An Investigation of the Loop Oscillations after a Solar Flare

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

The paper deals with an interesting problem, that of loos oscillation after a solar flare, and presents clear observations of it. I recommend it is published after a few minor aspects are improved in the writing

 

Line 26: sentence is wrong

 

Line 62: sentence is incorrect

 

Line 101: how is this speed of  441 km/s calculated? 

 

Caption of fig 4: represents -> represent

 

In Eq 1 it is not clear to me if the authors could’ve chosen a fitting method that could imply fewer parameters. A short discussion about this point might be added. It looks to me a reasonable choice, but the authors should briefly discuss how well this function can fit the data

 

Line 252-255: these sentences could be rewritten better and without errors

Comments on the Quality of English Language

English shall be improved a little. Please double check especially the use of plural and third person, beside some sentence that is not well written.

Author Response

The reply is in the attachment. Thank you very much!

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Report on the paper “Investigation of the loop oscillations after solar flare" by Jun Xu et al.

 

This manuscript presented observations of decaying oscillations in positions or brightness of coronal loops during two flares and a jet occurred. The observed 12 oscillations were characterized using some parameters. In addition, two oscillations that have the brightness variations were investigated using a differential emission measure diagnosis and were interpreted as density oscillations.

 

Below I list some comments and questions that the author should answer before publication of this paper.

 

 

 

My major comments are as follows.

 

Q1 - all

 

Readers can see the analysis and the results. However, what is not clear in this manuscript is what new discovery is. Please clearly describe what new thing is presented, using appropriate references.

 

Q2 - Introduction

 

What scientific question does this study address? It is not clear in this manuscript. In the second paragraph, significant controversy over the driving mechanism of decay-less oscillations were mentioned. However, the decay-less oscillations are not objects that this article presents.   

 

Q3 - Page 4 Line 97

 

Do the all 11 oscillations that were seen in slit s1 - s5 have the same phase?

 

Q4 - Page 4 Lines 95 - 96, 101 - 102, Figure 3

 

It is not clear how the red arrows were derived. In figure 3, it is not clear to me many oscillations overlapping, and intersecting in these slits. Would you mind modifying the figure to show them more clearly?   

 

Q5 - Page 4 Lines 106 and 107

 

Why does the phase of the longitudinal oscillation deviate by 1/4 of the period? The fact that slit s6 is perpendicular to s1 - s5 is not enough explanation for me to understand. 

 

Q6 - Page 4 Lines 111,

 

The oscillation # 11 started about 10:12 when the jet started. The oscillation seems to be triggered by the jet.

 

Q7 - Page 6 Lines 135 - 137

 

Why does oscillation on a longer loop have a longer period? Do waves propagating along all loops from shorter one to longer one have similar phase velocity?

 

Q8 - Page 6, Lines 137 - 140

 

Why does oscillation in slit s5, which was the closest to the flare, have a small amplitude?

 

Q9 - Page 7, Line 154,

 

It is hard to see oscillatory signals at the center of brightest areas in figure 4 (a1). Would you mind modifying the figure 4 (a1) to show it more clearly?

 

Q10 - Page 7, Line 157 - 159,

 

It is concluded that the decaying transverse oscillation was caused by the change in material density in the loop. However, the DEM was integrated on the edge of the bright region, where the gradient of the brightness is high. Would it be possible that we are just seeing the density gradient, and the integration range changes in the position against the density gradient?

 

Q11 - Page 7 Figure 4 (c2) and lines 166 - 167,

 

The peak in Figure 4 (c2) seems to be significant. On the other hand, the change in temperature was presented as not obvious. If it is significant, please clarify in the manuscript.

 

Q12 - Page 7, Line 173

 

Given the error bars, I would not think that the peak at log T = 6.35 seems to be significant. Is it significant?

 

Q13 - Figure 1, and Page 8, Line 184 - 186,

 

In Figure 1(b), it is hard to see brightness along oscillation #4. It would be nice if you could show us the temporal variation of the brightness of loop oscillation 4 in Fig 1(c).

 

Q14 - Page 9 Line 226,

 

Oscillation #1 and #4 have large ratio of decaying time to oscillation period. Were they included in the calculation for the average ratio?

