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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Status: open (until 12 May 2024)
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RC1: 'Comment on angeo-2023-38', Anonymous Referee #1, 01 Feb 2024
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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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