Interplanetary Shock Wave From Unknown Source Causing Strong Geomagnetic Storm

GEOMAGNETIC STORM: A G2-class (Kp=6) geomagnetic storm is in progress following the arrival of an interplanetary shock wave on May 31st. The source of the shock is not known; it might have been a minor CME that left the sun without drawing attention to itself. The impact sparked auroras across many northern-tier US states. This photo, for instance, comes from Christopher Griffith in Baxter, Minnesota:

"I wasn't expecting to see any lights, but right before the midnight it broke loose and the sky lit up," says Griffith. "Sadly the clouds quickly filled in my little window, and the auroras were gone. Just thankful for what I got so see!" Elsewhere in the USA, auroras were sighted as far south as Colorado, Maryland, Iowa, Wisconsin, andNebraska.

High-latitude sky watchers should remain alert for auroras tonight as Earth's magnetic field continues to reverberate from the impact. NOAA forecasters estimate a 40% chance of polar geomagnetic storms on June 1st.

source: http://spaceweather.com/

Space Weather Message Code: ALTK06
Serial Number: 308
Issue Time: 2013 Jun 01 1502 UTC

ALERT: Geomagnetic K-index of 6
Threshold Reached: 2013 Jun 01 1500 UTC
Synoptic Period: 1500-1800 UTC
Active Warning: Yes
NOAA Scale: G2 - Moderate
Potential Impacts: Area of impact primarily poleward of 55 degrees Geomagnetic Latitude.
Induced Currents - Power grid fluctuations can occur. High-latitude power systems may experience voltage alarms.
Spacecraft - Satellite orientation irregularities may occur; increased drag on low Earth-orbit satellites is possible.
Radio - HF (high frequency) radio propagation can fade at higher latitudes.
Aurora - Aurora may be seen as low as New York to Wisconsin to Washington state.

source: http://www.swpc.noaa.gov/alerts/archive/current_month.html