Near-UV Pulsations in the Aurora Region Measured by Orbital Telescope TUS during High-Intensity and Long-Duration Continuous AE Activity

Round 1

Reviewer 1 Report

This manuscript studies the particles characteristics of pulsating auroras using the joint data between the TUS and DEPRON data on board the Lomonosov satellite and EMFISIS RBSP data. The research results give us a further understanding of particles characteristics of pulsating aurora during the periods with a high-intensity, long-duration, continuous AE activity.

The research results are meaningful. However, as a paper on aurora research, the author should present the aurora data corresponding to the events in the article completely, rather than simply replace them with a paper citation.

I suggest that this manuscript should be major revised. The author should add the aurora data analysis corresponding to the event in the text.

Author Response

We are very grateful for referees' careful reading of this paper and valuable comments and remarks. We are agreed with the most of the suggestions. Below are point-to-point answers to referees’ comments.

This manuscript studies the particles characteristics of pulsating auroras using the joint data between the TUS and DEPRON data on board the Lomonosov satellite and EMFISIS RBSP data. The research results give us a further understanding of particles characteristics of pulsating aurora during the periods with a high-intensity, long-duration, continuous AE activity.

The research results are meaningful. However, as a paper on aurora research, the author should present the aurora data corresponding to the events in the article completely, rather than simply replace them with a paper citation.

I suggest that this manuscript should be major revised. The author should add the aurora data analysis corresponding to the event in the text.

ANSWER:

We have added the following information about the events:

It was found out that these 64 events has pulsations in a range of 1–10~Hz and typical size of luminous regions about 10 km. The pulsation amplitude is about 10\% of the background level. Events are distributed between 55° and 71° of magnetic latitudes which corresponds to a source location in a magnetosphere between 4 and 9 Earth radii. 70% of events occur on L-shells less than 6. This indicates a connection with the structures of the inner magnetosphere, namely the outer radiation belts.

For some events where it was possible data of ground-based all-sky cameras were analyzed. In particular, 11 coincidences with the cameras of the THEMIS network were found, and it was shown that in all cases an active auroral glow is observed in the region of observation of events by the TUS detector~\citep{ klimov2022uv}. Two cased of joint observations with Polar Geophysical Institute (Apatity) all-sky cameras were analyzed. A total of TUS six events were measured inside a field of view of the ground-based cameras and in all cases a wide region of pulsating auroras was observed~\citep{ klimov2022joint}.

Reviewer 2 Report

Report on: Near-UV pulsations in aurora region measured by orbital telescope TUS during high-intensity and long-duration

continuous AE activity

Pavel Klimov 1* , Vladimir Kalegaev 1, Ksenia Sigaeva 1,2, Alexandra Ivanova 1,2, Grigory Antonyuk 1, Viktor

Benghin 1 and Ivan Zolotarev 1

 

This paper aims to show a relationship between electron fluxes of > 0.1-1 MeV in the outer radiation belt and near-UV pulsations in the auroral region.

 

The abstract includes: “64 events with pulsations were registered … Most of the events occur mainly in the region of the outer radiation belt of the Earth near the maximum of electron fluxes with energies >100 keV and >1 MeV according to the instruments of the Meteor-M and Lomonosov satellites respectively…Apparently, particles of these energies and higher cause a pulsating emission relatively deep in the atmosphere”

 

For each satellite the measurements of electron fluxes were shown relative to the times of 2 events. Statements were made that in either 60% of cases (Lomonosov) or nearly all (Meteor-M) there was a correlation. This seems to me to be the main point of the paper, so I would have thought it appropriate to show more cases so that the reader can confirm the authors’ analysis.

 

It is also shown that most of the events correlate with peaks in AE activity during HILDCAA events. The first paragraph of the discussion suggests that these can produce relativistic electrons. I assume that the next four paragraphs explain this, but I cannot follow the argument. They introduce additional phenomena to the paper (LBC, PA and PPA) and additional instruments (SAMPEX, EISCAT, Arase and Polar satellite plasma wave instrument). Statements were made about previous studies with these without explaining how they relate to the current work. This leads to a vague conclusion that “the observations of NUV pulsations by the TUS detector in the region of an increased flux of precipitating relativistic electrons during long-term geomagnetic activity fit into the described picture and confirm the earlier observations.”

