the results might be more convincing if there were one or two examples of the By - NLC relationship. For example, they could show a time series of the relevant measurements where we can see that the NLC properties indeed do change concurrent with By variations.

The main problem with this study is that the Authors do not present a believable mechanism that would explain the connection between IMF By and NLCs. They very casually invoke cloud microphysics as a possible explanation, but do almost nothing to explore a plausible pathway. Regarding the microphysics of NLC/PMCs, there are many published studies that could offer some clues here. First off, are the candidates for ice nucleation, which include sulfate droplets, proton hydrates, and meteoric smoke particles (Rapp and Thomas, 2006; Duft et al., 2016), in addition to homogeneous nucleation (Murray and Jensen, 2009). More recent studies indicate that meteoric smoke is contained within NLC particles (Havnes and Næsheim 2007; Hervig et al., 2012), making it perhaps the most likely candidate. Note also that ice - ice coagulation is generally considered unimportant in NLCs. It is relevant that model studies show that increasing the number of ice nuclei can reduce the size of ice particles in PMCs (Megner, 2010), and that changing the ice nucleation rate can alter the concentration and size of NLC particles (Wilms et al., 2016). These later papers may be of particular interest to the present study, and there are certainly more papers to consider than are listed here. The present study would be much more convincing if the Authors present a survey of the relevant literature, and derive a convincing pathway by which the IMF can impact NLC. 

It is applicable to this study that the CIPS particle size and IWC results can be used to calculate the column number density of ice particles (i.e., the # of ice particles in the vertical column, #/cm2). This quantity may prove enlightening, especially if you are considering microphysical processes. For example, if ice nucleation is suspect, then the concentration of ice crystals may be expected to change.

Specific Comments

line 23: Here you should introduce the term polar mesospheric cloud (PMC), and state that PMC and NLC are essentially the same phenomena.  In the rest of the paper it would be preferred to use only one term, NLC or PMC, but not both. 

line 24: You could state "140K or lower", temperatures of <120K have been observed. 

line 24: The sentence starting “The long-term trends…” is long and could be 2 sentences.

line 33: It is not the water vapor and temperature of NLCs, but rather the water vapor and temperature in the NLC region.

line 77: Define the acronym IWC

line 95: Start a new sentence at the semicolon.

lines 116-118:  Is there a reference that supports this claim?  Alternately can you include a figure (perhaps a scatter plot) that demonstrates these relationships?

figure 6: The axis label should be frequency of occurrence

line 174: This sentence is confusing. In particular the phrase “by setting the albedo of NLCs varying by 5×10-6 sr-1,” is not clear. 

line 186: This sentence continues to line 194, and is far too long. In addition, the ideas here are not expressed clearly.

line 192: This statement is unclear. For example, by “the growth of coagulation” do you mean “growth by coagulation”? The next idea, that ice particle coagulation would enhance the formation of ice nuclei, is nonsense. Ice nuclei in the upper mesosphere are likely meteoric smoke particles (there are recent references that discuss this that you should include). Perhaps if ice particle charge had the opposite polarity as smoke particles, then there would be an attraction. In any case. the ideas here are potentially important and need to be more clearly expressed.

line 207: Note that Lynan-alpha radiation also varies on an 11-year cycle. 

Responses of CIPS/AIM Noctilucent Clouds to the Interplanetary Magnetic Field by Liang Zhang, Brian Tinsley, Limin Zhou.

The manuscript describes an analysis of space based observations of Noctilucent Clouds, also called Polar Mesospheric Clouds.

Observations between 2007 and 2017 are used and a correlation study with the IMF is performed on a day-to-day basis. The paper is well structured and reads in most parts well.

The analysis has a couple of major flaws that make the results questionable:

Tides and observational effects:

Tides at the cloud altitude are known to have a large effect on cloud occurrence and brightness, and other properties. Orbit changes and changes in the local time of the ascending and descending node might affect the correlation coefficients. A discussion is needed.

Microphysics:

The authors provide no detailed discussion about microphysical aspects that are well elaborated in literature (e.g., Rapp and Thomas, 2006 and references therein). Instead, they mention “coagulation”, which is less relevant (unimportant) for mesospheric clouds.

For example, IWC, brightness, and radius have a strong relation to each other. Since the detection threshold of CIPS depends on the particle size, it should be discussed how this affects the small particle size cutoff and its changes (e.g. Fig. 6).

Electron densities:

A discussion about the state of knowledge on IMF effects on the electron density at cloud altitudes is needed. E.g. in case IMF effects are longitude dependent, the results may be different for ascending and descending nodes. Since the electron density is relevant for particle charging in the dusty plasma environment, it is a key parameter.

A discussion of radar echoes associated with icy particles (PMSE) is completely missing. These radar echoes are caused/affected by electron density fluctuations and icy particles. Following the authors “‘IMF By - ionospheric potential - NLCs microphysics - NLCs brightness’”, they are likely more directly affected than NLCs.

Specific comments:

Line 106: Due to the large number of noisy lines in Figure 1, a correlation is not visible. A more convincing display would help.

Line 112: Figure 2 does not provide uncertainties. How significant are the year-to-year changes shown?

Line 126: It may be more convincing if negative lag days are also shown in Fig. 3.

Line 127: “In previous studies of the link between Ly-α and NLCs, the proposed mechanism involving photodissociation, heating, or circulation all required longer time”: What causes the “longer time”, for example, for photodissociation? A more detailed discussion/references may help.