
G3 (Strong) geomagnetic storming periods were observed on 28 September, 2017. G3 levels were reached at 28/0559 UTC (28/0159 ET) and again at 28/0805 UTC (28/0205 ET). These geomagnetic storming levels were reached during prolonged and pronounced periods of favorable interplanetary magnetic field (IMF) orientation; combined with elevated solar wind speed near 700 km/s due to the influences of a coronal hole high speed stream (CH HSS). Prior to the escalation of activity to G3 storming levels, three concurrent periods of G2 (Moderate) storming occurred. For additional information regarding the CH HSS and resultant geomagnetic storming - read our forecast discussion. Also, review our SWPC webpage for the latest forecasts and alerts/warnings.