Peer review of: Electron Bernstein waves driven by electron crescents near the electron diffusion region (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
 
This article is dense and the figures are dense. Nonetheless it 
convincingly makes its point about the origin and magnitude, hence 
consequences, of the EBW in the current reversal region. 
 
The authors can make a lot of improvements to the presentation to 
make the article more accessible. Some of the comments below relate to 
that. There are a few discrepancies in the descriptions of certain 
phenomena, and a few places where essential information is omitted. 
Others of the comments below relate to those. 
 
Comments on the figures: 
 
Remove as many traces from line plots, leaving only the essential ones needed to make your 
argument. In many cases all three components of B, J, and V are shown when only one, usually the L-
component, is discussed in the text. The extra traces are not an advantage and obscure the main 
point. 
 
Remove unnecessary lines from the electron distribution plots. Remove any not needed for the 
argumentation, such as, for example, in Fig. 4egi do not show both k and -k directions, one of these 
suffices. 
 
Separate Fig. 3a as its own figure. First of all, it's referred to much earlier in the text than Fig. 3. 
Second, being printed so small makes it misleading. For example, Fig. 3a suggests that neutral line 
and current boundary should coincide, but this is not true in the case study. It would be nice if a 
larger version of the Figure could show this. 
 
Show zero as a dotted line in the cases where zero-crossings are 
important (if unnecessary traces have been removed, this will be possible). 
 
Include the vertical red and black lines indicating neutral sheet 
and separatrix in Fig. 2, to better connect it to Fig. 1. 
 
In Fig. 4d, light colored traces do not appear to be discussed in text and should be removed. The 
mini-panels and associated color-coding should be described in the caption. 
 
Comments on the text: 
 
Last sentence of abstract is unclear: what is meant by "from EDR to IDR"? The observations are in 
the EDR, what can they say about the IDR? The discussion section p. 11-12 does not seem to address 
anything related to this phrasing in the abstract. 

On p. 4, Fig 4a is referred to, where Fig 3a is meant, but I urge breaking that into its own separate 
Figure. 
 
Bottom p. 4, indicate that the lower hybrid waves are "not shown" 
(unless they are---are they evident in the Delta-E panel i? If so, point this out.) 
 
Middle of p. 6, current obtained from plasma data. Some discussion is needed here of the energy 
range of the plasma data, and whether it captures all or most of the current, and how you know 
that. 
 
Bottom p. 6, suggest adding "which are the Hall current peaks" after "respectively", to explicitly tell 
the reader the physical significance of the yellow bars. 
 
p 7, caption for (d), it appears some kind of low-pass filter has been applied? (otherwise, the wave 
field shown in Fig 3, nearly 50 mV/m, would dominate?) Add discussion of any applied filtering. 
 
p 7 4 lines from bottom, text refers to beams as parallel to B, figure shows M. In text, remind reader 
B is (mostly) along M. 
 
p 8, and Fig. 3: here, I assume some kind of high-pass filter has been applied to E, hence it's called 
"high-frequency E." Please give filter details. 
 
Fig. 3 and text: Explain how the yellow region is defined. Is it strictly on wave power or something 
else? Is it arbitrary "by eye" or please give a quantitative criterion defining it. 
 
State in text or caption what exact time interval was used for the spectral/MVA analyses in Fig. 3 
(e.g., was it the yellow region?) 
 
There is some significant variation of Nf_ce across the yellow region in Fig. 3ef. Tell in text how the 
specific values of Nf_ce indicated by dashed lines in Fig 3g were determined. 
 
Top of p 8: text refers to perp drift as ExB/B^2, Fig. 3 refers to same drift as V. Help the reader out 
here, use the same nomenclature in both places or put V_perp=ExB/B^2 in the text. 
 
Same place: E_perp >> E_par appears to hold only in the yellow region, text implies everywhere. 
 
Middle p 8 "which is close to the reconnecting field direction L" 
indicate not shown (unless it is?) Tell how close it is. 
 
Top of p 9: There seems to be a discrepancy between text and figure caption concerning the 1-d 
electron distribution cuts shown in Fig. 3f (and presumably also those in 3hj). Figure indicates it's the 
k-direction, text indicates it's the direction to the centroid of the crescent. If the latter, more 
information needs to be given about how this centroid was determined. 

Starting p. 9, the direction k starts being refered to. Presumably this means the direction of the wave 
propagation? It needs to be defined and told how it was determined (from MVA perhaps?). 
 
On p. 9 toward bottom, Where Liouville mapping trajectories are introduced, a better explanation 
needs to be given of what these are. Reference should be made here to the appendix, so the reader 
knows it exists. 
 
It is unclear to me what is meant by "the non-uniform shifting of the phase space densities." Does 
this have something to do with the relation of the red curve to the black curve in Fig. 4d? 
 
Middle p. 10 "...along a phase velocity direction pointing towards the crescents." Indicate this is not 
shown (unless it is? in which case indicate how it is shown.) 
 
Middle p. 10 and Fig. 4l: Fig. 4l appears to show max growth rate of the EBW very close to the 
cyclotron harmonics, whereas in the data, the wave power peaks in between the harmonics (Fig. 3g). 
Some discussion is needed to assess this and whether this is a significant discrepancy between 
theory and experiment. 
 
Bottom p. 10 "trap a large fraction: It doesn't seem too hard to give the approximate fraction (half? 
quarter? three quarters?) 
 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
This paper reported an interesting magnetic reconnection electron diffusion region (EDR) encounter 
case by the Magnetospheric Multiscale spacecraft (MMS), in which they found large-amplitude 
electron Bernstein waves. The results is very important for space physics community as well as 
fusion plasma, and the paper is well organized. Therefore, I support to paper to be published as 
current form. Nevertheless, I have some minor and (optional) suggestions which may helps for the 
paper. 
 
