Evidence for the Magnetoionic Nature of Oblique VHF Reflections from Midlatitude Sporadic-E Layers
Round 1
Reviewer 1 Report
This study reveals the true reflection mechanism of the midlatitude Es using the polarization behavior of radio wave signals. This is an interesting study and it could improve the understanding of the propagation of radio waves in the Es layer.
Minor issues
According to the secant law, if the Es layer could reflect the signals about 50 MHz, take the link UK-Hungary for example, the circle distance is about 1500km (please see Table 1 and Figure 12). It is assumed that the altitude of Es is about 90 km, and then it requires the critical frequency of the Es about 6 MHz by equation (1). The 110 km corresponds to about 7.5 MHz.
f_oblique=f_vertical/sin(atan(h_Es/750 km)) (1)
In Figure 12, the critical frequency of the Es is from 3.3 MHz to 5.6 MHz. It requires the altitude of the Es is about 85 km for the maximum value (5.6 MHz) if the Es layer could reflect the signals about 50 MHz.
However, authors state that the altitude of midlatitude Es is at altitudes of 90-130 km. I suggest that authors could explain it in the revised manuscript.
In conclusion, I suggest that the manuscript could be published after minor revision.
Author Response
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Author Response.pdf
Reviewer 2 Report
The article "Evidence for the Magnetoionic Nature of Oblique VHF Reflections from Midlatitude Sporadic-E Layers" by C. Deacon&al. is a thorough presentation of a radio measurement campaign aimed to investigate the polarization features of signals reflected from midlatitude sporadic-E clouds, so as to infer robust information about the reflection mechanism. The design of the measurement system (including a couple antennas of a seven-element loop-fed Yagi-Uda design) is extensively described - together with its calibration - in a previous work (doi: 10.1109/TAP.2020.3044654).
The experimental approach described in the present paper is detailed and systematic, and the topic fits the scopes of the journal. The subject is of practical interest for the Community, since sporadic-E formation heavily impacts radio communications. Also, this phenomenon has been raising the interest of the Space Weather Community for the last decade due to evidence of correlation between probability of Es occurrence and high values of Kp magnetic index upon major disturbances of solar origin.
I'm pretty fine with the manuscript in its current form, since concepts, data and deductions are clearly exposed with no unnecessary verbosity.
English language is excellent. My only suggestion is to split the long sentence at lines 106-109 into two separate portions to improve readability.
A comparison of these experimental results with outputs from modeling will represent a desirable extension of work in the near future.
I do recommend publication in Atmosphere.
Author Response
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Reviewer 3 Report
Review on " Evidence for the Magnetoionic Nature of Oblique VHF Reflections from Midlatitude Sporadic-E Layers"
This study investigates the polarization properties of radio signals reflected from Es layers which were received at a station in the UK using 50 MHz amateur radio beacons as signal sources during a measurement campaign in the summer of 2018. In all cases the received signals were elliptically polarized indicating that signals were reflected from midlatitude Es by magnetoionic double refraction. The topic is interesting, I agree with the authors that "Although there is a clear consensus about the principal mechanisms of formation of sporadic-E, the process by which oblique VHF radio wave reflection from Es clouds occurs has never been clearly established". The text is well written. The used method and the results are clear, furthermore the discussion of the results is adequate, too. Only some minor changes need to be made before the manuscript will be accepted for publication.
1) Comment to Figure 3. I suggest to show the geomagnetic field on the map since it is important for the section 3.2.
2) In more places in the text the authors write about separate Es events. However, it is not well defined what is a single or separate sporadic E event. The definition of an Es event is based on ionosonde data? In this case which ionosonde has been taking into account? How the number of Es events relates to the times shown in Table 1?
3) Comments to Figure 6 – 11. How do you define the sample? It seems to be defined based on time according to the caption of the Figures but it should be defined in the main text, too. Why did you choose exactly 6000 sample/s rate?
4) Comments to Figure 6 – 11. and/or the related text. I suggest to write the number of investigated Es events for every station in the text or in the caption of the Figs if you wrote it for certain cases (e.g. for Faroe Islands).
5) L337-338 “which was made up of regular extracts from a single sporadic-E event which was over 90 minutes long” My question is related to our previous comment, how do you know that it was related to “a single sporadic-E event which was over 90 minutes long”?
6) Comment to Figure 12. On the plot you show the critical frequencies of Es layers (numbers in circles) determined by different European ionosondes. As you wrote it was a long lasting Es layer/activity in Western Europe, and the foEs parameter showed large variation from ionogram to ionogram at the different stations during that afternoon. So for which time did you show/take into account the measured foEs parameters? Furthermore, I suggest to indicate the magnetic north or the geomagnetic field on the map.
7) L506 “over multiple sporadic-E events” Again the question arose what do you consider as a sporadic E event?
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
This is a very interesting work using amateur radio beacons as signal sources to analyze the reflection by sporadic E layers, Es. The study of Es is really welcome for the scientific community studying not only signal propagation, but ionospheric physics and space weather as well.
I consider that this work can be accepted for publication in this journal in its present form. I only have one minor comment:
If a HF signal reflects from the ionosphere, it is because when it enters the ionosphere from below, due to the ionospheric refractive index the signal is "bend" until it finally reflects from a given layer which can be in the E or F region. And this is explained by magnetoionic theory. Why do you mention as an option “scattering” or “specular reflection”.
I think that specular reflection is just a simplification involving the refraction in the ionosphere, which is a weakly ionized plasma embedded in the Earth’s magnetic field. This field in fact is the responsible for the double refraction. And as far as I know this always happens to an electromagnetic wave entering the ionosphere, that is it splits into an ordinary and an extraordinary wave. Why do you mention it as if it were a different process which could be responsible of the reflection? In fact, you mention all this in lines 478-484.
And regarding scattering, are there any examples of radar HF sky-waves which are reflected and detected back at surface due to scattering?
Very minor comment:
Page 1, line 30: “Because if its ...” should be “Because of its ...”
Author Response
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Author Response File: Author Response.pdf
Reviewer 5 Report
The authors present a study on the sporadic E phenomenon over central Europe performed by ground-based VHF Reflections analysis.
The documents is well written and the experimental setup is clearly described. All figures are necessary and of high quality. However, I feel there is a lack of physical interpretation. The sections contain detailed explanation on the technical properties of the received reflected signals. As a reader, I'm also interested in some information on the properties of the sporadic E clouds. What causes, e.g., a large spread in the axial ratio or the Major axis tilt? I would recommend to extent the Results and Discussion section with more detailed information of the sporadic E layer properties.
little typo in line 30: Because of...
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 5 Report
The authors have made significant changes to the manuscript in response to my previous review. These changes are satisfactory, and have helped to improve the manucript and make it even more easy to understand. I am satisfied with the revisions and recommend the publication of the manuscript in the present form.
