Plasma transport into the duskside magnetopause caused by Kelvin–Helmholtz vortices in response to the northward turning of the interplanetary magnetic field observed by THEMIS

Yan, Guang Qing; Parks, George K.; Cai, Chun Lin; Chen, Tao; McFadden, James P.; Ren, Yong

doi:https://doi.org/10.5194/angeo-38-263-2020

Articles | Volume 38, issue 1

https://doi.org/10.5194/angeo-38-263-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/angeo-38-263-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 38, issue 1

Regular paper

|

25 Feb 2020

Regular paper |

| 25 Feb 2020

Plasma transport into the duskside magnetopause caused by Kelvin–Helmholtz vortices in response to the northward turning of the interplanetary magnetic field observed by THEMIS

Guang Qing Yan, George K. Parks, Chun Lin Cai, Tao Chen, James P. McFadden, and Yong Ren

Download

Final revised paper (published on 25 Feb 2020)
Preprint (discussion started on 19 Jul 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

- Printer-friendly version

- Supplement

RC1: 'manuscript review', Anonymous Referee #1, 09 Sep 2019
- AC1: 'Response to Anonymous Referee #1', Guang Qing Yan, 25 Oct 2019
RC2: 'Review report on the manuscript "Plasma transport into the duskside magnetopause caused by Kelvin–Helmholtz vortices in response to the northward turning of the interplanetary magnetic field observed by THEMIS"', Anonymous Referee #2, 17 Oct 2019
- AC2: 'Response to Anonymous Referee #2 (angeo-2019-103)', Guang Qing Yan, 25 Oct 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (15 Nov 2019) by Anna Milillo

AR by Guang Qing Yan on behalf of the Authors (26 Nov 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Nov 2019) by Anna Milillo

RR by Anonymous Referee #2 (24 Dec 2019)

RR by Anonymous Referee #1 (26 Dec 2019)

Suggestions for revision or reasons for rejection

Review of the manuscript entitled “Plasma transport into the duskside magnetopause caused by Kelvin-Helmholtz vortices in response to the northward turning of the interplanetary magnetic field observed by THEMIS” by Yan et al.

First of all, I would like to thank the authors for their work to respond to all the reviewers’ comments. They have taken all the comments into account. Nevertheless, I have to admit that I still have doubts about the identification of the KH vortices and about the interpretation of the measurements with respect to the plasma transport across the magnetopause, facilitated by the KHI. In my opinion, the paper should be published if all the conclusions drawn from the data and analysis are formulated in a much more careful and cautious way.

Specific comments:

The authors now state the eigenvalue ratios associated with the MVA; this is very good. It is, however, not surprising that those ratios are quite small, due to the small variations in the magnetic field across the magnetopause. Contrary to what is stated in the manuscript, the MVA results are considered reliable when the ratio is above 10, and acceptable when it is above 4. The latter condition only holds for 2 of the 8 analyzed intervals. Hence, local boundary normal directions obtained from MVA may well be considered only partially reliable or even unreliable in this case. Unfortunately, these MVA-based directions are still the main argument for the KH vortex identification.

In the revised manuscript, some higher-speed lower-density plasma measurements are indeed identified, after lowering the magnetosheath velocity to 134 km/s by taking the average over some magnetosheath intervals. However, I would say that the original estimate of 180 km/s as magnetosheath velocity at the position of TH-A was more accurate, as it was probably observed further out in the “undisturbed” magnetosheath (see panels 4 and 5 of Figure 2). Note that the existence of high-speed low-density plasma does not ensure that the observations pertain to rolled-up vortices. The same plasma features can also come from magnetopause surface waves that are not amplified by the KHI. It is also not unimaginable that those non-KH surface waves may have fine structure yielding the double peaks observed by TH-E.

Consequently, the existence of KH vortices is suggested by not very reliable MVA-based directions and a few inconclusive high-speed low-density plasma observations. Rotation features in the bulk velocity and magnetic field deviations can also come from surface waves that are not subject to the KHI. The fact that the periodic magnetopause oscillations started with the northward turning of the IMF supports at least the assumption that the magnetopause surface waves were amplified/driven by the KHI – herein I agree with the authors. An argument against the existence of rolled-up vortices is the observation location, far upstream of the terminator. Rolled-up vortices are known to form at and beyond the terminator; they should collapse further down the flank/nightside magnetopause. The manuscript claims that in this particular case the vortices form, fully developed, and collapse before even reaching the terminator.

Regardless of whether there are KH vortices or not, it is an interesting question if secondary processes at the magnetopause led to the transport of magnetosheath plasma into the magnetosphere. Contrary to what is stated in the response to the reviewers’ comments letter and in the manuscript (e.g., line 178, line 200), the coexistence of hot and cold ions/electrons does not prove any local transport of particles into the magnetosphere. Coexisting plasma populations could have been already present as part of a pre-existing LLBL, which the authors admittedly cannot exclude. Consequently, in my opinion, there is neither “unambiguous” (original manuscript) nor “clear” (revised manuscript) evidence of plasma transport based on these “coexisting plasma” measurements; they do not provide evidence for plasma transport in this case.

Indirect evidence for plasma transport may only come from the evolution between periodic and dispersed magnetopause observations from TH-A to TH-E. The authors argue that the differences between the spacecraft come from the collapse of the KH vortices between them; this is possible but may not be most likely (see above). The same observations may potentially be more easily explained by different positions/distances of the respective spacecraft with respect to the magnetopause (wave), and by a pre-existing LLBL. Changing spacecraft distances could also explain the later periodic magnetopause oscillations observed by TH-E but not by TH-A. In this alternative scenario, in the absence of secondary processes at the magnetopause, local plasma transport would not be expected.

What can we conclude at the end? Are there KH vortices that form at the dayside magnetopause and collapse between TH-A and TH-E, dayside of the terminator? Maybe, maybe not. Most probably it can neither be fully proven nor excluded. Is there transport of magnetosheath plasma into the magnetosphere as a result of secondary processes at the magnetopause? Again, maybe, maybe not. It can neither be proven nor excluded. And here lies the essence of my criticism: The conclusions in the manuscript are formulated way too strongly, as if there were no possibility of doubt or alternative explanation. Should the manuscript the published, I would strongly encourage the authors to tone down all the conclusions and discuss possible alternative explanations.

Hide

ED: Publish subject to minor revisions (review by editor) (07 Jan 2020) by Anna Milillo

AR by Guang Qing Yan on behalf of the Authors (19 Jan 2020) Author's response Manuscript

ED: Publish as is (27 Jan 2020) by Anna Milillo

AR by Guang Qing Yan on behalf of the Authors (28 Jan 2020)

Short summary

We present (1) K–H vortices in direct response to the northward turning of the interplanetary magnetic field (IMF); (2) solar wind transport into the magnetosphere caused by the K–H vortices, involving both ion and electron fluxes; and (3) typical portraits of the ion and electron fluxes in the regions of plasma transport. The unique characteristics may complement existing observations and enhance our understanding of the K–H vortices and transport process.