the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Oct 2025
Observations of Fragmented Aurora-like Emissions and Picket Fence on the Poleward Edge of the Auroral Oval
Abstract. We analyzed fragmented auroral-like emissions (FAEs) and picket fence structures observed in northern Scandinavia during a magnetic storm on January 1, 2025. The analysis is based on ground-based high-sensitivity optical observations and in-situ measurements from the Swarm satellites. While FAEs and picket fences have previously been reported in the polar cap and subauroral region, respectively, this study presents the first observation of both phenomena in auroral latitudes, near the poleward edge of the oval. Ground-based camera observations revealed that some FAEs exhibited field-aligned structures and appeared simultaneously at multiple longitudinally separated locations. Furthermore, the FAEs appeared to follow the motion of red auroras, suggesting that the background electric field structure and spatial gradients in the electron density may influence their formation. Consistent with previous studies, the generation of FAEs is considered to be due to local acceleration of electrons in the ionosphere rather than electron precipitation from the magnetosphere. While we could not clearly identify the generation mechanisms, the morphological diversity observed in this event suggests that multiple plasma instabilities may be involved in the generation of both FAEs and picket fence structures.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 28 Nov 2025)
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RC1: 'Reviewer Comment on egusphere-2025-4560', Anonymous Referee #1, 07 Nov 2025
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AC1: 'Reply on RC1', Sota Nanjo, 21 Nov 2025
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The attached PDF file contains our reply to RC1, including figures. For your convenience, the text portion is copied below.
We deeply thank the reviewer for the careful and constructive assessment of our manuscript. We are also grateful for the rapid review, which enabled us to improve the paper efficiently. Below are our point-by-point responses to the comments and questions. Since this response includes figures, we would appreciate it if you could read the PDF version attached.
> Line 4, 280, 357. The authors should note that the figures in Partamies et al. (2025) do show green emissions below the continuum emissions, for example in their Figure 1a, 11a, and 12c. Their spectrograph data shows evidence of strong 557.7 nm emissions. While the simultaneous presence of the continuum and picket fence does not appear to be Partamies et al.’s focus, this information, which is available within your author team, should be acknowledged, and the claim of “first observation” should be removed.
Thank you for pointing out our insufficient explanation. We will explicitly acknowledge that Partamies et al. (2025) reported continuum emissions together with co-located green emissions in the introduction. However, all events discussed by Partamies et al. (2025) are dayside cases observed at Svalbard, whereas our study analyzes nightside events observed in Scandinavia, so the local-time context differs. Because this MLT difference may be useful for future discussion of generation mechanisms, we will make the distinction explicit by adding the term “nightside” throughout the manuscript. In addition, the statement at Line 280 concerns a feature not reported by Partamies et al. (2025)—namely, the simultaneous occurrence of FAEs at multiple longitudinally separated locations in auroral latitudes—and we would like to keep this statement.
> Line 175 The authors claim that the picket fence is field-aligned. It appears to be true for some pickets, but a significant part of the picket fence is not field-aligned. See the figure in the PDF version of the comments. The ones marked in red are tilted away from the magnetic field lines. (a) The authors should mention that a significant part of the picket fence is not field-aligned. (b) The magnetic field lines depend on the assumed emission altitude. Please describe the assumption used to draw the magnetic field lines, and discuss uncertainties of the field-alignment considering uncertainties in the altitude.
Thank you for the helpful comment, and we apologize if our previous figure caused confusion. Our manuscript reports three events (on the same night) separately. For Events 2 and 3, we do not find any field-aligned FAEs. In contrast, for Event 1, we consider that the structures are essentially field-aligned. We did not extract only those FAEs that happened to align within randomly extended structures; rather, the sequence shows orientations parallel to the local field in general.
