Locating Earth Disturbances Using the SDR Earth Imager
Round 1
Reviewer 1 Report
Comments for the “Locating Earth disturbances using the SDR Earth Imager” by Sharif et al. 2022
This paper using SDR to image the Earth’s atmosphere. This is a really good work. However, I have some minor comments, see below.
1. At subsection 1.1, the ionospheric disturbances need more explanation or references. The mechanism included should be described,
For instance,
a). the detailed interaction between southward IMF Bz and geomagnetic field (Dungey, J. W. (1961). Interplanetary magnetic field and the auroral zones. Physical Review Letters, 6(2), 47–48. https://doi.org/10.1103/PhysRevLett.6.47).
b). The positive and negative disturbances in ionosphere during the geomagnetic storm (Liu et al., 2016, JGR 10.1002/2015JA021832.).
c). The roles of subauroral polarization streams on the disturbances in ionosphere (Wang et al., 2012, JGR, 10.1029/2012JA017656; Zhang et al., 2022 GRL, 10.1029/2022GL098623).
d). The disturbances in thermosphere could in turn affect the ionosphere (Zhang et al., 2017, GRL, 10.1002/2016JA023307).
e). The effects of solar flares on the ionosphere (e.g., Liu et al. 2021, Nature Physics, 10.1038/s41567-021-01203-5).
2. There have a number of works on the effects of Tonga volcano on the ionosphere. This might be better to be introduced and added, such as, Aa et al (2022), JGR 10.1029/2022JA030527; Harding et al. (2022), GRL e2022GL098577; Themens et al. (2022) GRL e2022GL098158.
3. Should you tell us the aim of your work at the end of introduction?
4. line 95, why the text is highlighted?
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This paper covers an interesting topic but promises more than it delivers. It would be better if it was rewritten with more emphasis on the method of locating ionospheric (not Earth - the source can be discussed later) waves (dot disturbances) using the SDR Earth Imager (including an example of the method from this paper), rather than trying to extrapolate one case of few measurements to a general conclusion. For instance, to confirm the source of waves to assumed power plants, more measurements in different geomagnetic conditions and different local time should be made to determine the consistency of direction on each receiving station. If possible, it would be good to know when the powerplants have scheduled yearly maintenance to determine if the waves are nor appearing in such periods. Also, characterization ionospheric waves in different ionospheric layers would be more detailed if several transmitting frequencies were used, reflecting from different ionospheric layers with different electron density. Maybe some of these ideas are not feasible to be conducted at the authors’ site, but they could be put in the method description for others to try on.
Section 1.1 puts a lot of weight on ionospheric disturbances originating from Earth opposed to space-weather-related disturbances. The description of space weather effects should be expanded, or the title changed to relate only to Earth-originating natural sources of disturbances. Also, ionosphere has its continuous dynamics and not all variations could be considered as disturbances. Authors should describe this dynamics more in the introduction.
Line 109 specifies a range of frequencies for the transmitter, but throughout the paper it seems that only one frequency was used. Sentence in lines 108 and 109 should be clearer about this matter.
Sections 2.2, 2.3 and 2.4 have a lot of repeating information and must be merged into one section, with Tables 1 and 2 also merged in a single table.
The sentence in lines 161-163 mentions the array detector's center. Is it a geometric center? If yes, why is not the grid origin set to this point? Figure 3 seems fine, but Figure 2, and related Figures 8 and 9 do not have the geometric center and grid origin in the same point.
Figure 9 is mentioned in the text before Figure 8. Their numbers and positions in the text should be replaced.
The sentence in lines 235-237 has error in stating cardinal and inter-cardinal directions (North-East).
Figures 13 and 14 should have subfigures marked as a) and b).
The Discussion states “There has been indisputable scientific evidence clearly indicating a link between man-made Earth disturbing events and the ionosphere response.” It is not clear if authors are referring to the results of this paper or some previous research, but in this paper such indisputable evidence is not present. Also, space as the source of disturbances is mentioned in the previous sentence, even though it is not mentioned in the results and analysis.
In the results presented in this paper there are no statistics on wave direction variability throughout the measurement time, which would be necessary to define the direction of the waves. The number of measurements is not specified, nor its timespan, local time etc. It is not clear why was this exact frequency used, nor its connection with electron density and the reflection ionospheric layer height.
Smaller remarks:
In line 9 the author’s name is repeated before his affiliation.
Sentence in lines 9 and 10 is not clear.
Sentence in lines 52-54 misses a reference.
Degree markings in line 98 and 99 are incorrect.
