Not every supposed northern light is a real aurora

One image each of a normal and an enhanced aurora

A normal aurora (left) consists of glowing curtains of red, green and blue. An “enhanced aurora” (right) contains a thin, brighter layer that appears much greener. The latter may be caused by the same physical process as the “Steve” and “picket fence” phenomena. © Vincent Ledvina,

The green, red and purple shimmering curtains of the northern lights in the northern and southern hemispheres of the earth are among the most famous phenomena in the night sky. Less common are the mysterious purple stripes called “Steve” and a common companion – the glowing green “picket fence.” These two phenomena look like auroras at first glance, but they are likely produced under different, unusual physical conditions, a new study suggests.

Auroras - also called northern lights - are temporary light phenomena in the sky. The fascinating lights arise when particles from solar storms hit the Earth's protective magnetic field. The particles excited by solar energy then travel along the Earth's magnetic field lines towards the two poles. The magnetic field there is thinner and permeable, so that the sun's particles hit oxygen and nitrogen molecules in the upper atmosphere at an altitude of around 1,000 kilometers and stimulate them. When the gas molecules then relax again, they glow. Oxygen emits green and red light, nitrogen mainly blue light and a little red. The colorful, shimmering curtains of the northern lights that are created in this way can extend for thousands of kilometers across northern or southern latitudes.

What are “Steve” and the “picket fence”?

In addition to this spectacle, two other colorful natural spectacles can also be observed in our sky from time to time, which usually occur at the same time. They are referred to as "Steve" and his "picket fence" - based on a hedge and a fence from a children's film. Steve glows in an arc in a wide spectrum of colors around the color purple, the stripes of the picket fence vibrate intensely green. No blue light is visible during either process. Although experts have acknowledged since 2018 that these phenomena are visually different from ordinary auroras, experts have still assumed that both auroras and Steve and the picket fence are caused by the same physical processes. However, exactly what creates their different colors has been a mystery.

Researchers led by Claire Gasque from the University of California have now investigated this and are using their results to question common assumptions. Accordingly, Steve and the picket fence only look like auroras at first glance, but are different celestial events. According to Gasque's calculations, the luminous phenomena Steve and the picket fence form significantly further from the poles than auroras. They may even occur at the equator. At these latitudes, electric fields that run parallel to the Earth's magnetic field at a relatively low distance from the Earth's surface apparently produce the lights of the picket fence.

Theoretically impossible electric field discovered

Such electric fields should theoretically not exist because they should actually create a short circuit and disappear. However, Gasque and her colleagues came to the conclusion that at an altitude of around 110 kilometers, a weak electric field of around 100 millivolts per meter could actually run parallel to the Earth's magnetic field. The researchers report that unusual conditions occur in this area, such as charged plasma with a lower density and neutral oxygen and nitrogen atoms. These conditions could potentially have an insulating effect and prevent the electric field from shorting out.

This electric field could be responsible for the green light phenomenon of the picket fence, according to the study. Because it could stimulate existing electrons, which then – just like 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 with lower energy than in the auroras. This is expressed in a different color spectrum than that of the northern lights, as the scientists explain. The picket fence phenomenon also got its name because the glow changes characteristically, as if it were being turned on and off. Gasque attributes this to wave-like fluctuations in the electric field.

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

Rocket should provide clarity

The theory of the creation of false auroras emerged solely from calculations. In order to test their hypothesis experimentally, the researchers next want to send a rocket from Alaska through these light events. Measuring devices on board will then measure the prevailing electric and magnetic fields for the first time and examine them in more detail. The scientists first want to examine hybrid forms of ordinary auroras with picket-fence-like components, so-called “enhanced auroras,” because these occur much more frequently than pure picket fences. Further rockets will then also examine the conditions at Steve and the picket fence at latitudes further away from the poles in order to be able to compare the conditions of the two phenomena with the normal auroras.

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

Source: Claire Gasque (University of California) et al., Geophysical Research Letters, doi: 10.1029/2023GL106073

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