Saturday, October 22, 2011, started like most any other day, lots of activity of varying size and shape was occurring on the Sun. Most people looking at the Sun that day remember the spectacular lightbulb shaped CME that occurred from just behind the Northwest limb.
Here is the eruption and flare.
and the resulting CME.
But it turns out the really interesting event originated from a filament on the solar disk just to the left of the more spectacular eruption. Just before the eruption that produced the lightbulb CME and filament eruption almost went unnoticed. The only real sign was after the filament lifted off. The magnetic field that tied the filament down, like a series of ropes tying down a long skinny balloon, was ripped away from the surface spreading outward. Where they were connected to the surface and pull away they left 2 expanding ribbons.
Once the filament left the solar surface it formed an irregular shaped CME directed toward Earth. It can been seen here in SOHO/LASCO and it occurred just before the lightbulb CME.
It turns out that the filament contained a knot of magnetic field that was southward directed. This partially countered the northward magnetic field of Earth. This combined with the CMEs strong compression of the magnetosphere created a strong geomagnetic storm that generated aurora to low magnetic latitudes. This is why aurora were seen as far south as Alabama, USA. Aurora were seen around the world including in over 30 US states. Here is a great video illustrating what happened by Dr. Keith Strong of NASA’s Goddard Space Flight Center.
Many observers saw a red aurora. Red auroras are rarer. There is a nice discussion of red aurora by Carla Helfferich.
Here are just a few of the many Aurora photos taken by observers.
Many more can be found at the October 2011 aurora gallery from spaceweather.com.