Peer review of: Loss-cone instability modulation due to a magnetohydrodynamic sausage mode oscillation in the (2019)

RR2
Reviewers' comments:
Reviewer #1 (Remarks to the Author):
The authors have the manuscript sufficiently improved. I recommend publication.

Reviewer #2 (Remarks to the Author):
I thank the authors for being responsive to referee input, which was extensive. While the revision addressed many points raised by the referees, it also raises new questions. In addition, for this referee, the primary objection to the work remains: that the interpretation of the observations is more complex than the data can justify and relies on too many assumptions and guesses to be convincing.
I fully acknowledge that this is an interesting and important observation but, in my opinion, the authors over-interpret the data. I cannot recommend the paper for publication in its present form. I urge them to confine their attention to a clear presentation of the data and a fairly brief and critical assessment of what the data might imply about energy transport, release, and modulation.
Additional comments:
Intro:
p. 2 - "This energy release results in the acceleration of particles to relativistic energies and the generation of high intensity bursts of radio emission." Well, EM radiation across the spectrum including, notably, hard X-ray emission.
p. 2 - "The radio emission is thought to be generated by a plasma instability..." Perhaps at this particular wavelength. At frequencies above 1-2 GHz plasma emission rarely plays any role.

p. 2- "Direct high cadence radio imaging of the behaviour of electron beams during a pulsation event has the ability
to confirm one (or some combination) of these mechanisms." Has the "potential" to confirm one or more mechanism.
General comment: it is puzzling to me that the authors do not make use of RHESSI observations. (They do use FERMI data in their earlier paper on this event.) There is coverage during the time in question. It would be useful to establish whether there is HXR emission in the non-flaring active region or whether the HXR emission in the flaring active region has any connection to the distant pulsating radio source. The same applies to ancillary radio data from, for example, Ondrejov.
Results:
p. 6- "possible cyclotron mechanism or loss-cone instability" The cyclotron mechanism refers to a resonance; a loss-cone provides free energy that could drive either plasma radiation or cyclotron maser radiation.
I don't understand the argument that the type III "most likely" injected energetic electrons into the quiet AR. They type III appears to be directed out into the corona and perhaps the IPM. Is there evidence that it is bidirectional? The rise in radio flux in the quiet AR appears to be well underway by the time the type III occurs.
p. 7&8- The case for reverse drift components - and for downward electron beams - is not very convincing and may be more apparent than real. Were the spectral data corrected for the instrument bandpass?
p. 9- Why do the authors claim that electrons accumulate at the loop footpoint? Downward-moving electrons will either precipitate or mirror back up to greater heights.
Referring to Fig. 5a, the systematic change in the source centroid with an apparent speed of ~700 km/s is striking. Why is the motion in all bands confined to the observed direction? How does the detailed timing relate to the EUV wave, which has a speed that is only 310 km/s?

p. 10&11: If the variation in the position of source maxima is apparent (p.10), it is not necessary to quote a speed of 0.1c (p.11).
The authors have embraced the presence of electron beams with very little observational evidence. I am also puzzled by the statement: "This shows that while there is pulsing energisation of electrons at P, the electrons accumulate at the adjacent footpoints close to R and Q ...". Energisation due to what?
p. 11- Referring to Fig. 6 the authors claim to identify a magnetic loop in which a sausage mode MHD oscillation could be supported. In panel (a), how does the (systematic) source motion shown in Fig. 5 relate to the NLFFF extrapolation? Is the NLFFF extrapolation consistent with the presence of the loop identified in EUV and shown in Fig. 6c? How do fast electrons gain access to the loop?
p. 13- The authors estimate the density in the loop using a DEM analysis of the AIA data. Is this necessary? They are trying to establish "whether sausage modes could be responsible for the radio pulsations". If the radio emission is due to fundamental plasma radiation the density in the source is ~6x10^8/cm^3, a factor of a few less than they estimate based on the DEM. How does the DEM- based estimate relate to the assumption of a density of 10^9 at the base of the loop?
p. 14- Using an average extrapolated magnetic field strength seems hazardous. Do the authors have any way to determine and propagate uncertainties in estimated quantities here?
Bottom: The authors argue (in the supplementary material) that the pulsations are not well described by an L-V system, yet they comment on p. 9 that the "light-curve of the pulsations shows a
sawtooth like pattern ... (this sawtooth characteristic is unusual, as [MHD] pulsations usually have a symmetric rise and decay phase)." This discrepancy between the observations and the MHD interpretation is not adequately addressed.
p. 15- "Observations of such large spectral slopes are rare". Insofar as narrowband emissions are often seen, this statement is not accurate. The "only emission that can explain such spectra..."? The references given are generic and do not really show that ECM amplification of Z-mode is the only possible mechanism.

p. 16- The authors claim that the observed pulsations are highly X-mode polarized. On the other hand, they claim that the reverse-drift emissions are beam-driven fundamental plasma radiation, which should be O-mode polarized. Is such a difference in polarization observed?
p. 17- "roughly coincides" - please be more precise and show, for example, in Fig. 1 of the supplementary material the start time and end time during which the EUV wave could be interacting with the quiet AR.
"10:54 UT" --> "12:54 UT"
p. 18&19- the words "most likely" or "likely" are over-used because the various suggestions have not been compellingly demonstrated.
The authors again acknowledge the expectation that an MHD sausage mode would yield a symmetric pulse profile. They then comment that "In this instance given the involvement of electron acceleration and the asymmetric pulse profiles, the modulation may be indirect." What does this mean? How does it yield the observed sawtooth pattern?
The point made above that the ECM source and the electron beam emission should be oppositely polarized is relevant here, too.
Reviewer #3 (Remarks to the Author):
The authors managed to revise the paper according to my comments, and I am happy to recommend the acceptance.