Review Reports - The Physical Image of Various Solar Radio Emissions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviewer's comments and suggestions are in the attached pdf file

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Comments are included in the attached pdf file

Author Response

The complete physical image of various solar radio emissions

Review

The manuscript presents an integral view of different phenomena that may result in solar radio emissions. Various sources, mechanisms, and parameters that contribute to solar radio emissions are discussed. Specific focus is given to presenting a clear view of the processes of formation and the various interactions involved, which can then help to disentangle the complex resulting picture and isolate the specific contributions to the detected radio signals.

The manuscript is a review paper that provides a comprehensive overview of the subject based on previous work by the authors and others, rather than presenting original results. The manuscript is conceptually well planned and composed, and it mostly provides a clear description of the relevant processes in the field, giving an informative and thorough overview that is well supported by references.

There is, however, room for improvement in terms of specific formulations and clarity of language. Some sentences, and in some cases whole paragraphs, have cumbersome structures and use imprecise formulations, both grammatically and in terms of the scientific terminology used. This reduces the clarity and flow of the text. We believe that addressing these issues would non-negligibly improve the readability and clarity of the manuscript and would allow the presented ideas to be conveyed more effectively.

Reply: Firstly, the authors greatly appreciate the reviewer's comments and suggestions. In the revised version, we have made detailed revisions to the whole text, added two figures (Figure 1 and Figure 4), corrected some inappropriate expressions in the original text, and corrected all spelling errors. Below are answers to all the questions in the review report.

Below are some, but not all, examples of such passages, together with suggestions for how they could be improved.

Line 193: The following passage (and the one after that) could be rephrased for better clarity and flow:

“Tan et al. (2025) proposed that the chromosphere should be heated by (is (probably) heated by? could be heated by? “should” here could be ambiguous) waves, including acoustic waves, MHD waves, and Alfvén waves. The corresponding spectral pattern is expected to consist of spike groups (perhaps something like “corresponding spectral pattern is characterized by groups of spikes” would read more naturally? Similarly in the remainder of the paragraph) at millimeter and short-centimeter wavelengths. In contrast, the TR and corona should be heated mainly by the MGP mechanism, with a partial contribution from magnetic reconnection. The corresponding spectral patterns are expected to include groups of fiber bursts, sporadic type III pairs, or spike pairs at long-centimeter, decimeter, and meter wavelengths.”

In addition, the acronym MHD is not defined when it is first introduced in the above passage.

Reply: in the revised version, we made such corrections.

Line 204:

“2.2 The solar radio bursts (SRB)

Unlike QRE, SRB typically has a very high brightness temperature and is rapidly changing. Correspondingly, their formation mechanism and physical conditions are much more complex and closely related to the plasma physical processes.”

SRB is used as a plural in the section title ("Solar Radio Bursts"), while it is used as a singular in the text ("SRB typically has..." and "is rapidly changing"). The authors should decide whether SRB is intended as a singular or plural acronym. If it is intended as a singular noun, SRBs should be used for the plural. Alternatively, the text could consistently refer to SRB in the plural, e.g. "SRB have..." and "are rapidly changing," as the subsequent text also appears to use the term in the plural.

Reply: We corrected them in the revised version.

Line 212:

“Considering the temperature of coronal plasma loops above intense active regions exceeding 2 or 3 million K, obviously higher than the surrounding quiet regions (1 or 2 million K), and the magnetic field is distinctly stronger than the vicinity”

This sentence should be rephrased, as its current form is confusing. The use of "Considering..." suggests that a conclusion should follow, but the sentence ends without such a conclusion. For example:

“Coronal plasma loops above intense active regions have temperatures exceeding 2–3 million K, significantly higher than those of the surrounding quiet regions (1–2 million K), and are associated with considerably stronger magnetic fields.”

The sentence following the above one could also be rephrased for better clarity and flow.

Reply: Thanks very much. We adopt this sentence to replace the original confusing description in the revised version.

