The Morphology of Poleward Moving Auroral Forms

Goertz, Anton Oscar; Partamies, Noora; Whiter, Daniel; Baddeley, Lisa

doi:https://doi.org/10.5194/egusphere-2022-296

Preprints

https://doi.org/10.5194/egusphere-2022-296

Preprints

13 May 2022

| 13 May 2022

The Morphology of Poleward Moving Auroral Forms

Anton Oscar Goertz, Noora Partamies, Daniel Whiter, and Lisa Baddeley

Abstract. We investigated the morphology of poleward moving auroral forms (PMAFs) qualitatively by visual inspection of all sky camera (ASC) images and quantitatively using the arciness index. The PMAFs in this study were initially identified with a meridian scanning photometer located at Kjell Henriksen Observatory (KHO), Svalbard, and analyzed using ASC images taken by cameras at KHO and in Ny-Ålesund, Svalbard. We present a detailed six-step evolution of PMAF morphology in two dimensions. This evolution includes (1) an equatorward expansion of the auroral oval and an intensification of auroral brightness at the open-closed boundary, (2) the appearance of an arc-like structure in the oval, (3) poleward and possible west/eastward propagation, (4) azimuthal expansion events, (5) rebrightening of the PMAF and eventual (6) fading away. While there have been previous studies commenting on PMAF morphology this is the first work dedicated to the morphological evolution of PMAFs and it includes more detailed discussion and novel aspects, such as the observation of initial merging of 557.7 nm auroral patches to form a PMAF. Moreover, the morphology of PMAFs is quantified using the arciness index, which is a number describing how arc-like auroral forms appear in ASC images. We present the results of a superposed epoch analysis of arciness in relation to PMAF occurrence. This analysis uncovered that arciness increases suddenly during the onset of a PMAF event and decreases over the PMAF lifetime to return to its baseline value once the event has concluded. This behavior may be understood based on changes in the morphology of PMAFs and the auroral oval and furthermore may be used to identify PMAFs from arciness data.

Received: 05 May 2022 – Discussion started: 13 May 2022

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Journal article(s) based on this preprint

21 Mar 2023

Morphological evolution and spatial profile changes of poleward moving auroral forms

Anton Goertz, Noora Partamies, Daniel Whiter, and Lisa Baddeley

Ann. Geophys., 41, 115–128, https://doi.org/10.5194/angeo-41-115-2023,https://doi.org/10.5194/angeo-41-115-2023, 2023

Short summary

Anton Oscar Goertz et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-296', Anonymous Referee #1, 06 Jun 2022

Reviewer report of “The Morphology of Poleward Moving Auroral Forms” by Goertz et al.

Summary:

This paper describes the morphology of poleward moving auroral forms (PMAFs) based on the image data from all-sky cameras and tests the possibility of use of the arcness index for PMAF event identification. As for the morphology of PMAFs, they showed mostly known typical features but found a new feature, merging of auroral patches/arcs into larger scale PMAFs. They suggest that it might be explained by the localized dayside magnetopause reconnection. As for the arcness index, they concluded that the automation of PMAF detection using arcness index is difficult.

Although this paper may potentially include some new points, it is not suitable for publication for this journal unless substantial reorganization for the reason described in the below major comments.

Major comments:

1. One of the major concerns is that the purpose/motivation and new aspects of this paper are unclear. The manuscript does not explain why they examined the morphology PMAFs and why they could examine the morphology “in detail” (e.g., due to new data/method). The term “in detail” is very ambiguous, and the critical point is what those details are. As the authors say in the abstract, previous studies have examined PMAF morphology. The authors should focus more on what is different data/methods and new finding compared with those previous studies.

2. The title “Morphology of Poleward Moving Auroral Forms” is too general, and such a title is suitable for a review paper or the first report on morphology. They argued that they found a new feature, merging auroral patches/arcs into larger-scale PMAFs. If it is a really new finding, this paper should focus on it, and the title should be, for example, “Merging of auroral patches/arcs into large-scale PMAFs”. More analysis and discussion of the generation mechanism of this phenomenon are needed. Also, I wonder whether this feature is a new typical feature or a particular case of PMAFs?

3. The analysis of the arcness index is not needed for this paper since it looks like a different topic. Also, it did not help find a new morphology of PMAFs.

4. Some of their arguments are not supported by data. See the below specific comments.

Specific comments:

P5 section 4.1: The authors abruptly summarize the general morphological evolution without showing any data supporting it. Multiple examples, at least 3-4 cases, showing the general morphological evolution should be displayed before the summary. Also, what is new points that previous works have not been reported?

Fig3: I cannot see the arc moved poleward. Is this really PMAF event? The author should quantify the velocity of the move of aurora. Also, latitude-longitude grids are needed to identify right direction.

p9 L2: Where are data supporting “no relationship between PMAF occurrence time and class.”?

Fig6: Why the author uses several events not all the events?

Fig7: Why values of arcness are not shown in the figure?

P12 L14: Where are data supporting “the general evolution of arciness relating to PMAF occurrence.”?

Citation: https://doi.org/10.5194/egusphere-2022-296-RC1
- AC2:
  'Reply on RC1', Anton Goertz, 25 Aug 2022
  
  Please find our responses in the attached .pdf file. Thank you
  
  Citation: https://doi.org/10.5194/egusphere-2022-296-AC2
  - AC3: 'Reply on AC2', Anton Goertz, 09 Dec 2022
    
    Here are our responses to RC1's comments:
    Author Response to RC1
    Major comments:
    1. One of the major concerns is that the purpose/motivation and new aspects of this paper are unclear. The manuscript does not explain why they examined the morphology PMAFs and why they could examine the morphology “in detail” (e.g., due to new data/method). The term “in detail” is very ambiguous, and the critical point is what those details are. As the authors say in the abstract, previous studies have examined PMAF morphology. The authors should focus more on what is different data/methods and new finding compared with those previous studies.
    
    Thank you for bringing this up. The motivation behind this paper is that there has not been a study dedicated to the morphological evolution of PMAFs. There have only been previous studies with different focuses that commented on the morphology of PMAFs, however no detailed (as in, in-depth) study of the subject. We have listed the exact advances and novel aspects this paper introduces in response to your first specific comment. We will rework the introduction section to include the information provided in that response.
    