 

Q15 - Page 9, 234 - 241,

 

What is the magnetic field strength derived from the oscillation 2 using eq. (4)?

 

 

Here are the minor comments.

 

Q16 - Page 1, Line 4

 

Please clarify what DEM stands for.

 

Q17 - Page 1, Line 5

 

The sentence is misleading. It reads that all observed oscillations result from the density fluctuation. But, two of them showed brightness oscillations and one of them were investigated with the DEM analysis. Please modify the sentence.

 

Q18 - Page 1, Line 6

 

Same as the above. It reads that all observed oscillations have brightness fluctuations.

 

Q19 - Page 1, Lin 16

 

Several kilometers is too thin to be resolved with the current telescope. According to reference [3], the ‘several’ could be written as ‘100’.

 

Q20 - Figure 1 and 2

 

This is a suggestion. It would be better if figure 1 and figure 2 are swapped. Readers would like to see figure 2 in advance to know where the slits are.

 

Q21 - Page 2 Line 64

 

How was the end time defined? At the end time 10:19 UT, the X-ray flux was still decreasing and not yet reaching the X-ray flux before the flare happened.

 

Q22 - Page 2 Line 67

 

Can we see the jet in all AIA channels? Was it seen in the AIA 1700 A channel?

 

Q23 - Page 2 Line 69

 

Please rewrite “Each pixel of images is 0.6 arcsec” as “Spatial sampling is 0.6 arcsec”. I am not sure if we need this information here or not.

 

Q24 - Page 2 Line 73

 

Please rewrite ‘moving’ as ‘movie’.

 

Q25 - Page 2 Line 77,

 

SUTRI could not spatially resolve the coronal loops. Why was SUTRI mentioned in this manuscript? What information did SUTRI provide?

 

Q26 - Page 2 Lines 83 and 84

 

The sentence ’The red dashed line …’  is not needed to be written in the main text, because it is in the caption of figure 1.

 

Q27 - Page 2 Lines 88, Page 4 Lines 97 - 99,

 

The fitting results should be written in Section 3.1.

 

Q28 - Page 4 Lines 91 and 92,

 

Slit s6 is parallel to the loop system. Not all six slits are perpendicular to the loop system. Would you modify the words ‘, which are as perpendicular as possible to the loop system’?

 

Q29 - Page 4 Line 103

 

Is it ‘parallel’ instead of ‘perpendicular’?

 

Q30 - Page 7, Line 170

 

‘More than’ does not clearly describe the length of the error bars. What is the exact length of the error bars? Please mention it exactly in stead of ‘more than twice the standard deviation’.

 

Q31 - Page 7, Figure 4 (b1),

 

Is the title in the vertical axis ‘EM’ instead of ‘DEM’?

 

Q32 - Figure 1 and 4, captions

 

How ware the integration curves derived?

 

Q33 - Page 8, Lines 191 - 192

 

Please unify the unit of time. ’s’ or ‘minutes’ 

 

Q34 - Page 8, Line 202

 

Where can we see the oscillation peak? In Fig4 (b1)? If so, ‘peaks’ can be ‘periods’.

 

Q35 - Page 9, Line 205

 

What does the sentence ‘The result is oscillation 2 curve’ mean?

 

Q36 - Page 9 Line 214,

 

Oscillation #10 have 1.5 periods and not have at least two periods. On the other hand, the sentence reads that all oscillations last for at least two periods. Would you modify it?

 

Author Response

The reply is in the attachment. Thank you very much!

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper presents the analysis of transverse oscillations of coronal loops in imaging data, supplemented by the DEM analysis. Together with the displacement of the oscillating loop in the plane of the sky, the authors detected the oscillatory signals in the emission intensity and DEM. I disagree with the interpretation of the latter effect in terms of the oscillations of density. In my opinion, a much simpler interpretation is the modulation of the emission intensity and DEM by the variation of the line-of-sight column depth (see my comments to l. 150 and 175 below). The empirical results presented in the paper are original and important, and are worth publishing. However, I suggest the authors to revise the interpretation of the observed phenomenon. In addition, the paper would benefit from the modification summarised below.