 

In the conclusions the main point in the abstract is confirmed, but then confused by the addition of “Previously the SAMPEX satellite and the Polar satellite measurements found a correlation between observations of REMs and VLF chorus waves”, so it is not clear whether “These particles” in the final sentence refers to the Lomonosov and METERO-M2 observations or the SAMPEX observations.

 

My overall assessment is that correlation is shown between electron fluxes in the outer radiation belt and the near UV pulsations, but it would be better if more of the relevant measurements were presented and analysed. Most of the Discussion is disconnected references to previous work that do not form a coherent discussion. 

 

.

 

 

Specific comments:

 

Line 102: recovery phase of week magnetic storm

What does “week” mean?

 

Lines 103-105: TUS events are shown by red verical lines. All of them are close and follow after the AE local

maximums.

This is not obvious to me. For example, the first vertical line for Dec 26 coincides with the start of the local maximum.

 

Lines 124-126: One can see the intensive wave activity with frequency close to electron gyro frequency fc during observed TUS events.

I can see enhanced activity near the middle one of 3 lines. What is the relationship between the 3 lines and the gyro frequency f_c? What is different about the middle line?

 

Lines 220-222: Thus, the observations of NUV pulsations by the TUS detector in the region of an increased 

flux of precipitating relativistic electrons during long-term geomagnetic activity fit into the described picture and confirm the earlier observations.

This is vague. 

 

Line 228: Spatial and temporal structure of the events were described in [14].

This should not be presented in the conclusions section.

 

Lines 238-241: Previously the SAMPEX satellite and the Polar satellite measurements found a correlation between observations of REMs and VLF chorus waves.

This should not be presented as a conclusion.

 

Lines 240-241: These particles penetrates and cause a pulsating glow deep in the atmosphere relatively to a usual region of PsA (80 – 250 km).

 

What is the depth of the pulsating glow? 

Comments for author File: Comments.pdf

Author Response

We are very grateful for referees' careful reading of this paper and valuable comments and remarks. We are agreed with the most of the suggestions. Below are point-to-point answers to referees’ comments.

This paper aims to show a relationship between electron fluxes of > 0.1-1 MeV in the outer radiation belt and near-UV pulsations in the auroral region.

 

The abstract includes: “64 events with pulsations were registered … Most of the events occur mainly in the region of the outer radiation belt of the Earth near the maximum of electron fluxes with energies >100 keV and >1 MeV according to the instruments of the Meteor-M and Lomonosov satellites respectively…Apparently, particles of these energies and higher cause a pulsating emission relatively deep in the atmosphere”

 

For each satellite the measurements of electron fluxes were shown relative to the times of 2 events. Statements were made that in either 60% of cases (Lomonosov) or nearly all (Meteor-M) there was a correlation. This seems to me to be the main point of the paper, so I would have thought it appropriate to show more cases so that the reader can confirm the authors’ analysis.

 

 

It is also shown that most of the events correlate with peaks in AE activity during HILDCAA events. The first paragraph of the discussion suggests that these can produce relativistic electrons. I assume that the next four paragraphs explain this, but I cannot follow the argument. They introduce additional phenomena to the paper (LBC, PA and PPA) and additional instruments (SAMPEX, EISCAT, Arase and Polar satellite plasma wave instrument). Statements were made about previous studies with these without explaining how they relate to the current work. This leads to a vague conclusion that “the observations of NUV pulsations by the TUS detector in the region of an increased flux of precipitating relativistic electrons during long-term geomagnetic activity fit into the described picture and confirm the earlier observations.”

 

In the conclusions the main point in the abstract is confirmed, but then confused by the addition of “Previously the SAMPEX satellite and the Polar satellite measurements found a correlation between observations of REMs and VLF chorus waves”, so it is not clear whether “These particles” in the final sentence refers to the Lomonosov and METERO-M2 observations or the SAMPEX observations.

 

ANSWER:

The mention of SAMPEX and Polar was deleted from conclusions since they are not related to the result of the paper. The particles analyses in the current work were measured only by Lomonosov and METEOR-M2 satellites

 

My overall assessment is that correlation is shown between electron fluxes in the outer radiation belt and the near UV pulsations, but it would be better if more of the relevant measurements were presented and analysed. Most of the Discussion is disconnected references to previous work that do not form a coherent discussion.

