Detection of Solar Flares from the Analysis of Signal-to-Noise Ratio Recorded by Digisonde at Mid-Latitudes

Round 1

Reviewer 1 Report

Review on "Detection of Solar Flares from the Analysis of Signal-To-Noise Ratio Recorded by Digisonde at Mid-latitudes "

This study presents a new ground-based method to detect the ionospheric absorption variation caused by solar flares using the signal to noise ratio (SNR) measured by a DPS4D digisonde during flare events and comparing it to the reference days. The method has been applied on the data measured at the Ebro Observatory (midlatitude) during 262 X-, M-, and C- class events occurred in the period 2011-2014. The method detected 77 flares from the 262 events occurred during the investigated period. However, the authors analyzed the different reasons for the lack of the detection and determined three physical constraints that flares must accomplish to produce absorption effects in the ionosphere, thus to be detectable by the method. The topic is important because solar flares can cause shortages and blackouts in HF radio communications and the riometers, dedicated to measure the ionospheric absorption, are operating at higher latitudes. Therefore, a reliable method using the data of digisondes, which are available at low- and midlatitudes as well, would be very relevant. The text is well written. The introduction provides sufficient background and the research design is appropriate. Furthermore, the results are clear and their discussion is adequate, too.

Nevertheless, some questions have to be answered especially related to the method before the manuscript will be accepted for publication.

General comments:

1.) To the “Data and Method of Detection” section: Because it is a non-conventional use of the digisonde data and SAO-X I miss a technical description of the used data. How the amplitude and the SNR can be extracted from SAO-X and more importantly the extracted files how can be interpreted? Actually, it is not described/detailed in the manual (https://digisonde.com/pdf/Digisonde4DManual_LDI-web.pdf) and/or in the paper Reinisch et al. (2005) cited by the authors. It is important to know if one wants to repeat the developed method and apply it on data measured by another DPS4D digisonde as it is suggested by the authors. Therefore, I suggest to upload a technical description of the method as a supplementary material.

2.) To the “Data and Method of Detection” section: Which record has been analyzed? The first detection point after the flare or that one when the time difference was minimal? It will be clear later in the manuscript but it has to be clearly define here as well according to my opinion.

3.) “Data and Method of Detection” section, Line 177-179: “Thus, to confirm a detection, we imposed that the event had to be under -20dB for at least four consecutive frequencies” with 0.25 MHz frequency step? It is not evident for me here. The investigated frequencies started from the first detected freq. (so from fmin)? Or it is not important that the -20dB difference should be at the frequencies close to the fmin?

4.) To the “Data and Method of Detection” section. Did you take into account a time window after the peak time of the flare? Or do you analyze one record for one flare? How did you handle the strong X-class flares when a longer total radio fadeout can occur?

5.) To the Eq. 2. and the solar latitude/altitude angle: The authors use different term for ?? in the manuscript. They define ?? as "local solar latitude angle" in line 240 and by Eq. 2. However later they refer for ?? as elevation angle or solar altitude angle. It is a bit confusing for the reader. What is the correspondence between ?? defined by the Eq. 2. and the solar altitude/elevation angle or zenith angle? Probably a figure about these used angles, the used coordinate system can be useful because the findings related to the solar latitude/altitude angle are important part of the manuscript.

6.) To the Fig. 6. and the analysis related to that. We can see the asymmetric feature of the variation of the X-ray flux on The Fig. 6. However, the ionospheric absorption variation caused by the flares is even more asymmetric as it can be seen in the temporal changes of the fmin parameter before and after the flares. I wonder that the method developed by the authors can show a similar variation of the ionospheric absorption in time if they would investigate 5 min resolution data?

Minor comments, typos:

- Line 212-216: “By checking the SGAS, we noticed 43 events under these adverse conditions. Notwithstanding this, the method was able to detect the effect of 77 solar flares out of 219 not affected by adverse ionospheric conditions.” These sentences are not clear for me.

- Number of the sections: 3.1 ; 3.2; 3.3…

- Note to the Table 1. Whether the M4.3 and M1.2 flares can not be detected if the method would take into account the first record after the peak time of the flare? There is not high changes in the ionospheric absorption before the peak time of the flare, at least we see that in the variation of the fmin parameter detected by the ionosondes.

- Line 384. “solar flare event set events by using Equation 4” Too many words in this part of the sentence, I think.

- To the Table 2. Suggestion: use the same size for +/- 1 in that table what you use for the top and bottom indices in the other cases

- Caption of the Fig. 10. I suggest to write E_(eff) and H_(eff) after the  geoeffective hard X-Ray (E_(eff) ) and geoeffective hard X-Ray radiant exposure (H_(eff)) in brackets. It can help the understanding of the reader.

Author Response

Please see the attachment.

In the pdf we answered, in italics, point by point the comments of the reviewers.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper propose a new method to detect solar flare effect on ionosphere sounding measurements. And it has been applied on a local ground observation equipment in mid-latitude. In the introduction, the author summarized previous studies shortages, which reflects the necessaity of carrying out a new method to detect solar flare effect. The following content is also well organized and written. The results and application can be a potentially important contribution to the measurements and theory in Aeronomy. Therefore, I recommend this paper to be published in present form. 

Author Response

Please see the attachment.

In the pdf we answered, in italics, point by point the comments of the reviewers.

