December 19, 1980 Red Aurora
Although all data from far-flung stations are not yet available, it is possible to piece together a description of the unusual auroral display of December 19, 1980 using reports from those who saw the display and instrumental data acquired at Fairbanks and Fort Yukon, Alaska. Aside from the rarity of the blood-red auroras, this display is of special interest because it induced very large currents in the electrical transmission line between Nenana and Fairbanks between 5:00 am and 7:00 am on the morning of December 19 (Alaska standard time).
Black and white pictures taken each minute by an all-sky camera at Fort Yukon showed that a normally appearing auroral arc--probably a green one--developed directly overhead that location just after 7:00 p.m. on the evening of December 18, 1980. Several arcs and bands swirled overhead Fort Yukon during the next hour, not an unusual occurrence at that auroral zone location. The aurora then quieted down.
The first hint that this might be an unusual night was visual sighting of aurora directly overhead Anchorage shortly after midnight. This in itself was not really unusual, since auroras do often appear over Anchorage; when they do, it means a larger-than-average display is underway.
At 1:43 am, the aurora over Fort Yukon started anew, and by 2:30 a.m. the arcs were showing unusually tall ray structures; perhaps they were more reddish than usual. Things started to get exciting about 3:45 am, and the aurora came on like gangbusters over the whole of the southern half of Alaska. However, the best of the action appears to have been seen in the Anchorage area and the areas easterly of it over into British Columbia. The aurora was seen overhead Seward between 5:15 am and 6:30 am and overhead Ketchikan near 6:20 am.
A red glow was seen on the north horizon in the Anchorage area near 4:00 am. There was deep red aurora to the west of Clear, Alaska at 6:45 am, and then truly red aurora mixed in with auroras of green and yellowish hues spread over Anchorage-area skies in the hour or two after 6:30 am. The last aurora seen was being entirely obscured by sunlight by 9:00 am.
A number of people living within a hundred miles of Anchorage saw long ray structures, of mixed colors, stretching up toward the center of the sky and converging to a point called the magnetic zenith. In the Anchorage area this point is somewhat to the southeast of the true zenith which is directly overhead the observer. Called the corona, this rayed auroral structuring signifies that the aurora is overhead, since the coronal appearance cannot happen otherwise.
Several persons righty noted that the red portions of this aurora were of unusual purity of color and quite different from the reddish hues that sometimes tinge the bottoms of normal, active green auroras. The deep blood-red color of the unusual all-red auroras is truly pure, as is light from a laser, since it is not a mixture of colors. Instead it is, in this case, composed of light of only two red wavelengths, at 6300 and 6364 Angstroms.
Most of the aurora we see is a mixture of many colors. There usually is much bluish light near 3900 Angstroms, but our eyes are poor in the blue, so we detect it poorly. Our eyes see best in the green, and there usually is strong auroral light at 5577 Angstroms (a yellowish green color) hence we see green aurora frequently.
The rare all-red auroras and the tall ray structures or tall featureless masses in which the red auroras appear are caused by a preponderance of incoming particles (electrons and protons) having lower speeds than those that cause the common garden-variety green auroras usually seen. Moving too slowly to penetrate down to an altitude near 100 km (60 miles) where green auroras are created, the slow-moving particles lose most of their energy to oxygen atoms in the high atmosphere, at altitudes in the range 200 to 400 km. The interaction of the incoming particles with the oxygen atoms gives the pure red light at 6300 and 6364 Angstroms.
By contrast, the red tinge sometimes seen at the bottoms of green auroras is a mixture of many different shades of red caused by the incoming particles striking oxygen and nitrogen molecules lying comparatively low in the atmosphere, at altitudes 60 to 90 km.
