Northern Lights Now – The late September geomagnetic activity resulting from a large coronal hole has exceeded initial expectations reaching G3 storm levels and helping aurora hunters world-wide capture staggering views. Solar wind speeds have been between 650 and 750 km/s for just over 24 hours now. Periods of high density and negative Bz, and quickly fluctuating Bz during that time pushed KP values above 6.67 for several hours.
The timing worked well for aurora hunters from Northern Europe across Northern North America. Clouds disrupted viewing in the UK and New England, but many locations saw vivid displays of Green, Red and Purple overnight.
Wendy T shared this great set of 4 images
Thanks Everyone. Heres' a few from last night (not off back of camera but straight off!) Fingers crossed for more tonight.. pic.twitter.com/zr7IEKeyGA
Northern Lights Now – On December 20th and 21st of 2015 the third largest geomagnetic storm of solar cycle 24 treated aurora hunters to 30 hours of dancing lights. The long duration of the storm gave nighttime photographers in North America two opportunities to see the northern lights through gaps in the clouds. Aurora reports on Twitter filled the NLN feed with images first from Wisconsin, then Alberta, Alaska, New Zealand, Northern Europe, Austria, Germany, England, Ireland, Iceland and then the North America again. Here is a chart of the official NOAA/SWPC recorded KP values from Boulder during the storm:
This solar storm started from two events on the Sun’s surface. The first was a long duration C6.69 flare at nearly dead center in the Earth strike zone. The second was a filament eruption to the south and east of the first eruption. Both events produced CMEs. Read more about the pair of eruptions NLN’s initial blog post on this storm.
Giants Causeway in Northern Ireland by Roy Smith Photo:
The CMEs from these two storms arrived later than initially predicted. Initial predictions were that the leading edge of the CME would reach Earth early in the day on December 19. The CME’s initial interplanetary shock was detected in ACE satellite data around 1520 GMT. Those 15 hours were time that many nighttime photographers wished they were sleeping instead!
Once they arrived, the two storms hit in sequence, not quite merging. As the storms played out, both had strongly negative Bz. Negative Bz is an aurora hunter’s dream. Once the field shifts south, a good show is sure to come – but we never know Bz until Earth is in the CME cloud. Space Weather scientists are still anticipate a long time before Bz can be accurately predicted in advance of a CME arrival. For now, forecasters assume arriving CMEs plasma clouds have a roughly 50/50 chance of being oriented with a Bz south.
In the Winter Solstice Storm of 2015, once the Bz shifted south, it stayed strongly south for 32 hours from 02:30GUTC on the 20th through 1030UTC on the 21st. During that time, the Bz deflection remained around -16 to -18 nT. Interestingly, after the initial shock, solar wind speeds stayed relatively low at below 450km/s for the duration of the storm. Had solar wind speeds been stronger, it’s possible that G3 level storming might have occurred. The slow wind speeds probably increased the duration of the storm (if the CME was moving faster, it would have completed it’s pass by Earth more quickly).
With a special shoutout to @VirtualAstro who helped surface some of these, here are some of our favorite images from this worldwide display of northern lights:
Swirls of green glow behind snow covered pine trees in Alaska by David W. Shaw