Furthermore, the paper requires a serious lingual revision. I'd suggest to author to seek help of a native-English colleague. 

l. 33: "there is still significant controversy over the driving mechanism of decayless oscillations" - I'd not call it a "controversy", as some of the mechanisms, e.g., the harmonic footprint driver and the KH vortex model, have been clearly excluded, while other require observational validation. Please rephrase.

l. 42: "the hot plasma" - I'd suggest the author to avoid the use of the term "hot", as typically the kink oscillations are detected in the emission associated with the temperature of about 1 MK, while the term "hot" is usually applied to the hotter plasmas, > 5 MK, i.e., SUMER oscillations of hot loops. Please rephrase.

l. 62: "There are two solar flare continuously taken place at AR N11E40" - not clear what is meant by "continuously". Perhaps, sequentially?

l. 69: "there is a group of coronal loop system" --> either "a group of coronal loops" or "a coronal loop system".

l. 72- 73: "The enormous energy eruption results into these loops unstable and oscillating in the AIA moving". Why is the "energy eruption" enormous? "AIA moving" - movie? Do you see the loop destabilisation, or just a displacement from an equilibrium which results in an oscillation?

Figure 2 should be shown ahead of Figure 1b-d.

l. 82: "four distinct oscillations detected and marked by 1, 2, 3, 4" - I understand that you mean oscillations in four distinct threads which form the bundle (the loop). If it is correct, please rephrase.

l. 89: "they have a period of about 4.25 minutes" - do you mean that the different threads are seen to oscillate with the same period?

Fig. 1c, caption: What is meant by the "integration curves"? Is it the variation of the emission intensity in the oscillating loop? (This term appears also below, please introduce it in the text). 

l. 93: "S6 is a slit along the coronal loop" - I understand that the loop shows transverse oscillation. Does the slit make movements together with the oscillating loop?

l. 98: I am confused. Here the oscillation period is 6.7 min, while in l. 89 it is 4.25 min. Please explain.

l. 99: "Due to the different lengths of the inner and outer loops, the oscillation of the inner loop begins before that of the outer loop." - please elaborate this point. Why should the inner loops start oscillating earlier than the outer loops?

l. 117: "In total, 12 individual oscillations are detected" - how is this statement related to the statement in l. 82 ("four distinct oscillations detected")?

Table 1: Please explain in the caption what are the parameters shown in different columns. 

Eq. (1): Indeed, traditionally, the damping pattern of a kink oscillation is assumed to be exponential. I think it is all right to make the same assumption in this study. However, the recent work of Zhong+'23 (2023MNRAS.525.5033Z) demonstrated that the damping pattern is rather superexponential. I suggest the authors to mention it somewhere in the manuscript. 

l. 126 and below: "drifting motion of the oscillation", "the initial position of the oscillation" - do you mean the motion and position of the oscillating loop or thread?

l. 129: Which fitting technique was used?

l. 131: Please indicate that the velocity amplitude is projected on the plane of the sky. Also, please use either v_m or V_m for the amplitude, not both.

l. 149: Please define what "EM" is.

l. 150: "The EM map shows the distribution of material density" - The emission is optically thin, and hence the increase in EM can also show the column depth of the emitting volume, along the line-of-sight. Please clarify.

l. 157, l. 193: A related question. "indicating that this decaying transverse oscillation is precisely caused by the change in material density in the loop." - Why not by the variation of the column depth, as it was suggested in Cooper+'03 (2003A&A...397..765C) and demonstrated in, e.g., Verwichte+'09 (2009ApJ...698..397V). In the latter paper it was stated: "... intensity variations associated with the transverse loop oscillations are reported for the first time. They are shown to be caused by the effect of line-of-sight integration."

l. 211: "We studied the decaying transverse oscillation in the coronal loop" - in a bundle of loops?

l. 221: "a large solar flare induced the decaying and twisting oscillation" - a "twisting" oscillation has not been mentioned anywhere above. Is it a correct term?

l. 222: "decaying oscillations are usually driven by solar activities such as solar flares, EUV waves, coronal currents, or coronal rain" - actually, in the vast majority of detected cases, decaying kink oscillations are excited by the low coronal eruptions (see your reference [16] discussed in the next paragraph).

l. 241: The paragraph seems to be incomplete. Was is the value of the magnetic field estimated by this technique?

l. 251: "This fact indicates that the thermal plasma are frozen in the loops and oscillating with together." - is this behaviour not obvious in the large magnetic Reynolds plasma such as the corona?

l. 254: "both EM and temperature display the oscillations with a similar period" seems to contradict the statement in l. 250, "there is no any oscillating signals on the time-distance map of temperature". 

l. 256: Please see my comment to l. 150-157.

