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, f₀F2 and h_maxF2, 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 f₀F2 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 f₀F2 peak amplitude reaches the sizes 20-100% of f₀F2 values. Similarities of delta f₀F2 variations on two chains are: (i) the delta f₀F2 peak mentioned above occurs simultaneously as a rule before disturbances on both chains; (ii) similar delta f₀F2 variations are observed on all of the mid-latitude (or high-latitude) ionosondes of each of the chain. Delta h_maxF2 variations are approximately the opposite of the f₀F2 variations. Their amplitudes fall in the range 15 - 25% from the average h_maxF2 value. This investigation is useful and important for the short-term prediction of space weather states.