GLOBAL IONOSPHERIC EFFECTS OF THE OCTOBER 1989 GEOMAGNETIC STORM

Based on a large data base from 40 ionosonde stations distributed worl dwide and 12 total electron content stations, a case study is made on the global behavior of ionospheric responses to the great magnetic sto rm of October 1989. The magnetic storm was triggered by a solar flare with the largest class of X13/4B and started with a sudden storm comme ncement (ssc) at 0917 UT on October 20. After the initial phase the st orm underwent two periods of maximum activities in the following 2 day s. Low-latitude auroras were sighted and reported in widely separated areas in both northern and southern hemispheres. In response to these magnetic and auroral activities the ionosphere showed remarkable effec ts. Depending on the local time of ssc occurrence, the ionospheric res ponse differed appreciably. Impressive changes were long-lasting, larg e-scale effects, such as the severe depressions of foF2 at higher lati tudes, the temporary suppression of the equatorial anomaly and large h orizontal gradients at certain latitudes. Also observed were positive storm effects of short duration during the post-sunset period in respo nse to the onset of both ssc and main phase of the magnetic storm. The se two positive storm effects showed different patterns suggesting dif ferent causal mechanisms. In addition, global propagation of large-sca le traveling ionospheric disturbances (TIDs) was seen during 2 nights, identified by dramatic rises of h'F with periodic fluctuations. The e quatorward propagation velocities of the TIDs varied between 330 m/s a nd 680 m/s for the east Asia region.