Preprints
https://doi.org/10.5194/angeo-2023-38
https://doi.org/10.5194/angeo-2023-38
11 Jan 2024
 | 11 Jan 2024
Status: a revised version of this preprint is currently under review for the journal ANGEO.

Investigation of the October effect in VLF signals

Marc Hansen, Daniela Banyś, Mark Clilverd, David Wenzel, and M. Mainul Hoque

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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Marc Hansen, Daniela Banyś, Mark Clilverd, David Wenzel, and M. Mainul Hoque

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on angeo-2023-38', Anonymous Referee #1, 01 Feb 2024
    • AC1: 'Reply on RC1', Marc Hansen, 05 Jun 2024
  • RC2: 'Comment on angeo-2023-38', Anonymous Referee #2, 02 May 2024
    • AC2: 'Reply on RC2', Marc Hansen, 05 Jun 2024
Marc Hansen, Daniela Banyś, Mark Clilverd, David Wenzel, and M. Mainul Hoque
Marc Hansen, Daniela Banyś, Mark Clilverd, David Wenzel, and M. Mainul Hoque

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Short summary
The VLF amplitude does not show a symmetrical behavior over the year, which would be expected from its dependency on the solar position. The VLF amplitude rather shows a distinctive sharp decrease around October, which is hence called the “October effect”. This study is the first to systematically investigate this October effect, which shows a clear latitudinal dependency.