Finnish photographer Pekka Parviainen
is known worldwide for his photographs of atmospheric and astronomical phenomena. This all-sky view shows the converging streamers that typify an auroral corona.
The earliest known references to auroras date back to the Old Testament, and in the 2,500 years since then the northern lights have fascinated casual skywatchers and scientists alike. But probably fewer than 5 percent of the Earth's inhabitants during this period have ever seen an aurora, since auroras usually appear only at high northern and southern latitudes, far from the world's population centers.
However, dedicated skywatchers who keep their eyes open have a much better chance than the public at large.
The solar wind transforms the Earth’s magnetic field into a weathervane, a long 'tear-drop' cavity confined by the solar wind and the magnetic field it carries with it. The bow of our magnetosphere bears the brunt of the solar gale and manages to splash the solar ions around and behind us. From time to time, some plasma reaches the Earth and enlivens the perpetual auroral circles around the magnetic poles.
Sky & Telescope: Gregg Dinderman.
When and Where to Look
Zones of auroral visibility. This map of the Northern Hemisphere shows the percentage of nights during which one can expect to see an aurora, weather permitting. The statistical auroral oval (white) is centered on the hemisphere's geomagnetic pole.
Courtesy Robert Eather.
Until recently there was no way to predict an aurora. You just had to spend many hours out under the stars and wait to get lucky. Today, however, you can do better.
The aurora is a more or less permanent feature of Earth's high latitudes. It is usually confined to two large, thin ovals encircling the Earth's north and south magnetic poles. Nighttime satellite pictures routinely show these so-called auroral ovals as halos of light crowning the top and bottom of the globe (an example appears at lower right).
One can talk about the auroral oval in two ways. The "statistical oval" is its average location at a given level of magnetic activity; the "instantaneous oval" is the aurora's actual location at a particular moment. The statistical oval can be used to estimate the likelihood of seeing auroras from a particular site. On any given night, the instantaneous oval often falls elsewhere.
A Northern Hemisphere coastline map superimposed on a 12-minute ultraviolet image of the instantaneous auroral oval taken from space on November 8, 1981.
Courtesy Louis A. Frank, University of Iowa.
To complicate matters further, auroral activity is spasmodic and often culminates in a period of spectacular activity called a substorm
. Substorms usually occur around local midnight. They cause the instantaneous oval to rapidly expand poleward.
The auroral ovals are two halos of light (the aurora borealis and the aurora australis) about 110 kilometers above the Earth and centered on the magnetic poles. The Earth rotates under these halos about its geographic axis. This means that at far northern latitudes, near the oval's ordinary "home" location, some simple rules of thumb apply. In central Alaska or northern Scandinavia, on most nights you can expect to rotate directly under the oval in the midevening hours, be inside it (poleward of it) around midnight, and pass under it again before dawn. Thus one expects to see the aurora move from the northern sky shortly after dark, to overhead at midevening, and to the south around midnight, then back overhead again in the early hours of the morning.
At the lower latitudes where most of us live, however, we are usually so far equatorward of the auroral oval that we can't see it at all. When we can, we almost always have to look near the north horizon (for observers in the Northern Hemisphere). Because the oval extends farthest toward the equator around the middle of the night, that is the best time for would-be aurora watchers at temperate latitudes to look.
During magnetic storms the auroral oval expands to lower latitudes and we can see a display earlier in the evening and higher in the sky or even overhead. During a rare large storm, the oval may expand so far toward the equator that it can be seen from the tropics, while skywatchers at midnorthern latitudes actually find themselves facing south.
A curtain aurora is a rayed band looped in dramatic folds.
Courtesy Donald E. Cartier Sr.
Fortunately for today's aurora watchers, magnetic storms can often be forecast a day or two in advance. The tip-off is a solar flare near the center of the Sun's disk or a coronal mass ejection aimed toward Earth. A worldwide array of observatories constantly monitors the Sun for such events. The Sun always gives off a solar wind, composed mostly of electrons and protons, which interacts with the Earth's magnetic field to light the auroral oval. A solar eruption can make the solar wind come thicker and faster, with gusts that buffet the Earth's magnetosphere and power up the auroral oval accordingly. These wind enhancements take one or two days to reach us, allowing time to plan.