    Furthermore, it is not necessarily the methods and data that are inherently different from previous studies, with the exception of our analysis of the arciness index in relation to PMAF occurrence, but rather the depth of analysis.
    
    2. The title “Morphology of Poleward Moving Auroral Forms” is too general, and such a title is suitable for a review paper or the first report on morphology. They argued that they found a new feature, merging auroral patches/arcs into larger-scale PMAFs. If it is a really new finding, this paper should focus on it, and the title should be, for example, “Merging of auroral patches/arcs into large-scale PMAFs”. More analysis and discussion of the generation mechanism of this phenomenon are needed. Also, I wonder whether this feature is a new typical feature or a particular case of PMAFs?
    
    Thank you for your comment and suggestion of a new title. We chose this title since this paper is the first paper dedicated to the morphology of PMAFs. And you are right, this topic has been commented on multiple times in previous studies. However, each of those studies were not focused on PMAF morphology.
    
    Moreover, while a major new finding of our paper is the discovery of auroral patches merging before the start of a PMAF event, this is not the only new finding, as there are other important aspects we discuss. Additionally, the arciness analysis is also an important part of our paper which validates our description of PMAF morphology. Thus, it does not seem appropriate to call our paper “Merging of auroral patches into large-scale PMAFs”, as that title does not fully describe the breadth of our paper, since it introduces many new points beyond the merging of auroral patches into PMAFs. However, we are surely open to changing the title, provided the new title encompasses all parts of our study. We suggest 'Morphological evolution and spatial profile changes of poleward moving auroral forms'.
    
    We observed the merging of auroral patches into a large-scale arc in less than half of PMAFs in this study. We will make sure this is clearly mentioned in the manuscript.
    
    3. The analysis of the arcness index is not needed for this paper since it looks like a different topic. Also, it did not help find a new morphology of PMAFs.
    
    We believe the arciness analysis is a valuable part of this paper. Arciness has been used multiple times in the literature [Partamies2022, Partamies2014, Partamies2017, Partamies2017a, Partamies2015] and has been established as a useful tool to quantify auroral morphology. The major advantage arciness brings to the table is that it is a completely objective measure of auroral morphology. Beyond that, due its quantitative nature, arciness allows us to investigate the morphology of PMAFs in a statistical manner using superposed epoch analyses. Since we are able to explain the evolution of arciness based on our description of PMAF morphology, this validates our report on PMAF morphology.
    
    Specific comments:
    P5 section 4.1: The authors abruptly summarize the general morphological evolution without showing any data supporting it. Multiple examples, at least 3-4 cases, showing the general morphological evolution should be displayed before the summary. Also, what is new points that previous works have not been reported?
    
    We are happy to rearrange section 4.1 to show our summary of the morphological evolution of PMAFs after showing fig. 2 and fig. 3, which combined give three examples of PMAF morphology. Furthermore, fig. 6 gives a fourth example of the morphological evolution of PMAFs. Thus, we believe there are enough specific examples in our manuscript to support our description.
    
    The novel points this paper introduces are numerous small advances and one or two more significant conclusions rather than a single major finding. These new findings are further verified by the use of the arciness analysis, the results of which can be understood based on our description of PMAF morphology.
    
    This paper contributes to the literature by reporting novel aspects on PMAF morphology. One of the novel aspects we report is the observation that PMAFs can form following the merging of distinct auroral patches into a singular arc-like structure, the PMAF. In the literature, the beginning of a PMAF is typically described as an appearance of an auroral arc or a the intensification of the equatorward boundary of the dayside auroral oval. We propose a possible explanation for this phenomenon, which relates to the occurrence of localized dayside magnetopause reconnection, or the localized differences in reconnection efficiencies.
    
    Another new point we report is the intensification of the auroral oval happening at the open-closed boundary, as opposed to the equatorward boundary of the auroral oval. While this difference might seem semantic in nature, because those two domains overlap around magnetic noon, they are not identical. Further away from noon (9-10 and 14-15 MLT), the open closed boundary is inside the dayside auroral oval and PMAFs at those magnetic local times do not emerge from the equatorward boundary of the dayside auroral oval. The equatorward boundary of the auroral oval between dusk/dawn and magnetic noon is on closed field lines, as there can be auroral precipitation from the plasma sheet. The open-closed boundary is poleward of the equatorward boundary and it determines the location at which PMAFs first appear, as they exist exclusively on open field lines.
    
    Furthermore, we also observed PMAFs to frequently expand along the north-south dimension as they propagate poleward, specifically near the end of their lifetime. This has also never been reported before.
    
    We compare some of our conclusions to the conclusion made by Sandholt and Farrugia (2007). They reported a strong dependency in the occurrence of PMAF categories 1 and 2 on IMF By and magnetic local time, where under positive IMF By PMAF2s would almost exclusively occur in the pre-noon sector, while under the same IMF conditions observations of PMAF1s would strongly outnumber those of PMAF2s in the post-noon sector. We have found no such relationship. In our data, we observe approximately 70% PMAF2s and 30% PMAF1s, with no apparent preference for pre-noon or post-noon occurrence under any IMF conditions.
    
    Sandholt and Farrugia also reported observations of poleward boundary intensifications, as opposed to equatorward boundary intensifications, for PMAFs occurring in the post-noon sector under IMF By>0 and in the pre-noon sector under IMF By<0. While we did not observe intensifications of the auroral oval at its poleward boundary, we did frequently observe intensifications inside the auroral oval and hence poleward of the equatorward boundary. This relates to our previous finding of PMAFs emerging from the open-closed boundary, rather than the equatorward boundary of the auroral oval.
    
    They were also the first to report that PMAF2 events would have a higher green auroral emission component after the rebrigthening event close to the end of the PMAF lifetime. We can confirm this finding based on our data and interpret it according to the proposed re-brightening mechanism proposed by [Fasel1992].
    
    Fig3: I cannot see the arc moved poleward. Is this really PMAF event? The author should quantify the velocity of the move of aurora. Also, latitude-longitude grids are needed to identify right direction.
    
