Effects of solar flares on the ionosphere as shown by the dynamics of ionograms recorded in Europe and South Africa
<p>We have investigated the solar flare effects on ionospheric absorption with the systematic analysis of ionograms measured at midlatitude and low-latitude ionosonde stations under different solar zenith angles. The lowest recorded ionosonde echo, the minimum frequency (<span class="i...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-08-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/37/747/2019/angeo-37-747-2019.pdf |
| Summary: | <p>We have investigated the solar flare effects on
ionospheric absorption with the systematic analysis of ionograms measured at
midlatitude and low-latitude ionosonde stations under different solar zenith
angles. The lowest recorded ionosonde echo, the minimum frequency (<span class="inline-formula"><i>f</i><sub>min</sub></span>, a
qualitative proxy for the “nondeviative” radio wave absorption occurring
in the D-layer), and the d<span class="inline-formula"><i>f</i><sub>min</sub></span> parameter (difference between the value of
the <span class="inline-formula"><i>f</i><sub>min</sub></span> and the mean <span class="inline-formula"><i>f</i><sub>min</sub></span> for reference days) have been considered. Data
were provided by meridionally distributed ionosonde stations in Europe and
South Africa during eight <span class="inline-formula"><i>X</i></span>- and <span class="inline-formula"><i>M</i></span>-class solar flares in solar cycle 23.
Total and partial radio fade-out was experienced at every ionospheric
station during intense solar flares (> <span class="inline-formula"><i>M</i></span>6). The duration of the
total radio fade-out varied between 15 and 150 min and it was highly
dependent on the solar zenith angle of the ionospheric stations.
Furthermore, a solar-zenith-angle-dependent enhancement of the <span class="inline-formula"><i>f</i><sub>min</sub></span> (2–9 MHz) and d<span class="inline-formula"><i>f</i><sub>min</sub></span> (1–8 MHz) parameters was observed at almost every station.
The <span class="inline-formula"><i>f</i><sub>min</sub></span> and d<span class="inline-formula"><i>f</i><sub>min</sub></span> parameters show an increasing trend with the enhancement
of the X-ray flux. Based on our results, the d<span class="inline-formula"><i>f</i><sub>min</sub></span> parameter is a good
qualitative measure for the relative variation of the “nondeviative”
absorption, especially in the case of the less intense solar flares, which do
not cause total radio fade-out in the ionosphere (class < <span class="inline-formula"><i>M</i></span>6).</p> |
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| ISSN: | 0992-7689 1432-0576 |