the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Nov 2024
First observations of continuum emission in dayside aurora
Abstract. We report the first observations of continuum emission at the poleward boundary of the dayside auroral oval. Spectral measurements of high-latitude continuum emissions resemble those of STEVE, with light characterised by colours such as white, pale pink or mauve. The emission enhancement spans the entire visible wavelength range. However, unlike STEVE, the high-latitude dayside continuum emission events tightly follow the auroral particle precipitation often forming field-aligned rays and other dynamic shapes. Some dayside emissions appeared as wide arcs or cloud-like structures within the red-emission dominated dayside aurora. Our spectral measurements further suggest that the broad band continuum emission may extend into the near-infrared regime. Similar to the STEVE emission, low-Earth orbit measurements of plasma flow in the region of continuum emission show a strong horizontal cross-track velocity shear. Ground-based radar and optical observations provide evidence of both plasma and neutral heating, as well as upwelling, in connection to the continuum emissions. We conclude that the interplay of different heating mechanisms may be an important factor in generating high-latitude continuum emissions.
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RC1: 'Comment on egusphere-2024-3669', Anonymous Referee #1, 07 Jan 2025
This manuscript reports on a phenomenon referred to as "continuum emission," which appears as pink or purple light. While the color is reminiscent of the known phenomenon STEVE, the differences in its occurrence location and morphological characteristics ensure its novelty. The paper introduces observational results from various instruments, including a commercial digital camera, spectrometer, incoherent scatter radar, Fabry-Perot interferometer, the DMSP satellite, and OH imager, all of which are explained in detail. While the mechanism underlying all presented cases requires further clarification through future observations and modeling, the authors discuss various plausible explanations. For example, a flow shear in plasma shown in Figure 11, which is also observed in STEVE and its dawnside counterpart, is a valuable discovery that brings us closer to understanding the generation mechanism of continuum emissions. I do not have significant disagreements with the content of the manuscript. However, I believe the following revisions would make the paper more accessible and comprehensible to readers:
1. When comparing the spectra of continuum and the background in Figure 4, how about including a comparison with the spectrum of sunlit aurora as well? While Figures 1A and 12C clearly illustrate the visual color differences between continuum and sunlit aurora, a more precise comparison using spectra could help to understand their differences.
2. To make the comparison between Figures 1A and 1B clearer, could you add numerical labels corresponding to the scan angles in Figure 1B onto the slit region (white rectangle) in the all-sky image (Figure 1A)?
3. Although the direction is explained in the caption of Figure 1A, it would be helpful to indicate "N," "S," "E," and "W" directly in Figure 1A to make it visually clearer.
4. I understand that Figure 1B is not calibrated for sensitivity, but adding a color bar might still be helpful for readers to understand the scale.
5. Please add a label to the vertical axis of Figure 4.
6. The time labels in Figures 8C and 8F are too small to read. Could you enlarge them?
7. Please add a label to the vertical axis of Figure 12B.
Citation: https://doi.org/10.5194/egusphere-2024-3669-RC1 -
AC1: 'Reply on RC1', Noora Partamies, 07 Mar 2025
The authors thank the reviewer 1 for careful reading of the manuscript and helpful comments to improve it. Below are out point-by-point answers to the suggestions.
1. When comparing the spectra of continuum and the background in Figure 4, how about including a comparison with the spectrum of sunlit aurora as well? While Figures 1A and 12C clearly illustrate the visual color differences between continuum and sunlit aurora, a more precise comparison using spectra could help to understand their differences.
This is a good idea that has been discussed. We want to add a spectrum that is taken from the same image/spectrogram as our continuum and background sky spectra to avoid any changes in the sky or auroral conditions, and this seemed most feasible to do for the event in Figure 12, because the sunlit aurora structures are narrower and fainter in the event in Figure 4. We have therefore added a sunlit spectrum to Figure 12B, and its corresponding location to Figure 12A, just north of the continuum. Compared to the continuum spectrum the sunlit spectrum shows much less background and equally strong red emission. Compared to the background sky spectrum the sunlit one shows more enhanced the blue and also green emission lines. The enhanced blue component within the dayside red aurora is the signature of the sunlit conditions.
2. To make the comparison between Figures 1A and 1B clearer, could you add numerical labels corresponding to the scan angles in Figure 1B onto the slit region (white rectangle) in the all-sky image (Figure 1A)?
Approximate spectrograph pixel values of 40, 80, 120 and 160 have been added into Figure 1A for reference.
