What matters most is "where," or as they say in the real estate
business--location, location, location. Your best bet is Fairbanks,
Alaska; you may also see northern lights in Winnipeg, Canada, or even
International Falls, Minnesota, but they could only be near the northern
horizon.

Scientists call the phenomenon the "polar aurora." Earlier it was
named "aurora borealis" meaning "northern dawn" in Latin, since in
Europe it was mostly seen as a glow near the northern horizon. However,
it also occurs near the south pole, so "polar aurora" is now preferred.
Aurora is caused by electrons energized in the Earth's magnetic
environment, the magnetosphere, and guided earthward along magnetic
field lines (lines of force) along which they slide, a bit like beads
sliding along a wire. They move at about 1/5 the speed of light and in
many ways they resemble electrons beamed at the screen inside the
picture tube of a TV receiver. Where picture-tube electrons produce
light when they hit the screen, auroral electrons do so when they hit
the fringes of the atmosphere, about 100 kilometers (60 miles) up.

The field lines on which this happens are the ones coming down along
a circle centered near the magnetic pole ("auroral circle" or "auroral
oval"), and that is where the accelerated electrons end up. The circle
can expand and contract, but it usually passes near Fairbanks.

When? Don't expect to see aurora in the Alaskan tourist season, during
the summer, it just does not get dark enough. Just as near-polar winter
nights are long and dark, summer days are long and bright, and even
after the Sun sets, twilight persists. September has some darkness,
October much more--March and April are also OK, and so is winter, if you
do not mind the cold. When? The brightest auroras come from the night
side of the magnetosphere, so you should see them around midnight, or
later because of Alaska's time zone. If you stay in a hotel, ask the
desk clerk to wake you when a good display occurs.

What about the 11-year sunspot cycle? Occasionally, but especially in
years of peak sunspot activity, the Sun sends out blobs of hot gas,
which hit the magnetosphere and agitate it, causing "magnetic storms."
At such times the "auroral circle" expands and reaches the lower 48
states of the US and mid-latitude Europe. The agitation also produces
fine auroras, as happened on 5 November 2001, when the US was favorably
located. However, since Fairbanks is located near the normal auroral
circle, auroras are observed there throughout the 11-year cycle. The
main factor is the slanting of interplanetary magnetic field lines
(which come from the Sun)--southward slant, auroras likely, northward
slant, not so much. That slant varies randomly, though satellites
monitor it, and you can read their latest reports on the web.

You find much more in the various chapters and links of "The
Exploration of the Earth's Magnetosphere",
http://www.phy6.org/Education/Intro.html . That collection includes
sections on the aurora, magnetic field lines, the event of 5 November
2001 and one about "space weather," with links telling where to find
predictions of the likelihood of aurora.
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David P. Stern is a physicist retired from Goddard, where he studied the
Earth's magnetosphere.