Ionospheric effects preceded the October 2003 Halloween storm

The goal of this study is to reveal some ionospheric forerunners which occur prior to the expansion phase of any storm or substorm. Another task is to compare variations of two main ionospheric parameters, f/sub 0/F2 and h/sub max/F2, on two chains of ionosondes located in Europe and North America during the interval October 16-28, 2003. This interval precedes the Halloween storm and includes its onset. Analysis is based on ionospheric data of the SPIDR system. The key findings are as follows. A positive peak of delta f/sub 0/F2 occurs some hours before onset of a magnetic storm or substorm, where delta is a deviation from the quiet value. This peak can serve as a forerunner of disturbance development. Delta f/sub 0/F2 peak amplitude reaches the sizes 20-100% of f/sub 0/F2 values. Similarities of delta f/sub 0/F2 variations on two chains are: (i) the delta f/sub 0/F2 peak mentioned above occurs simultaneously as a rule before disturbances on both chains; (ii) similar delta f/sub 0/F2 variations are observed on all of the mid-latitude (or high-latitude) ionosondes of each of the chain. Delta h/sub max/F2 variations are approximately the opposite of the f/sub 0/F2 variations. Their amplitudes fall in the range 15 - 25% from the average h/sub max/F2 value. This investigation is useful and important for the short-term prediction of space weather states.