Extreme geomagnetically induced currents

We propose an emergency alert framework for geomagnetically induced currents (GICs), based on the empirically extreme values and theoretical upper limits of the solar wind parameters and of d B /d t , the time derivative of magnetic field variations at ground. We expect this framework to be useful for preparing against extreme events. Our analysis is based on a review of various papers, including those presented during Extreme Space Weather Workshops held in Japan in 2011, 2012, 2013, and 2014. Large-amplitude d B /d t values are the major cause of hazards associated with three different types of GICs: (1) slow d B /d t with ring current evolution (RC-type), (2) fast d B /d t associated with auroral electrojet activity (AE-type), and (3) transient d B /d t of sudden commencements (SC-type). We set “caution,” “warning,” and “emergency” alert levels during the main phase of superstorms with the peak Dst index of less than −300 nT (once per 10 years), −600 nT (once per 60 years), or −900 nT (once per 100 years), respectively. The extreme d B /d t values of the AE-type GICs are 2000, 4000, and 6000 nT/min at caution, warning, and emergency levels, respectively. For the SC-type GICs, a “transient alert” is also proposed for d B /d t values of 40 nT/s at low latitudes and 110 nT/s at high latitudes, especially when the solar energetic particle flux is unusually high.