3. Although the direction is explained in the caption of Figure 1A, it would be helpful to indicate "N," "S," "E," and "W" directly in Figure 1A to make it visually clearer.
Small arrows indicating the direction to north and east has been added onto Figure 1A to support the caption text.
4. I understand that Figure 1B is not calibrated for sensitivity, but adding a color bar might still be helpful for readers to understand the scale.
Figure 1B is indeed showing uncalibrated intensity, so it is just counts, similar to the values on Y axis of Figure 4 and 12B. We have added the colour bar to Figures 1B and 12A, and also normalised the count values in all figures of spectrograph data, so that they just show the relative brightness enhancement. This has been clarified in the figure captions and axis labels have been added to support the figure caption texts.
5. Please add a label to the vertical axis of Figure 4.
Sure, an axis label has been added. That is now “Normalised brightness”.
6. The time labels in Figures 8C and 8F are too small to read. Could you enlarge them?
Absolutely. Figure 8C and 8F time and panel labels have been enlarged significantly.
7. Please add a label to the vertical axis of Figure 12B.
An axis label has been added. That is now “Normalised brightness”.
Citation: https://doi.org/10.5194/egusphere-2024-3669-AC1
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AC1: 'Reply on RC1', Noora Partamies, 07 Mar 2025
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RC2: 'Comment on egusphere-2024-3669', M.J. Kosch, 24 Feb 2025
This is a well-written and comprehensive paper describing a new type of aurora, namely, continuum emission poleward of the dayside auroral oval. The paper is novel and therefore worthy of publication. I recommend publication after a few minor revisions to improve clarity:
Figures 9 and 10: Please add error bars to both panels.
Figure 15: The phenomenon described is simply not visible in the figure. If it is very faint, I suggest adding arrows to aid the reader.
Figure 17: A non-expert reader may easily become confused with the normal aurora present. Please add an arrow to point out the continuum event.
Citation: https://doi.org/10.5194/egusphere-2024-3669-RC2 -
AC2: 'Reply on RC2', Noora Partamies, 07 Mar 2025
We thank reviewer 2 for helpful suggestions to improve the manuscript. The revised version of the manuscript includes implementation of all comments.
Figures 9 and 10: Please add error bars to both panels.
The error bars are included in the revised version of the manuscript, where then, for clarity, we only include two different heights for ESR data. These are the top and bottom heights from the same range as before, to show the magnitude of the vertical variability.
Figure 15: The phenomenon described is simply not visible in the figure. If it is very faint, I suggest adding arrows to aid the reader.
Good point, thank you. This is done.
Figure 17: A non-expert reader may easily become confused with the normal aurora present. Please add an arrow to point out the continuum event.
Indeed. The region of interest has been marked in the revised version of the manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-3669-AC2
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AC2: 'Reply on RC2', Noora Partamies, 07 Mar 2025
Status: closed
-
RC1: 'Comment on egusphere-2024-3669', Anonymous Referee #1, 07 Jan 2025
This manuscript reports on a phenomenon referred to as "continuum emission," which appears as pink or purple light. While the color is reminiscent of the known phenomenon STEVE, the differences in its occurrence location and morphological characteristics ensure its novelty. The paper introduces observational results from various instruments, including a commercial digital camera, spectrometer, incoherent scatter radar, Fabry-Perot interferometer, the DMSP satellite, and OH imager, all of which are explained in detail. While the mechanism underlying all presented cases requires further clarification through future observations and modeling, the authors discuss various plausible explanations. For example, a flow shear in plasma shown in Figure 11, which is also observed in STEVE and its dawnside counterpart, is a valuable discovery that brings us closer to understanding the generation mechanism of continuum emissions. I do not have significant disagreements with the content of the manuscript. However, I believe the following revisions would make the paper more accessible and comprehensible to readers:
1. When comparing the spectra of continuum and the background in Figure 4, how about including a comparison with the spectrum of sunlit aurora as well? While Figures 1A and 12C clearly illustrate the visual color differences between continuum and sunlit aurora, a more precise comparison using spectra could help to understand their differences.
2. To make the comparison between Figures 1A and 1B clearer, could you add numerical labels corresponding to the scan angles in Figure 1B onto the slit region (white rectangle) in the all-sky image (Figure 1A)?
3. Although the direction is explained in the caption of Figure 1A, it would be helpful to indicate "N," "S," "E," and "W" directly in Figure 1A to make it visually clearer.
4. I understand that Figure 1B is not calibrated for sensitivity, but adding a color bar might still be helpful for readers to understand the scale.