    Thank you for pointing this out. The poleward motion is in fact hard to make out from the ASC image series. This is partly due to the fact that the westward motion is dominant over the poleward propagation. Here is a keogram that includes the event time frame. The black lines show approximate locations of two PMAF events that are visible in the ASC image series in fig. 2. We can include the keogram in our manuscript.
    
    p9 L2: Where are data supporting “no relationship between PMAF occurrence time and class.”?
    
    Thank you for this question. We have not included any raw data on PMAF class for each event in the manuscript. As an example, here are the PMAFs of event list 2 sorted by PMAF class and start time (times are in UT, MLT ≈ UT + 3):
    
    PMAF1: (06:31, 06:56, 07:11, 07:25, 07:51, 08:12, 08:19, 08:52, 09:25, 09:29)
    
    PMAF2: (06:16, 06:25, 06:50, 07:16, 07:35, 07:46, 07:56, 08:00, 08:39, 09:06, 09:17, 09:35)
    
    Evidently, there is no preference for either class in the pre-/post-noon sector in our data, as has been reported by [Sandholt2007]. We will gladly include this data in our manuscript.
    
    Fig6: Why the author uses several events not all the events?
    
    Thank you for catching this. These plots were made with all events. However, you are right that the use of the word 'several' is confusing here, and has been changed to 'all'.
    
    Fig7: Why values of arcness are not shown in the figure? P12 L14: Where are data supporting “the general evolution of arciness relating to PMAF occurrence.”?
    
    That paragraph refers to the general evolution of arciness over the course of PMAF events, and how that evolution relates to the general evolution of the morphology of PMAFs. We use fig. 7 as an example to map actual ASC images to approximate times in the arciness evolution (fig. 6) and to give another example of PMAF morphology. Hence, the data is contained in fig. 6. We do not show arciness values for the ASC images in fig. 7 as it is merely an example, and cannot be used to validate the results of a statistical analysis, the results of which are plotted in fig. 6.
    
    References
    
    [Fasel et al., 1992] Fasel, G. J., Minow, J. I., Smith, R. W., Deehr, C. S., and Lee, L. C. (1992). Mul-
    
    tiple brightenings of transient dayside auroral forms during oval expansions. Geophysical Research
    
    Letters, 19(24):2429?2432.
    
    [Partamies et al., 2015] Partamies, N., Juusola, L., Whiter, D., and Kauristie, K. (2015). Substorm
    
    evolution of auroral structures. Journal of Geophysical Research: Space Physics, 120(7):5958?5972.
    
    [Partamies et al., 2017a] Partamies, N., Weygand, J. M., and Juusola, L. (2017a). Statistical study of
    
    auroral omega bands. Annales Geophysicae, 35(5):1069?1083.
    
    [Partamies et al., 2017b] Partamies, N., Whiter, D., Kadokura, A., Kauristie, K., Tyssøy, H. N., Mas-
    
    setti, S., Stauning, P., and Raita, T. (2017b). Occurrence and average behavior of pulsating aurora.
    
    Journal of Geophysical Research: Space Physics, 122(5):5606?5618.
    
    [Partamies et al., 2022] Partamies, N., Whiter, D., Kauristie, K., and Massetti, S. (2022). Local time
    
    dependence of auroral peak emission height and morphology. Ann. Geophys. Discuss.
    
    [Partamies et al., 2014] Partamies, N., Whiter, D., Syrjaesuo, M., and Kauristie, K. (2014). Solar
    
    cycle and diurnal dependence of auroral structures. Journal of Geophysical Research: Space Physics,
    
    119(10):8448?8461.
    
    [Sandholt and Farrugia, 2007] Sandholt, P. E. and Farrugia, C. J. (2007). Poleward moving auroral
    
    forms (PMAFs) revisited: responses of aurorae, plasma convection and birkeland currents in the
    
    pre- and postnoon sectors under positive and negative IMF by conditions. Annales Geophysicae,
    
    25(7):1629?1652.
    
    Citation: https://doi.org/10.5194/egusphere-2022-296-AC3
RC2:
'Comment on egusphere-2022-296', Anonymous Referee #2, 27 Jun 2022

Reviewer report of “The Morphology of Poleward Moving Auroral Forms” by Anton Goertz, Noora Partamies, Daniel Whiter, and Lisa Baddeley

In this study, the authors observed carefully the spatial and temporal morphological profiles of poleward moving auroral forms (PMAFs) based on the data obtained from the scanning photometer and all-sky camera image data and found new feature on PMAFs; the merging of auroral patches into a singular arc-like structure, which can be considered that auroral structures with the order of ~ 100 kilometers can be led to form a PMAF within a large-scale physics. This merging might be addressed by the localized dayside magnetopause reconnection.

This reviewer considers that some new features previously unrevealed can be explained by this study, but this paper does not yet reach the publication level in a present form because of following reasons.

Major comments:

1) On the use of terminology of “morphology”;

This reviewer considers that this terminology is frequently used to point out “form” and “shape” of thing in our field. If you say, “the morphology of Poleward Moving Auroral Forms”, readers may think that PMAFs have various kinds of shape or form. However, the authors discuss that PMAFs, which temporally and spatially changed, in this paper. The implication of “morphology” used in this study seems to be wrong. Then, this reviewer suggests replacing “morphology” with the other word, such as “temporal and spatial (profile) changes”.ãIn particular, this reviewer strongly felt that this section did state “temporal and spatial changes of auroral arcs (or PMAFs, but it actually remains question whether or not the whole process as shown in Figure 2 is “PMAFs”). The title should also be changed. This reviewer’s candidate is “Temporal and Spatial (Profile) Changes of Poleward Moving Aurora Forms: Observations Based on All-sky Camera and Scanning Photometer (at Svalbard)”.

2) The database compiling;

In this study, the authors promote the discussion on the PMAFs using two databases; one is the statistical database of PMAFs which were detected from 2003 to 2008, and another is the unusual and multiple PMAF events occurred on 18^th December 2017.

The 1^st database includes each individual PMAF event occurred in a day from 2003 to 2008? If only one PMAF event usually occurs in a day, the 2^nd database should be identified as “anomalous” event. If so, can the authors discuss these two databased within the same work frame? The physics, such as formation mechanism, IMF conditions, and background magnetic field/plasma characteristics during the 2^nd PMAF event might be different from the usual PMAFs (the 1^st PMAF database)?

