Dayside magnetopause reconnection and flux transfer events: BepiColombo earth-Flyby observations

Sun et al.

This paper reports on observations by BepiColombo as it passed through the dayside magnetopause of the terrestrial magnetosphere during its Earth gravity-assist.  Magnetometer measurements show signatures consistent with six flux transfer events (FTEs), signatures of magnetic reconnection between the magnetosheath and terrestrial fields.  The nature of FTEs, as a primary source of solar wind-magnetosphere coupling, is of perennial interest.  Although only a single pass of the dayside magnetopause occurred and the number of events observed is relatively small, the fact that they spacecraft involved is BepiColombo further increases the topicality of the paper.  There are novel aspects to the observations (e.g. the apparent coalescence of two FTE flux ropes), and the conclusions drawn from them appear accurate.  Hence I recommend the paper for publication in Annales Geophysicae.

There are a few minor typos and grammatical issues, listed below, that the authors may wish to address before the paper is published.

Line 161: “next.” > “next section.”

Line 168: “was cleared resulted” > “clearly resulted from”

Line 192: “was” > “has”

Line 211: “shall” > “should”?

Line 219: “reconnection the magnetosheath” > “reconnection associated with the magnetosheath”?

Line 222: “detail” > “detailed”

Summary

The manuscript reports experimental observations made by the BepiColombo mission during an Earth flyby on 10 April 2020. It focusses on the crossing of the Earth’s magnetopause, and uses data from the MPO magnetometer and MMO electron spectrometer. A series of flux ropes are identified in the data, and their qualitative motion is compared with models of the expected X-line location, showing good agreement. A flux rope model is also used to calculate various properties. Evidence is presented showing that one flux rope in facts consists of two discrete structures separated by a thin current sheet, and it is argued that coalescence via magnetic reconnection is occurring.

Overall the manuscript makes a new and important contribution to our knowledge and understanding of flux ropes on the Earth’s magnetopause and subject to minor corrections I would recommend publication. This is based on a need to clarify certain aspects of the analysis, and also soften some of the statements, particularly on the existence or otherwise of active reconnection at the thin current sheet seen in the proposed coalescence structure. The comments below expand on this in more detail, and are presented in the order they appear in the manuscript. In addition I would also recommend a careful proof-reading of the manuscript to address some mistakes in English and grammar which I have not listed.

Detailed comments

Line 35. A caveat to this paragraph is that the transport of flux means they are closed to the ionosphere at one end.

Line 49. It would be worth citing recent studies on this topic by e.g. Toledo Redondo et al. https://doi.org/10.1029/2021JA029506

Line 57. I am not sure it is not quite right to say that e.g. Oieroset et al. and Kacem et al. observed coalescence of flux ropes. There is so-called secondary reconnection, but the analysis suggests it is of a type where interlinked flux tubes are in tension against each other and then reconnecting.

Line 70. Add a comment to mention that MMO and MPO are attached and so for the purposes of the flyby there is one spacecraft/observation point.

Line 80. Maybe mention the local time here, or the exact location in GSE

Line 101. In the overview figure it may help to plot the magnetic field in the boundary normal coordinate system as well. This would help show the existence of the flux ropes even more clearly, and allows the reader to see the polarity in B_N (negative/positive for southward and positive/negative for northward) more clearly.

Line 126. The location of the model X-line in Figure 2a is nicely consistent with the observations, but I found Figure 2b and 2c harder to understand. In Figure 2c there is no predicted X-line? In Figure 2b it is highly tilted, and so while I agree that it would lead to northward motion in the general sense, there would also presumably be lateral motion with the reconnection exhausts pointing northward and dawnward. This should be clarified.

Line 142. Is it necessary to assume a specific speed for the fit, or just that it is moving at constant speed? I could understand that the speed is necessary to get the estimate of flux content.

Line 149. The model gives interesting extra information about the flattening of the flux ropes. Apologies if I missed this, but can you add some text to discuss how the flux ropes are flattened, is it in the direction of motion, or along the normal to the current sheet. Also is the flattening significant? Is there much deviation from a circular profile. I think these points would be of interest to other readers studying this problem.

Line 166. This is a very nice observation. For completeness, are you able to model FR#A and FR#B independently – is it possible to say anything about how well aligned they are?

