Abstract

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 dB/dt, 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 dB/dt values are the major cause of hazards associated with three different types of GICs: (1) slow dB/dt with ring current evolution (RC-type), (2) fast dB/dt associated with auroral electrojet activity (AE-type), and (3) transient dB/dt 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 dB/dt 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 dB/dt 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.