The Large Imaging Spectrometer for Solar Accelerated Nuclei (LISSAN): A Next-Generation Solar γ-ray Spectroscopic Imaging Instrument Concept

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1 In line 65, a reference is missing.

2 In section 4.2, the discussion related Figure 5 needs improvement. For instance, first, it was mentioned that “adequate cleaning algorithms” are needed, and later “the CLEAN algorithm” was run for processing the dirty image. What is “the” CLEAN algorithm?

3 In section 4.3.1, a mitigation strategy for a potential technical issue was mentioned. However, how would this mitigation strategy affect performance should be briefly explained. This is not a big issue, but it is helpful to make the introduction more logical. 

Author Response

1. In line 65, a reference is missing.

    

   Response: This is a link to another paper submitted to this special issue and so can be resolved as part of the published process.

    

2. In section 4.2, the discussion related Figure 5 needs improvement. For instance, first, it was mentioned that “adequate cleaning algorithms” are needed, and later “the CLEAN algorithm” was run for processing the dirty image. What is “the” CLEAN algorithm?

    

   Response: We agree that we did not make this clear. The CLEAN algorithm is a specific image reconstruction algorithm commonly used in X-ray and radio indirect imaging. In the caption of Figure 5, we now call it the “CLEAN image reconstruction algorithm” and added two references that provide more detail on it.

    

3. In section 4.3.1, a mitigation strategy for a potential technical issue was mentioned. However, how would this mitigation strategy affect performance should be briefly explained. This is not a big issue, but it is helpful to make the introduction more logical. 

    

   Response: Agreed. We have added the following text to Section 4.3.1:

   “The only performance penalty associated with this measure would be a reduction in the field of view from 13” to 10”, as the material's lower density would require thicker grids to achieve the same absorption. However, this still fulfils the field of view requirement in Table 1.”

Reviewer 2 Report

Comments and Suggestions for Authors

The article describes clearly and effectively the scientific background that motivates the study, and expose and explain the experimental solutions proposed. 

I found a missing link to a reference at line 65.  

I have just a couple of general questions on the project:

1. At line 234 you discuss of a possible active shielding. If not inserted, what is the expected background from high energy cosmic rays?

2. Your system requires a cooling. Which cooling strategy would you recommend ad how would it impact the mission lifespan?

3. Regarding the SiPMs ageing with dose, have you evaluated the dose foreseen during the mission life? How much lower is to the previous missions, where the dark current increase was observed?

4. In the end you discuss of a possible multi-instrument mission. Do you already have in mind which kind of measurements would improve your understanding of the problem/provide complementary information/cross calibration in caso of a similar choice?

Author Response

1. I found a missing link to a reference at line 65.

   Response: This is a link to another paper submitted to this special issue and so can be resolved as part of the published process.

2. At line 234 you discuss of a possible active shielding. If not inserted, what is the expected background from high energy cosmic rays?

   Response: We have redrafted this paragraph to add more detail, including a background estimate based on experience from past missions. However, more detailed modelling of the background will be required during the detailed design of LISSAN.

3. Your system requires a cooling. Which cooling strategy would you recommend ad how would it impact the mission lifespan?

   Response: The cooling requirements are driven by the front-end electronics and detectors. Both require around 0C. Therefore a passive cooling strategy should suffice. This is highlighted in section 4.4.

4. Regarding the SiPMs ageing with dose, have you evaluated the dose foreseen during the mission life? How much lower is to the previous missions, where the dark current increase was observed?

   Response: We estimate a fluence of less than 10^9 (67 MeV equivalent) protons/cm2 for three years of mission.  Previous radiation studies conducted by CNES at the Louvain-la-Neuve synchrotron have shown that the Hamamatsu SiPM should cope sufficiently with this level of radiation. We have added these details to the end of Section 4.3.2.

5. In the end you discuss of a possible multi-instrument mission. Do you already have in mind which kind of measurements would improve your understanding of the problem/provide complementary information/cross calibration in case of a similar choice?

   Response: Yes, we do. In fact there is another paper being submitted to this special issue on the SPARK mission concept that includes LISSAN. We have added a sentence at the end of the conclusions directing interested readers to that paper. (Reference not yet working as that paper is also in review.)

Reviewer 3 Report

Comments and Suggestions for Authors

The paper by Daniel F. Ryan and collaborators aims to describe the concept of the LISSAN space mission which will be capable of observing gamma-ray emissions with better time, energy, and spatial accuracy in comparison to existing missions. The paper is very well written and can serve as a useful source of information about the mission. I was able to find some things that should be updated or corrected, however, all the corrections are minor, therefore I recommend accepting the paper after the minor revision.

Here is the list of suggestions and corrections:

s18 space-weather -> space weather

s20-22 "These are the locations, where ... where .... where..." I suggest decreasing the usage of "where" words and rewriting the sentence.

s26-27 "solar energetic particle events (SEPs)". SEPs is an abbreviation for solar energetic particles, but not events.

s35 "accelerated ions in the range 1-100 MeV/nucl can be detected ... ". What about higher energies? Is it possible to detect them?

s40 "Accelerated electrons and hot plasmas (>5 MK)". What is the energy range for accelerated electrons? Is the same as the temperature of the plasma? How does this T correspond to energy?

s65 "[SPARK; ? ]". Probably missing a citation.

Figure 2. What is the function of the dashed line?

Figure 2. Missing dot after citation.