3) The data structure;

“Depending on the exact lifetime of each individual PMAF we used 1-2 minute time bins of the arciness data for the SEA. This corresponds to 3-4 time bins per event lifetime with about 5-10 data points per bin per PMAF.” The relation between the time resolution of PMAF all-sky image data and arciness time bins is unclear and so complicated. Please explain more clearly with an illustration or rewrite this sentence more detailed.

4) Dayside reconnection evidence;

The authors assert through this study that PMAFs can closely be connected (linked) with dayside reconnection based on the previous studies. However, in this study, you do not show any clear observational evidence for the occurrence of dayside magnetic reconnection associated with PMAFs. The reconnection evidence can be obtained from in-situ space-based and remotely ground-based observations, such as the HF radar arrays (SuperDARN radars). At least, the authors should show some examples (data) of dayside reconnection evidence, if the PMAFs are associated with the dayside magnetospheric processes.

5) Statistics of arciness;

The tendency as shown in Figure 6 is varied depending on the IMF and solar wind conditions? Although the authors show the average profiles of IMF and solar wind plasma, actually, the PMAF events should occur under various solar wind conditions. If the authors try to examine statistical characteristics of arciness, the PMAF data under the specific or average IMF-By and -Bz and solar wind plasma conditions (as seen in Table 1) were used? Although the authors tell that “SEA was employed to analyze the behavior of narrow-band arciness during the 23 PMAFs in the first event list.” and “the same analysis was conducted with the second event list, which includes 18 events that occurred on a single day”, these PMAFs (23 events in first event list and 18 events in the second list) were occurring under the similar solar wind and IMF conditions or average solar wind conditions as shown in Table 1?

Minor comments:

1) In Abstract and everywhere: What is the definition of “open-closed boundary”? Is it the same region as the poleward edge of the main aurora oval?

2) Table 1; Why don’t the authors show the average value of plasma number density (N_p)? This reviewer considers that the solar wind density is more effective parameter in auroral phenomena than the solar wind temperature (T_sw).

3) Section 4.1; Here, the authors tried to state the profile changes of PMAFs, but this reviewer feels just like reading several sentences as written in the research note. In these items, there are some PMAF signatures that have already well-known. The reviewer recommends re-organizing or re-structuring this section. In order to concisely and shortly show these series of spatial and temporal PMAF change flows, how about illustrating these using the block diagrams?

4) “The merging of auroral patches into a singular structure is interpreted in two different ways depending on the scale of the auroral patches.” “Patches on the order of tens of kilometers separated by similarly sized regions devoid of 557.7nm aurora may be the ionospheric manifestation of inhomogeneities in the spatial distribution of solar wind particles.”

Can you provide the associated references? Or these are your considerations? If the latter case, why can you consider these?

5) What is “PMAF1 category” and “PMAF2 category”? What do PMAFs 1 and 2 have the significant characteristics?

6) What is the definition of “re-brightening events”? Please explain these phenomena more clearly.

7) Figures 2, 3 and 4; The explanations of these two figures are complicated. The author should show the time on the top of each panel, such as “(a) 6:15:13 UT (b) 6:15:35 UT…”. In particular, in Figures 2 and 3, the title should be put. For example, “ASC images on 18^th December 2017”. In relation to this, this reviewer recommends that the authors should put a movie of ASC during the time intervals when you are discussing here (18^th December 2017) as “supplementary information”.

8) Figure 4; What is (are) the color code (colored regions and curves) assigned? The highness of arciness index? If so, please put a color bar to easily understand what color shows. Please explain more clearly how the colored region shown in the center column, and colored curves in the right column were calculated.

9) Figure 5; This reviewer cannot find the red part. Maybe you changed the color from red to black?

10) Figure 6; What do the labels from (a) to (f) seen in Figures 6 mean? This reviewer cannot find the explanations (notations) on these labels in the manuscript. Maybe, these labels are related to Figure 7?

11) In relation to 10) and Figure 7; Why do the authors need to independently show the PMAF’s images here? Readers must become confused. If the authors want to discuss the arciness index variations associated with the PMAF’s evolution, they should discuss this with a combination of Figure 6 with Figure 7. This reviewer recommends re-organizing these figures and associated sentences (paragraphs).

12) Section 5 (pp.13, L5 and L11); morphological evolution of PMAFs --> temporal and spatial profile changes of PMAFs.

              PMAF morphology --> A series of temporal and spatial PMAF changes

13) Section 5 (pp.13, L12); which --> where or that

14) Section 4.2 (pp.12, L16) and Section 5 (pp.13, Ls 28 – 31); This reviewer considers that the automation of PMAF detection might has already been started to be developed based on machine learning technique (e.g., Convolutional Neural Network; CNN). Do you have any opinions on the event search using machine learning? If yes, you also should discuss the relation between your opinion and machine learning technique in the manuscript.

There are several recent reports that, with a help of machine learning technique, the auroras detected by the all-sky camera can automatically be categorized. The corresponding links are shown as follows.

1.https://www.nature.com/articles/s41598-022-11686-8

2.https://tromsoe-ai.cei.uec.ac.jp/#/

3.https://www.sciencedirect.com/science/article/pii/S1364682622000797?via%3Dihub#fig1

This reviewer considers that your research results and principle of the PMAF event search can be implemented to these algorithms. However, on the other hand, independently, will the authors build some system to automatically detect the PMAFs in near future?

----------------------------------------

The PDF format version of this reviewer's report was attached. Please also refer to this file.

Citation: https://doi.org/10.5194/egusphere-2022-296-RC2
- AC1: 'Reply on RC2', Anton Goertz, 25 Aug 2022
  
  Please find our responses in the attached .pdf file. Thank you
  
  Citation: https://doi.org/10.5194/egusphere-2022-296-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-296', Anonymous Referee #1, 06 Jun 2022

Reviewer report of “The Morphology of Poleward Moving Auroral Forms” by Goertz et al.

