Polar substorm on 7 December 2015: preonset phenomena and features of auroral breakup

Safargaleev, Vladimir V.; Kozlovsky, Alexander E.; Mitrofanov, Valery M.

doi:https://doi.org/10.5194/angeo-38-901-2020

Articles | Volume 38, issue 4

https://doi.org/10.5194/angeo-38-901-2020

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

https://doi.org/10.5194/angeo-38-901-2020

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

Articles | Volume 38, issue 4

Regular paper

|

28 Jul 2020

Regular paper |

| 28 Jul 2020

Polar substorm on 7 December 2015: preonset phenomena and features of auroral breakup

Vladimir V. Safargaleev, Alexander E. Kozlovsky, and Valery M. Mitrofanov

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Final revised paper (published on 28 Jul 2020)
Preprint (discussion started on 25 Sep 2019)

Interactive discussion

Status: closed

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

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RC1: 'Review of "Polar substorm on 07 December 2015: pre-onset phenomena and features of auroral breakup"', Anonymous Referee #1, 08 Oct 2019
- AC1: 'Reply to Referee #1', Vladimir Safargaleev, 22 Jan 2020
RC2: 'Reviewer report', Anonymous Referee #2, 20 Dec 2019
- AC2: 'Reply to Referee 2', Vladimir Safargaleev, 22 Jan 2020

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (13 Feb 2020) by Minna Palmroth

AR by Vladimir Safargaleev on behalf of the Authors (16 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Mar 2020) by Minna Palmroth

RR by Anonymous Referee #3 (14 Apr 2020)

Suggestions for revision or reasons for rejection

Safargaleev et al. present an event study of a moderate substorm that took place around 17-18 UT on December 07, 2015 in the northern parts of the IMAGE magnetometer network. They describe the substorm and the preceding solar wind driving and magnetospheric activity by combining several satellite and ground-based datasets. They classify the substorm as of ``polar'' type, as it took place inside the oval close to its poleward boundary.

The main feature in the event study is to identify a sequence of 2 pre-onset negative bays with ~15 min interval in the ground magnetic field, trace them to 2 previous convection enhancements with similar time separation and ultimately connect them with 2 southward BZ excursions seen earlier in the IMF. The authors argue that these southward turnings and associated bursts of reconnection at the magnetopause caused global oscillations in the magnetospheric cavity, which may have triggered the substorm onset.

Description of the data and events leading to the substorm are clear, and the authors have already modified and clarified the manuscript according to previous comments by 2 reviewers. However, I share some of the concern raised by the reviewer #2, that the authors may over-interpret the data when discussing ``periodic'' phenomena in the IMF, magnetosphere and ionosphere. After all, there are only 2 southward turning, 2 convection enhancements and 2 negative bays. Their time intervals are similar and their relative timings are consistent, but both the intervals and relative timings have significant uncertainty. Nevertheless, the scenario presented by the authors seems possible and is worth of wider discussion.

In summary, I feel that there are still some points that should be clarified, as detailed below, but the manuscript can be accepted after minor modifications.

MAJOR COMMENTS

As noted previously by reviewer #2, it is strange to talk about periodic structures and periodic reconnection etc. when there are only 2 instances. I recommend that you reduce talk of periodicity and instead talk about 2 structures separated by ~15 minutes. I have made several detailed comments about this below (see minor comments), but have surely missed several points. Especially in Lines 368-372 you should remind the reader that your "periodicity" is based on 2 instances.

L70: I'm not sure if Mishin et al. (2001) discussed "quasi-sinusoidal" IMF BZ variations as a necessary condition for triggering a polar substorm. They did discuss the 2-step sequence of the substorm, but I don't think quasi-sinusoidal variations were discussed. Also, the examples in Mishin et al. (2001) seem to show a large variation in timing, not always ~15 minutes. Could you clarify this?

L120-122: As mentioned by reviewer #2, the relation between equivalent current and FAC is approximate. Also Palin et al write "...FAC can sometimes be identified by a quasi‐circular clockwise (counterclockwise) equivalent current vortex...". More specifically, in order to associate curl of the equivalent current with FAC, you need to assume that conductance gradients are parallel to the electric field, see e.g. section 2 in Amm (2002) for details. Therefore I'd recommend that you change in line 120 "are manifested by" --> "can often be associated with" and add reference to Amm et al. (2002).

