Spring-Fall Asymmetry in VLF Amplitudes Recorded in the North Atlantic Region: The Fall-Effect

E. L. Macotela, M. Clilverd, T. Renkwitz, J. Chau, J. Manninen, D. Banyś

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4 Citations (SciVal)


A spring-fall asymmetry is observed in daytime amplitude values of very low frequency (VLF) radio wave signals propagating over the North Atlantic during 2011–2019. We explore the processes behind this asymmetry by comparing against mesospheric mean temperatures and the semidiurnal solar tide (S2) in mesospheric winds. The solar radiation influence on VLF subionospheric propagation was removed from the daytime VLF amplitude values, isolating the fall-effect. Similarly, the symmetric background level was removed from mesospheric mean temperatures undertaking comparable analysis. During fall, all three analyzed parameters experience significant deviation from their background levels. The VLF amplitude variation during spring is explained by the seasonal variation in solar illumination conditions, while the fall-effect can be interpreted as a mean zonal wind reversal associated with both a S2 enhancement, and temperature reductions. Decreases in temperature can produce decreases in collision frequency, reducing VLF signal absorption, driving the observed VLF asymmetry.

Original languageEnglish
Article numbere2021GL094581
Number of pages10
JournalGeophysical Research Letters
Issue number16
Early online date30 Jul 2021
Publication statusPublished - 10 Aug 2021

Bibliographical note

Funding Information:
This work is supported by "AMELIE - Analysis of the MEsosphere and Lower Ionosphere fall Effect" (DLR project D/921/67286532). The authors thank Alexander Kozlovsky for providing his insights for this research work, in particular for looking mesospheric temperature data. MAC would like to acknowledge support from the UK Research and Innovation (UKRI-NERC) through National Capability Space Weather Observatory funding (NC-SS SWO). Open access funding enabled and organized by Projekt DEAL.

Publisher Copyright:
© 2021. The Authors.


  • D-region
  • mesospheric temperature
  • semidiurnal solar tide
  • VLF propagation
  • VLF signal absorption

ASJC Scopus subject areas

  • Geophysics
  • General Earth and Planetary Sciences


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