Line 220:

“Actually, almost all large active regions are composed of a large number of magnetized plasma loops, whose foot-points undergo complicated movements such as approaching, separating, shearing, twisting, etc. driven by the convection of the photosphere, resulting in interactions such as collisions, shearing, twisting, etc. at the top of these loops extending into the corona, and forming a large number of local, short-lived, small-scale current sheets in the corona. Around these small-scale current sheet regions, non-thermal electrons can be accelerated to excite sporadic radio type I bursts superimposed on the type I storm background through coherent emission processes (plasma emission, or electron cyclotron maser emission, Melrose 2017).”

This passage could be rephrased for better clarity and flow. The sentence is very long and contains several consecutive processes and constructions, making the sequence of physical processes somewhat difficult to follow. In addition, similar processes (e.g. shearing and twisting) are mentioned both as footpoint motions and as subsequent interactions. The passage could be divided into several shorter sentences, with a clearer distinction between the photospheric motions, the resulting stressing and interaction of coronal magnetic structures, the formation of current sheets, and the subsequent particle acceleration and radio emission.

Reply: In the revised version, we rewrote this paragraph into: “Actually, almost all large active regions are composed of a large number of magnetized plasma loops, whose foot-points undergo complicated motions, such as approaching, separating, rotation, vortex and oscillations driven by the photospheric convections, resulting in interactions, such as kink, collisions, shearing and twisting around the upper part of these loops, and forming a large number of local, short-lived, small-scale current sheets in the corona. Around these coronal small-scale current sheets, non-thermal electrons can be accelerated to excite sporadic radio type I bursts superimposed on the background of type I storm through coherent emission processes.”

Line 230:

“For radio bursts with frequency drifting characteristics, the frequency drifting rate can be used to determine the velocity of the driver of emission source region,”

It is unclear which velocity is being described here. The phrase "velocity of the driver of emission source region" is ambiguous and could be interpreted in several ways. If the intended meaning is the propagation velocity of the radio-emitting source region, this should be stated explicitly. In the context of Type II bursts, if the source region is assumed to be associated with a propagating shock, it may be more appropriate to refer to the quantity as the shock propagation velocity.

Furthermore, the formula used to describe the velocity should either be derived in the manuscript or supported by an appropriate reference. The assumptions behind the relation should also be stated, in particular the assumption of the plasma emission mechanism and the use of a particular coronal plasma density model.

Reply: In the revised version, we rewrote this sentence as: “For radio bursts, the frequency drifting rate can be used to determine the velocity of the emission source (Tan 2013)”. Here, we added a reference of (Tan 2013) where presented a detailed derivation of Equation 2.

Line 270:

“Firstly, we must distinguish that Type IV bursts and the fine spectral structures superimposed on their background are two different types of explosive processes.”

The formulation "their background" is somewhat imprecise. If the intended meaning is that the fine spectral structures are superimposed on the continuum background associated with the Type IV burst, this should be stated more explicitly. The distinction between the Type IV burst itself and the fine structures superimposed on its emission should be described more precisely.

Reply: In the revised version, we rewrote this sentence as: “Firstly, we must distinguish type IV bursts from the fine spectral structures superimposed on them. They are actually two different types of emissions.”

Line 303:

In the following text, "explain" could perhaps be replaced with "illustrate." It is difficult for an image to "explain" complex phenomena such as those discussed here - "illustrate" would be more appropriate in this context.

Reply: In the revised version, we made these corrections.

Line 344:

The acronym ARE does not appear to be defined. Please define the acronym when it is first introduced (if it is intended to mean, for example, "Active Region Eruption" or something similar).

These are just some examples that could give authors an idea how to improve the text generally.

Reply: Here, it is a spelling mistake. The previous ARE should be QRE. We corrected it in the revised version.

Other smaller corrections:

Line 95: "dulk" → "Dulk"

Reply: In the revised version, we corrected it.

Line 165: "; while" → ", while"

Reply: Reply: In the revised version, we corrected it.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors, nice review!

Just a few comments

Is there a connection of the various types with other Wavelenghts?
What is the frequency of occurrence of these radio flares vs Flare intensity (e.g. X-class or M-class flares)?

A few more specific comments.