 

ANSWER:

It is difficult to agree with the referee statement. The discussion starts with HILDCAA events description and its connection to the acceleration of charged particles. And some correspondence to the TUS measurements are discussed. Then the REM are introduces, and it is a result of acceleration which is active during HILDCAA. And the from the one hand are the result of HILDCAA and from the other hand – possible source of atmospheric emission (see, modeling of Miyoshi et al 2020). REM has the same L-shell distribution as the TUS events. So, we are trying to describe a self-consistent picture of various processes and phenomena which can explain TUS NUV pulsating events appearance.

 

 

Specific comments:

 

Line 102: recovery phase of week magnetic storm

What does “week” mean?

ANSWER:

This is misprint, we mean “weak”. Weak storm with maximum intensity less than 100 nT

 

Lines 103-105: TUS events are shown by red vertical lines. All of them are close and follow after the AE local maximums.

This is not obvious to me. For example, the first vertical line for Dec 26 coincides with the start of the local maximum.

We deleted this sentence since it is difficult to demonstrate.

We replaced this sentence with: “All of them occur during continuous auroral activity and correspond to the substorm activations”.

 

Lines 124-126: One can see the intensive wave activity with frequency close to electron gyro frequency fc during observed TUS events.

I can see enhanced activity near the middle one of 3 lines. What is the relationship between the 3 lines and the gyro frequency f_c? What is different about the middle line?

ANSWER:
The description of three lines added to the text and figure caption. The middle line is 0.5f_c.

 

Lines 220-222: Thus, the observations of NUV pulsations by the TUS detector in the region of an increased flux of precipitating relativistic electrons during long-term geomagnetic activity fit into the described picture and confirm the earlier observations.

This is vague. 

ANSWER:

Slightly corrected the sentence but it is the direct consequence of discussion (see the answer above):

 

Thus, NUV pulsations observed by the TUS detector can be originated from energetic particle precipitation during long-term auroral activity and described by mentioned above sequence of processes.

 

Line 228: Spatial and temporal structure of the events were described in [14].

This should not be presented in the conclusions section.

DELETED

 

Lines 238-241: Previously the SAMPEX satellite and the Polar satellite measurements found a correlation between observations of REMs and VLF chorus waves.

This should not be presented as a conclusion.

 DELETED

 

Lines 240-241: These particles penetrate from the magnetosphere to low altitudes and cause a pulsating glow deep in the atmosphere relatively to a usual region of PsA (80 – 250 km).

What is the depth of the pulsating glow?

ANSWER: It is impossible to give a definite answer only using the monocular measurements in nadir direction. Electrons with energies of 100 keV will produce maximum ionization and emission around altitude of 80 km, 800 keV electrons – 60 km (see for example Turunen 2009). But we can’t state that measured electrons directly related to the emission since we don’t have such temporal resolution of charged particles detectors to search for the coincidences if temporal structures of particles flux and UV emission.

Reviewer 3 Report

Regarding the work, it is an interesting paper and one that will contribute to advancing the study of pulsating auroras. 

There are not many background comments, just comments on two chapters that could contribute to the paper's clarity, mainly in the "Introduction" and "Materials and Methods."

Concerning the introduction, the authors can improve by enriching the text with a more complete and comprehensible description of pulsations auroras by referring to different types of auroras, for instance. Different types of pulsation auroras are referred to in the section "discussion," but it will be interesting to mention them in the "introduction." A good discussion can be found in Grono & Donovan, 2020. Also, this section can better explain the relationships between Chorus waves and pulsation auroras (see Hill & Al., 2019, for instance).

Regarding the chapter "Materials and Methods," there is an imbalance: while the materials are widely described (and sometimes excessively, for instance, the second period of data is unnecessary since it was not considered for the study), the methods used are not so clearly exposed. Some justifications need to be included: as an example, for period one, 32 NUV pulsations were registered, and 21 presented two characteristics. What about the other 11? Similar missed justifications occur for the three period.

 

Author Response

We are very grateful for referees' careful reading of this paper and valuable comments and remarks. We are agreed with the most of the suggestions. Below are point-to-point answers to referees’ comments.

Regarding the work, it is an interesting paper and one that will contribute to advancing the study of pulsating auroras. 

There are not many background comments, just comments on two chapters that could contribute to the paper's clarity, mainly in the "Introduction" and "Materials and Methods."

