When is an aurora not an aurora? Phenomena called “Steve” and “picket fence” masquerade as aurora

When is an aurora not an aurora?  Phenomena called “Steve” and “picket fence” masquerade as aurora

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The shimmering green, red and purple curtains of the northern and southern lights – the aurora borealis – may be the most famous phenomenon that lights up the night sky, but even more mysterious are the violet-white streaks called STEVE and their recurring companion, a glowing star. Green “picket fence”.

First identified in 2018 as being different from the common aurora, however, Steve – an ironic reference to the benign name given to a creepy fence in a 2006 children’s film – and the associated picket fence are thought to be caused by the same physical processes. . But scientists were left scratching their heads as to how these glowing emissions were produced.

Claire Gaskey, a graduate student in physics at the University of California, Berkeley, has proposed a physical explanation for these phenomena that is very different from the processes responsible for known aurorae. She teamed up with researchers at the campus’ Space Science Laboratory (SSL) to suggest that NASA launch a rocket into the heart of the aurora to see if she’s right.

Vibrant aurorae and glowing phenomena like STEVE and picket fences are becoming more common as the Sun enters the active period of its 11-year cycle, and November has been a good month for STEVE observations in northern latitudes. Because all of these transient luminous phenomena are caused by solar storms and coronal mass ejections from the Sun, approaching solar maximum is an ideal time to study rare events like STEVE and picket fences.

Gaskey described the physics behind the picket fence in a paper published last month in the journal Geophysical Research Letters We will discuss the results on December 14 in an invited lecture at the American Geophysical Union meeting in San Francisco.

She calculated that in the region of the upper atmosphere south of that where the aurora forms, electric fields parallel to the Earth’s magnetic field could produce the color spectrum of the picket fence. If true, this unusual process has implications for how physicists understand the flow of energy between Earth’s magnetosphere, which surrounds Earth and protects it from the solar wind, and the ionosphere at the edge of space.

“This would upend our model of what creates the light and energy in the aurora in some cases,” Gaskey said.

“The really interesting thing about Claire’s research is that we’ve known for a couple of years that Steve’s spectrum tells us there’s some very strange physics going on,” Brian Harding said. “But we didn’t know what it was.” Co-author of the paper and research assistant physicist SSL. “Clare’s research has shown that parallel electric fields are able to explain this strange spectrum.”

The paper was a side project of Gaskey’s Ph.D. The thesis that focuses on the relationship between events such as volcanoes on the Earth’s surface and phenomena in the ionosphere 100 kilometers or more above our heads.

But after hearing about Steve — now short for Strong Thermal Emission Velocity Enhancement — at a conference in 2022, she couldn’t resist looking into the physics behind Steve and the picket fence.

“It’s really cool,” she said. “It’s one of the biggest mysteries in space physics right now.”

Steve’s physics and picket fence

Common auroras are produced when the solar wind energizes particles in the Earth’s magnetosphere, often at altitudes higher than 1,000 kilometers above the surface. These energetic particles wrap around Earth’s magnetic field lines toward the poles, where they collide and excite oxygen and nitrogen molecules in the upper atmosphere. When those molecules relax, oxygen emits specific frequencies of green and red light, while nitrogen generates a slightly red, but primarily blue, emission line.

The resulting curtains of shimmering color can extend for thousands of kilometers across northern or southern latitudes.

However, STEVE does not display individual emission lines, but rather a broad band of frequencies centered around the violet or violet color. Unlike the aurora borealis, neither Steve nor the picket fence emit blue light, which is generated when more energetic molecules collide with nitrogen and ionize it. Steve and picket fence also occur at latitudes lower than the aurora borealis, perhaps even as far south as the equator.

Some researchers have suggested that STI is caused by ion fluxes in the upper atmosphere, referred to as subauroral ion drift, or SAID, although there is no well-accepted physical explanation for how SAID generates the colored emissions.

