Abstract
The article presents the results of the study of ionospheric effects of the extreme geomagnetic storm of May 25/26, 1967 (Dst = −387 nT) obtained using ionospheric data measured at five ionospheric stations located in the Central Asian region. The geomagnetic storm with an initial phase lasting approximately 9 hours began at ~20 UT on May 25, which for the region under consideration corresponds to nighttime (at 00:01 LT) on May 26. The negative ionospheric disturbance lasting about 28 hours, which began at all stations under consideration during the onset of the main phase of the geomagnetic storm, was characterized by a decrease in the critical frequencies of the ionospheric F2 layer foF2 by 2.2 times compared to the foF2 level under quiet geomagnetic conditions. The “G condition” observed during the negative phase of the ionospheric storm, when foF2 foF1, lasted for about 11 hours and indicated significant changes in the thermospheric parameters responsible for the formation of the F-region in the middle latitudes. The anomalous formation of the nighttime E-layer observed at all ionospheric stations is discussed in the context of the physical mechanism of energetic particle precipitation at middle and low latitudes. The increased values and wave-like variations of the critical frequencies of foF2 observed during the recovery phase of the geomagnetic storm are interpreted in terms of their being caused by traveling atmospheric disturbances (TAD). The black out event associated with the complete absorption of the sounding signal observed at stations in the Central Asian and Far Eastern regions on May 28 is associated with the impact of the high-intensity solar cosmic ray flux recorded during this period on the lower ionosphere. The ionospheric storm of May 25/26 can serve as a model of the ionospheric response to an extreme disturbance of the Earth's magnetic field.