<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>

Comments on manuscript "Investigation of the ionospheric absorption response to flare events during the solar cycle 23 as seen by European and South African ionosondes"The analysis of the absorption induced by the solar flares was performed in this paper. The ionosonde data located at different latitudes were considered. The methods of the fmin and dfmin were applied.

<p>The impact of individual meteors on the lower ionosphere (90-150 km height) has been investigated during wintertime meteor showers using measurements of two DPS-4D Digisondes installed at Sopron (47.63°, 16.72°) and at Pruhonice (50°, 14.5°). The optical measurements of meteors have been performed by a zenith camera installed next to the digisonde at Sopron. It provided the opportunity to compare high cadence ionograms measured during meteor showers parallel with the optical data to determine the plasma trails of individual meteors. Campaign measurements with two ionograms/minute have been performed at Sopron station during the Leonid (16-18 November) and Geminid (10-15 December) meteor showers in 2019. Furthermore, skymaps (1/min) detected by the Digisonde at Sopron during the campaign were also investigated.</p> <p>In the 20-25% of the observed meteors faint, short-lived (20-120 sec) Es layers were detected on the ionograms during and after (< 2 min) the optical record, which are typical signal of individual meteor trails on the ionogram based on previous studies. There was no observed Es activity at the same height on the ionograms detected before and after these events. Furthermore, the direction of the echo can be also defined on the ionograms of the DPS-4D Digisonde thanks to the multi-beam observation technique. The direction of the detected Es layers agreed well with the optical observations in most of the cases. The maximum frequency of the observed faint layers (foEs) varied between 1,6 and 4,5 MHz, while their height was between 85 and 136 km. Points on the skymaps were also detected at the time of the faint Es layers in 40 % of the cases. The height and direction of the observed points agreed with these parameters of the plasma traces on the ionograms.</p> <p>Comparing the ionograms with the closest ionosonde observation at Pruhonice station at the same time, we could conclude that the detected faint Es layers were local plasma irregularities, no Es activity at the same height was observed there. This strengthens the hypothesis that the observed trails on the ionograms represents the echo of the optically recorded meteors.</p> <p> </p>

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