Geomagnetic storms of remarkable intensity may have a potential effect on many aspects of human life, including health effects due to radiation hazards, disruption of electrical systems like communications appliances, navigation systems, satellites… The biological mechanisms altered by geomagnetic storms are explored in ref.10, and geomagnetic activity has been recently related to several human conditions as hypertension11, multiple sclerosis12,13,14 or cancer15. However, it is still unclear to what extend and under what threshold these health effects appear or the risk increases. Regarding communications, in the period under study the most intense event happened on March 1989, reaching a peak Dst under −600 nT, Fig. 1, which caused the collapse of the Hydro-Québec power grid, and resulted in the loss of electrical power to six million people16. Another event on July 15-16, 2000 with a peak Dst under −300 nT did not cause any terrestrial damage. Therefore, it seems reasonable to infer that the threshold to expect immediate observable consequences might be between these two values.