Statistical analysis of geomagnetic storms and their relation with the solar cycle

Geomagnetic storms can be modeled as stochastic processes with log-normal probability distribution function over their minimum D_st index value measured during the main phase of each event. Considering a time series of geomagnetic storm events between 1957 and 2019 we have analyzed the probability of occurrence of small, moderate, strong and extreme events. The data were separated according to solar cycle (SC) and solar cycle phases and fitted through maximum likelihood method in order to compare rates of occurrence of the last Solar Cycle (SC24) with previous ones. Our results show that for D_st < - 100 nT events in SC24 are similar to those in SC20, obtaining ∼42 vs 21 median rate storms per cycle with 95% confidence intervals using Bootstrap Method. As SC24 has been the least active solar cycle in over 200 years, we conclude that this method tends to overestimate geomagnetic storms occurrence rates even for small events.