To clarify this point, we provide a revised Figure 5 below and will state the assumptions to calculate the field lines: the lowest points (footpoints) of the field lines are placed at geographic latitude 69.6°, altitude 110 km, and geographic longitudes 19.0°–21.0° in 0.2° steps. As you noted, some FAEs in individual snapshots do not appear perfectly parallel to the model field lines. However, when following the sequence, as the FAEs propagate from west to east, their orientation tracks the change in the model field-line inclination across the FoV (features that are oblique on the western side become more nearly vertical where the local field-line direction is more vertical). Such a trend is not consistent with mechanisms that are independent of the magnetic-field direction. While some snapshots indeed show FAEs not perfectly parallel to the model field lines, the systematic alignment supports the view that a field-aligned mechanism essentially contributes to their formation. Accordingly, we wish to avoid the characterization that a ‘significant part’ of the FAEs is non-field-aligned.
> Line 190-202. Several issues exist regarding the results from Swarm. (a) Swarm did not cross FAE, and therefore the FAC, density and temperature from Swarm cannot be used to discuss the generation conditions of FAE. Swarm provides background plasma conditions at best. This limitation should be mentioned explicitly. (b) Downward region 2 FACs on the red aurora is inconsistent with the converging electric field deduced from Figure 3. The converging electric field should be connected to upward FACs, likely where the region 1 is. (c) Looking at Figure 4a, Swarm crossed the latitude of the FAE before 19:10:30 UT. The FAE location in Figure 6 should be corrected. (d) It is difficult to compare Figure 6 and 4a. Add more tick marks along the satellite trajectory of Figure 4a. Then make the location of the red aurora and FAE in Figure 6 consistent with Figure 4a.
Thank you very much for these helpful suggestions. We have revised Figure 4a and Figure 6 and reinterpreted the Swarm data accordingly, as summarized below.
(a) Scope of Swarm measurements for this study.
We agree that FAEs occurred in a very confined region and that Swarm did not cross them directly. We will state explicitly that Swarm’s measurements in this case are used only as background conditions, not to infer FAE generation directly.(b) FAC sense over the red aurora.
As seen in the revised Figure 4a, the red aurora (marked by red arrows) extends longitudinally and overlaps the Swarm tracks. Swarm A and C appear to cross the red aurora near 19:11:15 UT and 19:11:00 UT, respectively. In the revised Figure 6, this interval would correspond to the electron-density enhancement labeled "Red aurora?". In our first preprint this was labeled "FAEs?", but we now withdraw that labeling and interpret the enhancement as a crossing of the red aurora. This region corresponds to upward Region 1 FACs (see Figure 6b), addressing the inconsistency you noted.(c) FAE latitude vs. Swarm timing.
The revised Figure 4a shows that Swarm passed the latitude where FAEs later appeared at about 19:10:30 UT. However, as you correctly point out, the FAEs were spatially offset from the Swarm track. We therefore avoid using Swarm measurements to deduce FAE properties in the formal revision.(d) Figure labels.
To improve comparability, Figure 4a now includes diamonds every 30 s along the Swarm trajectories, and Figure 6 has matching vertical grid intervals (every 30 s). In addition to the figures, the main text will be revised to ensure consistency with these interpretations. We are grateful for your comments; they helped us correct our interpretation and clarify the manuscript.Data availability:
The Watec ASIs data used in this study are publicly available at http://esr.nipr.ac.jp/www/optical/watec/skb/rawdata2/. The qCMOS camera data are also available at http://gwave.cei.uec.ac.jp/~hosokawa/public/flagments/. The ASIS data are available at: https://asis.aeronomie.be/papers. The spectral riometer data are currently being processed for public release, and will be made available in the formal revision of the manuscript.
Minor comments:> Line 4, 278, 357 Line 4 and 357 state that FAEs are in the oval, but line 278 states the FAEs are poleward of the auroral oval. Please be consistent.
We will revise Lines 4, 278, and 357 to consistently describe the FAE region as near the poleward edge of the auroral oval.
> Line 24 Please provide references that show the picket fence is “usually” field-aligned. I’m only aware of the case studies by Semeter et al. If there are no references showing the usual field-aligned occurrence of the picket fence, this sentence should be rewritten to “Semeter et al. suggested that the picket fence is field-aligned.”
We will revise it as suggested.
> Line 38 Nanjo et al. (2024) did not demonstrate that the emission similar to STEVE is aurora. “Aurora” should be removed.
We will revise "auroral emissions" to "emissions" as suggested.
> Section 2 should provide references to each of the instruments, unless this is the first paper that uses the data from the instruments.