L259-260: The vortices are in an area where there are few magnetometers. Uncertainty in the equivalent currents increases in these areas, and in my experience there may be spurious vortex-like structures over oceans, where there are no magnetometers. Therefore I'd suggest you to re-phrase the sentence and say something like "The vortices seen in the equivalent current are consistent with downward FAC at the poleward side of the coiling structure and upward FAC equatorward of it.".

L204: Do you mean that the polar patch caused the shift in the electrojet, or that they just happened to occur at the same time?

L386-390: Is there a peak in the power spectra around 15 minutes, or did you just select the 0.8-1.7 mHz band based on earlier observations of structures separated by ~15 minutes? If the latter is true, then it would be good to check the power spectra. Looking at the magnetograms in Fig2 it seems that there could be stuff also at shorter periods.

MINOR COMMENTS

L11: Repetition period --> time interval

L31: PBIs is --> PBIs are & trigger --> triggers

L46: Pulkinen --> Pulkkinen

L71: by the --> by a

L86: Heikila --> Heikkila

L94: onset so that such --> onset, so such

L108: 15 min oscillations --> 2 structures separated by 15 min

L143: event was --> event took place & No a --> No

L185: As it was mentioned --> As mentioned

L193: So that, the increase --> This increase

section 3.2: Check references to panels in Fig 4, some of them may refer to the old version.

L201: that gives --> estimated from

L216: which --> whose

L239: repetition of variation --> interval between the two negative bays

L276-284: I had to read this couple times to get the point. I recommend you re-write it, for example: "The bottom panel in Fig.8b shows variation in the H magnetic field component at the low-latitude stations Alibag (ABG, 18.5°N, 72.9°E; geomagnetic latitude 11.65°N) located near midnight and at the dayside station San Juan (SJG, 18.1°N, 293.8°E; geomagnetic latitude 28.79°N). The increase of H-component at low latitudes in all MLT sectors is traditionally connected with the enhancement of solar wind dynamic pressurem, while decrease or disruption of the cross-tail magnetospheric current contributes to the Dst variation mainly on the nightside (Maltsev et al. 1996; Huang et al. 2004). Thus the very different magnetic field behavior seen at ABG and SJG support current disruption of the cross-tail current."

L304: of the same periodicity --> with the same time separation

L306: 15-minutes periodicity --> 15-minute separation.

L330: at latitudes --> at magnetic latitudes

L363: 15-min periodicity --> 15-min time separation

L372: period --> interval

L387: 0.8 - 1.7

L401: periodic --> two

L417 and elsewhere: use consistently either arcX or arc X

L453: repetition period --> time separation

L454: periodic reconnection --> two bursts of reconnection

L457: periodic erosion --> repeated erosion

L466: In accordance with --> According to

L467: turned out to be sensityve to the --> detected

L469 satellite --> satellite's

Fig2: Numbers in the panel showing AE index are too small. Simiarly the white text in the high resolution SOD and BAB keograms is difficult to see.

Fig 3b: Is the colorbar supposed to have another set of numbers on the left side, or why there is text "electrons" and "ions" in blue and red and numbers only in red?

Fig 5 caption: arives --> arriving

Fig 8 caption: SGN --> SJG

REFERENCES
Amm et al (2002) https://dx.doi.org/10.1029/2002JA009472

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RR by Anonymous Referee #1 (21 Apr 2020)

ED: Publish subject to revisions (further review by editor and referees) (21 Apr 2020) by Minna Palmroth

AR by Vladimir Safargaleev on behalf of the Authors (04 May 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 May 2020) by Minna Palmroth

RR by Anonymous Referee #3 (04 Jun 2020)

ED: Publish as is (22 Jun 2020) by Minna Palmroth

Short summary

Comprehensive analysis of a moderate substorm was performed using optical observations inside the auroral oval and in the polar cap, combined with data from satellites, radars, and ground magnetometers. The onset took place near the poleward boundary of the auroral oval that is not typical for classical substorms. The data fit to the near-tail current disruption scenario of the substorm onset. The role of the 15 min oscillations in the IMF Bz component in the substorm initiation is discussed.