Long‐term occurrence probabilities of intense geomagnetic storm events

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Abstract
A quantitative assessment of the occurrence probability of intense geomagnetic storms (peak Dst < −100 nT) has been investigated by analyzing the Dst index time series database from 1957 to 2001. The main purpose was to derive two parameters, the probable intensity ST and the occurrence frequency λt, that can act as proxies for long‐term space weather quantities. The intensity ST represents the expected maximum storm level with an occurrence rate of 1/T (a−1, where a is years) and has been derived from the probability density function (PDF) of extreme (∣Dst∣ > 280 nT) storms. The mathematical tool to determine this type of PDF is the extreme value modeling, which exhibits more accurate statistics for extreme behavior. Our results estimate S60 ≈ 589, indicating that the March 1989 storm (the event with the largest ∣Dst∣ in the database) corresponds to an event expected to occur only once every 60 a. The other parameter λt gives the average occurrence rate of storm events. We have tested the null hypothesis that the storm occurrence pattern can be modeled as a Poisson process represented by λt, where different λt exist for the active and quiet periods of the solar cycle. Ordinary χ2 tests of goodness of fit can not reject this hypothesis, except within the periods that include extremely frequent occurrences. The rate λt is approximately 2.3 (0.7) per 3 months in the active (quiet) period. A future practical application of this work is that the resultant Poisson probability will enable us to calculate the expected damage due to storms, which represent potential risks in space activities.

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