First off, the aurora actually did happen and right on time. The big wave rolled over the Scandinavian countries and Iceland during the late afternoon hours for North America. By the time darkness encroached on this side of the Atlantic, the show was quickly ebbing. An updated, Saturday evening forecast from the Space Weather Prediction Center (SWPC) in Boulder reflected this but also predicted a second surge during the early morning hours Sunday. Sadly, that never happened. In fact, we didn't even cracked the G1 minor storm level overnight.
I feel badly for anyone who drove up in anticipation of the fickle lights. The forecasters do the best they can. Using data from space-based observatories such as NASA's Solar Dynamics Observatory, the Solar and Heliospheric Observatory and DSCOVR, the pros can observe solar events, estimate their magnitude and speed and predict their arrival at Earth. The one thing they can't know is the Bz or magnetic orientation and strength of the arriving bundle of solar plasma, a mix of protons and electrons.
That material comes bundled with some of the sun's magnetic field. Without getting technical, if that field points north (like the north pole of a magnet) as it blows past the Earth, it glides by without interacting with our planet. But if the field points south, it's very effective at coupling with Earth's north-pointing magnetic field — similar to how the north and south poles of an ordinary magnet will clack together — and finding its way down into the upper atmosphere.
Both electrons and protons strike nitrogen and oxygen atoms high in the ionosphere, but the electrons are the key driver of the aurora because because they so much lighter compared to protons. Connection to Earth's field accelerates them to speeds up to 45 million miles an hour. They slam hard into the atoms and energize them. When the atoms "relax," that energy is released as the bits of primarily red and green light that together create the aurora.
We only know the impending "storm cloud's" magnetic strength and direction about an hour or so out. That's when NASA's DSCOVR satellite, parked at the L1 Lagrange Point about a million miles (1.5 million km) ahead of Earth, samples the material and provides direct knowledge of the Bz. Space weather scientists have to walk a fine line. They can determine the size and power of an incoming CME but can't truly know its impact until nearly the last moment.
Remember, it's not just aurora watchers who use the agency's forecasts. Power grid and oil pipeline firms as well as satellite operators critically depend on alerts and forecasts. Fluctuations in Earth's magnetic field and changing ground voltages during big storms can damage equipment and electronics.
Electrons streaming down the ionosphere 60 miles (96 km) overhead can produce intense electrical currents in the range of several hundreds of thousands of amperes. These currents can induce voltages across the ground that can overload poorly protected power grids or insufficiently "hardened" satellite electronics. Better to be prepared with a potentially inaccurate forecast than none at all.
So what's the latest forecast? W-e-l-l, the SWPC predicts a G3 storm to occur from about 4-7 p.m. CDT Sunday night, Oct. 31. If you live in eastern Canada and Maine you might see the tail end of the show. After a pause in activity, things will kick into gear again from about 10 p..m. to 1 a.m. when a G1 minor storm is expected. We have a partly cloudy forecast for my region tonight, so I'll update you on what's happening on Astro Bob's Astronomy for Everyone on Facebook. Good luck and have heart.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.