Thank you for your suggestion.
Ground-based magnetometer (IMAGE): This is part of the IMAGE magnetometer network. We will add the reference in the formal revision (10.1029/2008JA013682).
ASIS and qCMOS camera: These instruments were not developed exclusively for this study, but they have only recently become operational, and a dedicated instrument paper has not yet been published. We believe the technical specifications currently described are sufficient for interpreting the data used here.> Line 137 Auroral explosion is not a widely used term in auroral physics. Change this to “poleward expansion.”
We will revise it as suggested.
> Figure 3 and 4. It is unclear why Figure 3 presents many images without FAE. FAE is shown in only one image in Figure 4 with a gap in time from Figure 3. Please show more images between 19:08:33 and 19:11:06 UT, and describe how the red aurora changes during the FAE appearance.
Thank you for the comment. In Figure 3, we show multiple frames to illustrate the motion of the red aurora, as indicated by the white arrows. The frames between 19:08:33 UT and 19:11:06 UT that connect Figure 3 and Figure 4 are already provided as Video A1 (Supplement). As can be seen there, the red aurora exhibits no pronounced evolution during this interval; therefore, adding more still images would not materially improve clarity. We will add a brief sentence in the text describing this point.
> Line 221 Describe what assumptions were used to determine the latitude.
We did not determine the latitude by assumption. Rather, we chose the latitude slice for which the features captured from Abisko and Skibotn align most closely in projection, minimizing their separation in the longitude–altitude plane. We will explain this in the text.
> Line 278. Change “poleward side of” to “near the poleward edge of.” The poleward side means near but poleward of the auroral oval, but it is not what the authors say in the conclusion.
We will revise it as suggested.
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RC3: 'Reply on AC1', Anonymous Referee #1, 24 Nov 2025
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I appreciate the authors for considering my original comments and explaining proposed revisions. The proposed revisions for the Swarm data interpretation, data availability, and the minor comments seem appropriate. However, I’m not convinced by the responses about the comments on the prior work and the field-aligned structures. My follow-up comments are described below:
(1)
> all events discussed by Partamies et al. (2025) are dayside cases observed at SvalbardThis is not entirely true because their event in Figure 14c is on the nightside (19 MLT). Figure 14c shows picket fence-like emissions.
I’m not sure why Svalbard vs. Scandinavia can claim the uniqueness of the paper. Both papers show observations near the poleward edge of the auroral oval, providing a similar geophysical context. The latitude is different, but the oval location changes with magnetic activity.
Considering these, I still urge that the observations by Partamies et al. (2025) should be introduced and acknowledged. Although the point of your paper is the nightside observations, the common nature of the emissions near the poleward edge of the oval should be stated clearly.
(2)
> In contrast, for Event 1, we consider that the structures are essentially field-aligned. We did not extract only those FAEs that happened to align within randomly extended structures; rather, the sequence shows orientations parallel to the local field in general… As you noted, some FAEs in individual snapshots do not appear perfectly parallel to the model field lines. However, when following the sequence, as the FAEs propagate from west to east, their orientation tracks the change in the model field-line inclination across the FoVI don’t think that revised Figure 5 demonstrates this point. I have copied my markups to revised Figure 5 (see the PDF version of the comment). My previous comment pointed out that a significant part of the picket fence is not field aligned, and this version of the figure still shows the same issue. Although I agree that the orientation tracks the changes of field line orientation overall, the misalignment of a significant part of the picket fence does not support the conclusion that the picket fence for Event 1 is field-aligned.
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AC3: 'Reply on RC3', Sota Nanjo, 27 Nov 2025
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Thank you very much again for your quick and thoughtful feedback on our response. Since this response includes several figures, we would appreciate it if you could read the attached PDF version.
