The aurora borealis (northern lights) appears in many forms—from patches or scattered clouds, to arcs, streamers or rippling curtains. Scientists attribute the vibrant colors to excited gases emitted in the Earth’s atmosphere; oxygen, for example, gives off a green color.

I think of February and March as a miniseason, that of northern lights. This time of year provides an opportune moment to turn your gaze to winter’s night skies. If you live in the Northern Hemisphere, it’s possible that you’ll catch a glimpse of the aurora borealis, the beautiful, dancing ribbons of light that have captivated people for millennia and been named one of the Seven Natural Wonders of the World.

The northern lights are created when energized particles from the sun slam into Earth’s upper atmosphere at speeds of up to 45 million miles per hour. Luckily, our planet’s magnetic field protects us from the onslaught. As Earth’s magnetic field redirects the particles toward the poles—there are southern lights, too—the dramatic process transforms into a cinematic, atmospheric phenomenon that dazzles and fascinates scientists and sky-watchers alike.

But winter’s night skies hold other wonders, too. While auroras occur at high latitudes, the associated phenomena called “STEVE” and the “picket fence” occur farther south and at lower altitudes. Their emissions also differ from that of auroras. Now, a physics graduate student has proposed a physical mechanism behind these mysterious sights. And on the moon, a new geological epoch may be dawning: the Lunar Anthropocene.


While the aurora australis (southern lights) is as stunning as the northern lights, they are seen in less accessible places near the South Pole, such as in Antarctica, Australia, New Zealand or Tasmania.

Inscrutable STEVE

While the shimmering green, purple and red “curtains” of the northern and southern lights—the aurora borealis and the aurora australis—may be the best-known phenomena lighting up the nighttime sky, the title of most mysterious may go to the mauve and white streaks called “STEVE” (Strong Thermal Emission Velocity Enhancement) and its frequent companion, a glowing, green “picket fence.”

First recognized in 2018 as distinct from the common auroras (northern and southern lights), STEVE is a rare ionospheric, optical phenomenon characterized by a narrow mauve arc extending thousands of miles across the subauroral sky. Associated with STEVE are the vibrant green streaks known as the “picket fence,” which often appear at lower altitudes after the mauve arc develops and occasionally persists after it fades. Both were thought to be caused by the same physical processes, although scientists scratched their heads about how these glowing emissions were produced.

But now, a University of California, Berkeley, graduate student in physics, L. Claire Gasque, has proposed a physical explanation for these phenomena that is totally different from the processes responsible for the well-known auroras. Her paper on the subject was published in the science journal Geophysical Research Letters in November 2023.


STEVE is an aurora-like glow that often occurs with northern lights, yet it is a distinct phenomenon. It’s characterized by a narrow, mauve arc that extends for hundreds of miles.

Vibrant auroras and glowing phenomena such as STEVE and the picket fence are becoming more common as the sun enters the active period of its 11-year cycle, and November 2023 was a good month for STEVE observations in the northern latitudes.

Gasque calculated that in a region of the upper atmosphere farther south than that in which auroras form, electric fields parallel to Earth’s magnetic field could produce the exotic look and spectrum of the picket fence.

If correct, this unusual process has implications for how physicists understand the energy flow between Earth’s magnetosphere, which surrounds and protects Earth from the solar wind, and the ionosphere at the edge of space.

It would also upend how scientists model what creates the energy and the light in the auroras in some cases.


Sometimes, STEVE is accompanied by smudges of green lines, nicknamed a “picket fence” owing to its appearance.

Perplexing picket fence

The common auroras are produced when the solar wind energizes particles in Earth’s magnetosphere, often at altitudes higher than 620 miles above the surface. These energized particles spiral around Earth’s magnetic field lines toward the poles, where they crash into and excite nitrogen and oxygen molecules in the upper atmosphere. When those molecules relax, oxygen emits specific frequencies of green and red light; while nitrogen primarily generates a blue emission line, with a bit of red. The colorful, shimmering curtains that result can extend for thousands of miles across the northern or southern latitudes.

STEVE, however, displays not individual emission lines, but a broad range of frequencies centered around mauve or purple. And unlike auroras, neither STEVE nor the picket fence emit blue light. They also occur at lower latitudes than the aurora borealis, potentially even as far south as the equator.

In the past, some researchers had hypothesized that STEVE is caused by ion flows in the upper atmosphere, referred to as “subauroral ion drift,” or SAID, though there’s no widely accepted physical explanation for how SAID could generate the colorful emissions.