Purpose of line 95 is unclear.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
Review of “Locating Earth disturbances using the SDR Earth Imager” by Sharif et al., 2022
This work aims to investigate properties of waves in the ionosphere F-layer using phase information extracted from radio signals reflected by the F ionization layer and capture by two high resolution cameras, namely the University of New Mexico’s Long Wavelength Array’s LWA-1 and LWA-SV. The novel approach proposed can help identifying wave’s phase shift, frequency and wavevector. However, the use of these information to establish the wave source, important for the conclusion of the paper, need clarification.
There are many unclear points in the procedure used and some of the main results provide questionable results. As such, this work needs extensive revision prior to consideration for publication. Below are more specific comments.
Major comments
Line 38 “space weather events including solar flares and sunspots” -> Sunspots by themselves are not space weather events. The phenomena that happen in association to sunspots (flares, CME, etc..) lead to space weather events. Also, appropriate references regarding waves generated by space weather events should be included.
Line 49-50 “A number of researchers …” -> This statement without references is not appropriate. Possible examples are https://www.nature.com/articles/s41598-021-88125-7 and https://www.mdpi.com/2072-4292/12/20/3299?type=check_update&version=2
Line 50-51 “gravity waves … have waves that affect the ionosphere” -> Please clarify this statement. Gravity waves generate other waves? If so, what kind of waves? Again, without appropriate references, it is not clear what is the meaning of the sentence.
Line 52-53 What/who is the “one observer”? Again, missing references.
Line 141 There is ambiguity in the description of the results. Figure 5 is named “waterfall plot”, but the figure shows the spectrogram (also known as dynamic spectrum) in a limited frequency range for data from three antennas. Are the waterfall plot and the spectrogram the same entity in this context?
Line 143-144. “The relative unwrapped phase difference of the antenna reference of this channel …” There are many unclear points. Phase difference between what? Which antenna? Which channel? Please clarify this sentence.
Line 145-148 What do the author means by “distinguish the carrier frequency from the … Fourier Transform”? Frequency and STFT are two different entities. Also, the parameters listed seems like the ones used to perform a STFT to estimate a spectrogram. Usually, when performing a STFT the data in the running interval are weighted with a certain window (Hann, Hamming, etc …). Here, a Hamming window is applied to the spectrogram. Is the window applied in time, frequency, or both? If this is the case, what are the parameters of the window (number of points, width in time and/or frequency)?
Line 161 It is unclear how the relative phase is evaluated from the comparison of each antenna with the one used as reference. Is the phase evaluated from the time difference (∆t) and the frequency (f) as ∆φ=2πf*∆t? Also, the time difference based on the arrival of the carrier wave is well defined for the first detection of the radio wave from each antenna. However, how is the time difference defined in the rest of the time interval and, consequently, the phase difference? I might have misunderstood the procedure used for this step. Maybe a dynamic cross-phase analysis between the data of pairs of antennas was used? Please clarify.
Line 179-180 “the rest of the rectangles of the mesh account to be zero” -> While adding zero outside the area covered by the antennas has little effect on the 2D FFT (this operation is usually referred to as zero-padding), adding zero inside that area can results in spikes and introduction of spurious results. For example, in Figure 9, there are series of antennas with similar large values, based on the color scale, separated by empty bins. While this suggest that those areas have little variation around a non-zero phase value, adding a zero in the empty bins would create spurious fluctuations. An alternative approach could be the use of interpolation to fill the values in the empty bins inside the area covered by the antennas.
Line 180-181 The center frequency, the carrier bin, and their relation is not described. In addition, the values reported are different from the values used in Figures 11-14.
Figures 11-14. The 2D FFT reported in the figures show the same time, carrier frequency, center frequency, and file name for LWA-SV and LWA-1, respectively. However, the 2D FFT results shown are different: Figure 1a compared to Figure 1c; Figure 12a vs 13a vs 14a. Since the result for each LWA are relative to the same spatial phase image how can the results be different?
Line 267-269. This is an important point of this work: the use of the wavevector to identify the source of the wave. This approach is ambiguous due to the previous comment on Figures 11-14. It is unclear why there are different wavevectors (due to different 2D FFT results) at the same time. If the results correspond to different times than the results would indicate an evolution of the wave spatial structure in time, but this would not support the local origin of the wave. In addition, since the 2D FFT provide peaks at location symmetric with respect to the origin to indicate the occurrence of a wave, the intersection of the wavevector will always be the origin. In conclusion, it is not convincing that this procedure provides the source region of the wave without further information and clarifications. In the discussion (line 295), a triangulation approach is mentioned, even though this is not discussed in the body of the text.
Line 302-304 Other than the possible source of the wave above the power generating station, there are no other evidence that suggest the relation of the ionospheric disturbances with the power station. Without clarifying the previous comment, the conclusion of this work are also questioned.
