Ionospheric effects preceding 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 hmaxF2, on two chains of ionosondes located in Europe and North America during the interval 26–28 October 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 chain. Delta hmaxF2 variations are approximately the opposite of the f0F2 variations. Their amplitudes fall in the range 15–25% from the average hmaxF2 value. This investigation is useful and important for the short-term prediction of Space Weather states.