Fig. 1 what is the O-Mode?
L32 explain CMEs acronym
L80-L82 (e.g.) please adopt the same or "type" or “Type” all over along the text
L86 Which is the “fundamental plasma frequency”?
L101 What are the “reverse-sloped (RS) type III bursts”
L104, L122 different character size? (it might be just the PDF)
L149: could you quote the different energies?

Author Response

Replies to review 2

Dear authors, nice review!

Just a few comments

Is there a connection of the various types with other Wavelengths?

Reply: It is not difficult to see from Figure 2 that, except for quiet radio emission (QRE) that appears in almost all frequency range, other types of radio emission occur in different frequency ranges. The reason for this is that they are generated through different radiation mechanisms, corresponding to different physical processes and source regions. This is also a fundamental argument emphasized in this article. At the same time, different types of radio bursts also have corresponding signals in observations of other wavelengths. For example, radio type I bursts are often associated with bright structures in extreme ultraviolet (EUV) imaging observations above intensive active region, type II bursts are generally associated with shock waves in EUV imaging observations, type III bursts are often closely related to hard X-ray bursts, and type IV bursts are associated with bright structures in the imaging at high-temperature spectral lines of EUV in flaring active regions.

What is the frequency of occurrence of these radio flares vs Flare intensity (e.g. X-class or M-class flares)?

Reply: Radio bursts occur during most flare events. Generally, big radio events and shock-related bursts are more likely for strong flares, such as M-class and X-class flares. As for the very strong flare events, almost all will produce radio burst phenomena, for example, almost all X-class flare events are accompanied by radio bursts with very complex spectral structures.

A few more specific comments.

Fig. 1 what is the O-Mode?

Reply: In the revised manuscript, we have changed the original Figure 1 to the current Figure 2, removing some inaccurate and unnecessary information.

L32 explain CMEs acronym

Reply: The CME is an acronym of coronal mass ejections. We have made corresponding supplements in the revised version.

L80-L82 (e.g.) please adopt the same or "type" or “Type” all over along the text

Reply: In the revised version, we have corrected them.

L86 Which is the “fundamental plasma frequency”?

Reply: In the revised version, we rule out the word “fundamental”.

L101 What are the “reverse-sloped (RS) type III bursts”

Reply: in the revised version, we added “the normal type III burst means the frequency drifting rate is negative, while the RS type III burst mean the frequency drifting rate is positive”

L104, L122 different character size? (it might be just the PDF)

Reply: in the revised version, we have unified the font and character size throughout the entire text.

L149: could you quote the different energies?

Reply: Yes. Here we quote the different energies of electrons, the low energy means the nonrelativistic electrons with the factor , the mild energy means the mildly relativistic electrons with , and the high energy means relativistic electrons with .

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Solar radio emissions show considerable diversity in all of their aspects so any attempt for a unified treatment of them is welcome. The paper by Tan et al. is an effort in that direction but there are a few issues that the authors need to address before I recommend publication of their manuscript. My concerns are as follows.

1) Page 1. There is no radio emission coming from the photosphere. Even the high-frequency 239 GHz ALMA emission has been found to originate from around the temperature minimum.

2) In Figure 1 it is written "Quiet radio emission Broadband continuum ... O-mode". Assigning O-mode to quiet radio emission coming from free-free mechanism is totally wrong since that emission is slightly stronger in the X-mode. The same is true for gyro-resonance emission above sunspots.

3) Why do the authors consider the slowly varying component as part of the quiet radio emission? Traditionally (e.g. see the old books by Kundu, Zheleznyakov, etc) the slowly varying component forms a separate category in distinction from the quiet Sun component and the sporadic component. The choice for treating it separately stemmed from its unique temporal scales; shorter than the quiet Sun emission but longer than that of bursts.

4) In page 3, line 89, it is written that type II emission "...is always associated with fast coronal mass ejection (CME) or high speed plasma jet". The word "always" is not appropriate since counter-examples have been reported in the literature (e.g. see [Nindos et al. 2011, A&A, 531, A31], [Magdalenic et al. 2012, ApJ, 746, 152]).

5) Page 3, line 101: the authors refer to RS bursts without defining them first (the definition appears a few lines below). It should be the other way around: first define them and then discuss their drift rates.

