Abstract
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.
Original language | English |
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Pages (from-to) | 12370-12377 |
Number of pages | 8 |
Journal | Journal of Geophysical Research: Space Physics |
Volume | 122 |
Issue number | 12 |
DOIs | |
Publication status | Published - Dec 2017 |
Bibliographical note
Funding Information: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). CHIANTI is a collaborative project involving George Mason University, the University of Michigan (U.S.), and the University of Cambridge (UK). Data availability is described at the following websites: https://thesavnet724.wordpress.com/ data-download/ (SAVNET), http://www. sgo.fi/Data/VLF/VLF.php (Kannuslehto), http://lasp.colorado.edu/lisird3/data/ composite_lyman_alpha/ (LISIRD Lyman α), and https://umbra.nascom. nasa.gov/sdb/yohkoh/ys_dbase/ (GOES X-ray).
Publisher Copyright:
©2017. American Geophysical Union. All Rights Reserved.
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). CHIANTI is a collaborative project involving George Mason University, the University of Michigan (U.S.), and the University of Cambridge (UK). Data availability is described at the following websites: https://thesavnet724.wordpress.com/ data-download/ (SAVNET), http://www. sgo.fi/Data/VLF/VLF.php (Kannuslehto), http://lasp.colorado.edu/lisird3/data/ composite_lyman_alpha/ (LISIRD Lyman α), and https://umbra.nascom. nasa.gov/sdb/yohkoh/ys_dbase/ (GOES X-ray).
Keywords
- ionospheric disturbances
- ionospheric sensitivity
- lower ionosphere
- minimum fluence
- VLF technique
ASJC Scopus subject areas
- Space and Planetary Science
- Geophysics