Summary:

This work presents a new method for identifying magnetic flux ropes from global simulation results. The advantages of this method is that it only requires the magnetic field information and does not require prior knowledge of existing structures like current sheets, reconnection lines, or the magnetopause surface. The authors outline the methodology and present an application with the hybrid Vlasov code Vlasiator.

The presentation of this work is very clear and provides a novel contribution to an important topic in space science. The identification of flux ropes in simulation is beneficial to the study of magnetospheric physics, solar physics, and any other space environment where magnetic reconnection occurs.

Overall, the methods, results and conclusions are well written. In particular, the methodology is both straightforward and appears quite effective at accomplishing the goal of flux rope identification. Additionally, the comparison with the topological identification of Alho et al. 2024 provides a meaningful point of validation that the dayside identified structures are indeed Flux Transfer Events.

Recommendation:

Below are detailed comments, my recommendation is to accept with minor revisions. Primarily, I think the authors could be more quantitative with the results that are presented. By providing more quantification the conclusions can be strengthened and this work will be easier to compare with future studies.

Detailed Comments:

Line 41: The authors use the term flux rope for both FTE and plasmoid, but what about Kelvin Helmholtz vorticies? They are not mentioned until the very last paragraph in the conclusions. If they are, or are not included in the ‘flux rope’ definition here it should be mentioned.

Figure1: Have you looked at the boundary points in terms of the points which fail only one of either R+ or R-? They would be the edges of the flux rope structure and might be regions of interest themselves.

Line 115: Why ‘significantly larger than the domain size’? I would assume if the traced point leaves the domain then surely the point is not flux rope.

Line 145: How is the fraction of omitted cells decided? Is it random, or geometric conditions set by the user?

Line 165: What radius are the field aligned currents coupled from? The data availability statement notes the large size of the output data, but it may beneficial to upload a simulation parameter input file ( or text file summarizing the simulation input parameters) for future comparisons.

Figure 2: This is where a quantified measure could be helpful. In the snapshot it’s clear there are more points identified with a larger Rcutoff, and at least one flux rope is missing between 3Rc and 7Rc. What I would like to see is what is the total flux rope volume for each of these panels? This could then be included in the video or even shown as a time series with a curve for each Rc setting. It could help justify the selection of Rcutoff = 7Rc.

Figure 4: The + and circles in panels e and f are difficult to distinguish between.

Line 245: With both the yellow circle O points and the green flux rope points, perhaps the coverage of the O points (within some distance threshold) could be reported here and used as validation. It could be reported as a percentage over time, again for the different choices of Rcutoff to show that 7Rc performs the best.

Figure 5: The satellite traces may show a better structure crossing if put into LMN coordinates.

Figure 6: I believe the X and Y scaling (horizontal axes) is different between some panels in the same column. This made it difficult to understand the point about the cross section shrinking. It would improve comparison between panels in a single column to have the scales the same.

Figure 7: Great use of perspective to summarize the flux rope detection results. Again, could be improved with an indication of how many flux ropes were found for each contour level. If identifying contiguous flux ropes is not currently possible, at least the total number of points (or volume of all points?).

Line 349: Figure 7 demonstrates that no dayside flux ropes survive to X=0, but is there a gradient or a sudden cutoff? If it is reconnection eroding the structure, maybe larger FTEs make it further downstream before vanishing?

Line 351: “any section of the flux rope presenting a magnetic field configuration anti-parallel to the lobe magnetic field will erode away due to magnetic reconnection.” Consider rewording slightly. The results only show a single flux rope that has its anti-parallel portion eroded, while every flux rope with such conditions could experience such erosion, it was not shown that this occurs in every instance. If the results do show this, then that should be included explicitly in the results section.

Line 356: Can it be estimated from these results what percentage of flux ropes are vanishing over the poles vs surviving downstream? Does it match some geometric ratio of the portion of the dayside X line length?

Line 377: Similar to above, it’s mentioned that ‘such a prediction would be perilous’. While I agree that a single simulation should not be over-extrapolated, the results can certainly be reported. How many low latitude flux ropes were found for this time interval? How does that compare to the number that vanished over the poles? The answer will be limited to this simulation setup, but nonetheless interesting.

Line 397: Does this imply that there may be holes that form in the field of flux rope points? Will this then make it more difficult to identify individual contiguous flux rope structures?

Line 449: How would this method tell the difference between a rolled up KH vortex and a low latitude flux rope which has been carried downstream?

Review Report

Title: Global evolution of flux transfer events along the magnetopause from the dayside to the far tail

Authors: Yann Pfau-Kempf et al.

