University of Oulu

Macotela, E. L., Raulin, J.‐P., Manninen, J., Correia, E., Turunen, T., & Magalhães, A. (2017). Lower ionosphere sensitivity to solar X‐ray flares over a complete solar cycle evaluated from VLF signal measurements. Journal of Geophysical Research: Space Physics, 122, 12,370– 12,377.

Lower ionosphere sensitivity to solar X‐ray flares over a complete solar cycle evaluated from VLF signal measurements

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Author: Macotela, Edith L.1; Raulin, Jean-Pierre2; Manninen, Jyrki1;
Organizations: 1Sodankylä Geophysical Observatory, University of Oulu, Sodankyla, Finland
2Centro de Radio Astronomia e Astrofisica Mackenzie, Universidade Presbiteriana Mackenzie, Sao Paulo, Brazil
3National Institute for Space Research, Sao José dos Campos, Sao Paulo, Brazil
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 0.5 MB)
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Language: English
Published: American Geophysical Union, 2017
Publish Date: 2019-09-24


The daytime lower ionosphere behaves as a solar X‐ray flare detector, which can be monitored using very low frequency (VLF) radio waves that propagate inside the Earth‐ionosphere waveguide. In this paper, we infer the lower ionosphere sensitivity variation over a complete solar cycle by using the minimum X‐ray fluence (FXmin) necessary to produce a disturbance of the quiescent ionospheric conductivity. FXmin is the photon energy flux integrated over the time interval from the start of a solar X‐ray flare to the beginning of the ionospheric disturbance recorded as amplitude deviation of the VLF signal. FXmin is computed for ionospheric disturbances that occurred in the time interval of December–January from 2007 to 2016 (solar cycle 24). The computation of FXmin uses the X‐ray flux in the wavelength band below 0.2 nm and the amplitude of VLF signals transmitted from France (HWU), Turkey (TBB), and U.S. (NAA), which were recorded in Brazil, Finland, and Peru. The main result of this study is that the long‐term variation of FXmin is correlated with the level of solar activity, having FXmin values in the range (1 − 12) × 10−7 J/m2. Our result suggests that FXmin is anticorrelated with the lower ionosphere sensitivity, confirming that the long‐term variation of the ionospheric sensitivity is anticorrelated with the level of solar activity. This result is important to identify the minimum X‐ray fluence that an external source of ionization must overcome in order to produce a measurable ionospheric disturbance during daytime.

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Series: Journal of geophysical research. Space physics
ISSN: 2169-9380
ISSN-E: 2169-9402
ISSN-L: 2169-9380
Volume: 122
Issue: 12
Pages: 12,370 - 12,377
DOI: 10.1002/2017JA024493
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: The SAVNET project was funded by FAPESP (Proc. 06/02979), and received partial funds from CNPq (482000/2011‐2), and Centre National de la Recherche Scientifique (CNRS).
Copyright information: © 2017. American Geophysical Union. All Rights Reserved.