Line 178. The existence of the thin current sheet separating the two flux ropes is clear and therefore a site where reconnection can occur. But I think the statements should be softened a bit here, because on the basis of the data alone, it is not 100% certain that reconnection is occurring, as there is no complementary evidence. I know that jets cannot be observed, but is there any evidence for e.g. Hall magnetic field signatures or other structure that would point to active reconnection ongoing? With a guide field of 0.28 this signature would be somewhat distorted but should be visible, and could be related to the more-negative deviation in B_M where B_L reverses. In the electron data is there any evidence for localised heating etc. (although this would likely be a weak signature and maybe difficult to observe in the 4s cadence data)? Also is it possible to use the electron data to understand the connectivity of the plasma through the whole observation of FR#A and FR#B.

This paper presents some first observations from BepiColombo, taken as it crossed the terrestrial magnetopause during its Earth flyby. The authors present observations of six flux transfer events (FTEs). They conclude that Bepi crossed the magnetopause near the dayside reconnection line, which moved such that a combination of northward-travelling and southward-travelling FTE flux ropes were observed. They also infer the occurrence of a coalescence event whereby two smaller flux ropes merge, supporting theoretical arguments that FTE flux ropes can grow in this manner.

The paper is succinct and generally clear; if substantiated, the conclusions make a contribution to the body of knowledge, and it is nice to see some early results from BepiColombo. I am not aware of northward and southward-moving flux ropes having been observed on the same magnetopause crossing, and the idea of flux rope coalescence is relatively new. I just have two concerns on which I would appreciate it if the authors can provide further reassurance or clarification:

1) As plotted in Figure 1, these are not the clearest examples of flux transfer events, though this could be due to the temporal scale of the plot (relative to the scale of the signatures) and the fact that the data are plotted in GSM, rather than boundary normal coordinates. [Only one event is shown in boundary normal coordinates, in Figure 3.] Although the normal direction quoted on line 94 is predominantly along the GSM X direction, small differences can obscure signatures, and the fact that Bx seems to alternate between periods of about +5 nT and about -10 nT does suggest that Xgsm does not approximate the local magnetopause normal particularly well. For example, the signature for the first event seems to be more of a step function from a generally-negative to a generally-positive Bx orientation, and (as far as I can make out from the figure), the enhancement in magnetic field magnitude precedes the Bn/Bx reversal, rather than being centered on it (as would be expected for a flux rope). This does inject some doubt into the identification of this event, and that in turn undermines the conclusion of the X-line moving across Bepi. The magnetosheath field is highly structured, and a skeptic might wonder if this first signature is simply associated with some rotation of the magnetosheath field, rather than the passage of a flux rope. The remaining events are too small and close together to be able to see confidently in Figure 1, with the exception of the penultimate event. Can I suggest the authors plot all six events in a boundary normal coordinate frame, to improve confidence that these events are reliably identified? Hopefully this will rebut any skepticism, but if the signatures are as unclear in a boundary normal coordinate system, it might be worth considering whether there is any further evidence in support of the first event, particularly, being an FTE (or reducing the strength of the claim about the motion of the X-line past Bepi).

2) The authors have shown evidence of a magnetic shear, and hence current sheet, but as I understand it, evidence of active reconnection relies on the ratio of the normal/tangential field components. The authors note that uncertainties in the normal direction will influence this (line 185) - is there any other evidence that the authors can present to support this conclusion? (Or can they quantify the uncertainty on the minimum variance direction and how that translates into uncertainty on the magnetic field component normal to the current sheet?)

Aside from the above, I had a few minor comments:

Line 18: "flux rope" -> "flux ropes"

Lines 45 and 46: "reconnections occur" -> "reconnection occurs"

Line 71: "and" missing after Heyner reference

Line 73: outbound/inbound should be outboard/inboard?

Line 137: xmin and xmax are the locations of what? And what is meant by "along with nmin and nmax"?

Line 140: What is the physical meaning of chi?

Line 154: I am confused by the text structuring here, as Figure 3 is introduced but you then go on to talk about flux ropes (at 00:26:06 and 00:26:26 UT) that are not shown in the figure. Should the sentence introducing Fig 3 move to the next paragraph?

Line 161: Word missing after "next"

Line 163: There seem to be some words missing from this sentence

Line 164: I am inferring that the two successive bipolar signaures mentioned here correspond to two green arrows in Figure 1, but please clarify.

Line 168: "was clearly resulted" - does not make sense