1. It may be better to include a bit more introduction about electron Bernstein waves, which could 
bring a broad interest. 
 
2. Page 6. it maybe better to provide an explanation why the width of Hall current is much smaller 
than the ion inertia scale? 
 
3. Page 6. line 3 of the last paragraph, density -> phase density 
 
 

Reviewer #3 (Remarks to the Author): 
 
NATURE PAPER LI ET AL , “ELECTRON BERNSTEIN WAVES…”  
This paper is on an interesting topic that potentially makes a contribution to the relative 
importance of wave-particle vs finite Larmor radius effects in enabling magnetic reconnection 
in nearly collisionless plasmas. The authors do not marshall their arguments well from 
the beautiful measurements. Many figures are presented as having self evident properties. 
Others reveal internally contradictory properties. 
 
As an example: Pitch angle pictures are shown as relief maps implying two independent variables 
with the 3rd the color coding of intensity, and then refers to them as showing agyrotropy. A typical 
pitch 
angle portrait of this type suppresses gyrophase of the observed particles to make this plot 
one of energy vs acos(V*B/|V|/|B|); such a plot by definition cannot reveal agyrotropy! 
If it is a map vs gyro phase, then some other variable in velocity space is being suppressed, 
since to show that particles of common pitch angle in the fluid rest frame must be contrasted in 
intensity 
vs gyro phase suppressing all other pitch angles. Unfortunately the authors have clearly indicated 
what this figure shows and or how it does so. 
Otherwise Figure 2f-h is about, the authors need to state that clearly, including discussion 
how the other parts of velocity space were suppressed. 
 
Later it was stated that the locales of the observations reported were where crescent distributions 
were observed. Since it is the apparent thesis of this paper that the crescents are the cause of the 
electron Bernstein waves it is reasonable for the reader to be shown data that (a) show the 
crescents 
rather than referring to their existence, and (b) show the persistence of crescents across the domain 
of the radiation, with a demonstrable plottable measure of crescent presence. Currently an interval 
is simply declared to be region where agyrotropic distributions are seen. There are a number of scalar 
measures of agyrotropy that could be used to document what interval in the data quantitatively 
reflect the stated properties that at present are declared as a fact. The author’s seem to be unaware that 
recording such agyotropy in conjunction with other magnetic geometry indicators may already be 
enough to all reconnection to proceed. There seems to be interest only in the waves for this purpose 
without considering their role as causal to reconnection or reflexive of the gyro-mechanics that 
caused the agyrotropy to form in the beginning. This situation is complicated by the heavy aliasing of the 
plasma measurements by the time variations of the waves being reported. This heavy aliasing 
also clouds the instability analysis reported to explain the EBW. 
 
The preparation for the dispersion analysis also appears sketchy to this reviewer. Shouldn’t the 
sufficiency of the eVDF for this modeling be judged based on reproducing the entire measured 
distribution function before doing an initial value instability analysis? The present text appeals to having 
matched the perpendicular cut of the distribution as a sufficient test for the initial value simulation. 
Shouldn’t the modeled input crescent distribution for instability analysis exhibit its faithfulness by 
plotting f_disp vs f_obs as a Y vs X scatter plot on log, log paper using every velocity pixel measured 
to demonstrate the realism of the input for the analysis for the dispersion solver. 
 
What is the dispersion solver used? It should be referenced. 
 
Other places: undescribed pitch angles pictures contain overplotted 3 orthogonal L,M,N directions 
superposed on the 2 independent variables to draw conclusions about who is pointed in which directions. 
 
Flat topped electron distributions in the magnetosheath are common; the paper interprets the 
flattening of the distribution as evidence for the role of EBW in regulating the reconnection process. Because the 
initial value distribution function is not shown, there is hardly a before and after demonstration that 
the diffusion process was important. 
 
Isn’t the 7.5ms FPI electron observations heavily aliased in the EBW interval of 16ms? If so how is 
there adequate information for showing that it is unstable using initial value instability measures? Clearly 
the waves are large amplitude, but the argument about their being important for formation of the 
diffusion region or field line erosion escapes this reviewer. The instability analysis attempted presupposes 
the agyrotropic distribution is created first and the EBW evolves from it. Isn’t the formation of the 
agyrotropic distribution of this type already capable of disrupting frozen flux conservation? To advance the 
argument it would appear necessary to comment on the relative importance of Div P effects seen 
and those of these waves. From the abstract one gets the impression that the paper had assayed 
this competition; disappointingly it has not. 
 
 
p 9 typescript: “ The crescent shaped electron distribution [not shown] is close to the ExB direction” 
This is difficult to swallow since the distribution lives in 3-dimension velocity space and is sub sonic, 
so in what sense is this 4 surface close to a direction? 
 
 
Recommendation: The 25 authors can surely marshal the experimental argument more cogently 
than in this submitted manuscript. To the principal author: give the submitted manuscript to 3 
subgroups of authors for comment about its clarity. Take their comments seriously and revise 
to alleviate issues like those above. 
 
This is an interesting topic, but that does not mean its is publishable in its 
present form. The exposition of the paper’s experimental data seems less conversant with clear 
description of the necessary plasma supporting information. The graphics for, and diagnosis of the 
crescent distribution is absent or presently unintelligible. Seems unlikely that authors Rager- Giles on 
the authorship list have approved the submitted article. The article’s claim for importance in 
the electron diffusion region has not separated its role as progenitor or reactor to the agyrotropy, 
contrary to the impression of the last line in the abstract.