Review of MS “High bandwidth measurements of auroral Langmuir waves with multiple antennas” by C. Moser, J. LaBelle, and I. H. Cairns

This manuscript describes the HIBAR rocket experiment. The manuscript very nicely describes the measurements of Langmuir/upper-hybrid wave fluctuations that coincide with the whister/lower-hybrid wave fluctuations. Figure 2 displays two events where the high-frequency fluctuations are detected by the rocket. Figure 3 shows the corresponding low-frequency fluctuations (presumably with the wave angle quasi perpendicular to the ambient magnetic field, hence, interpreted as being associated with the lower-hybrid frequency cutoff). Figures 4 and 5 constitute the enhanced plots of the high-frequency spectrum, in particular, those cases that display the narrow Langmuir wave line and the so-called the diffuse spectrum. The manuscript devotes a significant portion on the discussion of the polarization of the measured electric field.

The authors present the analysis via Tables 1 and 2 where the readers may glean that the exact nature of the high-frequency fluctuations is uncertain. It appears that the main focus of Section 3 is on determining whether the measured high-frequency fluctuations are quasi-parallel Langmuir wave or quasi-perpendicular, or oblique, Z/upper-hybrid waves. If the authors can state this physical motivation at the very beginning of Section 3, then it would help readers understand the main thrust of Section 3 a bit more clearly. The authors do mention the motivation of this analysis, but almost at the end of Section 3, line 160, equations 1 & 2. It would be much more helpful for the readers if they mention the purpose of E_||/E_perp analysis at the beginning (in words) and rephrase in more quantitative details (through equations 1 & 2) at the end of Section 3, and further elaborated in subsequent subsections (as they are laid out herein).

In subsection 3.1, the authors recapitulate the fundamental linear wave properties of plasma normal modes in over- and under-dense plasma. In subsection 3.2, they present the most interesting three-way analysis. Figure 7 appears to represent quite a sophisticated effort both in terms of extracting the necessary input parameters from data and also in terms of numerical effort. The authors make convincing argument that the three-wave processes involving the low-frequency whistler/lower-hybrid branch might be at work. The authors, however, point out that the model electron distribution function, shown in Figure 8, did not produce positive growth rate for the high-frequency waves corresponding to Figure 7, as evidenced from Figure 8c. The authors seem to have come to a conclusion that the free energy source for the high-frequency waves are the high energy electrons, but their instrument does not directly detect these particles.

One question that naturally arise is the following: In generating Figure 7 dispersion surfaces the authors have constructed the frequency, k_|| and k_perp. This means that the correspond E_||/E_perp for the Langmuir and diffuse fluctuations can also be computed theoretically. How do they match up with observations, as laid out in the two Tables? are they consistent?

In the remaining part of Section 3, the authors discuss the wave intensities associated with the high- and low-frequency fluctuations and estimate the cumulative three-wave amplitudes involved in the decay process. However, this analysis is very speculative. Nevertheless, it might be useful for some future actual three-wave weak turbulence analytical calculation.

In conclusion, upon a careful reading of this manuscript my recommendation is to publish this work more or less in present form, but I suggest the authors to motivate Section 3 a bit more directly in the final version of their manuscript before publication, and to address the question I raised above.

Referee report

This paper discusses a data analysis of 5 short-duration auroral Langmuir wave burst events in the polar F-region ionosphere, consisted of a plasma line at frequencies ranging from 2470–2610 kHz with an associated diffuse feature occurring 5–15 kHz above this line, measured by a rocket flight with an apogee of 382 km. The aim of this study and the methodology adopted follow closely a recent paper by Moser et al. (2021) which reported a similar investigation based on the data from two sounding rockets with apogees of 1,042 and 756 km, respectively. The hypothesis of nonlinear wave interaction is tested using a dispersion solver, Wave in Homogeneous Anisotropic Multicomponent Plasma (WHAMP, Rönnmark 1982), to determine surfaces corresponding to the normal plasma modes: the Langmuir-Upper Hybrid and the Whistler-Lower Hybrid surfaces, that might take part in nonlinear 3-wave interaction. This paper concluded that the observed auroral Langmuir events can be explained by a Langmuir/z-mode wave coalescing with or scattering off the whistler/Lower Hybrid wave, confirming the results of two previous papers by Stasiewicz et al. (1996) and Bonnell et al. (1997). This paper is well-written and contains interesting results for understanding nonlinear space plasma processes in the aurora. It can be published after some minor revisions commented below.

1. The subject of Langmuir turbulence in the auroral ionosphere has been reviewed recently by Akbari, LaBelle and Newman (Front. Astron. Space Sci. 7, 617792, 2021) which should be inserted in the reference. In addition, a discussion should be added to explain the source of field-aligned electron beams that excite the auroral Langmuir waves, and if these substorm events are the ionospheric signatures of magnetic reconnection in the tail regions of the magnetosphere.

2. In addition to wave-wave processes involving Langmuir and Lower Hybrid waves, the authors should mention other nonlinear wave interaction studies in the auroral ionosphere. For example, Bohm et al. (JGR 95, 12157, 1990) showed that the most intense Langmuir and whistler waves measured by auroral rocket flights occur in association with Alfven waves. The theory of auroral Langmuir-Alfven-whistler events were studied by Chian et al. (A&A 290, L13, 1994) and Lopes and Chian (A&A 305, 669, 1996). In particular, these theoretical papers showed the feasibility of a 3-wave process involving the parametric decay of a Langmuir wave into a whistler wave and an Alfven electromagnetic ion-cyclotron wave which may apply to the observation of electromagnetic ion-cyclotron waves in a flickering aurora by Lund and LaBelle (JGR 17, 241, 1996).

3. Although the approach of this paper is based on the concepts of linear wave dispersion and weakly nonlinear wave-wave interactions, for the sake of completeness a discussion should be inserted to relate these linear and weakly nonlinear wave studies to the description of Langmuir intermittent turbulence consisted of coherent structures such as cavitons discussed by Akbari et al. (JGR Space Phys. 118, 3576, 2013) and phase-space vortices such as electron and ion holes discussed by Ergun et al. (Phys. Rev. Lett. 81, 826, 1998) and Schamel et al. (Phys. Plasmas 27, 062302, 2020).