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