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

RR3

Reviewers' comments:
Reviewer #2 (Remarks to the Author):
I am afraid the authors and I are not really converging. Below are a few additional comments on the various components of their scenario. If I've mischaracterized or misunderstood aspects of the scenario after three readings, it is indicative of how hard it might be for a typical reader to follow. I leave it to the editor on how they wish to proceed.
The authors suggest the following scenario to account for the observed radio imaging (NRH) and spectroscopic (Orfées) observations:
• An MHD sausage mode oscillation is excited in a coronal loop by the passage of an EUV wave driven out from a large flare in an adjacent active region
o I think the authors have fairly convincingly shown that the passage of the EUV wave is associated with the onset of the pulsations
• The oscillation leads to the periodic acceleration of electrons near a magnetic null point where intense 228 MHz emission is produced
o The weak pulsations in source Q could just be a sidelobe response to the strong pulsations in P (p. 7)
o In Fig. 5, panels a-c, where are P and Q relative to the candidate pulsating loop?
o The authors claim that the intense, pulsating 228 MHz emission is fundamental plasma radiation (p. 12). What produces (and confines) the underlying spectrum of plasma waves? What is the degree and sense of circular polarization of the pulsations? Same as it is for the emission near 210 MHz (Fig. 6)?
• The accelerated electrons stream down from P toward the foot points of adjacent loops at site Q; the downward streaming electrons produce reverse-drift type III radio emission
o Are the reverse-drift type IIIs fundamental plasma radiation? What is their polarization degree and sense?
o Is there any evidence for classical type IIIs drifting from 228 MHz to lower frequencies in the spectrum? What is the emission <200 MHz in Fig. 7 in the Supp. Materials?
o Looking at the Fig. 4, lower panel, it appears that there is a radio signature correlated with the type IIIs at frequencies up to 327 MHz. Can’t the 327 MHz data be filtered (remove the

background with a running boxcar or similar) in order to isolate the location, timing, and polarization of the purported reverse-drift type IIIs at 327 MHz source relative to P and Q?
o I asked the authors why they hadn’t used FERMI and RHESSI HXR observations. They now make use of HXR and microwave light curve data and claim that it is well correlated with the observed meter-wavelength phenomena. But what about RHESSI maps? It would be extremely interesting and important to see where an associated HXR source is located relative to P and Q.
• At site Q, a loss cone is set up that is unstable to the production of Z-mode waves that are subsequently converted to electromagnetic waves – the emission is imaged at 228, 298, and 327 MHz
o I find the section beginning on p. 13 and continuing (emission mechanism and loss-cone instability modulation) to be very confusing. The claim is that a “secondary mechanism” must be in play to account for the strong narrow-band emission that peaks at ~210 MHz and the authors suggest a loss-cone instability. Isn’t that source P? What sets up a loss cone there?
o Bottom of p. 14: the ratio of the electron plasma frequency to the electron gyrofrequency is given as 1.6. The statement is made that the ratio is 2 for harmonic emission. Not 3.2? Harmonic plasma radiation is near 2 times the plasma frequency.
o The authors suggest that z-mode waves are amplified by the loss-cone and then coalesce, producing x-mode harmonic radiation. They also point out the fact that the 228-327 MHz emission seems to be associated with loop footpoints at source Q. Are the authors claiming loss-cone emission at both points P and Q and that it is harmonic plasma radiation? It is also hard to see how this is consistent with statements about the 228 GHz source on p. 12 since the harmonic emission at source Q would have a lower density than the fundamental emission at source P.
o It is hard to see the relevance of several of the references cited in support of the above claims (e.g., the regimes considered by the references relevant to polarization [51-53] don’t seem to be relevant to conditions in the source.
• The loss cone is periodically quenched by the periodic introduction of fresh electrons streaming down from the pulsating acceleration site P
o It seems the authors argue that the loss-cone-driven emission is relatively broadband (100 MHz) in Q. Either the source is extended (plasma radiation from a range of sites) or it is intrinsically broadband, perhaps due to conditions described by Winglee and Dulk (1986).
o Why is the emission <200 MHz also subject to sudden reductions?