Summary:

This paper describes the morphology of poleward moving auroral forms (PMAFs) based on the image data from all-sky cameras and tests the possibility of use of the arcness index for PMAF event identification. As for the morphology of PMAFs, they showed mostly known typical features but found a new feature, merging of auroral patches/arcs into larger scale PMAFs. They suggest that it might be explained by the localized dayside magnetopause reconnection. As for the arcness index, they concluded that the automation of PMAF detection using arcness index is difficult.

Although this paper may potentially include some new points, it is not suitable for publication for this journal unless substantial reorganization for the reason described in the below major comments.

Major comments:

1. One of the major concerns is that the purpose/motivation and new aspects of this paper are unclear. The manuscript does not explain why they examined the morphology PMAFs and why they could examine the morphology “in detail” (e.g., due to new data/method). The term “in detail” is very ambiguous, and the critical point is what those details are. As the authors say in the abstract, previous studies have examined PMAF morphology. The authors should focus more on what is different data/methods and new finding compared with those previous studies.

2. The title “Morphology of Poleward Moving Auroral Forms” is too general, and such a title is suitable for a review paper or the first report on morphology. They argued that they found a new feature, merging auroral patches/arcs into larger-scale PMAFs. If it is a really new finding, this paper should focus on it, and the title should be, for example, “Merging of auroral patches/arcs into large-scale PMAFs”. More analysis and discussion of the generation mechanism of this phenomenon are needed. Also, I wonder whether this feature is a new typical feature or a particular case of PMAFs?

3. The analysis of the arcness index is not needed for this paper since it looks like a different topic. Also, it did not help find a new morphology of PMAFs.

4. Some of their arguments are not supported by data. See the below specific comments.

Specific comments:

P5 section 4.1: The authors abruptly summarize the general morphological evolution without showing any data supporting it. Multiple examples, at least 3-4 cases, showing the general morphological evolution should be displayed before the summary. Also, what is new points that previous works have not been reported?

Fig3: I cannot see the arc moved poleward. Is this really PMAF event? The author should quantify the velocity of the move of aurora. Also, latitude-longitude grids are needed to identify right direction.

p9 L2: Where are data supporting “no relationship between PMAF occurrence time and class.”?

Fig6: Why the author uses several events not all the events?

Fig7: Why values of arcness are not shown in the figure?

P12 L14: Where are data supporting “the general evolution of arciness relating to PMAF occurrence.”?

Citation: https://doi.org/10.5194/egusphere-2022-296-RC1
- AC2:
  'Reply on RC1', Anton Goertz, 25 Aug 2022
  
  Please find our responses in the attached .pdf file. Thank you
  
  Citation: https://doi.org/10.5194/egusphere-2022-296-AC2
  - AC3: 'Reply on AC2', Anton Goertz, 09 Dec 2022
    
    Here are our responses to RC1's comments:
    Author Response to RC1
    Major comments:
    1. One of the major concerns is that the purpose/motivation and new aspects of this paper are unclear. The manuscript does not explain why they examined the morphology PMAFs and why they could examine the morphology “in detail” (e.g., due to new data/method). The term “in detail” is very ambiguous, and the critical point is what those details are. As the authors say in the abstract, previous studies have examined PMAF morphology. The authors should focus more on what is different data/methods and new finding compared with those previous studies.
    
    Thank you for bringing this up. The motivation behind this paper is that there has not been a study dedicated to the morphological evolution of PMAFs. There have only been previous studies with different focuses that commented on the morphology of PMAFs, however no detailed (as in, in-depth) study of the subject. We have listed the exact advances and novel aspects this paper introduces in response to your first specific comment. We will rework the introduction section to include the information provided in that response.
    
    Furthermore, it is not necessarily the methods and data that are inherently different from previous studies, with the exception of our analysis of the arciness index in relation to PMAF occurrence, but rather the depth of analysis.
    
    2. The title “Morphology of Poleward Moving Auroral Forms” is too general, and such a title is suitable for a review paper or the first report on morphology. They argued that they found a new feature, merging auroral patches/arcs into larger-scale PMAFs. If it is a really new finding, this paper should focus on it, and the title should be, for example, “Merging of auroral patches/arcs into large-scale PMAFs”. More analysis and discussion of the generation mechanism of this phenomenon are needed. Also, I wonder whether this feature is a new typical feature or a particular case of PMAFs?
    
    Thank you for your comment and suggestion of a new title. We chose this title since this paper is the first paper dedicated to the morphology of PMAFs. And you are right, this topic has been commented on multiple times in previous studies. However, each of those studies were not focused on PMAF morphology.
    
    Moreover, while a major new finding of our paper is the discovery of auroral patches merging before the start of a PMAF event, this is not the only new finding, as there are other important aspects we discuss. Additionally, the arciness analysis is also an important part of our paper which validates our description of PMAF morphology. Thus, it does not seem appropriate to call our paper “Merging of auroral patches into large-scale PMAFs”, as that title does not fully describe the breadth of our paper, since it introduces many new points beyond the merging of auroral patches into PMAFs. However, we are surely open to changing the title, provided the new title encompasses all parts of our study. We suggest 'Morphological evolution and spatial profile changes of poleward moving auroral forms'.
    
    We observed the merging of auroral patches into a large-scale arc in less than half of PMAFs in this study. We will make sure this is clearly mentioned in the manuscript.
    
    3. The analysis of the arcness index is not needed for this paper since it looks like a different topic. Also, it did not help find a new morphology of PMAFs.
    
    We believe the arciness analysis is a valuable part of this paper. Arciness has been used multiple times in the literature [Partamies2022, Partamies2014, Partamies2017, Partamies2017a, Partamies2015] and has been established as a useful tool to quantify auroral morphology. The major advantage arciness brings to the table is that it is a completely objective measure of auroral morphology. Beyond that, due its quantitative nature, arciness allows us to investigate the morphology of PMAFs in a statistical manner using superposed epoch analyses. Since we are able to explain the evolution of arciness based on our description of PMAF morphology, this validates our report on PMAF morphology.
    
    Specific comments:
    P5 section 4.1: The authors abruptly summarize the general morphological evolution without showing any data supporting it. Multiple examples, at least 3-4 cases, showing the general morphological evolution should be displayed before the summary. Also, what is new points that previous works have not been reported?
    