6) Page 4, lines 162-168: how do the authors define spatial large scales and spatial small scales in radio sources?

7) Page 5, lines 180-182: magnetic fields associated with the network have been omitted.

8) Page 5. lines 184-189: this paragraph gives the impression that high-energy non-thermal electrons are always present above active regions. That's not correct; such electrons appear as a result of transient processes.

9) Page 5, lines 212-218: in the presence of strong magnetic fields (i.e. above sunspots), gyroresonance emission is produced at microwaves. The connection between gyroresonance and type I emission is not rigorously established.

10) In figure 3 some fiber bursts are associated with quiet Sun emission. But as the authors themselves admit (page 9, lines 391-392) fiber observations in quiet Sun areas are not available. Such a disclaimer should probably be included in Figure 3 (for example with a different color).

11) Gyroresonance emission above sunspots has been omitted from Figure 3. By the way, why do the authors use the term "cyclotron emission" which appears rarely in the literature and not the mainstream term "gyroresonance"?

Author Response

Page 1. There is no radio emission coming from the photosphere. Even the high-frequency 239 GHz ALMA emission has been found to originate from around the temperature minimum.

Reply: in the revised version, we removed all inaccurate and unnecessary information.

In Figure 1 it is written "Quiet radio emission Broadband continuum ... O-mode". Assigning O-mode to quiet radio emission coming from free-free mechanism is totally wrong since that emission is slightly stronger in the X-mode. The same is true for gyro-resonance emission above sunspots.

Reply: In the revised version and figures, we removed all inaccurate and unnecessary information.

Why do the authors consider the slowly varying component as part of the quiet radio emission? Traditionally (e.g. see the old books by Kundu, Zheleznyakov, etc) the slowly varying component forms a separate category in distinction from the quiet Sun component and the sporadic component. The choice for treating it separately stemmed from its unique temporal scales; shorter than the quiet Sun emission but longer than that of bursts.

Reply: In the original manuscript, we consider the slowly varying component (SVC) as a part of quiet radio emission (QRE) mainly because their change time-scales are too long compared to solar radio burst (SRB), so they can be regarded as part of the background emission. However, in order to follow the conventions, we still introduce and discuss SVC as a separate type of solar radio emission in the revised manuscript, which is reflected in sections 1.2 and 2.2, as well as in Figures 1, 2, 4, and 5.

In page 3, line 89, it is written that type II emission "...is always associated with fast coronal mass ejection (CME) or high speed plasma jet". The word "always" is not appropriate since counter-examples have been reported in the literature (e.g. see [Nindos et al. 2011, A&A, 531, A31], [Magdalenic et al. 2012, ApJ, 746, 152]).

Reply: In the revised version, we change this paragraph into: “Multi-wavelength observations show that most radio type II bursts are closely related to fast coronal mass ejections (CME) or high speed plasma jets, while the others may be associated with some impulsive flares (Nindos et al. 2011, Magdalenic et al. 2012).”

Page 3, line 101: the authors refer to RS bursts without defining them first (the definition appears a few lines below). It should be the other way around: first define them and then discuss their drift rates.

Reply: In the revised version, we added the following definition: “Here, when the sign is minus ‘-’ it means the normal type III burst with negative frequency drifting rate, and when the sign is plus ‘+’ it means the reverse-sloped (RS) type III burst with positive frequency drifting rate”

Page 4, lines 162-168: how do the authors define spatial large scales and spatial small scales in radio sources?

Reply: Here, we added the definitions: the large scale is defined as size exceeding several thousand km at least, such as flaring loops, coronal loops, active regions, and even the whole solar atmosphere, while the small scale is defined as size generally smaller than one thousand km, such as the local volume of plasma instabilities, magnetic islands, and the magnetic reconnection sites, or current sheets in the solar atmosphere.

Page 5, lines 180-182: magnetic fields associated with the network have been omitted.

Reply: in the revised version, we added the description as: the network and internetwork magnetic fields.

Page 5. lines 184-189: this paragraph gives the impression that high-energy non-thermal electrons are always present above active regions. That's not correct; such electrons appear as a result of transient processes.

