February 25, 2024

Not every supposed northern light is true aurora

The shimmering green, red and purple curtains of the northern lights in Earth's northern and southern hemispheres are one of the most famous phenomena in the night sky. Less common are the fuzzy purple stripes called “steves” and their common companion, the glowing green “picket fence.” A new study suggests that these two phenomena look like the aurora borealis at first glance, but are likely produced under different and unusual physical conditions.

The aurora borealis – also called the northern lights – is a temporary light phenomenon in the sky. The brilliant lights are created when particles from solar storms strike the Earth's protective magnetic field. The solar excited particles then travel along the Earth's magnetic field lines toward the poles. The magnetic field there is thinner and more permeable, so that the sun’s molecules collide with oxygen and nitrogen molecules in the upper atmosphere at an altitude of about 1,000 kilometers and stimulate them. When the gas molecules relax again, they glow. Oxygen emits green and red light, and nitrogen emits mainly blue and a little red light. The colorful, shimmering curtains of northern lights created in this way can extend for thousands of kilometers across northern or southern latitudes.

What is “Steve” and “Picket Fence”?

In addition to this spectacle, two other colorful natural scenes can also be seen in our skies from time to time, usually occurring at the same time. They are referred to as “Steve” and his “picket fence” – based on a fence and picket fence from a children’s movie. The arc-shaped steve glows with a wide range of colors around the purple, and the picket fence lines vibrate an intense green. There is no visible blue light during either process. Although experts have acknowledged since 2018 that these phenomena are visually different from regular aurora borealis, experts still assume that both Steve's aurora and the picket fence are caused by the same physical processes. However, the exact reason behind their different colors has remained a mystery.

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Researchers led by Claire Gaskey from the University of California, California, have investigated this and are using their findings to question common assumptions. Accordingly, Steve and the picket fence look like the aurora borealis only at first glance, but they are two different celestial events. According to Gaskey's calculations, the luminous phenomena STEVE and Picket Fence form significantly further away from the poles than the aurora borealis. It may occur even at the equator. At these latitudes, electric fields acting parallel to the Earth's magnetic field at a relatively low distance from the Earth's surface appear to produce picket fence lights.

Detecting an electric field is theoretically impossible

In theory, such electric fields should not exist, because in reality they should create a short circuit and disappear. However, Jaski and her colleagues came to the conclusion that at an altitude of about 110 kilometers, a weak electric field of about 100 millivolts per meter can act parallel to the Earth's magnetic field. The researchers reported that unusual conditions occur in this region, such as low-density charged plasma and neutral oxygen and nitrogen atoms. These conditions likely have an insulating effect and prevent the electric field from shorting out.

According to the study, this electric field could be responsible for the phenomenon of the green light of the picket fence. Because it can stimulate existing electrons, which – just as with the northern lights – stimulate oxygen and nitrogen molecules in the atmosphere. When these gas molecules return to their original state, they emit energy in the form of light and glow – but in this case at a wavelength of lower energy than that of the aurora. This is expressed in a different color spectrum than the northern lights, scientists explain. The picket fence phenomenon also gets its name because the glow changes characteristically, as if it is being turned on and off. Gaskey attributes this to wave-like fluctuations in the electric field.

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“This is a completely different mechanism than any aurora we have studied or known about before,” Gaskey says. Because unlike the aurora borealis, picket fences are not generated directly by solar energy particles. However, researchers point out that solar storms can disrupt the atmosphere and thus indirectly contribute to picket fence formation. Based on their calculations, Jaski and her team suspect that Steve was also created through processes similar to picket fencing, but ultraviolet light is released in the process. “The study shows that parallel electric fields can explain Steve's strange light spectrum,” says Gaskey's colleague and co-author Brian Harding.

The missile must provide clarity

The theory of creating false auroras arose solely from calculations. In order to test their hypothesis experimentally, the researchers next want to send a rocket from Alaska through these light events. The on-board measuring instruments will then measure the prevailing electric and magnetic fields for the first time and examine them in more detail. Scientists first want to examine hybrid forms of regular aurora with picket fence-like components, called “enhanced aurora,” because these occur much more frequently than pure picket fences. More rockets will then also examine conditions in STEVE and Picket Fence at latitudes away from the poles so they can compare conditions for the two phenomena with normal aurora.

“There will be a lot of studies in the future about how these electric fields got there, what waves are or aren't associated with them, and what that means for greater energy transfer between Earth's atmosphere and space,” Harding says. “This study is just the first step in this understanding.” Scientists hope the experiments will also provide a better understanding of the chemistry and physics of the upper atmosphere, Earth's magnetic field, and the ionosphere at the edge of space. .

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Source: Claire Gaskey (UCLA) et al., Geophysical Research Letters, doi: 10.1029/2023GL106073