Response of the middle-latitude ionosphere to the extreme geomagnetic storms of the 23rd solar cycle

Ionospheric response to the extreme geomagnetic storms of the 23rd solar cycle in Irkutsk (52.3° N, 104.3° E) was analyzed with data of the Irkutsk Incoherent Scatter Radar (ISR) and digital vertical-sounding ionosonde DPS-4. Divergence of parameters from the undisturbed level (i.e., from monthly medians or values obtained on a quiet day) was considered as an ionospheric response. The values of the electron density maximum (NmF2) and electron temperature at a height of 350 km (Te) were chosen as the parameters. Interpretation of the ionospheric response was made within the framework of the concept of thermospheric storm and penetration of the magnetospheric electric field.