When is an aurora not an aurora? Phenomena called 'Steve' and 'picket fence' are masquerading as auroras

The most enigmatic phenomena that light up the night sky are the mauve and white streaks known as Steve and their frequent partner, a blazing green "picket fence." The auroras, or northern and southern lights, are noted for their shimmering green, red, and purple curtains.

Though Steve (a playful nod to the innocuous name given to a terrifying hedge in a 2006 children's movie) and its picket fence were first identified in 2018 as different from the typical auroras, they were believed to be created by the same basic processes. However, the mechanism generating these luminous emissions baffled experts.

A physics doctoral student at the University of California, Berkeley named Claire Gasque has recently put up a scientific theory for these occurrences that is entirely distinct from the mechanisms underlying the well-known auroras. In order to determine whether she is right, she has joined up with scientists at the Space Sciences Laboratory (SSL) on campus to suggest that NASA send a rocket into the center of the aurora.

As the sun moves into the active phase of its 11-year cycle, bright auroras and illuminating phenomena like Steve and the picket fence become more frequent. November proved to be a strong month for Steve sightings in the northern latitudes. Steve and the picket fence are good examples of unusual events that may be studied during the approaching solar maximum since all of these fleeting light phenomena are caused by solar storms and coronal mass ejections from the sun.

Gasque will explain the findings in an invited session at the American Geophysical Union meeting in San Francisco on December 14th. Gasque detailed the physics underlying the picket fence in a study that was published last month in the journal Geophysical Research Letters.

She estimated that the color spectrum of the picket fence may be produced by electric fields parallel to Earth's magnetic field in a region of the upper atmosphere that is farther south than the aurora formation area. If this novel method is accurate, it will affect the way physicists see the flow of energy between the ionosphere at the edge of space and the magnetosphere that envelops and shields Earth from the solar wind.

"This would upend our modeling of what creates light and the energy in the aurora in some cases," Gasque stated.

"What makes Claire's study so intriguing is that, as we've known for a few years, the Steve spectrum indicates that some really unusual physics is occurring. As an SSL assistant research physicist and co-author of the report, Brian Harding stated, "We simply didn't know what it was." "Claire's paper showed that parallel electric fields are capable of explaining this exotic spectrum."

The work was a side project from Gasque's doctoral dissertation, which focuses on the relationship between Earthly occurrences such as volcanoes and phenomena occurring in the ionosphere, which is located at least 100 kilometers above our planet.

However, she couldn't help but investigate the physics underlying Steve and the picket fence after learning about it at a conference in 2022 (Steve is now an abbreviation for Strong Thermal Emission Velocity Enhancement).

"It's pretty awesome," she said. "It's one of the biggest mysteries in space physics right now."

Steve and picket fence physics

When solar wind energizes particles in Earth's magnetosphere, frequently at elevations greater than 1,000 kilometers above the surface, common auroras are created. These charged particles travel in a spiral around the lines of Earth's magnetic field, heading for the poles, where they collide and energize molecules of nitrogen and oxygen in the upper atmosphere. Oxygen releases certain green and red light frequencies as the molecules relax, while nitrogen mostly produces a blue emission line with some red light.

The resultant curtains of color and shimmering light can stretch hundreds of kilometers across the northern or southern latitudes.

Steve, on the other hand, exhibits a wide variety of frequencies focused on purple or mauve rather than separate emission lines. Nor does the picket fence, nor Steve, emit blue light, which is produced when the most energetic particles strike and ionize nitrogen, unlike auroras. In addition to the aurora, Steve and the picket fence also occur at lower latitudes—possibly as low as the equator.

Although there isn't a widely accepted scientific explanation for how subauroral ion drift, or SAID, may produce the multicolored emissions, some researchers have hypothesized that Steve is created by ion fluxes in the high atmosphere.

Suggestions that low-altitude electric fields parallel to Earth's magnetic field could be the source of the picket fence's emissions piqued Gasque's interest. This was previously believed to be impossible because any electric field aligned with the magnetic field should quickly short out and disappear.

Gasque then demonstrated, using a widely used physical model of the ionosphere, that electrons could be accelerated to an energy that would excite nitrogen and oxygen and produce the light spectrum seen from the picket fence at a height of approximately 110 km by a moderate parallel electric field of about 100 millivolts per meter. Unusual circumstances in that region, including a higher concentration of neutral oxygen and nitrogen atoms and a lower density of charged plasma, may serve as insulation to prevent the electric field from shorting out.

The picket fence's spectrum reveals a lot more green than one may anticipate. Furthermore, none of the blue that results from nitrogen ionization is present, according to Gasque. "What that's telling us is that there's only a specific energy range of electrons that can create those colors, and they can't be coming from way out in space down into the atmosphere, because those particles have too much energy."

Instead, according to her, "the light from the picket fence is being created by particles that have to be energized right there in space by a parallel electric field, which is a completely different mechanism than any of the aurora that we've studied or known before."

Harding and her believe that comparable mechanisms may also generate Steve. The kind of UV emissions that would result from this process are also predicted by their calculations, and they may be verified in order to validate the novel theory on the picket fence.

The on-off glow that gives the phenomena its picket fence-like appearance is probably caused by wave-like changes in the electric field, according to Gasque, even if her calculations don't specifically address it. Additionally, solar storms' disruption of the atmosphere is likely what causes Steve and the picket fence, as well as the typical aurora, even if the particles driven by the electric field are most likely not solar in origin.

Picket fence-like light is shown by enhanced auroras.

The next stage, according to Harding, is to use these phenomena to launch a rocket from Alaska and gauge the direction and intensity of the magnetic and electric fields. That is what SSL scientists specialize in creating and developing equipment for. Numerous of these equipment are now on spacecraft that circle the sun and Earth.

The initial aim would be an enhanced aurora, which is a regular aurora with emissions resembling picket fences imbedded in it.

"This brilliant layer that is included into the regular aurora is essentially the enhanced aurora. The colors are comparable to the picket fence in that they have more green from oxygen and more red from nitrogen, with less blue. Though they are far more prevalent than picket fences, it is hypothesized that they are likewise produced by parallel electric fields," Gasque stated.

According to her, the idea is not just to "fly a rocket through that enhanced layer to actually measure those parallel electric fields for the first time," but also to launch a second rocket higher to measure the particles in order "to distinguish the conditions from those that cause the auroras." She eventually dreams of a rocket that will pass through Steve and the picket fence exactly.

This fall, Harding, Gasque, and associates presented NASA with a compelling rocket campaign, and they anticipate hearing back from the agency about its selection in the first half of 2024. Gasque and Harding saw the experiment as a proposal in accordance with NASA's Low Cost Access to Space (LCAS) program, which supports such initiatives, and as a significant step toward understanding the chemistry and physics of the upper atmosphere, the ionosphere, and the Earth's magnetosphere.

"It's fair to say that there's going to be a lot of study in the future about how those electric fields got there, what waves they are or aren't associated with, and what that means for the larger energy transfer between Earth's atmosphere and space," said Harding. It's really unknown to us. The first link in the chain of that knowledge is Claire's paper."

Gasque conveyed gratitude for the assistance she received in deciphering the answer from experts in the fields of stratospheric and intermediate ionosphere research.

"With this collaboration, we were able to make some really cool progress in this field," she stated. "Honestly, it was just following our nose and being excited about it."

Harding is co-authored with three other people: D. Megan Gillies from the University of Calgary in Alberta, Canada; Justin Yonker from the Applied Physics Laboratory at Johns Hopkins University in Laurel, Maryland; and Reza Janalizadeh from Pennsylvania State University in University Park.