5. Please add a label to the vertical axis of Figure 4.
6. The time labels in Figures 8C and 8F are too small to read. Could you enlarge them?
7. Please add a label to the vertical axis of Figure 12B.
Citation: https://doi.org/10.5194/egusphere-2024-3669-RC1 -
AC1: 'Reply on RC1', Noora Partamies, 07 Mar 2025
The authors thank the reviewer 1 for careful reading of the manuscript and helpful comments to improve it. Below are out point-by-point answers to the suggestions.
1. When comparing the spectra of continuum and the background in Figure 4, how about including a comparison with the spectrum of sunlit aurora as well? While Figures 1A and 12C clearly illustrate the visual color differences between continuum and sunlit aurora, a more precise comparison using spectra could help to understand their differences.
This is a good idea that has been discussed. We want to add a spectrum that is taken from the same image/spectrogram as our continuum and background sky spectra to avoid any changes in the sky or auroral conditions, and this seemed most feasible to do for the event in Figure 12, because the sunlit aurora structures are narrower and fainter in the event in Figure 4. We have therefore added a sunlit spectrum to Figure 12B, and its corresponding location to Figure 12A, just north of the continuum. Compared to the continuum spectrum the sunlit spectrum shows much less background and equally strong red emission. Compared to the background sky spectrum the sunlit one shows more enhanced the blue and also green emission lines. The enhanced blue component within the dayside red aurora is the signature of the sunlit conditions.
2. To make the comparison between Figures 1A and 1B clearer, could you add numerical labels corresponding to the scan angles in Figure 1B onto the slit region (white rectangle) in the all-sky image (Figure 1A)?
Approximate spectrograph pixel values of 40, 80, 120 and 160 have been added into Figure 1A for reference.
3. Although the direction is explained in the caption of Figure 1A, it would be helpful to indicate "N," "S," "E," and "W" directly in Figure 1A to make it visually clearer.
Small arrows indicating the direction to north and east has been added onto Figure 1A to support the caption text.
4. I understand that Figure 1B is not calibrated for sensitivity, but adding a color bar might still be helpful for readers to understand the scale.
Figure 1B is indeed showing uncalibrated intensity, so it is just counts, similar to the values on Y axis of Figure 4 and 12B. We have added the colour bar to Figures 1B and 12A, and also normalised the count values in all figures of spectrograph data, so that they just show the relative brightness enhancement. This has been clarified in the figure captions and axis labels have been added to support the figure caption texts.
5. Please add a label to the vertical axis of Figure 4.
Sure, an axis label has been added. That is now “Normalised brightness”.
6. The time labels in Figures 8C and 8F are too small to read. Could you enlarge them?
Absolutely. Figure 8C and 8F time and panel labels have been enlarged significantly.
7. Please add a label to the vertical axis of Figure 12B.
An axis label has been added. That is now “Normalised brightness”.
Citation: https://doi.org/10.5194/egusphere-2024-3669-AC1
-
AC1: 'Reply on RC1', Noora Partamies, 07 Mar 2025
-
RC2: 'Comment on egusphere-2024-3669', M.J. Kosch, 24 Feb 2025
This is a well-written and comprehensive paper describing a new type of aurora, namely, continuum emission poleward of the dayside auroral oval. The paper is novel and therefore worthy of publication. I recommend publication after a few minor revisions to improve clarity:
Figures 9 and 10: Please add error bars to both panels.
Figure 15: The phenomenon described is simply not visible in the figure. If it is very faint, I suggest adding arrows to aid the reader.
Figure 17: A non-expert reader may easily become confused with the normal aurora present. Please add an arrow to point out the continuum event.
Citation: https://doi.org/10.5194/egusphere-2024-3669-RC2 -
AC2: 'Reply on RC2', Noora Partamies, 07 Mar 2025
We thank reviewer 2 for helpful suggestions to improve the manuscript. The revised version of the manuscript includes implementation of all comments.
Figures 9 and 10: Please add error bars to both panels.
The error bars are included in the revised version of the manuscript, where then, for clarity, we only include two different heights for ESR data. These are the top and bottom heights from the same range as before, to show the magnitude of the vertical variability.
Figure 15: The phenomenon described is simply not visible in the figure. If it is very faint, I suggest adding arrows to aid the reader.
Good point, thank you. This is done.
Figure 17: A non-expert reader may easily become confused with the normal aurora present. Please add an arrow to point out the continuum event.
Indeed. The region of interest has been marked in the revised version of the manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-3669-AC2
-
AC2: 'Reply on RC2', Noora Partamies, 07 Mar 2025
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