Reviewed by: Weijie Sun, Space Sciences Laboratory, University of California, Berkeley (weijiesun@berkeley.edu)

Summary: This manuscript investigates the global evolution of flux transfer events (FTEs) on the dayside magnetopause, their generation, propagation, and dissipation during reconnection with the nightside lobe field tailward of the cusp using the global simulation model Vlasiator. The study demonstrates that flux transfer events with an axial direction parallel to the flow velocity and without an anti-parallel component of the magnetic field can maintain their structures and propagate further downtail. The authors developed a technique to trace magnetic field lines using the hybrid-Vlasov model Vlasiator to identify flux transfer events and to achieve these conclusions.

General Comments: This is a well-conducted study on the evolution of flux transfer events in Earth's magnetosphere. The manuscript is well-written, and the figures are clear and informative. However, there are several areas where further clarification and revisions are needed to improve the manuscript's readability and accuracy. In addition, some terminology used is uncommon in the space plasma field and should be revised for better understanding and readability.

Specific Comments:

Line 1: The term "rolled-up" is used to describe magnetic flux ropes. While not incorrect, "helical magnetic field" is more commonly used and precise. Also, the term "longitudinal axis" is not clear and should be clarified as it can vary in different space plasma contexts.

Line 16: The word "twisting" is used to describe the magnetic field geometry of flux ropes. Again, "helical" is a more accurate and commonly used term.

Lines 18-21: The description of flux rope formation suggests they originate inside the Sun and pass through the solar surface. I do not think this is the case. References are needed if this is the case. On the other hand, this should include the possibility of formation by magnetic reconnection in the solar wind, as noted by Cartwright and Moldwin (2008) and Feng (2010).

Line 25: The phrase "When the magnetotail current sheet disrupts and reconnects, flux ropes form" should be revised to "When the magnetic field reconnects in the magnetotail current sheet, flux ropes can form."

Lines 52-53: May mention the automated method developed by Li et al. (2023) for detecting FTEs in Mercury's magnetosphere.

Figure 1: Consider including a panel showing the curvature radius along with the magnetic field line for better illustration.

Section 2.2. The simulation in this study was under a pure southward IMF. Therefore, the flux ropes resulted from this simulation likely do not have strong core field. How does the core field influence the criteria set up here?

On the other hand, is it possible to investigate the curvature radius in a flux rope event with a strong core field or a flux rope without core field in the simulation? As shown in Sun et al. (2019, 2019GL083301) and Smith et al. (2024), flux rope with strong core field corresponded to a maximum in curvature radius, while without strong core field a minimum in curvature radius.

Lastly, flux transfer events often correspond to coalescence, i.e., merging of neighboring flux transfer events, as shown in Sun et al. (2022, angeo-40-217-2022 and many other simulation and observation works). Is it possible for this technique to identify those events?

Line 162: Clarify the term "Neumann (copy)."

Line 168: Revise "for on" to "on."

Line 205: I think that I can identify the "long flux rope" and the "curved flux rope". However, it would be better to identify them in the Figure.

Line 226: Add citations for MGA and MDD.

Figure Captions (Figures 5 and 6): Clarify the abbreviation "resp."

Figures 5c to 5f, may include horizontal lines at y = 0.

Figure 6 pretty nicely shows the flux ropes!

Lines 280 to 281: Is it possible that the counter-rotating vortices of magnetic field lines are coalescing flux ropes as I mentioned earlier?

Line 290: "more inclined along the main diagonal of the (x,y)-plane" Is this place trying to say that the axis of the flux rope is mainly in the x-z plane?

In Figure 7, the contour for the x within 4 to 12 RE is larger than the contour for the x within 0 to 4 RE. I could not understand why. Could the authors explain more about this?

Line 341: Consider using "convected" instead of "advected." Same for other places.

Line 390: Clarify the term "one-dimensional axis."

Line 392: Explain the use of "said."

Line 434: Revise "priori" to "prior" or "previously."

Line 437: Clarify the term "agnostic."

Line 441: Consider using "antiparallel" instead of "shearing."

Line 443: Revise "shear component" to "antiparallel component."

Line 444: Consider using "property" instead of "integrity."

Line 447:  I think that with Vlasiator simulations, it is also possible to investigate the energizations of protons and electrons as well as the important of flux transfer events in transferring magnetic flux and particles in the space plasma physics (Section 2.1 in Sun et al., 2012, https://doi.org/10.1007/s11430-021-9828-0).