Figure 2. If I understood the text correctly, the figure describes the X17 event on 28 October 2003, however, it is written as "2003 October 3" in the caption.

s98 "be localised . Note" -> "be localised. Note"

Figure 3. The same issue with the date of the event as for Figure 2.

s130 "in this event". What does "this mean"? The section describes different events and definitely cannot describe the October 2003 event, since the paper on which the narrative is based was published in 1991

s132-134. The authors do not emphasize that ref 41 describes only one event. I believe the results of one measurement hardly can be extrapolated to a general case.

Figure 4 caption. "100''from" ->"100'' from"

Figure 5 caption. Please provide a description or ref to the CLEAN algorithm.

Comments on the Quality of English Language

-

Author Response

1. s18 space-weather -> space weather

   Response: Fixed.

2. s20-22 "These are the locations, where ... where .... where..." I suggest decreasing the usage of "where" words and rewriting the sentence.

   Response: Agreed. We have added missing punctuation and altered the start of the sentence:

   “...due to turbulent motions at the ``footpoints'' below. Here, the magnetic field passes between the lower atmosphere, where the plasma dictates the motion of the field (high-beta plasma), to the corona, where the field dictates the motion of the plasma (low-beta plasma).”

3. s26-27 "solar energetic particle events (SEPs)". SEPs is an abbreviation for solar energetic particles, but not events.

   Response: Agreed. We have changed it to “solar energetic particle (SEP) events"

4. s35 "accelerated ions in the range 1-100 MeV/nucl can be detected ... ". What about higher energies? Is it possible to detect them?

   Response: We agree this sentence is ambiguous. The higher energy ions refer to those with energies above 100MeV/nucl which can be detected via the decay products of the pions they produce. We have altered the sentence after the one quoted by the referee:

   “Accelerated ions with energies greater than 100 MeV/nucleon can be detected via the decay of secondary pions…”

5. s40 "Accelerated electrons and hot plasmas (>5 MK)". What is the energy range for accelerated electrons? Is the same as the temperature of the plasma? How does this T correspond to energy?

   Response: We agree that this is ambiguous. X-ray emission from flare-accelerated electrons is not described with a temperature. In our original text, we meant for the “(>5MK)” to define what we mean by “hot”. We have reversed the order in which these are mentioned in the text, so it is clear that the 5MK only refers to the hot plasma.

   Regarding the energy range of accelerated electrons, thermal emission dominates at lower energies. Non-thermal emission dominates at higher energies when there are a sufficiently large number of interactions between accelerated electrons and the ambient medium. The precise energy at which the spectrum transitions from one regime to another is different for each flare, and for different times during a flare. It depends on the temperature and amount of emitting thermal plasma, as well as the number of accelerated electrons and their distribution. But typically the transition occurs in the range 15-20 keV. In addition to the above-mentioned change, we have clarified this with the following edit to the same paragraph:

   “Hot plasmas can therefore be distinguished by their Maxwellian-shaped (thermal) spectrum, which dominates at lower energies (<~20 keV), and accelerated electrons can be distinguished by their power-law-shaped (non-thermal) spectrum, which tends to dominate at higher energies (>~15 keV)”

6. s65 "[SPARK; ? ]". Probably missing a citation.

   Response: This is a link to another paper submitted to this special issue and so can be resolved as part of the published process.

7. Figure 2. What is the function of the dashed line?

   Response: The dashed lines and shading denote different phases of the flare. The right-most dashed line denotes the end of the decay phase of ion gamma-ray emission. We have added this explanation to the caption, and also included the colour in the text where these phases are mentioned.

8. Figure 2. Missing dot after citation.

   Response: Fixed.

9. Figure 2. If I understood the text correctly, the figure describes the X17 event on 28 October 2003, however, it is written as "2003 October 3" in the caption.

   Response: Thank you for spotting this. Your understanding is correct. We have fixed the typo in the caption of Figure 2.

10. s98 "be localised . Note" -> "be localised. Note"

    Response: Fixed

11. Figure 3. The same issue with the date of the event as for Figure 2.

    Response: Agreed. Fixed.

12. s130 "in this event". What does "this mean"? The section describes different events and definitely cannot describe the October 2003 event, since the paper on which the narrative is based was published in 1991

    Response: We agree this is ambiguous. Here we mean the event studied by Murphy et al. 1991. We have rewritten this section of the parapgraph to make this clearer:

    “One study computed both narrow and broad gamma-ray line spectra for the most abundant elements and accelerated ions in the solar atmosphere \citep{Murphy:1991}. It compared elemental abundances of a single flare observed with SMM/GRS with similar abundances measured in space for large proton flares and impulsive 3He events. Although uncertainties were large for most elements, \citep{Murphy:1991} found that Mg and Fe were significantly enhanced (>3–4 sigma) relative to C and O, similar to impulsive 3He-rich events.”

13. s132-134. The authors do not emphasize that ref 41 describes only one event. I believe the results of one measurement hardly can be extrapolated to a general case.

    Response: We agree with the referee’s comment here. The fact that we can’t extrapolate one measurement to the general case emphasises the need for an instrument like LISSAN. To make this clear, we have added the following sentence to the paragraph:

    “However, since this study only examined one event, it is unclear whether this result is representative of ion acceleration in flares.”

14. Figure 4 caption. "100''from" ->"100'' from"

    Response: Fixed.

15. Figure 5 caption. Please provide a description or ref to the CLEAN algorithm.

    Response: Agreed. We have added two references that provide a description of the CLEAN algorithm in the caption of Figure 5.