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Lower Ionosphere Sensitivity to Solar X‐ray Flares Over a Complete Solar Cycle Evaluated From VLF Signal Measurements
Author(s) -
Macotela Edith L.,
Raulin JeanPierre,
Manninen Jyrki,
Correia Emília,
Turunen Tauno,
Magalhães Antonio
Publication year - 2017
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2017ja024493
Subject(s) - ionosphere , physics , solar flare , flare , amplitude , solar cycle , solar minimum , ionospheric absorption , flux (metallurgy) , solar cycle 24 , sensitivity (control systems) , astrophysics , geophysics , optics , solar wind , plasma , quantum mechanics , materials science , metallurgy , electronic engineering , engineering
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 (F Xmin ) necessary to produce a disturbance of the quiescent ionospheric conductivity. F Xmin 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. F Xmin is computed for ionospheric disturbances that occurred in the time interval of December–January from 2007 to 2016 (solar cycle 24). The computation of F Xmin 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 F Xmin is correlated with the level of solar activity, having F Xmin values in the range (1 − 12) × 10 −7 J/m 2 . Our result suggests that F Xmin 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.