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AC3: 'Reply on RC3', Sota Nanjo, 27 Nov 2025
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RC3: 'Reply on AC1', Anonymous Referee #1, 24 Nov 2025
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AC1: 'Reply on RC1', Sota Nanjo, 21 Nov 2025
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RC2: 'Comment on egusphere-2025-4560', Anonymous Referee #2, 12 Nov 2025
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This manuscript reports the simultaneous observations of fragmented aurora-like emissions (FAEs) and picket fence structures at auroral latitudes near the poleward edge of the auroral oval. Using coordinated optical measurements from Skibotn, Norway, and Swarm satellite data during the January 1, 2025 magnetic storm, the study characterizes their morphology, motion, and spatial distribution. The FAEs displayed diverse forms—some aligned with magnetic field lines while others were unaligned and propagated with nearby auroral structures. The results extend the known occurrence region of FAEs and picket fences. Reviewer 1 has provided a detailed review of this manuscript, and I agree with most of the suggestions.
My main additional question and concern are about the method used to determine whether the FAEs are field-aligned. The alignment of FAEs in space is three-dimensional, whereas both the camera view and the T89 model mapping are two-dimensional. Even if the field-line mapping appears aligned with the FAEs in a 2D projection, this does not necessarily mean the FAEs are actually parallel to the field lines in 3D space, given the lack of information about the third dimension. This method can demonstrate that the FAEs are not field-aligned in Events 2 and 3, but it seems insufficient to conclude that the FAE structures are field-aligned in Event 1. In this case, it might be premature to draw the conclusion stated in Lines 358–359.
Minor issue:
Several DOI links in the reference list are incorrect, as they begin with a duplicated prefix “https://doi.org/https://doi.org…”
Citation: https://doi.org/10.5194/egusphere-2025-4560-RC2 -
AC2: 'Reply on RC2', Sota Nanjo, 21 Nov 2025
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The attached PDF file contains our reply to RC2, including figures. For your convenience, the text portion is copied below.
Thank you for the careful and constructive assessment, and for the quick turnaround. Below are our point-by-point responses to your comments and questions. As this reply includes figures, please refer to the attached PDF for proper viewing.
> My main additional question and concern are about the method used to determine whether the FAEs are field-aligned. The alignment of FAEs in space is three-dimensional, whereas both the camera view and the T89 model mapping are two-dimensional. Even if the field-line mapping appears aligned with the FAEs in a 2D projection, this does not necessarily mean the FAEs are actually parallel to the field lines in 3D space, given the lack of information about the third dimension. This method can demonstrate that the FAEs are not field-aligned in Events 2 and 3, but it seems insufficient to conclude that the FAE structures are field-aligned in Event 1. In this case, it might be premature to draw the conclusion stated in Lines 358–359.
Thank you for this important comment. This issue was partially raised by Referee #1 as well, so some of our response below necessarily overlaps.
We agree with your concern: model magnetic field lines are defined in three dimensions, whereas images by a camera are two-dimensional. Any projection of aurora from one site therefore requires an assumed emission height.
That said, the relevant heights are constrained. The green emissions typically peak near 110 km (e.g., Whiter et al., 2023). Dreyer et al. (2021) also inferred that FAEs occur at altitudes around 110 km. Also in our data, green emissions with morphology similar to Event 1 occur within 110–140 km (see Figure 8). Based on this, we drew model field lines started at altitude 110 km, geographic latitude 69.9°, and longitudes 19.0°–21.0° at 0.2° steps. As shown in the figure below, these field lines converge slightly below the FoV center (the magnetic zenith), and the FAEs in panels (b–i) appear approximately parallel to the modeled local field-lines and would converge toward a similar point. Importantly, when we vary the starting height to 100 km or 140 km, the magnetic-zenith location is essentially unchanged, and the alignment of the FAEs with the local field lines remains the same. In other words, the conclusion that the FAEs tend to follow the local field-line direction holds across a reasonable range of altitudes.
We are not identifying strict 3D distributions of emissions. Our point is that the FAEs are consistent with alignment to the local modeled field lines within a reasonable emission-height range. We will state this limitation in the main text in the formal review.
Minor issue:
We will also correct DOI links. Thank you very much for checking.Reference:
Whiter, D. K. et al.: The altitude of green OI 557.7 nm and blue N2+ 427.8 nm aurora, Ann. Geophys., 41, 1–12, https://doi.org/10.5194/angeo-41-1-2023, 2023.