======

Comments on the Quality of English Language

The manuscript requires lingual editing. 

Author Response

The reply is in the attachment. Thank you very much!

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Report on the paper “Investigation of the loop oscillations after solar flare" by Jun Xu et al.

 

Thank you for significantly revise the manuscript. I list some more comments and questions that the authors should answer before publication of this paper.

 

My major comments are as follows.

 

Q1 - all, relating to the Q1 and 2 in the previous report,

 

Verwichte et al. 2009, ApJ, 698, 397 presented the first observations of simultaneous intensity and displacement oscillations in a coronal loop. Are the measurements of the similar wave periods in intensity and the displacements of the coronal loops the new discovery that the manuscript presents? If so, please cite papers presenting simultaneous intensity and displacement oscillations in coronal loops as many as possible, and summarize wave periods they presented.

 

Q2 - Introduction, relating to the Q2 and Q14 in the previous report,

 

As authors answered to the Q2 in the previous report, oscillations #1 and # 4 can be classified as decay-less oscillations. Please discuss the decay-less oscillations in section 4 ‘Discussions and Conclusions’. It would be nice if the author could explain why they didn’t decay even though they were triggered by the flare.

 

Q3 - Page 1 line 10, Figure 4 (a2), page 9, line 240, relating to Q11 in the previous report,

 

What is the conclusion about the oscillation in temperature? I am confused by the answer for Q11 in the previous report.

 

Relating to this, there are bright columns at times 10:12, 10:20, and 10:28 in Figure 4 (a2). They might make a significant oscillation power in Figure 4 (c2). Do they relate to the oscillation? Or systematic error?

 

 

Q4 - Page 3 lines 111 - 112,

 

The different lengths of the inner and outer loops is not enough for the referee to understand why the starting time of the oscillations are different. Would you explain more?

 

Q5 - Page 5 lines 160 - 162,

 

Please list the phase in Table 1 to present that the loops are oscillating almost in the same phase.

 

 

Q6 - Page 5 lines 163 - 165, relating to the Q7 in the previous report,

 

Why are the phase speeds of the kink waves similar in the same loop system? Please cite the literature (doi:10.1007/s11214-021-00847-2) that the authors referred in the answer to Q7 in the previous report, to explain why oscillations on a longer loop have a longer period.

 

 

Q7 - Page 5 lines 165 - 168, relating to the Q8 in the previous report,

 

Please explain why s5 does not have the largest displacement amplitude in this manuscript.

 

 

Q8 - Page 8, line 213 - 215 and page 9 lines 232 - 234, relating to the Q10 in the previous report,

 

Here, the manuscript would like to present brightness or EM oscillation ‘inside’ the loop system, but not brightness at ‘edge’ of the loop system. However, some oscillation trajectories trace the edge of the loop system. The referee think that the integration does not represent the oscillations inside the loop system. Because the trajectories were manually determined, how much the bright region is covered by the integral range can change in oscillation phase. The referee would suggest the following methods for the edges of the loops system. It would be nice if the authors have better idea.

 

At each time step, we can get distance-brightness profile. Let us call x-b profile. Take derivative x-b profile with respect of x. Fit the derivative with a single Gaussian function and get the X at the peak and the Gaussian width. Get the brightness Y at the bright side of 3 sigma positions, e.g., the X at the Gaussian peak + or - 3* the Gaussian width. The Y can represent the brightness inside the coronal loop associated with the oscillation in the edge displacement.

