Skywatchers in the northern U.S., Canada and across the Scandinavian countries may have a shot at seeing a modest display of northern lights tonight through tomorrow night. Streams of high-speed solar material flowing from a hole in the sun’s magnetic canopy are expected to arrive later tonight prompting the Space Weather Prediction Center to forecast a G1 geomagnetic storm or in plain English, minor auroras at higher latitudes.
Coronal holes are a regular feature of the sun’s fiery hot atmosphere. Normally, magnetic fields are closed loops that lock down the sun’s gases; holes are regions where the fields open directly into space. Released from their magnetic shackles, electrons and protons in the sun’s atmosphere stream freely into space.
The solar wind, a dilute soup of electrons and protons, normally blows by the planet at 250 miles per second (400 km/s) but coronal holes send tempests with speeds up to 500 miles per second (800 km/s). When a good-sized hole develops and the sun’s rotation brings it into line with Earth, our protective magnetosphere can gets a pounding.
Speedy particles can squeeze or directly enter the magnetosphere and spark auroras.
I’m often asked where’s the best place to see the aurora. The simple answer is to go north. The further poleward you live, the better your chances of seeing northern lights. In the northern border states auroras occur with regularity around the time of solar maximum, when the sun peaks in storm activity. The current solar max – the least stormy since 1906 – happens this summer.
There’s another consideration. Since the aurora forms a ring centered on the geomagnetic poles rather than on the geographical north and south poles, your chances for spotting it improve if you’re closer to the geomagnetic pole.
For instance, both Denver, Colo. and Madrid, Spain have nearly the same 40 degrees north latitude, but Denver’s in a much better location for an occasional auroral visit because the magnetic pole resides on the “North American side” of the globe.
You can predict the likelihood of northern lights for your location by finding your magnetic latitude. This is much like your regular latitude but as it relates to the magnetic pole instead of the geographic pole. Returning to our example, the magnetic latitude of Madrid is 33 degrees while that of Denver is 48. As far as the aurora is concerned, Denver’s much farther north and in a better position to see a display.
I’m always chattering on here about the Kp index, an indicator of geomagnetic or potential auroral activity. It’s rated on a scale of 0 to 9 and updated every 3 hours. The typical threshold for seeing the aurora from Duluth, Minn. (magnetic latitude 56 degrees) is a “4″. If you live in central Canada or northern Norway it can be as low as “3″ or “2″.
Another activity level indicator is the POES satellite auroral index rated from 1 to 10+. Here in Duluth, when the level reaches “8″ chances are good we’ll see at least a little green haze off to the north. For Denver it has to climb to “10+”.
The space weather people have put together a very useful Aurora Tips page you can use to find your own magnetic latitude. A table and drop-down menu list a number of cities, but if you can’t find yours, click on one of the four Kp maps below and then click on your location to get your number. All the maps are also available on the Tips page:
With magnetic latitude in hand, you can now use the tables on the site to know at what level the POES and Kp indices need to reach for you to justify losing sleep for the northern lights. Bear in mind that all these methods are indicators only and don’t absolutely guarantee you will or won’t see northern lights. Other factors like moonlight, weather nature’s unpredictability are always at play.
With the moon now past full and rising late, let’s cross our fingers the aurora will pay a visit soon.