From ARRL de WD1CKS@VERT/WLARB to QST on Sun May 28 13:17:25 2017
NOAA's Space Weather Prediction Center (SWPC) said strong (G3) geomagnetic storm conditions occurred overnight. The SWPC issued a G3 alert at 0419 UTC on May 28, due to activity associated with a coronal mass ejection (CME) on May 23.
"Although the solar wind speed is relatively slow, the embedded magnetic field had a prolonged period of southward Bz (a magnetic field component) that is causing the geomagnetic activity," the SWPC alert said. Moderate (G2) and strong (G3) storm warnings were valid until 0900 UTC on May 28, and the forecast calls for only a slight chance of minor storming for the remainder of May 28.
The geomagnetic activity generated aurora borealis displays that were visible along the northern tier of the US, including the upper Midwest and parts of Northern New England. A G3 storm can affect the electric power grid, cause intermittent HF radio conditions and affect satellite navigation as well as low-frequency navigation systems. In such conditions, aurora displays have appeared as far south as 50° geomagnetic latitude. It also can increase drag on low-Earth orbit satellites.
Spaceweather.com reported that the CME hit Earth's magnetic field on May 27.
"At first, for several hours, the seemingly weak impact barely disturbed our planet's magnetic environment," Spaceweather said. "Then the storms began. As Earth passed through the CME's wake, energetic particles poured through a crack in Earth's magnetosphere, sparking strong G3-class geomagnetic storms and bright auroras."