 

Q9 - Page 7 lines 180 - 181,

 

If the column depth does not change, the DEM does not change much. The assumption that is used in this study seems to be that the length of the loop system along the line of sight does not change in time. As it is discussed in Section 4, it is not always true in the solar atmosphere. Please write something like 

 

If the length of the loop system along the line of sight would be constant in time and space, then an EM map could present the distribution of the plasma density. We discuss the assumption in Section 4.

 

Q10 - Page 7 lines 190 - 191,

 

Here, the oscillation in displacements of the loops is discussed. The oscillation in EM along the oscillation trajectory is not discussed. Therefore, the transverse oscillation is caused by displacement of the loops, but not by the change in material density in the loop. Please rewrite what causes the transverse oscillations.

 

Q11 - Page 7, line 205, relating to the Q12 in the previous report,

 

According to the answer to Q12 in the previous report, the peak at the log T = 6.35 is not significant. Please do not mention about the peak at log T = 6.35.

 

Q12 - equations 2 - 4,

 

The referee cannot find the same equations as equations 2 - 4 in the reference 60 - 64. Please cite a paper to describe how to derive the equations.

 

 

Q13 - Page 8 lines 206 - 208, and Figure 5,

 

The integral range of temperature is between Log T = 5.6 and 6.0. This is the temperature in the transition region, but not coronal temperature. I think the results from the DEM analysis present the oscillation in density in the transition region, but not the coronal loops. Please change the integration range to between log T = 6.0 and 6.6 to show the coronal loop oscillation.

 

Q14 - Page 10, equation (2),

 

Does the spatial wavelength of the oscillation along the coronal loop have the same length of the circumference of the loop？

 

 

 

 

Here are the minor comments.

 

 

Q15 - Page 1 line 6

 

Does ‘interval’ mean ‘period’?

 

Q16 - Page 1 line 29

 

Does ‘model’ mean ‘mode’?

 

Q17 - Page 2 line 71

 

Thank you for swapping Figure 1 and 2. Please introduce Figure 1 before introducing Figure 2(a).

 

Q18 - Page 2 lines 86 - 90, relating to the Q25 in the previous report,

 

SDO/AIA images have been widely used and they are reliable. I don’t think that the SUTRI images are needed to give further evidence of the loop system. If they are needed, please describe more information about the group of loops provided by the SUTRI observations.

 

Q19 - Page 3 line 99,

 

Because the fitting function was presented in Section 3 and the reader can estimate the oscillation period by looking at Figure 2(b), please remove ‘After fitting these positions, ’.

 

Q20 - Page 3 line 104 - 105,

 

Please remove the sentence ‘S1 to S5 are perpendicular ….’, because it provide the same information as the previous sentence.

 

Q21 - Page 5 line 129,

 

Please remove ‘and the jet’, because there is the possibility that the jet triggered the oscillation.

 

Q22 - Page 5 line 136,

 

‘Darkest pixels’ are located out of the group of the loops, e.g., around 38 arcsec in vertical axis of Fig. 3(a). ‘The brightest pixels as the center of these oscillations or pixels at edges of groups of the oscillating coronal loops’ can be better to explain the method.

 

Q23 - Page 5 lines 148 - 156

 

Do the sentences mean that the super exponential damping model is the better model for this study?

 

Q24 - the caption of Figure 5,

 

Please remove the words ‘more than’.

 

Q25 - Page 9 lines 236 - 237,

 

The words ‘oscillation peaks’ confuses me. The ‘oscillation peaks’ reads like peaks in power spectra. How about rewriting as ‘…, it can be seen that oscillation 2 has oscillation in EM as well.’

 

Q26 - Page 9, line 253,

 

What does ‘This’ represent?

 

Q27 - Page 10, line 260,

 

What is ‘coronal currents’? Is it plasma flows in the corona? Or electric currents? 

 

Q28 - Page 10, lines 261 - 262

 

Is the first flare ‘the low coronal eruptions’? If the height of the eruption would not be important, please remove the word ‘low’.

 

Q29 - Page 10, line 278,

 

What are ’N’ and ‘A’?

 

Q30 - Page 10, line 284,

 

What is the reference for Zimovets et al.? It does not seem to be connected to any of the references.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf