The aurora borealis, or northern lights, is a common sight at north polar latitudes. The name aurora borealis comes from Latin and Greek. Aurora was the ancient Roman goddess of the dawn, and the ancient Greek name for the north wind is “boreas.” The corresponding occurrences in the southern polar skies are the aurora australis.
Extreme aurora light shows are caused by a coronal mass ejection, or CME, from the sun that causes a severe geomagnetic storm. Such storms are not uncommon. The sun spits out hot electromagnetic plasma from sunspots all the time, and the burps from within the blazing interior of the sun’s nuclear furnace vary in intensity. Fortunately, most CMEs are not aimed at Earth, so the vast majority of them miss us.
The plasma from a CME creates the aurora by doing naturally to Earth’s atmosphere what we do artificially to produce neon signs and television pictures.
When atoms and gas molecules at low pressure are energized by electrically charged particles, they absorb certain amounts of energy that correspond to transitions of electrons to higher energy levels within the atoms.
Eventually the energized electrons fall back to a lower energy level, emitting light with the same bar code of wavelengths that they absorbed. We see the emitted light as colors that are a mixture of the various wavelengths. Each gas has a unique spectral signature.
For example, neon gas emits almost entirely wavelengths of red light, so it is used in red signs.
To make a neon sign, electricity energizes a glass tube containing neon gas at extremely low pressure. Electrons stream through the tube, causing the neon atoms to jump up and down between energy levels and emit light.
Although we call all such glowing tubes “neon” lights, the different colors are mixtures of various gases designed to produce the desired colors.
It is not only during CMEs that the sun emits plasma particles. The particles continuously emanate in all directions at very high speeds — an average of almost 1 million mph! This “solar wind” pervades the solar system and pushes the tails of comets away from the sun, among other things.
Earth’s magnetic field deflects the relatively steady solar wind to form a magnetic shock wave called the magnetosphere.
The solar wind flows around the magnetosphere like a stream flowing around a rock. It stretches and elongates the magnetosphere into a raindrop shape with a long tail trailing away from Earth on the side away from the sun.
The accelerated solar wind ions spiral along Earth’s magnetic field lines, constrained like beads on a wire. In the upper atmosphere, the magnetic field lines converge toward the north and south magnetic poles, where they become nearly perpendicular to the surface. Energized ions traveling along the field lines collide with atoms and molecules of oxygen and nitrogen in the thin upper atmosphere and produce light.
Oxygen emits green and, less frequently, deep red light. Nitrogen is responsible for blue or purplish-red light that produces the purple-tinged lower borders and rippling edges of the aurora.
From Earth the aurora appears as colorful curtains and rays of light that dance across the sky, but from space they appear as bright symmetric ovals centered on the northern and southern magnetic poles and extending outward toward the equator, gradually diminishing in intensity.
We will likely not see the aurora as far south as Hawaii, but their shows have enchanted viewers at higher latitudes from prehistoric times. It is a real treat to see it from the far north as I did from northernmost Norway last month.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