Gaskey’s interest was sparked by suggestions that picket fence emissions could be generated by electric fields at low altitudes parallel to the Earth’s magnetic field, a situation thought to be impossible because any electric field in line with the magnetic field would quickly shorten and disappear.

Using a common physical model of the ionosphere, Gaske later showed that a moderately parallel electric field — about 100 millivolts per meter — at an altitude of about 110 km could accelerate electrons into energy that would excite oxygen and nitrogen and generate a light spectrum. Observed from the picket fence. The unusual conditions in that region, such as a lower density of charged plasma and more neutral oxygen and nitrogen atoms, can act as insulation to prevent the electric field from falling short.

“If you look at the scope of the picket fence, it’s a lot greener than you would expect. There’s no blue color that comes from nitrogen ionization,” Gaskey said. “What this tells us is that there is only a specific energy band of electrons that can create those colors, and they can’t come from space into the atmosphere, because those particles have too much energy.”

Instead, she said, “The light emitted by the picket fence is generated by particles that must be energized out there in space by a parallel electric field, a mechanism very different from any aurora we have studied or known before.”

She and Harding suspect that Steve himself may be produced by related processes. Their calculations also predict what kind of ultraviolet emissions this process will produce, which can be verified to verify the new hypothesis about the picket fence.

Although Gaskey’s calculations don’t directly address the intermittent glow that makes the phenomenon look like a picket fence, it’s likely due to waveform differences in the electric field, she said. While the particles accelerated by the electric field may not be from the Sun, the mixing of the atmosphere by solar storms may excite Steve and the picket fence, as do the common aurora borealis.

Enhanced aurora shows a fence-like glow

Harding said the next step is to launch a rocket from Alaska through these phenomena and measure the strength and direction of the electric and magnetic fields. SSL scientists specialize in designing and building tools that do just that. Many of these instruments are aboard spacecraft now orbiting the Earth and the Sun.

First, the target will be what’s known as an enhanced aurora, which is a regular aurora that has fence-like emissions built into it.

“The enhanced aurora is basically this bright layer incorporated into the normal aurora. The colors are like a picket fence in that there’s not as much blue, there’s more green from oxygen and red from nitrogen. The hypothesis is that these are done,” Gaskey said. They are also created by parallel electric fields, but are much more common than picket fencing.”

The plan is not only to “fly a rocket through that boosted layer to actually measure those parallel electric fields for the first time,” she said, but also to send a second rocket to measure molecules at higher altitudes, “to differentiate between conditions.” Of the ones that cause the aurora.” Ultimately, she hopes to get a rocket that flies straight through Steve and the picket fence.

Harding, Gaske and colleagues proposed such a rocket campaign to NASA this fall and expect to hear back about its selection in the first half of 2024. Gaske and Harding see the experiment as an important step in understanding the chemistry and physics of the upper atmosphere. Earth’s ionosphere and magnetosphere, and a proposal consistent with NASA’s Low Cost Access to Space (LCAS) program for projects like this.

“It’s fair to say that there will be a lot of studies in the future about how those electric fields got there, what waves are or aren’t associated with them, and what that means for the larger energy transfer between Earth’s atmosphere and space,” Harding said. “We really don’t know. Clare’s paper is the first step in this understanding.”

Gaske appreciated the contributions of people who study the ionosphere, or mesosphere, and stratosphere, whose ideas helped her find the solution.

“Thanks to this collaboration, we have been able to achieve some great progress in this area,” she said. “Honestly, it was just following our noses and getting excited about it.”

In addition to Harding, other co-authors are Reza Janalizadeh of Pennsylvania State University in University Park, Justin Junker of the Applied Physics Laboratory at Johns Hopkins University in Laurel, Maryland, and Dr. Megan Gillis of the University of Calgary in Alberta. Canada.

more information:
the. Claire Gaske et al., It’s Not Easy Being Green: Kinetic Modeling of the Observed Emission Spectrum at Steve’s Fence, Geophysical Research Letters (2023). doi: 10.1029/2023GL106073

Magazine information:
Geophysical Research Letters

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