Multi-instrumental investigation of the solar flares impact on the ionosphere occurring in December 2006

Abstract

<p>The sudden increase of X-radiation and EUV emission following solar flares causes additional ionization and increased absorption of electromagnetic (EM) waves in the sunlit hemisphere of the Earth’s ionosphere. The solar flare impact on the ionosphere above Europe on 05 and 06 December 2006 was investigated using ground-based (ionosonde and VLF) and satellite-based data (Vertical Total Electron Content (VTEC) derived from Global Navigation Satellite Systems (GNSS) observations and VLF measurements from the DEMETER satellite). Based on the geomagnetic indices Kp and Dst, 05 December was a quiet day, while there was a geomagnetic storm on 06 December 2006.</p><p>The total fade-out of the EM waves emitted by the ionosondes was experienced at all investigated stations during an X9 class flare on 05 December. The variation of the fmin parameter ( representing the minimum frequency of the echo trace observed in the ionogram, and is a rough measure of the “nondeviative” absorption) and its difference between the quiet period and during the flares have been analyzed. A latitude dependent enhancement of fmin (2-9 MHz) and Delta_fmin (relative change of about 150-300 %) was observed at every station at the time of the X9 (on 05 December) and M6 (on 06 December) flares.</p><p>Furthermore, we analyzed VTEC changes during and after the flare events with respect to the mean VTEC values of reference quiet days. During the X9 solar flare, VTEC increased depending on the latitude (2-3 TECU and 5-20 %). On 06 December, the geomagnetic storm increased ionization (5-10 TECU) representing a „positive” ionospheric storm. However, an additional peak in VTEC related to the M6 flare could not be detected.</p><p>We have also observed a quantifiable change in transionospheric VLF absorption of signals from ground transmitters detected in low Earth orbit associated with the X9 and M6 flare events on 05 and 06 December in the DEMETER data. Moreover, amplitude and phase of ground-based, subionospherically propagating VLF signals were measured simultaneously during the investigated flares to analyze ionosphere reaction and to evaluate the electron density profile versus altitude. For the X9 and M6 flare events we have also calculated the ionospheric parameters (sharpness, reflection height, etc.) important for the description and modeling of this medium under forced additional ionization.</p>

Investigating the effect of large solar flares on the ionosphere based on novel Digisonde data comparing three different methods

“…In parallel, if we look through the variation of the fmin parameter we can not recognise any changes of the diurnal pattern in connection with the geomagnetic storm, only the increased values at the time of the flare events are clear. It agrees well with the findings of previous studies, e.g., Barta et al, 2022, that the fmin parameter is sensitive to the changes caused by the solar flares, while the foF2 parameter can show the variation caused by the dynamical changes in the thermosphere-ionosphere (F-region) system during geomagnetic storms.…”

“…However, these large flares caused long-lasting total radio blackouts at all stations, thus we had detected signals only 30-120 min after the peak time. Nevertheless, the computed Ne changes are ∼100% at 74 km after C4.8-C5.1 class flares (Srećković et al, 2021;Barta et al, 2022) which agree with the changes (68%-102%) observed at 2.5 MHz after the C8.3 flare on 04 September, when there was no total radio blackout.…”

“…Chen et al (2021) used numerical modeling to investigate the effect of the X9.3 (06 September) and X8.2 (10 September) flares on the ionosphere and found that at the flare peak, not only the electron and temperature increased but also the E-region conductivity which causes further electrodynamic responses. Barta et al (2022) found a 0.5-0.7 MHz enhancement of the foE parameter (critical frequency of E-region) compared to the reference days at PQ and VT stations after an M6 flare. Furthermore, Sharma et al (2010) detected increased values (by 0.5 MHz) of the foE at Ahmedabad (India) low latitude station after the flare occurred on 12 May 1997.…”

“…This formula was used in Barta et al (2022) to investigate the fmin variation during flares. It is regularly used to analyze foF2 parameter changes during geomagnetic storms in the literature (e.g., Buresova et al, 2014;Berényi et al, 2018).…”

“…Furthermore, a finer logarithmic scale is also applied ranging from 0 to 9 within the above written classes. During flare events the enhanced radiation causes extra ionization and increased attenuation of the EM waves, leading to so-called short-wave fade-outs which last for tens of minutes or even hours (Rishbeth and Garriot, 1969;Davies, 1990;Tsurutani et al, 2009;Barta et al, 2019;Tao et al, 2020;Barta et al, 2022). Solar flares are also accompanied by energetic particles (protons and electrons from tens of keV to hundreds of MeV) propagating along the magnetic field lines, reaching the Earth at the polar region and causing further ionization and enhanced absorption there (Rishbeth and Garriot, 1969;Tsurutani et al, 2009).…”