the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Investigation of the October effect in VLF signals
Abstract. Subionospheric Very Low Frequency (VLF) radio signals are reflected by free electrons in the ionospheric D-region at about 60–90 km altitude and can propagate over long distances, which makes them useful for monitoring the state of the D-region or perturbations due to solar flares. At the D-region height, the ionosphere is mainly ionized by the solar Lyman-α radiation. The reflection characteristics of VLF signals depend on the state and dynamics of the D-region which is highly influenced by the Lyman-α radiation. Although the amplitude of the received terrestrial VLF signal changes as a function of solar zenith angle over the course of the year, the VLF amplitude shows a distinctive sharp decrease around October, which is hence called the “October effect”. This study investigates the occurrence of the October effect and its dependencies on latitude and longitude. We developed a method to detect the occurrence of the October effect in the long-term VLF data and derive key parameters characterizing (start and end date, intensity) the sudden decrease in the signal amplitude. This investigation using a network of VLF stations distributed over low, middle and high latitude regions shows that the occurrence of the October effect has a clear latitudinal dependency, occurring earlier in high-latitude regions than at mid-latitudes. No low latitude signature is found.
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RC1: 'Comment on angeo-2023-38', Anonymous Referee #1, 01 Feb 2024
Revieuw of: Investigation of the October effect in VLF signals, by Marc Hansen et al submitted to Annales geophisicae.
The paper study focuses on the October effect using the VLF signal propagation technique. In its form, the paper presented materials are not convincing and should be revised. So I reject the publication of the paper.
-) In the data processing, it is not clear what the authors did when the noon time coincided with ionospheric disturbance times such as solar flares, lightning, early events, geomagnetic storms…
-) The color code used in the presented results are not clear and from figure to other the color code changes. For example, in figure 3 the NDK-KILL path is cyan colored but in figure 4 is violet. I recommend unifying the color coding for each path and using more forms not only two filled triangles. Also, use open forms rather than filled forms.
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AC1: 'Reply on RC1', Marc Hansen, 05 Jun 2024
Dear Reviewer,
We thank the reviewer for careful reading and for all the constructive suggestions. Please find our answers with references to our manuscript below.
Our idea for this paper was to statistically investigate the October effect and its occurrences. Together with other researchers at our institute, we try to find the physical explanation of the October effect, which than is planned to be presented in an additional paper. For example, in the recently published paper [Wendt, V., Schneider, H., Banyś, D., Hansen, M., Clilverd, M. A., & Raita, T. (2024). Why does the October effect not occur at night? Geophysical Research Letters, 51, e2023GL107445. https://doi.org/10.1029/2023GL107445], it is discussed why the October effect does not occur in the nighttime. As the October effect is a broad topic, which includes investigating possible correlation to neutral dynamics in the atmosphere and combining all this to model the October effect with the Long-Wavelength Propagation Capability (LWPC), this paper aims to create a basis for this ongoing investigation.
Comment 1: “In the data processing, it is not clear what the authors did when the noon time coincided with ionospheric disturbance times such as solar flares, lightning, early events, geomagnetic storms…”
Answer 1: To further clarify what happens when the noon time coincides with ionospheric disturbance times, we would add a paragraph to explain how the method handles such events.
Comment 2: “The color code used in the presented results are not clear and from figure to other the color code changes. For example, in figure 3 the NDK-KILL path is cyan colored but in figure 4 is violet. I recommend unifying the color coding for each path and using more forms not only two filled triangles. Also, use open forms rather than filled forms.”
Answer 2: Thank you for noticing this ambiguity. The color code used in the presented results is confusing, we would change it in a way to unify the color coding, while still showing the different dependencies of latitude and longitude. Also we will switch to open and more clearer forms.
With kind regards,
The authors
Citation: https://doi.org/10.5194/angeo-2023-38-AC1
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AC1: 'Reply on RC1', Marc Hansen, 05 Jun 2024
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RC2: 'Comment on angeo-2023-38', Anonymous Referee #2, 02 May 2024
In this paper authors investigates the occurrence of the October effect i.e. rapid decrease in VLF signal. This paper is a continuation of the author's research published in a recently published paper Wendt, V., Schneider, H., Banyś, D., Hansen, M., Clilverd, M. A., & Raita, T. (2024). Why does the October effect not occur at night?. Geophysical Research Letters, 51(7), e2023GL107445. Although the topic is very current and important, the submitted text cannot be accepted in its current form.
In order to clarify some needed details about the data and methods (addition X, gamma influence on VLF, etc.) I suggest the authors to create a github project with them.
In Sec. 6. Conclusion I suggest the authors to present the most important conclusions as a bulleted list or similar.
Citation: https://doi.org/10.5194/angeo-2023-38-RC2 -
AC2: 'Reply on RC2', Marc Hansen, 05 Jun 2024
Dear Reviewer,
We thank the reviewer for careful reading and for all the constructive suggestions. Please find our answers with references to our manuscript below.
Our idea for this paper was to statistically investigate the October effect and its occurrences. Together with other researchers at our institute, we try to find the physical explanation of the October effect, which than is planned to be presented in an additional paper. For example, in the recently published paper [Wendt, V., Schneider, H., Banyś, D., Hansen, M., Clilverd, M. A., & Raita, T. (2024). Why does the October effect not occur at night? Geophysical Research Letters, 51, e2023GL107445. https://doi.org/10.1029/2023GL107445], it is discussed why the October effect does not occur in the nighttime. As the October effect is a broad topic, which includes investigating possible correlation to neutral dynamics in the atmosphere and combining all this to model the October effect with the Long-Wavelength Propagation Capability (LWPC), this paper aims to create a basis for this ongoing investigation.
Comment 1: “In order to clarify some needed details about the data and methods (addition X, gamma influence on VLF, etc.) I suggest the authors to create a github project with them.”
Answer 1: To further clarify the influence of x-ray and gamma radiation on the propagation of VLF waves, we will add a paragraph explaining the connection and physical effects.
As data from the GIFDS network is not publicly available, we would try to provide a possibility to download the used data. In order to clarify the used method, we would like to expand the Method section in the paper and explain it in more detail.
Comment 2: “In Sec. 6. Conclusion I suggest the authors to present the most important conclusions as a bulleted list or similar.”
Answer 2: Thanks for the hint to better get our conclusions across. We will add a bulleted list to the Conclusion section.
With kind regards,
The authors
Citation: https://doi.org/10.5194/angeo-2023-38-AC2
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AC2: 'Reply on RC2', Marc Hansen, 05 Jun 2024
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