Reply: In this paragraph, we clearly expressed that the non-thermal electrons are present in the atmosphere above solar quiet regions. They may generate from the heating processes, including wave heating, magnetic reconnection heating, and MGP heating occurring in the chromosphere, transition region, and even corona.

Page 5, lines 212-218: in the presence of strong magnetic fields (i.e. above sunspots), gyroresonance emission is produced at microwaves. The connection between gyroresonance and type I emission is not rigorously established.

Reply: Here, we discuss the emission in coronal loops above intensive active regions, their position is obviously higher than the flaring loops, and their magnetic fields should be stronger than the quiet regions (generally from several to decades Gauss, or say, < 30 Gauss). The interaction between the hot electrons and the magnetic field may generate GR emission at low harmonic numbers inevitably, and result in enhanced continuum emission in the frequency range of meter and decameter wavelengths (not in the range of microwave), namely radio type I storm.

In figure 3 some fiber bursts are associated with quiet Sun emission. But as the authors themselves admit (page 9, lines 391-392) fiber observations in quiet Sun areas are not available. Such a disclaimer should probably be included in Figure 3 (for example with a different color).

Reply: in the revised version, we use different blue arrows to indicate the possible fiber bursts: the solid blue arrows represent fiber bursts and the related MGP upflows observed in the plasma loops above active regions around solar flares, while the dotted blue arrows represent the possible fiber bursts of the MGP pumping electron upflows in the plasma loops above quiet regions which need to be confirmed by higher-resolutions and more sensitive observations.

Gyroresonance emission above sunspots has been omitted from Figure 3. By the way, why do the authors use the term "cyclotron emission" which appears rarely in the literature and not the mainstream term "gyroresonance"?

Reply: In order to follow mainstream, we have uniformly used the term gyroresonance (GR) in the whole manuscript.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Report on "The complete physical image of various solar radio emissions" by Tan et al.

I did not find any new information nor any new results in the paper. The manuscript is a mere tentative to review solar radio emissions but nothing that we don't know. Dm and meterwave solar emissions are known for more than 40 years. Important references are missing: McLean and Labrum book "Solar Radiophysics"; Elgaroy book for Noise Storms; Monique Pick papers on Type IV. At some places there is a large confusion on the bandwidth of radio bursts. At other places there are wrong statements: p.5 2.2 Noise storms are not CE. They are due to plasma emission, totally polarized (O mode), while apparently well stated and described in p.2.

The claimed results in conclusion p.9 on a "complete physical image" just list properties that we already know.

A scientific paper needs to bring to the community new results, new information. Otherwise it is a review paper. For all these reasons I consider that the manuscript should be rejected.

Author Response

Reply to Review report 4

Report on "The complete physical image of various solar radio emissions" by Tan et al.

The claimed results in conclusion p.9 on a "complete physical image" just list properties that we already know.

A scientific paper needs to bring to the community new results, new information. Otherwise it is a review paper. For all these reasons I consider that the manuscript should be rejected.

Reply: Yes. This paper is actually an invited review by the guest editor, who requires us to provide a short and clear, brief and complete physical image covering all types of solar radio emissions for solar physicists, astrophysicists and even master and PhD students. In this work, we besides to list properties of all types of solar radio emissions that we have already known, also add the various kinds of spectral fine structures overlapping on the background of different types of solar radio emission, such as the non-thermal heating signals of solar chromosphere, transition region and corona. An important aspect is our first attempt to use an image to fully illustrate the formation process of different types of solar radio emissions and their correlations. In order to make this paper more complete, we have added two additional figures (Figure 1 and Figure 4) in the revised manuscript to illustrate the interrelationships between different types of solar radio emissions. All the figures in this paper are carefully crafted by the authors for the first time, and we hope that this review can help readers better understand solar radio astronomy.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed all the issues that I raised in my first report and now I am happy to recommend publication of their manuscript.

Author Response

The author greatly appreciates the reviewer's recognition of this manuscript. We have carefully checked the whole text again, corrected some sentences and grammar to make it smoother.