Storm sudden commencements and polar magnetic substorms

IGY sudden commencements of magnitude greater than 20 gammas simultaneously (±3 min) triggered magnetospheric substorms with a very high probability (∼90%). The magnitude dependence gives rise to a part of the large sunspot cycle variation of the probability of the occurrence of simultaneous events: 49% during the IGY and only 4% during the IQSY. Also, the degree of presubstorm disturbances (measured in terms of ΣKp), types of interplanetary discontinuities, and the direction of interplanetary magnetic field do not appear to be important parameters for ssc-triggered substorms. Therefore, these results may indicate that a sudden compression of the magnetosphere alone, if intense enough, can sometimes trigger substorms.

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