The authors here study the transport of highly energetic particles during ground level enhancement events, in the presence of stream interface transport conditions. In so doing they aim to elucidate the behaviour of particle transport coefficients in this scenario. While this is a potentially interesting study, I have some serious concerns about the methodology, as well as the results and the conclusions drawn therefrom, and do not think this manuscript is ready yet to be published. I raise the most serious concerns below, and follow up with some minor issues.

Major points

1. Turbulence is often, and correctly, invoked as the reason behind particle scattering in this study. However, even though it is often mentioned, absolutely no attempt is made to quantify turbulence levels before, during, and after the event in question. The authors should provide an analysis of magnetic variances and correlation scales (for appropriately-averaged intervals) during this event. This will quantify the claims made as to turbulence levels, and perhaps justify the possibly arbitrary assumptions as to the level of turbulence made in the modelling section of this study.
2. For clarity, and reproducibility, the model needs to be properly introduced. Referring to paper 1 is not sufficient. All equations should be given in full and explained. This includes the transport equation solved.
3. The authors ignore perpendicular diffusion. They must justify this, given the strong influence of this transport mechanism (see, e.g., Strauss et al., 2016, ApJ). They also need to discuss how this omission will influence their results. For instance: enhanced turbulence levels would reduce the parallel mean free path… but it would simultaneously increase the perpendicular mean free path, thereby altering their results should this be modelled self-consistently.
4. How do the authors justify their choice of radial dependence of \lambda_par? How would that chosen scaling compare with theory (see, e.g., Engelbrecht et al., 2022, SSRev)? How does the reference value of 0.3 au at Earth compare with other observations of the parallel mean free path (e.g. Lang et al. 2024, ApJ)? How do this study’s results compare with those of prior studies, like those of Droge et al., some of which are cited already?
5. Would not the local SI conditions lead to nonaxisymmetric perpendicular transport of the particles? See, e.g., Strauss et al., 2016, ApJ, for a discussion of this.
6. Figure 7 presents the biggest issue. Firstly, units should be given on the y-axis. But most importantly, how can diffusion coefficients be negative? This cannot be correct, and points to big problems with the model. This is also simply not discussed in the text.

Minor points

Line 21: faulty reference.

Line 54: what are “very increases”. I suspect a missing adverb.

Cannot access link: ftp://cr0.izmiran.rssi.ru/COSRAY!/FTP_GLE/.

Line 58: >1 GV, I surmise. Unit missing.

Reviewer #2:

General Comments:

The authors use a Monte Carlo simulation of particle transport to examine the effects of pitch angle scattering and changes in mean free path during increased turbulence in a stream interaction region.  They use GLE  72 from 10 September 2017 as a case study to see whether such transport modulation can obscure the characterization of the GLE as resulting from a flare (impulsive SEP) or a CME (gradual SEP), based solely on the GLE time profile.  As a consequence of their study, they conclude that the effects of transport  modulation through the SIR or through similar magnetosopheric turbulence are sufficient to obscure classification of the GLE based on time profile alone.  I think this is potentially interesting work.  In my reading, I found it difficult to interpret whether the text and the figures together sufficiently and quantitatively supported their conclusions. As such, I do not think this manuscript is ready for publication at this time.

Specific Comments:

• The authors mention characterizing GLE’s as arising from impulsive or gradual SEP events, based on rise and decay times, and reference Paper 1, but characteristic values for these times for are not given for the two types of GLEs.   What is the distribution of these times (figures would be useful), compared with the range of effect that the model shows for SIRs (perhaps beyond figure 8)?
• The model is mentioned in Section 3.1, but it is described insufficiently. Reproducing the Parker equation would be good.  What is the stochastic differential equation formalism?  Reproducing Paper 1 isn’t necessary, but summarizing the model would be very useful.
• It is difficult to determine from the text whether the model adequately reflects the SIR, as described in Section 2.2 and Figure 2. Was it intended to? Were the OMNIWEB data used as inputs?  This is where a more detailed summary of the model formalism might help, and if the model did indeed use some form of the OMNIWEB data as input, some indication on Figure 2 or another figure would help.
• Line 130: “Figure 3 validates the modeled intensity profile…”  It’s not clear how that figure validates the modeled profile vs. neutron monitor observations.  Based on the text and the caption, I would have expected the model profile equivalent to the measurements of Figure 1.  Is that the proton injection profile, i.e. is it the wrong figure?  How is time 0 defined?  Is the “intensity range” really that pale blue line?   I don’t see a band in the figure.
• Similarly, line 141: “In Figure 4, the mean particle intensity is shown along with percentiles bounding…”  I don’t see the percentile bounds in the figure.  An 8 hour rise is mentioned in line 143 but not shown in the figure, which just shows the decay phase.  The SIR occurs during the decay phase, so is the rise time important?
• Line 171: “in Figure 7, the pitch angle diffusion coefficient increases sharply across the stream interface…”  That is not at all clear from the figure itself.  Is the diffusion coefficient scale logarithmic?  How is “sharply” quantified?  What are the average values before and after the SIR onset?
• Lines 197-198: “We demonstrate the impact of even minor interplanetary structures on small GLEs, by quantifying mean free path increases of over 60% across the SI.”  This is the first time a 60% increase is mentioned in the paper, but line 7-8 says “In comparison to typical impulsive events, the particle mean free paths declined by approximately 35% during the 6-hour SI crossing.”  Did I miss something?

Technical or minor comments:

• Line 158: “Figure 5” should probably be Figure 4.
• There’s a section 3.1 but not a 3.2 or any other subsection in 3. Was a section 3.2 planned?
• Figure 2: It would probably be helpful to label the panels a through h.  Also, that vertical line isn’t exactly vertical.  Is the curve meaningful?