Investigation of the negative ionospheric response of the 8 September 2017 geomagnetic storm over the European sector

Occurrence features of Intermediate descending layer and Sporadic E observed over the higher mid-latitude ionospheric station of Moscow

“…Es layers subsequently intensify, also six hours apart, the rst at ~ 10 LT and the next one at ~ 16 LT, while they continue to move downwards from ~ 130 km to ~ 100 km within six hours with a much slower descending rate of around 5 km/h. Our results are consistent with previous studies that reported a similar quarter-diurnal periodicity in Es and IDLs over lower mid-latitude regions (Pignalberi et al, 2014; Haldoupis and Pancheva, 2006) and higher mid-latitudes (Oikonomou et al, 2022). The systematic appearance of the quarter-diurnal periodicity in Es and IDLs was also reported over equatorial and low latitude regions (Tong et al, 1988;Carter and Forbes, 1999;Earle et al, 2000;Niranjan et al, 2010;Christakis et al, 2009), and was attributed to the quarter-diurnal thermospheric tides, which though weaker than the semidiurnal and terdiurnal tides they are also a major contributor to the ionospheric-thermospheric dynamics (Kumari, 2021).…”

“…3. The semidiurnal periodicity in IDL occurrence predominates in spring (from February to April) and in autumn (September and October) which is consistent with the HTI results as well as with previous mid-latitude studies (Oikonomou et al, 2014(Oikonomou et al, , 2022. This prominent feature during equinoctial months and mainly in Spring equinox, is most probably associated with the ampli cation of semidiurnal thermospheric tides during this season (Forbes and Vial, 1989; Oberheide et al, 2011) and the possible enhancement of the bottomside F region electron density during spring (Yamazaki & Richmond, 2013).…”

“…The observed 12-hour periodicity in Es and IDLs occurrence is consistent with the results of previous studies (Haldoupis, 2012;Pignalberi et al, 2014;Oikonomou et al, 2014Oikonomou et al, , 2022 and is attributed to the downward phase propagation of the semidiurnal tides which dominate in the thermosphere throughout the year (Forbes, 1995). Apart from the prevailing semidiurnal periodicity, a terdiurnal periodicity is evident for winter in IDL and Es and during summer (Figs.…”

“…This 8-hour periodicity in Es and IDL traces was observed also during autumn and spring, though with much less intensity, with the aid of Spectrum analysis (see Section 3.2), despite the fact that is not evident in the respective HTI plots. A similar terdiurnal periodicity in Es was reported earlier over lower mid-latitude ionosonde stations (Nicosia in Cyprus, Milos and Athens in Greece) during summer (Haldoupis et al, 2004;Oikonomou et al, 2014Oikonomou et al, , 2022, as well as less frequently over the higher mid-latitude ionosonde station Pruhonice in Czech Republic by Mosna et al (2015). It has been demonstrated that the terdiurnal thermospheric tides are behind the formation of the 8-hour periodicity in IDL and Es (Haldoupis, and Pancheva, 2006) while the fact that the terdiurnal pattern in ionospheric layers are weaker than the semidiurnal one, is most probably due to the fact that terdiurnal tides are not as strong as semidiurnal tides at higher mid-latitudes to drive the formation of IDL and Es (Jacobi, 2012;Azeem et al, 2016).…”

“…In a recent study on IDL and Es occurrence characteristics over mid-latitude regions, we demonstrated the systematic manifestation of a shorter (4-hour) periodicity mainly in IDL during winter over three higher mid-latitude stations in Europe (Oikonomou et al, 2022). This section extends that investigation and concentrates to the new evidence of this short-scale periodicity over Moscow station.…”