Abstract:
A scheme is suggested and tested for forecasting severe space weather (SvSW) using solar
wind velocity (V) and the north-south component (Bz) of the interplanetary magnetic field (IMF)
measured using the ACE (Advanced Composition Explorer) satellite from 1998 to 2016. SvSW has caused
all known electric power outages and telegraph system failures. Earlier SvSW events such as the Carrington
event of 1859, Quebec event of 1989 and an event in 1958 are included with information from the
literature. Dst storms are used as references to identify 89 major space weather events (DstMin ≤ 100 nT)
in 1998–2016. The coincidence of high coronal mass ejection (CME) front (or CME shock) velocity ΔV
(sudden increase in V over the background by over 275 km/s) and sufficiently large Bz southward at
the time of the ΔV increase is associated with SvSW; and their product (ΔV × Bz) is found to exhibit a
large negative spike at the speed increase. Such a product (ΔV × Bz) exceeding a threshold seems
suitable for forecasting SvSW. However, the coincidence of high V (not containing ΔV) and large Bz
southward does not correspond to SvSW, indicating the importance of the impulsive action of large Bz
southward and high ΔV coming through when they coincide. The need for the coincidence is verified using
the CRCM (Comprehensive Ring Current Model), which produces extreme Dst storms (<DstMP>< 250 nT)
characterizing SvSW when there is coincidence.
