Minor geomagnetic storm in progress, weak negative CH HSS expected

According to the SWPC, the ACE satellite observed a pair of discontinuities in the solar wind field during the past 24 hours. 

The first discontinuity at 02:35 UTC on December 21 was weak and possibly indicated the arrival of a glancing blow from the December 17th Coronal Mass Ejection (CME).

At about 18:24 UTC, the second stronger discontinuity was observed. This second discontinuity probably signaled the arrival of the December 18 CME. Total field increased sharply from 9 nT to 17 nT while the Bz component dipped further southward to -13 nT. Wind speeds increased from approximately 350 km/s to about 450 km/s. Subsequent increases were also observed in density and temperature.

Geomagnetic K-index of 5 (G1-Minor) threshold was reached at 02:45 UTC on December 22.

Space Weather Message Code: ALTK05
Serial Number: 762
Issue Time: 2014 Dec 22 0245 UTC

ALERT: Geomagnetic K-index of 5
Threshold Reached: 2014 Dec 22 0245 UTC
Synoptic Period: 0000-0300 UTC
 
Active Warning: Yes
NOAA Scale: G1 - Minor

NOAA Space Weather Scale descriptions can be found at
www.swpc.noaa.gov/noaa-scales-explanation

Potential Impacts: Area of impact primarily poleward of 60 degrees Geomagnetic Latitude.
Induced Currents - Weak power grid fluctuations can occur.
Spacecraft - Minor impact on satellite operations possible.
Aurora - Aurora may be visible at high latitudes, i.e., northern tier of the U.S. such as northern Michigan and Maine.

Aurora forecast. Image credit: SWPC

The geomagnetic field was at quiet to active levels with isolated high latitude major storm periods yesterday and is expected to remain at unsettled to isolated minor storm levels (G1/Minor) through early December 22. Unsettled to active levels are expected to persist through the remainder of the UTC day.

Quiet to unsettled levels are expected on December 23 and 24 as a weak, negative polarity coronal hole high speed stream (CH HSS)  moves into a geoeffective position.

Featured image credit: SWPC