STORM WARNING (UPDATED): Among space weather forecasters, confidence is building that Earth’s magnetic field will receive a double-blow from a pair of CMEs on Sept. 12th. The two storm clouds were propelled in our direction by explosions in the magnetic canopy of sunspot AR2158 on Sept. 9th and 10th, respectively. Strong geomagnetic storms are possible on Sept. 12th and 13th as a result of the consecutive impacts. Sky watchers, even those at mid-latitudes, should be alert for auroras in the nights ahead. Aurora alerts: text, voice

EARTH-DIRECTED X-FLARE AND CME: Sunspot AR2158 erupted on Sept. 10th at 17:46 UT, producing an X1.6-class solar flare. A flash of ultraviolet radiation from the explosion (movie) ionized the upper layers of Earth’s atmosphere, disturbing HF radio communications for more than an hour. More importantly, the explosion hurled a CME directly toward Earth. The Solar and Heliospheric Observatory photographed the expanding cloud:

Updated: Radio emissions from shock waves at the leading edge of the CME indicate that the cloud tore through the sun’s atmosphere at speeds as high as 3,750 km/s. By the time it left the sun’s atmosphere, however, the cloud had decellerated to 1,400 km/s. This makes it a fairly typical CME instead of a “super CME” as the higher speed might suggest.

Even with a downgrade in speed, this CME has the potential to trigger significant geomagnetic activity when it reaches Earth’s magnetic field during the mid-to-late hours of Sept. 12th. NOAA forecasters estimate an almost-80% chance of polar geomagnetic storms on Sept. 12-13.

from: spaceweather.com