Ionospheric response over the Middle Asian region to the May 1967 geomagnetic storm

Ionospheric effects over the five Middle Asian ionospheric stations to the major space weather event of the May 25-26, 1967 geomagnetic storm with the Dst-index of-387 nT are studied based on ionosonde measurements that were not previously considered: Alma-Ata (43.25N, 76.92E), Karaganda (49.82N, 73.08E), Novokazalinsk (45.76N, 62.12E), Tashkent (41.33N, 69.62E), Ashkhabad (37.90N, 58.30E). The study presents a description of the geomagnetic storm effects on the ionosphere electron concentration from F-region down to the lower ionosphere, points out the storm effects in different ionospheric layers for the time period 22-31 May 1967. The observations indicate a major (>= 50%) and long lasting (>28 h) negative ionospheric disturbance appeared during night-and daytime simultaneously at all five stations. The daytime foF2 values on 26 May were practically at the nighttime foF2 level on this day and well below the background daytime foF2 level. A rapid increase in h'F associated with reductions in foF2 occurred over all stations during the development of the main phase of the magnetic storm. Large changes in the critical frequencies foE, foEs, and fmin that is possible linked to the enhanced X-ray flux, increased intensity in solar cosmic rays, and precipitation of energetic neutral atoms at the respective latitudes were observed during 23-26 May 1967.