Dreyer, J. et al.: Characteristics of fragmented aurora-like emissions (FAEs) observed on Svalbard, Ann. Geophys., 39, 277–288, https://doi.org/10.5194/angeo-39-277-2021, 2021
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AC2: 'Reply on RC2', Sota Nanjo, 21 Nov 2025
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The paper by Nanjo et al. reports the first observations of the FAEs and picket fence in auroral latitudes. The authors show that the FAEs are field-aligned and appear simultaneously at multiple locations. The FAEs follow the motion of red auroras. They suggest that the FAEs are caused by local acceleration of electrons. I think the observations in the paper are interesting and worth publication. However, the paper has several major issues, particularly regarding its claim about the first observation, field-aligned nature, Swarm data interpretation, and data availability. I suggest considering the comments described below before recommending publication in ANGEO.
Major comments
Line 4, 280, 357. The authors should note that the figures in Partamies et al. (2025) do show green emissions below the continuum emissions, for example in their Figure 1a, 11a, and 12c. Their spectrograph data shows evidence of strong 557.7 nm emissions. While the simultaneous presence of the continuum and picket fence does not appear to be Partamies et al.’s focus, this information, which is available within your author team, should be acknowledged, and the claim of “first observation” should be removed.
Line 175 The authors claim that the picket fence is field-aligned. It appears to be true for some pickets, but a significant part of the picket fence is not field-aligned. See the figure in the PDF version of the comments. The ones marked in red are tilted away from the magnetic field lines. (a) The authors should mention that a significant part of the picket fence is not field-aligned. (b) The magnetic field lines depend on the assumed emission altitude. Please describe the assumption used to draw the magnetic field lines, and discuss uncertainties of the field-alignment considering uncertainties in the altitude.
Line 190-202. Several issues exist regarding the results from Swarm. (a) Swarm did not cross FAE, and therefore the FAC, density and temperature from Swarm cannot be used to discuss the generation conditions of FAE. Swarm provides background plasma conditions at best. This limitation should be mentioned explicitly. (b) Downward region 2 FACs on the red aurora is inconsistent with the converging electric field deduced from Figure 3. The converging electric field should be connected to upward FACs, likely where the region 1 is. (c) Looking at Figure 4a, Swarm crossed the latitude of the FAE before 19:10:30 UT. The FAE location in Figure 6 should be corrected. (d) It is difficult to compare Figure 6 and 4a. Add more tick marks along the satellite trajectory of Figure 4a. Then make the location of the red aurora and FAE in Figure 6 consistent with Figure 4a.
Data availability. Some data in this research are only available upon request, which it does not meet ANGEO’s data policy: "Copernicus Publications requests depositing data that correspond to journal articles in reliable (public) data repositories...If the data are not publicly accessible at the time of final publication, the data statement should describe where and when they will appear...Nevertheless, authors should make such embargoed data available to reviewers during the review process in order to foster reproducibility." This data availability issue must be corrected before publication of this paper.
https://www.annales-geophysicae.net/policies/data_policy.html
Minor comments
Line 4, 278, 357 Line 4 and 357 state that FAEs are in the oval, but line 278 states the FAEs are poleward of the auroral oval. Please be consistent.
Line 24 Please provide references that show the picket fence is “usually” field-aligned. I’m only aware of the case studies by Semeter et al. If there are no references showing the usual field-aligned occurrence of the picket fence, this sentence should be rewritten to “Semeter et al. suggested that the picket fence is field-aligned.”
Line 38 Nanjo et al. (2024) did not demonstrate that the emission similar to STEVE is aurora. “Aurora” should be removed.
Section 2 should provide references to each of the instruments, unless this is the first paper that uses the data from the instruments.
Line 137 Auroral explosion is not a widely used term in auroral physics. Change this to “poleward expansion.”
Figure 3 and 4. It is unclear why Figure 3 presents many images without FAE. FAE is shown in only one image in Figure 4 with a gap in time from Figure 3. Please show more images between 19:08:33 and 19:11:06 UT, and describe how the red aurora changes during the FAE appearance.
Line 221 Describe what assumptions were used to determine the latitude.
Line 278. Change “poleward side of” to “near the poleward edge of.” The poleward side means near but poleward of the auroral oval, but it is not what the authors say in the conclusion.