Author Response File: Author Response.pdf

Reviewer 3 Report

Very interesting. The method could be very useful for solar flare detection. It would be worth publishing after major revisions:
(1) The description of the signal-to-noise (SNR) measurement that forms the foundation is incomplete and unclear, and should be added.
(2) It should also be explained why the method does not base its detection of solar flares on a direct measure of ionospheric absorption, e.g. by comparing the current signal amplitude (not SNR) with expected amplitudes based on historic information. Or maybe it does (the text is very unclear on this point), but then the term SNR is totally misplaced and the text should be corrected on this point.
(3) The reference list shows a substantial amount of self-citation. We congratulate the authors on all your work. However, is this the only relevant literature that could be found and are all these references to yourself the only literature supporting your work? What do other researchers conclude? Please review carefully and balance your self-citation, and discuss the work of others.
(4) Referring to the above: work of other authors, not listed in this paper, suggest that both signal amplitude and background noise drop due to ionospheric absorption caused by a solar flare. The authors have not clarified why then the SNR decreases. This should be discussed, as it is essential to the described method.
(5) Limitations of the measurement system and observation methods, and suggestions for possible improvements, are lacking in the summary and conclusions section.
Detailed feedback per line number:
8 “on the strength of ionospheric reflected radio signals.” Not strength, but signal-to-noise ratio, see title.
16 “and observed for daylight hours at EB040.” Not ‘for’, but ‘during’ daylight hours.
120-123 “seven observable parameters of reflected radio signals received from the ionosphere: frequency, range (or height for vertical incidence measurements), amplitude, phase, doppler shift and spread, angle of arrival and wave polarization.” These are 8 parameters, not 7.
126 “These amplitudes are used to obtain the SNR of a given radio wave.” According to #120-123 the amplitude of the received signal is observed, but not the noise level. To calculated the SNR, the noise level must be known as well. Please detail how the noise level measured. And explain if it is it measured on the same frequency or on adjacent frequencies? Is the noise level averaged over a longer time or is it measured real time?
124-126 “For this study, we considered amplitudes of the first echo radio wave reflected from the ionosphere, obtained by vertical incidence soundings and with ordinary wave polarization.” Why is the ordinary wave used and not the extraordinary wave? Is this an arbitrary choice, or is there a distinct motivation for that? And why are the not used both, would that not increase the reliability of the observations? Please explain.
132-135 “SAO-X makes possible to extract amplitudes for each reflected radio signal as well as the most probable amplitude, both in decibel (dB) units. Finally, SNRs are computed by subtracting the most probable amplitude from the signal amplitude.”
The “most probable amplitude” of what? Of a lot of observations? Why do you treat that as the noise level? Or is it the most probable amplitude when no signal is received? These two sentences are totally unclear. Please replace them with a proper explanation that the reader can understand and relate to. After all, this is the essential observation all your work builds on.
136-139 “Curto et al., 2018 shown that SNRs are strongly reduced in ionograms under solar flare conditions compared to the non-flare conditions, even causing a total wave fadeout. By using this effect, we built a method to detect the solar flare effects in the SNR by comparing the current SNR measurement with a SNR pattern for non-flare conditions.”
The authors support their work by self-citation. Please review and discuss other work in the field. For example: in <<Witvliet, B.A., van Maanen, E., Petersen, G.J. and Westenberg, A.J., 2016. Impact of a Solar X-Flare on NVIS Propagation: Daytime characteristic wave refraction and night-time scattering. IEEE Antennas and Propagation Magazine, 58(6), pp.29-37.>> not only the signal amplitude but also the ambient electromagnetic noise level drops due to increased ionospheric absorption caused by a solar flare. As this article assumes that the SNR drops, can the authors explain what causes the signal amplitude to always be reduced by more dB than the background noise level?
191-193 “Additionally, to extend the X-class flares sample, we added to the event set 4 more X-class flares occurred during 2011—2012 at daylight hours.” It should be “we added to the event a set of…”
197-198 “These false positives may be related with other phenomena that affect the ionosphere” Could it not be caused by a sudden increase in ambient electromagnetic noise decreasing the SNR?
232-234 “which provides 5- and 1-minute averaged flux for the wavelength bands of 0.05–0.4 nm (short channel), and 0.1–0.8 nm (long channel), in W/m2 units (Machol, 2016).” The terms “long channel” and “short channel” are not understood. Maybe the author means “long wavelength” and “short wavelength”? But they are overlapping and similar in wavelength, so it is weird to call them “long” and “short”. Please modify. A solution could be to delete these terms.
261-262 “The latter have a special impact in the flare detection when the ionospheric time sampling of measurements is coarse.” Vague formulation. When is it coarse, what sampling time? Please be specific. Also, what sampling time would the authors consider to be optimal?
539-622 “Summary and conclusions.” The summary and conclusions now only state what is good about the method. The observations are now done with a previously designed ionosonde. So, this is a (valuable) by-product of existing equipment, originally intended for another purpose. Could the authors also discuss what modifications to the observation equipment or the observation method would further improve the observations and the detection accuracy and latency? Would a stand-alone system especially for these observations yield better results if not, why not?

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

The point-by-point to the reviewer are in italics in the attached document

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Compliments to the authers for the revisions and the addendum. Interesting subject. I believe the article in it's present form merits publication.