    We are happy to rearrange section 4.1 to show our summary of the morphological evolution of PMAFs after showing fig. 2 and fig. 3, which combined give three examples of PMAF morphology. Furthermore, fig. 6 gives a fourth example of the morphological evolution of PMAFs. Thus, we believe there are enough specific examples in our manuscript to support our description.
    
    The novel points this paper introduces are numerous small advances and one or two more significant conclusions rather than a single major finding. These new findings are further verified by the use of the arciness analysis, the results of which can be understood based on our description of PMAF morphology.
    
    This paper contributes to the literature by reporting novel aspects on PMAF morphology. One of the novel aspects we report is the observation that PMAFs can form following the merging of distinct auroral patches into a singular arc-like structure, the PMAF. In the literature, the beginning of a PMAF is typically described as an appearance of an auroral arc or a the intensification of the equatorward boundary of the dayside auroral oval. We propose a possible explanation for this phenomenon, which relates to the occurrence of localized dayside magnetopause reconnection, or the localized differences in reconnection efficiencies.
    
    Another new point we report is the intensification of the auroral oval happening at the open-closed boundary, as opposed to the equatorward boundary of the auroral oval. While this difference might seem semantic in nature, because those two domains overlap around magnetic noon, they are not identical. Further away from noon (9-10 and 14-15 MLT), the open closed boundary is inside the dayside auroral oval and PMAFs at those magnetic local times do not emerge from the equatorward boundary of the dayside auroral oval. The equatorward boundary of the auroral oval between dusk/dawn and magnetic noon is on closed field lines, as there can be auroral precipitation from the plasma sheet. The open-closed boundary is poleward of the equatorward boundary and it determines the location at which PMAFs first appear, as they exist exclusively on open field lines.
    
    Furthermore, we also observed PMAFs to frequently expand along the north-south dimension as they propagate poleward, specifically near the end of their lifetime. This has also never been reported before.
    
    We compare some of our conclusions to the conclusion made by Sandholt and Farrugia (2007). They reported a strong dependency in the occurrence of PMAF categories 1 and 2 on IMF By and magnetic local time, where under positive IMF By PMAF2s would almost exclusively occur in the pre-noon sector, while under the same IMF conditions observations of PMAF1s would strongly outnumber those of PMAF2s in the post-noon sector. We have found no such relationship. In our data, we observe approximately 70% PMAF2s and 30% PMAF1s, with no apparent preference for pre-noon or post-noon occurrence under any IMF conditions.
    
    Sandholt and Farrugia also reported observations of poleward boundary intensifications, as opposed to equatorward boundary intensifications, for PMAFs occurring in the post-noon sector under IMF By>0 and in the pre-noon sector under IMF By<0. While we did not observe intensifications of the auroral oval at its poleward boundary, we did frequently observe intensifications inside the auroral oval and hence poleward of the equatorward boundary. This relates to our previous finding of PMAFs emerging from the open-closed boundary, rather than the equatorward boundary of the auroral oval.
    
    They were also the first to report that PMAF2 events would have a higher green auroral emission component after the rebrigthening event close to the end of the PMAF lifetime. We can confirm this finding based on our data and interpret it according to the proposed re-brightening mechanism proposed by [Fasel1992].
    
    Fig3: I cannot see the arc moved poleward. Is this really PMAF event? The author should quantify the velocity of the move of aurora. Also, latitude-longitude grids are needed to identify right direction.
    
    Thank you for pointing this out. The poleward motion is in fact hard to make out from the ASC image series. This is partly due to the fact that the westward motion is dominant over the poleward propagation. Here is a keogram that includes the event time frame. The black lines show approximate locations of two PMAF events that are visible in the ASC image series in fig. 2. We can include the keogram in our manuscript.
    
    p9 L2: Where are data supporting “no relationship between PMAF occurrence time and class.”?
    
    Thank you for this question. We have not included any raw data on PMAF class for each event in the manuscript. As an example, here are the PMAFs of event list 2 sorted by PMAF class and start time (times are in UT, MLT ≈ UT + 3):
    
    PMAF1: (06:31, 06:56, 07:11, 07:25, 07:51, 08:12, 08:19, 08:52, 09:25, 09:29)
    
    PMAF2: (06:16, 06:25, 06:50, 07:16, 07:35, 07:46, 07:56, 08:00, 08:39, 09:06, 09:17, 09:35)
    
    Evidently, there is no preference for either class in the pre-/post-noon sector in our data, as has been reported by [Sandholt2007]. We will gladly include this data in our manuscript.
    
    Fig6: Why the author uses several events not all the events?
    
    Thank you for catching this. These plots were made with all events. However, you are right that the use of the word 'several' is confusing here, and has been changed to 'all'.
    
    Fig7: Why values of arcness are not shown in the figure? P12 L14: Where are data supporting “the general evolution of arciness relating to PMAF occurrence.”?
    
    That paragraph refers to the general evolution of arciness over the course of PMAF events, and how that evolution relates to the general evolution of the morphology of PMAFs. We use fig. 7 as an example to map actual ASC images to approximate times in the arciness evolution (fig. 6) and to give another example of PMAF morphology. Hence, the data is contained in fig. 6. We do not show arciness values for the ASC images in fig. 7 as it is merely an example, and cannot be used to validate the results of a statistical analysis, the results of which are plotted in fig. 6.
    
    References
    
    [Fasel et al., 1992] Fasel, G. J., Minow, J. I., Smith, R. W., Deehr, C. S., and Lee, L. C. (1992). Mul-
    
    tiple brightenings of transient dayside auroral forms during oval expansions. Geophysical Research
    
    Letters, 19(24):2429?2432.
    
    [Partamies et al., 2015] Partamies, N., Juusola, L., Whiter, D., and Kauristie, K. (2015). Substorm
    
    evolution of auroral structures. Journal of Geophysical Research: Space Physics, 120(7):5958?5972.
    
    [Partamies et al., 2017a] Partamies, N., Weygand, J. M., and Juusola, L. (2017a). Statistical study of
    
    auroral omega bands. Annales Geophysicae, 35(5):1069?1083.
    
