When the northern lights creep far enough south to tantalize horizons in Illinois, for example, the display is more than just atmosphericit is also diagnostic. A glowing arc against a winter skyline indicates that the magnetic shield of Earth is hard at work, separating energy from the Sun into visible color.
The cause starts with solar activity filling space with electrically charged particles propelled outward by the solar wind. Most of this flux is turned back, but during times of intense geomagnetic activity, more energy is coupled into the Earth’s magnetosphere and strengthens the auroral ring that typically encircles high latitudes. Predicted maps depict this as an auroral oval, a glowing ring that circles the magnetic pole and expands and contracts in a breathing-like fashion.
What the eye sees as “dancing lights” is, in essence, a series of collisions in thin air. Electrons accelerate along magnetic field lines and fall into the upper atmosphere, where they collide with oxygen and nitrogen. These collisions “excite” atoms and molecules, boosting them into higher energy states, followed by the emission of photons as they return to lower states. The patterns of the curtains and waves correspond to magnetic field patterns, so the familiar waves are, in essence, a dynamic representation of the forces that are normally invisible in compass readings and satellite imagery.
The colors also encode information about altitude. Oxygen is usually represented by a light green color, while nitrogen is purple, blue, or pink. In very energetic environments, oxygen at high altitudes may be red. A common standard defines the lowest point of some aurorae at 80 miles above the Earth’s surface, while the highest point may be much higher, depending on the conditions. This creates a complex phenomenon, where different gases, altitudes, and excited state lifetimes are combined into a single image.
The question that most mid-latitude observers want to know is simple: how far south can it be seen? This is the uncertainty that is incorporated into the indices that are used in forecasting. The Kp index is a scale that converts global magnetic activity into a single number that is related to how far out the auroral oval will extend. A Kp of 5 is generally considered to be a value that is moderate enough to make the display brighter and extend visibility to lower latitudes, although the actual effect can vary depending on the conditions. The Geophysical Institute forecast products show a useful boundary with a “viewline” at its southern-most tip, suggesting where the aurora might be visible on the northern horizon.
There is also a strict visibility requirement: daylight will ruin it. The best odds are usually within a couple of hours of midnight, or near the dark edges of the night, just after sunset and just before sunrise, when the sky background is at its weakest. Even then, cloud cover can render the whole thing invisible, reducing a promising geomagnetic alignment to a night of nothing but reflected city lights.
The nature of what constitutes “seeing” has been altered by modern cameras. The digital sensor is capable of detecting dark detail and color that may be perceived as light or close to colorless by human night vision, which is due to the eye’s preference for light over color in night vision conditions. This is why steady, long-exposure images from smartphones are capable of detecting bands and hues that were not visible in real time, an engineering triumph that renders observation more sensitive than before.