Concerning the introduction, the authors can improve by enriching the text with a more complete and comprehensible description of pulsations auroras by referring to different types of auroras, for instance. Different types of pulsation auroras are referred to in the section "discussion," but it will be interesting to mention them in the "introduction." A good discussion can be found in Grono & Donovan, 2020. Also, this section can better explain the relationships between Chorus waves and pulsation auroras (see Hill & Al., 2019, for instance).

ANSWER:

[Grono &Donovan, 2020] classification was mentioned in the Introduction.

If we understand well Hill 2019 is the AGU abstract and it is difficult to judge about the results of research based on the abstract.

Hill, Shannon, et al. "The Relationship between Pulsating Aurora and Chorus Waves: Comparing Simultaneous Observations with Gakona All Sky Camera and the Van Allen Probes." AGU Fall Meeting Abstracts. Vol. 2019. 2019.

 

Regarding the chapter "Materials and Methods," there is an imbalance: while the materials are widely described (and sometimes excessively, for instance, the second period of data is unnecessary since it was not considered for the study), the methods used are not so clearly exposed.

Some justifications need to be included: as an example, for period one, 32 NUV pulsations were registered, and 21 presented two characteristics. What about the other 11? Similar missed justifications occur for the three period.

ANSWER:

The information about events was added in more details.

The aim of the section is to present the used data and equipment. Thus we suggest to rename the section as “Instruments and data”

Reviewer 4 Report

The manuscript "Near-UV pulsations in aurora region measured by orbital telescope TUS during high-intensity and long-duration continuous AE activity" by Pavel Klimov et al discusses the UV intensity pulsations in the auroral region using various satellite data (Lomonosov, RBSP/ Van Allen Probes, METEOR-M2). Authors show that most of the pulsations measured in the near-UV wavelength range occur either at the outer radiation belt maximum or in the high latitude region of quasi-trapped energetic electrons appearing as a result of geomagnetic activity and acceleration during the wave-particle interaction in the Earth's magnetosphere.

The paper contains noteworthy results that will be of interest to those who study pulsating aurora, its properties and the sources of this phenomenon. I can recommend this paper for publication in Remote Sensing if the several items and shortcomings are corrected and revised.

1. Authors write in detail about the RBSP/Van Allen Probes and METEOR-M2 satellites, but there is no information about the Lomonosov satellite, where the main instrument (the TUS detector) for studying pulsating aurorae was installed. One need to add information about the orbit, altitude, inclination and so on of the Lomonosov satellite.

2. Line 58. "The events which contain NUV pulsations in aurora region were selected and described in details in [14]." I think that here in the text it is necessary to briefly describe this item since this is important information for this study.

3. Caption of Figure 1. It should be added that TUS events are shown by red vertical lines here.

4. Line 118. "About 60% of events were measured in close relation with the ORB during periods of enhancement of high-energy electron fluxes." What was characteristic of the remaining 40%?

5. Authors indicated that  "TUS event measured at 12:39:14 UTC on 07.01.2017 (L-shell of the event is 4.4, MLT=23.3) and TUS event measured at 10:38:19 UTC on 08.01.2017 (L=4.9, MLT=23.1) lie inside these periods." I would recommend to display TUS events in Figure 3 as red lines, as has been done in other figures.

6. Line 122-124. "At these moments RBSP satellites were located approximately in magnetic conjunction with Lomonosov: RBSP-B satellite was at L=4.55 and MLT=23.5, while RBSP-A was at L=5.5, MLT=23.1." I did not understand this sentence. What are the moments? Active chorus wave generation processes or TUS events? Why are other values of L-shell and MLT indicated in the sentence above if TUS events are the same? You need to rewrite this part in more detail and more clearly.

7. Line 125. "...with frequency close to electron gyro frequency fc during..." I believe that it is worth pointing out the typical gyrofrequency values in this area.

8. Line 22. "..the auroral oval. [4] and in the midnight-morning MLT...".  Remove extra dot.

Author Response

The manuscript "Near-UV pulsations in aurora region measured by orbital telescope TUS during high-intensity and long-duration continuous AE activity" by Pavel Klimov et al discusses the UV intensity pulsations in the auroral region using various satellite data (Lomonosov, RBSP/ Van Allen Probes, METEOR-M2). Authors show that most of the pulsations measured in the near-UV wavelength range occur either at the outer radiation belt maximum or in the high latitude region of quasi-trapped energetic electrons appearing as a result of geomagnetic activity and acceleration during the wave-particle interaction in the Earth's magnetosphere.