    [Partamies et al., 2017b] Partamies, N., Whiter, D., Kadokura, A., Kauristie, K., Tyssøy, H. N., Mas-
    
    setti, S., Stauning, P., and Raita, T. (2017b). Occurrence and average behavior of pulsating aurora.
    
    Journal of Geophysical Research: Space Physics, 122(5):5606?5618.
    
    [Partamies et al., 2022] Partamies, N., Whiter, D., Kauristie, K., and Massetti, S. (2022). Local time
    
    dependence of auroral peak emission height and morphology. Ann. Geophys. Discuss.
    
    [Partamies et al., 2014] Partamies, N., Whiter, D., Syrjaesuo, M., and Kauristie, K. (2014). Solar
    
    cycle and diurnal dependence of auroral structures. Journal of Geophysical Research: Space Physics,
    
    119(10):8448?8461.
    
    [Sandholt and Farrugia, 2007] Sandholt, P. E. and Farrugia, C. J. (2007). Poleward moving auroral
    
    forms (PMAFs) revisited: responses of aurorae, plasma convection and birkeland currents in the
    
    pre- and postnoon sectors under positive and negative IMF by conditions. Annales Geophysicae,
    
    25(7):1629?1652.
    
    Citation: https://doi.org/10.5194/egusphere-2022-296-AC3
RC2:
'Comment on egusphere-2022-296', Anonymous Referee #2, 27 Jun 2022

Reviewer report of “The Morphology of Poleward Moving Auroral Forms” by Anton Goertz, Noora Partamies, Daniel Whiter, and Lisa Baddeley

In this study, the authors observed carefully the spatial and temporal morphological profiles of poleward moving auroral forms (PMAFs) based on the data obtained from the scanning photometer and all-sky camera image data and found new feature on PMAFs; the merging of auroral patches into a singular arc-like structure, which can be considered that auroral structures with the order of ~ 100 kilometers can be led to form a PMAF within a large-scale physics. This merging might be addressed by the localized dayside magnetopause reconnection.

This reviewer considers that some new features previously unrevealed can be explained by this study, but this paper does not yet reach the publication level in a present form because of following reasons.

Major comments:

1) On the use of terminology of “morphology”;

This reviewer considers that this terminology is frequently used to point out “form” and “shape” of thing in our field. If you say, “the morphology of Poleward Moving Auroral Forms”, readers may think that PMAFs have various kinds of shape or form. However, the authors discuss that PMAFs, which temporally and spatially changed, in this paper. The implication of “morphology” used in this study seems to be wrong. Then, this reviewer suggests replacing “morphology” with the other word, such as “temporal and spatial (profile) changes”.ãIn particular, this reviewer strongly felt that this section did state “temporal and spatial changes of auroral arcs (or PMAFs, but it actually remains question whether or not the whole process as shown in Figure 2 is “PMAFs”). The title should also be changed. This reviewer’s candidate is “Temporal and Spatial (Profile) Changes of Poleward Moving Aurora Forms: Observations Based on All-sky Camera and Scanning Photometer (at Svalbard)”.

2) The database compiling;

In this study, the authors promote the discussion on the PMAFs using two databases; one is the statistical database of PMAFs which were detected from 2003 to 2008, and another is the unusual and multiple PMAF events occurred on 18^th December 2017.

The 1^st database includes each individual PMAF event occurred in a day from 2003 to 2008? If only one PMAF event usually occurs in a day, the 2^nd database should be identified as “anomalous” event. If so, can the authors discuss these two databased within the same work frame? The physics, such as formation mechanism, IMF conditions, and background magnetic field/plasma characteristics during the 2^nd PMAF event might be different from the usual PMAFs (the 1^st PMAF database)?

3) The data structure;

“Depending on the exact lifetime of each individual PMAF we used 1-2 minute time bins of the arciness data for the SEA. This corresponds to 3-4 time bins per event lifetime with about 5-10 data points per bin per PMAF.” The relation between the time resolution of PMAF all-sky image data and arciness time bins is unclear and so complicated. Please explain more clearly with an illustration or rewrite this sentence more detailed.

4) Dayside reconnection evidence;

The authors assert through this study that PMAFs can closely be connected (linked) with dayside reconnection based on the previous studies. However, in this study, you do not show any clear observational evidence for the occurrence of dayside magnetic reconnection associated with PMAFs. The reconnection evidence can be obtained from in-situ space-based and remotely ground-based observations, such as the HF radar arrays (SuperDARN radars). At least, the authors should show some examples (data) of dayside reconnection evidence, if the PMAFs are associated with the dayside magnetospheric processes.

5) Statistics of arciness;

The tendency as shown in Figure 6 is varied depending on the IMF and solar wind conditions? Although the authors show the average profiles of IMF and solar wind plasma, actually, the PMAF events should occur under various solar wind conditions. If the authors try to examine statistical characteristics of arciness, the PMAF data under the specific or average IMF-By and -Bz and solar wind plasma conditions (as seen in Table 1) were used? Although the authors tell that “SEA was employed to analyze the behavior of narrow-band arciness during the 23 PMAFs in the first event list.” and “the same analysis was conducted with the second event list, which includes 18 events that occurred on a single day”, these PMAFs (23 events in first event list and 18 events in the second list) were occurring under the similar solar wind and IMF conditions or average solar wind conditions as shown in Table 1?

Minor comments:

1) In Abstract and everywhere: What is the definition of “open-closed boundary”? Is it the same region as the poleward edge of the main aurora oval?

2) Table 1; Why don’t the authors show the average value of plasma number density (N_p)? This reviewer considers that the solar wind density is more effective parameter in auroral phenomena than the solar wind temperature (T_sw).

3) Section 4.1; Here, the authors tried to state the profile changes of PMAFs, but this reviewer feels just like reading several sentences as written in the research note. In these items, there are some PMAF signatures that have already well-known. The reviewer recommends re-organizing or re-structuring this section. In order to concisely and shortly show these series of spatial and temporal PMAF change flows, how about illustrating these using the block diagrams?

4) “The merging of auroral patches into a singular structure is interpreted in two different ways depending on the scale of the auroral patches.” “Patches on the order of tens of kilometers separated by similarly sized regions devoid of 557.7nm aurora may be the ionospheric manifestation of inhomogeneities in the spatial distribution of solar wind particles.”