Minor comments
Line 35-36 “This region is divided into three layers based on ion densities” -> Please specify the layers (i.e., D, E, F) and/or add a reference.
Line 36-37 “They are considered the most sensitive Earth entities, lighter than air” -> The subject of this sentence is unclear. In the present form, it seems to refer to the three layers, but then it is unclear what the authors means with “lighter than air”.
Line 47-48 Please specific which Earth’s ionization layer. Is the ionosphere F-layer?
Line 95 “New Mexeico” -> New Mexico
Line 76 The phase shift of what? Please clarify. If the phase shift corresponds to the definition at line 86-87, please use this sentence first and then add comments about the phase shift, including the one at lines 85-86. Also, note that the is a similar sentence at lines 90-91.
Sections 2.2–2.5 Some of these subsections comprise one or two sentences. Consider join them in a unique section including the properties of the LWAs, transmitter and their relative location.
Line 163 “respectfully” -> respectively
Figure 9 -> Possible typo. The step along the Y direction (5m) does not match the number of bins showed. The Y axis shows 40 bins that for 100 m corresponds to dy=2.5 m.
Equation 3 -> Imaginary unit j should be removed.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Even though some review points were addressed, the paper still seems unfinished. Even the answer to the reviewer contains conversation between the coauthors “Radwan, state the time span…” which was actually not taken into account in the revised version of the manuscript. That review point remains the main concern regarding this paper and needs to be answered:
- In the results presented in this paper there are no statistics on wave direction variability throughout the measurement time, which would be necessary to define the direction of the waves. The number of measurements is not specified, nor its timespan, local time etc.
Other comments:
The section “Space Weather Disturbances” is unnumbered and contains typos like “Anther” instead of “Another” etc.
Figures 2, 8 and 9 remained unchanged and their geometric center is not the coordinate system origin, even though authors stated this point as corrected.
Authors did not correct the error where they stated a wavevector had direction from North to East (as opposed to North-South in previous case). Vector origin and end must be 180° apart, so North-East combination is not possible, only North-South, West-East or Northeast-Southwest. Which of those was it?
There is a title “4. Discussion” which should be deleted as the correct title “4. Discussion of LWA-1 and LWA-2 Results” follows afterwards. Parts of the section 4 have different margins, line spacings and font sizes, with extra blank space before the Conclusion.
Author Response
Please see the attachment
Author Response File: Author Response.docx
Reviewer 3 Report
Review of “Locating Earth disturbances using the SDR Earth Imager” by Sharif et al., 2022
The authors have addressed most of my comments. I found this work publishable pending some minor comments listed below.
Line 43 – “including solar flares and sunspots” -> The authors provided very good additional points about space weather phenomena, but they have not fully addressed my previous comment. Please, either remove the word “sunspots” or clarify the role of sunspots, for example “including solar flares and coronal mass ejections originating in sunspots”.
Line 144-145 – “a magnetic storm is a sudden disturbance of the ionosphere caused by the solar wind.” -> A geomagnetic storm comprises more than just ionosphere disturbances. Please rephrase this sentence. For example: “a geomagnetic storm caused by the solar wind determines strong disturbances of the magnetosphere-ionosphere-thermosphere system.”
Line 268-269 – Thank you for clarifying the zero-padding step applied outside the image. However, it is not specified how the empty rectangles inside the image (as evident in Figure 9) were treated before applying the 2D FFT. Please clarify as this would benefit the reproducibility of the results and the understanding by the reader.
Figure 11 – While the authors have clarified the difference in wavevectors between the two LWAs, there is still an unclear point about the results on the same LWA. The 2D FFT reported in Figure 11 are both relative to a phase image from LWA-SV at the same time, carrier frequency, center frequency, and file name (as reported in the title of Figure 11a and 11c). A brief clarification in the caption would be sufficient for the reader to understand that there was a short time difference between the two. If possible, provide the time difference.
Figure 12-13-14 – Same as Figure 11. Clarify in the caption that there was a short time difference between 12a, 13a, and 14a (all relative to LWA-1). If possible, provide the time difference.
Some typos:
Lines 62-65 – Please rephrase these sentences. For example: “Dungey explored the dynamics of the magnetosphere system while interacting with solar wind plasma carrying a southward interplanetary magnetic field. The model discussed in this study clearly illustrated the link between auroras and neutral points in the magnetic field [10].”
Line 200 – received
Line 201 – inonspher
Line 247 – “respectfully” change to “respectively”
Line 367 – collcating
Line 373 – emminating
The authors removed the Imaginary unit j from equation 2 instead of equation 3. Please add it again in equation 2 and remove it from equation 3.
Author Response
Please see the attachment
Author Response File: Author Response.docx