The paper contains noteworthy results that will be of interest to those who study pulsating aurora, its properties and the sources of this phenomenon. I can recommend this paper for publication in Remote Sensing if the several items and shortcomings are corrected and revised.

1. Authors write in detail about the RBSP/Van Allen Probes and METEOR-M2 satellites, but there is no information about the Lomonosov satellite, where the main instrument (the TUS detector) for studying pulsating aurorae was installed. One need to add information about the orbit, altitude, inclination and so on of the Lomonosov satellite.

ANSWER:

We added the following information:

The satellite had a sun-synchronous orbit with altitude of about 550 km, inclination of 97.3$^\circ$, and orbital rotation period of $\sim$94~min. The polar type of the orbit made it possible to register the auroral emission.

Altitude was already presented in the paper.

2. Line 58. "The events which contain NUV pulsations in aurora region were selected and described in details in [14]." I think that here in the text it is necessary to briefly describe this item since this is important information for this study.

ANSWER: We added the brief description of the events.

3. Caption of Figure 1. It should be added that TUS events are shown by red vertical lines here.

ANSWER:

We added the information in Figure 1 caption

4. Line 118. "About 60% of events were measured in close relation with the ORB during periods of enhancement of high-energy electron fluxes." What was characteristic of the remaining 40%?

ANSWER:

This part of the text is rewritten to add more information about all types of events:

Comparison with simultaneous measurements of DEPRON instrument on board Lomonosov spacecraft allows to establish relationships between optical emissions and ORB sporadic fluctuations. There were found 25 joint measurements by both instruments. Among this measurements for 15 NUV pulsating events a significant increasing of charged particle fluxes was registered. Two characteristic cases were observed: 1) the position of the event coincides with the maximum of the ORB particle flux (or is located between the maximum and polar boundary) -- 10 measurements, 2) increasing of the charged particle fluxes appears poleward of the ORB and the events coincides with this bump -- 5 measurements. Examples of these events are demonstrated in Figure~\ref{DEPRON}. Satellite is moving from polar latitudes to equatorial ones. One can see location of the TUS events during outer radiation belt crossings at the pre-midnight MLTs. Thus, about 60~\% of events were measured in close relation with the ORB during periods of high-energy electron fluxes enhancement. For other events no charged particles flux increasing is observed. Two of them occur at very high latitudes (L>8), 8 in relatively low latitudes (in the region ORB, but no enhancements of particle fluxes are measured by DEPRON).

 

One can explain this disagreement by energy range of particles registered by DEPRON instrument. Electrons responsible for NUV pulsation can have the energies lower than that detected by DEPRON.

 

5. Authors indicated that "TUS event measured at 12:39:14 UTC on 07.01.2017 (L-shell of the event is 4.4, MLT=23.3) and TUS event measured at 10:38:19 UTC on 08.01.2017 (L=4.9, MLT=23.1) lie inside these periods." I would recommend to display TUS events in Figure 3 as red lines, as has been done in other figures.

ANSWER:

We have added the TUS events in figures and modified caption.

 

6. Line 122-124. "At these moments RBSP satellites were located approximately in magnetic conjunction with Lomonosov: RBSP-B satellite was at L=4.55 and MLT=23.5, while RBSP-A was at L=5.5, MLT=23.1." I did not understand this sentence. What are the moments? Active chorus wave generation processes or TUS events? Why are other values of L-shell and MLT indicated in the sentence above if TUS events are the same? You need to rewrite this part in more detail and more clearly.

ANSWER:

 

There are L and MLT of the TUS events which were detected during RBSP measurements shown in fig.3.

We corrected slightly the text.

 

7. Line 125. "...with frequency close to electron gyro frequency fc during..." I believe that it is worth pointing out the typical gyrofrequency values in this area.

ANSWER:

We added typical gyrofrequency values in this area to the text, as well as to the caption of figure 3 where gyro frequency is shown by white lines (f_c, 0.5 f_c, 0.1 f_c).

8. Line 22. "..the auroral oval. [4] and in the midnight-morning MLT...".  Remove extra dot.

CORRECTED

Round 2

Reviewer 1 Report

The authors have made revisions based on my previous comments. I have no new comments for this version. 

I think the current manuscript is suitable for publication in this journal.