Over the last several months there has been a consistent pattern where corona holes are the primary source northern lights activity prompting geomagnetic storm watches. The current storm is no exception. On December 7th a large, clearly defined coronal hole was pointed directly at Earth. The high speed solar winds shooting out from that area on the Sun are expected to arrive at Earth on December 10th and December 11th. With their arrival, there is a chance for active aurora. Here’s a look at the hole in both AIA 211 and 193 side by side:
As the high speed wind stream arrives early on December 10th, expect the overall Interplanetary magenetic field to first increase. It will likely increase to above 10Bt (you can monitor it live at spaceweatherlive). After that, watch for the solar wind speed to increase. There’s a chance that the solar wind speed in this storm could exceed 600 km/s, up from ambient levels around 300-350 km/s. Once those happen, the Earth’s magnetic field will be primed to respond to any disturbances traveling with the wind, and to negative z-component of the magnetic field. If the Bz shifts to the south (there’s about a 50/50 chance) for a sustained period of time, the KP will rise and there will be a good chance for Aurora.
SWPC’s is currently estimating that there will be several 3-hour periods during which the KP may increase to G1 storming level. This could happen anytime over the 48 hour watch period, but is most likely in the periods indicated on the NLN 3-day AuroraCast clock:
During the storm, follow @northlightalert on Twitter for updates (and shares of photographers’ awesome northern lights photographs) and monitor the KP live.
UPDATE December 5, 2015: The G1 watch has been extended to 72 hours. This long duration event could produce aurora at almost any time over the next three days. Keep an eye on the KP to know when it may be possible to see northern lights in your area.
The expected high speed solar wind stream from a large Earth-directed coronal hole has prompted SWPC to issue a G1 geomagnetic storm watch for Sunday and Monday December 6 and 7. The coronal hole responsible for the watch, CH34, is one of three currently active coronal holes on the visible Solar disk at the moment. Coronal hole 34 is the nearly circular transequitorial dark area annotated with an orange outline on this AIA 211 image taken yesterday by SDO:
The other two coronal holes are visible in the same image. CH33 is the larger northern hemisphere dark area that has already moved past the Earth strike zone. CH35 is the long coronal hole to the South and East (to the right) of CH34. Coronal holes 34 and 35 almost appear to be merging into a single large big-dipper shaped coronal hole. You can see the demarcation clearly on the NOAA Solar Synoptic Map – coronal holes are outlined with a solid line with a hash to the inside of the coronal hole:
CH35’s extension to the north and west is responsible for second day of the extended watch. As both holes grow, there is a larger area of coronal hole pointed towards Earth for a longer time. The current 3-day forecast is calling for two 3-hour periods of KP=5 (G1 storming), with a long period of potential for G4 storming in the other times. If the Bz sets up correctly, this could turn into a long duration G1 or possibly G2 event, so stay tuned and keep an eye on the KP. Here’s the current 3-day Auroracast:
SWPC has issued a G2 geomagnetic storm watch for October 7th and 8th and a G1 geomagnetic storm watch for October 9th. A positive polarity equatorial coronal hole will be producing a high speed solar wind that should be arriving late on Wednesday October 7th. On the previous rotation (Sept 8) this coronal hole produced a 2 day period of G1 and G2 storming. Here’s an image of the coronal hole on the previous rotation, and on the current rotation (Click for larger view):
Coronal holes on the equator of the sun take roughly 27 days to make a full rotation. During that time they are constantly changing and evolving. It is clear this coronal hole – the dark areas on the AIA 211 images above – has become larger. It is generaly a safe bet that if the previous rotation created a strong solar stream, the current rotation will as well.
Like on the last rotation, the predicted storm is coming at a good time for aurora hunters. The Moon will be in a waning crescent phase, so there shouldn’t be much light pollution from the moon. For viewers in the northern hemisphere there is between 30 and 90 minutes of additional dark hours as the nights are longer and days are shorter since the last rotation. This is what the phase of the moon should look like:
Predicting the timing of Aurora that come from a coronal holes is a little easier than that from CMEs. It is hard to determine the speed and orientation of a CME, but with a coronal hole there is a narrower window. That said, predicting the exact timing of any geomagetic storm is difficult, and predictions can be off by as much as 6-12 hours. As of this writing, the current timeline calls for a peak of the G2 storming to happen starting at the end of October 7 and continuing through the early hours of Oct 8 – with G1 storming continuing for up to 6 more hours. For the East Coast Time zone, this means Wednesday evening from sunset through 3:00-6:00AM. NLN’s current infographic for the timing of the storm’s arrival: