Graduate students believe phenomena named "Stevie" and "Fences" are disguised as auroras. Shining with green, red and purple light, the northern and southern lights - the auroras - may be the best-known phenomenon lighting up the night sky, but the most mysterious are the lavender and white stripes known as "steves" and their constant companions, glowing green "fences."
Steve - the benign name given to a scary hedge in a 2006 children's movie - was first recognized as distinct from the common aurora in 2018, although it is still believed that Steve and its associated hedges are caused by the same physical process. But scientists have been scratching their heads over how these luminous bodies are created.
Claire Gasque, a graduate student in physics at the University of California, Berkeley, has now proposed a physical explanation for these phenomena that is completely different from the well-known auroral production process. Working with researchers at the school's Space Science Laboratory (SSL), she suggested that NASA launch a rocket to the Aurora Center to see if her idea was correct.
As the Sun enters the active phase of its 11-year cycle, vibrant auroras and glowing phenomena like Stevie and the Fence are becoming more common, and November is a good month to observe Stevie in northern latitudes. Since all of these transient glowing phenomena are triggered by solar storms and solar coronal mass ejections, the upcoming solar maximum is an ideal time to study rare phenomena like Steve and Fence.
Gaske described the physics behind the fence in a paper published last month in the journal Geophysical Research Letters, and will be a guest speaker at the American Geophysical Union meeting in San Francisco on December 14 to discuss the results. She calculated that in regions of the upper atmosphere further south than where auroras form, electric fields parallel to the Earth's magnetic field could produce the hedgerow spectrum. If the calculations are correct, this unusual process could have implications for how physicists understand the flow of energy between Earth's magnetosphere and the ionosphere at the edge of space.
"In some cases, this will upend our modeling of what causes the energy in light and auroras," Gaske said. "What's really interesting about Claire's paper is that we've known for several years that Steve's spectrum was telling us that there was some very peculiar physics going on. We just didn't know what it was," said Brian Harding, a co-author on the paper and an assistant research physicist at SSL. "The paper shows that parallel electric fields can explain this strange spectrum."
The paper, a side project of Gaske's doctoral thesis, focuses on the connection between events such as volcanoes on the Earth's surface and ionospheric phenomena 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 researched Steve and the physics behind the fence. This is one of the biggest mysteries in space physics right now.
Steve and the Physics of Fences
Common auroras are produced when solar wind electrifies particles in Earth's magnetosphere. These charged particles spiral around Earth's magnetic field lines toward the poles, where they hit and excite oxygen and nitrogen molecules in the upper atmosphere. When these molecules relax, oxygen emits specific frequencies of green and red light, while nitrogen produces a little red light but mostly blue light.
The resulting colorful, shimmering curtains can stretch for thousands of kilometers at both north and south latitudes.
However, Steve shows not individual emission lines, but a broad range of frequencies centered around violet or lavender. Unlike auroras, neither STEVE nor Hedgerow emit blue light, which is produced by high-energy particles hitting nitrogen and ionizing it. Stevies and hedgerows also occur at lower latitudes than auroras, and may even occur south of the equator.
Some researchers have proposed that "STEV" is caused by a flow of ions in the upper atmosphere known as the ParaTaurus Ion Drift, but there is currently no accepted physical explanation for how the ParaTaurus Ion Drift produces the colorful radiation.
It has been suggested that the fence's radiation may be generated by low-altitude electric fields parallel to the Earth's magnetic field, but this scenario is considered unlikely because any electric field aligned with the magnetic field would quickly short-circuit and disappear.
Gaske then used ordinary physical models of the ionosphere to show that at an altitude of about 110 kilometers, a modest parallel electric field - about 100 millivolts per meter - could accelerate electrons to energies that excite oxygen and nitrogen and produce the spectrum observed from the fence. Abnormal conditions in this area, such as a lower density of charged plasma and more neutral atoms of oxygen and nitrogen, may act as an insulator to prevent electric field short circuits.
"If you look at the spectrum of the fence, it's much greener than you think. There's no blue from nitrogen ionization," Gaske said. "That tells us that only electrons in a certain energy range can produce these colors, and they couldn't have come from space or entered the atmosphere because these particles are so energetic. Instead, the light at the fence is produced by particles that have to be excited by parallel electric fields in space, which is completely different from the mechanism of any aurora we've studied or understood before."
She and Hardin suspected that Steve himself might have been created by a related process. Their calculations also predicted the type of ultraviolet radiation this process would produce, which could be examined to test new hypotheses about the fence.
While Gaske's calculations don't directly address the issue of the switch glowing that makes the phenomenon look like a fence, she said it's likely due to wave-like changes in the electric field. While the particles accelerated by the electric field may not come from the sun, disturbances in the atmosphere from solar storms could trigger STEVE and Fence, just like regular auroras.
Enhanced auroras show fence-like glow
The next step, Harding said, is to launch a rocket in Alaska through these phenomena and measure the strength and direction of the electric and magnetic fields. SSL scientists designed and built this instrument specifically. Many of these instruments are mounted on spacecraft orbiting the Earth and Sun.
The initial target is so-called enhanced auroras, which embed fence-like emissions within ordinary auroras.
"Enhanced auroras are basically bright layers embedded in normal auroras. They are similar in color to fences, with not as much blue and more green from oxygen and red from nitrogen. Our hypothesis is that these auroras are also produced by parallel electric fields, but they are much more common than fences," Gaske said. "
The researchers' plan is not only to "fly a rocket through that enhancement layer and actually measure these parallel electric fields for the first time," but also to launch a second rocket to measure the particles at higher altitudes "to distinguish the conditions that cause the auroras." Eventually, hopefully the rocket will go right through Steve and the fence.
This fall, Hardin, Gaske and colleagues proposed such a sounding rocket program to NASA and hope to receive a response on their options in the first half of 2024. Gaske and Harding believe the experiment is an important step toward understanding the chemistry and physics of the upper atmosphere, ionosphere, and Earth's magnetosphere, and is a proposal consistent with the Low-Cost Access to Space (LCAS) program sponsored by NASA for such projects.
"It's fair to say there's going to be a lot of future research on how these electric fields are generated, what waves they are associated with or not associated with, and what that means for the greater transfer of energy between Earth's atmosphere and space, which we really don't know at the moment, and Clare's paper is a first step toward that understanding," Harding said.
Gaske is grateful for input from people who study the mesosphere, or mesosphere, and the stratosphere, whose ideas helped her find the solution.
Compiled source: ScitechDaily