Lower Ionosphere Sensitivity to Solar X‐ray Flares Over a Complete Solar Cycle Evaluated From VLF Signal Measurements

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.

D‐Region High‐Latitude Forcing Factors

“…The present study shows that there are more different periodicities in the nighttime VLF amplitude variability than in daytime. This result is expected because during daytime solar illumination is the main source of ionization that controls signal variability, decreasing the D‐region ionospheric sensitivity to other external sources (Macotela et al, ). During nighttime, when direct solar illumination influence is excluded, the ionospheric sensitivity to other drivers increases (Raulin et al, ) allowing a wider range of physical and chemical processes to influence VLF propagation conditions.…”

“…Periodic variations of solar (Demirkol et al, ; Thomson & Clilverd, ) and terrestrial (Samanes et al, ) origin affect recorded VLF signals over long‐ or short‐time scales. Many studies show long‐term periodic variations of solar origin, such as the 11‐year solar cycle, in the lower ionosphere (e.g., Macotela et al, ; Thomson & Clilverd, ). However, limited studies have been done using VLF data recorded at high‐latitude regions, where long‐term geomagnetic activity might be expected to strongly influence the VLF signals (e.g., Clilverd et al, ).…”

Response of Earth's Upper Atmosphere and VLF Propagation to Celestial X‐Ray Ionization: Investigation With Monte Carlo Simulation and Long Wave Propagation Capability code

“…Here we investigate an observed solar flare, the modulating effect of which on VLF amplitude is already demonstrated in a previous publication (Macotela et al, ). VLF propagation path considered here consists of the TBB transmitter and the Kannuslehto VLF receiver located in northern Finland.…”

“…In the later part of the paper an observed signal amplitude modulation of VLF signal during a solar flare is studied. The signal variation is estimated from the observed result presented in Macotela et al () for the VLF propagation path consisting of the transmitter TBB (Turkey, 37.41°N, 27.32°E, frequency: 26.7 kHz) and the Kannuslehto VLF receiver (northern Finland, 67.74°N, 26.27°E).…”

The Effect of Ozone Shadowing on the D Region Ionosphere During Sunrise

“…Under conditions of low solar illumination, the ionospheric sensitivity to external influences increases (Macotela et al, ; Raulin et al, ) allowing a wider range of physical and chemical processes to modify VLF propagation conditions. On the other hand, the size of SPP shown in Figure a varies from 5° to 50°.…”