Can you provide the associated references? Or these are your considerations? If the latter case, why can you consider these?

5) What is “PMAF1 category” and “PMAF2 category”? What do PMAFs 1 and 2 have the significant characteristics?

6) What is the definition of “re-brightening events”? Please explain these phenomena more clearly.

7) Figures 2, 3 and 4; The explanations of these two figures are complicated. The author should show the time on the top of each panel, such as “(a) 6:15:13 UT (b) 6:15:35 UT…”. In particular, in Figures 2 and 3, the title should be put. For example, “ASC images on 18^th December 2017”. In relation to this, this reviewer recommends that the authors should put a movie of ASC during the time intervals when you are discussing here (18^th December 2017) as “supplementary information”.

8) Figure 4; What is (are) the color code (colored regions and curves) assigned? The highness of arciness index? If so, please put a color bar to easily understand what color shows. Please explain more clearly how the colored region shown in the center column, and colored curves in the right column were calculated.

9) Figure 5; This reviewer cannot find the red part. Maybe you changed the color from red to black?

10) Figure 6; What do the labels from (a) to (f) seen in Figures 6 mean? This reviewer cannot find the explanations (notations) on these labels in the manuscript. Maybe, these labels are related to Figure 7?

11) In relation to 10) and Figure 7; Why do the authors need to independently show the PMAF’s images here? Readers must become confused. If the authors want to discuss the arciness index variations associated with the PMAF’s evolution, they should discuss this with a combination of Figure 6 with Figure 7. This reviewer recommends re-organizing these figures and associated sentences (paragraphs).

12) Section 5 (pp.13, L5 and L11); morphological evolution of PMAFs --> temporal and spatial profile changes of PMAFs.

              PMAF morphology --> A series of temporal and spatial PMAF changes

13) Section 5 (pp.13, L12); which --> where or that

14) Section 4.2 (pp.12, L16) and Section 5 (pp.13, Ls 28 – 31); This reviewer considers that the automation of PMAF detection might has already been started to be developed based on machine learning technique (e.g., Convolutional Neural Network; CNN). Do you have any opinions on the event search using machine learning? If yes, you also should discuss the relation between your opinion and machine learning technique in the manuscript.

There are several recent reports that, with a help of machine learning technique, the auroras detected by the all-sky camera can automatically be categorized. The corresponding links are shown as follows.

1.https://www.nature.com/articles/s41598-022-11686-8

2.https://tromsoe-ai.cei.uec.ac.jp/#/

3.https://www.sciencedirect.com/science/article/pii/S1364682622000797?via%3Dihub#fig1

This reviewer considers that your research results and principle of the PMAF event search can be implemented to these algorithms. However, on the other hand, independently, will the authors build some system to automatically detect the PMAFs in near future?

----------------------------------------

The PDF format version of this reviewer's report was attached. Please also refer to this file.

Citation: https://doi.org/10.5194/egusphere-2022-296-RC2
- AC1: 'Reply on RC2', Anton Goertz, 25 Aug 2022
  
  Please find our responses in the attached .pdf file. Thank you
  
  Citation: https://doi.org/10.5194/egusphere-2022-296-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to revisions (further review by editor and referees) (21 Sep 2022) by Dalia Buresova

AR by Anton Goertz on behalf of the Authors (08 Oct 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (24 Oct 2022) by Dalia Buresova

RR by Anonymous Referee #2 (26 Nov 2022)

RR by Anonymous Referee #1 (13 Dec 2022)

ED: Publish subject to minor revisions (review by editor) (15 Dec 2022) by Dalia Buresova

Dear Dr. Goertz, dear co-authors,

Thank you for your work you did when improving the manuscript according to the referees’ comments and suggestions. As you know, the manuscript has underdone the second revision. After the revision, the submission’s status changed from "major revision" to "minor revision". Please, consider carefully the additional comments of the referee (please, see below) and send us the revised version with the changes highlighted. I believe that this will be a last step for the manuscript to be recommended for publication.

Kindest regards

Yours sincerely
D. Buresova

Referee's comments:
Comment on Response to major comments 1 and 2: I understand that this work more comprehensively investigates the spatial and temporal evolution of PMAF. If true, the authors do not need to argue that no detailed previous study of PMAF. It gives the impression that the authors are trying to force a difference by using ambiguous language. Because the revised manuscript includes more specific points constituting “detail”, I suggest that the author remove some ambiguous statements about comparison with the previous studies (see the specific comment as an example).

Specific comments on the revised manuscript:
P1 L15: The phase of “While there have been previous studies commenting on PMAF morphology” can be removed.
P2 L23: “commented on their morphological evolution.” can be removed. An important issue is not whether they commented on the morphology or not, but what they show the morphology.
P3 L1: What does MSP stand for?
P3 L41: be commented on in the discussion.---> be discussed later.
P3 P41:“It is also one of two reasons we analyze the two event lists separately in the quantitative part of this study.” I do not understand what this sentence means. What is the first reason?
P4 L30:”The first complete description of the morphological evolution…” can be removed. If it was the complete description, no need to be investigated in detail anymore.

Hide

AR by Anton Goertz on behalf of the Authors (20 Dec 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (28 Dec 2022) by Dalia Buresova

AR by Anton Goertz on behalf of the Authors (23 Jan 2023) Manuscript

Journal article(s) based on this preprint

21 Mar 2023

Morphological evolution and spatial profile changes of poleward moving auroral forms

Anton Goertz, Noora Partamies, Daniel Whiter, and Lisa Baddeley

Ann. Geophys., 41, 115–128, https://doi.org/10.5194/angeo-41-115-2023,https://doi.org/10.5194/angeo-41-115-2023, 2023

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Anton Oscar Goertz et al.

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Poleward moving auroral forms (PMAFs) are specific types of aurora that are believed to be the signature of the connection of Earth's magnetic field to that of the sun. In this paper we discuss the evolution of PMAFs in regards to their auroral morphology as observed in all-sky camera images. We interpret different aspects of this evolution in terms of the connection dynamics between Earth's and the sun's magnetic fields. This sheds more light on the magnetic interaction between sun and Earth.


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