STEVE may be spotted farther from the poles than the auroras. STEVE has been observed in Alaska and other northern states in the U.S., Australia, Canada, New Zealand and the United Kingdom. It generally lasts for 20 minutes to an hour.

Gasque’s interest was sparked by suggestions that the picket fence’s emissions could be generated by low-altitude electric fields parallel to Earth’s magnetic field, a situation thought to be impossible because any electric field aligned with the magnetic field should quickly short out and disappear. But using a common physical model of the ionosphere, Gasque subsequently showed that a moderate parallel electric field—around 300 millivolts per foot—at a height of about 68 miles could accelerate electrons to an energy level that would excite nitrogen and oxygen, and generate the spectrum of light observed from the picket fence.

Unusual conditions in that area, such as a lower density of charged plasma and more neutral atoms of nitrogen and oxygen, could potentially act as insulation to keep the electric field from shorting out. The spectrum from the picket fence is much greener than expected and has none of the blue that comes from the ionization of nitrogen. There’s only a specific energy range of electrons that can create those colors; so, they can’t be coming from way out in space down into the atmosphere because those particles have too much energy. Instead, the light from the picket fence is being created by particles that must be energized in space by a parallel electric field, which is a completely different mechanism than any of the auroras that have been studied or known before. STEVE may be produced by related processes.

Though Gasque’s calculations don’t directly address the on-off glow that makes the phenomenon look like a picket fence, it’s likely due to wavelike variations in the electric field, she states. And while the particles that are accelerated by the electric field are probably not from the sun, the scrambling of the atmosphere by solar storms possibly triggers STEVE and the picket fence, as it does the common auroras.


In the near future, researchers would like to launch a rocket through these night sky phenomena to measure the direction and strength of their electric and magnetic fields.

The next step, with help from NASA, will be to launch a rocket from Alaska through these phenomena and measure the direction and strength of the electric and magnetic fields. Initially, the target would be what’s known as an “enhanced aurora,” which is a normal aurora with picket fence-like emissions embedded in it. The theory is that these are also created by parallel electric fields, but they are a lot more common than the picket fence.

Messy moon

Human beings first disturbed moon dust on September 13, 1959, when the U.S.S.R.’s unmanned spacecraft Luna 2 alighted on the lunar surface. In the following decades, more than 100 other spacecraft have touched the moon—both crewed and not; sometimes landing and sometimes crashing. The most famous of these were NASA’s Apollo Lunar Modules, which transported humans to the moon’s surface to the astonishment of humankind.

In the coming years, missions and projects already planned will change the face of the moon in more extreme ways. And according to anthropologists and geologists at the University of Kansas, it’s time to acknowledge that humans have become the dominant force shaping the moon’s environment by declaring a new geological epoch for the moon: the Lunar Anthropocene.


The concept of a Lunar Anthropocene aims to raise awareness about our impact on the moon’s surface, as well as provoke thoughts on how to preserve historical artifacts.

The idea for the Lunar Anthropocene, say scientists in a paper published in the journal Nature Geoscience in December 2023, is much like that for the Earth’s Anthropocene, the time during which human activities on the planet have had such an environmental impact that it constitutes a distinct geological age. On Earth, the consensus is that the Anthropocene began in the 1950s when the Great Acceleration, a dramatic increase in human activity affecting the planet, took off. Similarly, on the moon, scientists argue that the Lunar Anthropocene has already commenced. Hopefully, the Lunar Anthropocene concept will help dispel the myth that the moon is an unchanging environment, barely impacted by humanity.

Cultural activities are already starting to outstrip the natural, geological processes on the moon, such as moving sediments. Typically, these processes include meteoroid impacts and other mass movement events. However, the impact of human landers, rovers and even footprints, say the scientists, is already significantly disturbing the sediments.

And, according to the authors, there is a lot of refuse from human missions to the moon, including “discarded and abandoned spacecraft components, bags of human excreta, scientific equipment and other objects (such as flags, golf balls, photographs and religious texts).” While many outdoor enthusiasts are familiar with Leave No Trace principles, they don’t seem to exist on the moon.

Some scientists believe that the materials and footprints we leave on the moon are significant, akin to an archaeological record that we should be committed to preserving. ©NASA

The lunar landscape is expected to look entirely different in 50 years. Multiple countries will have established a presence, leading to numerous challenges. The goal of naming a Lunar Anthropocene is to dispel the lunar-static myth and emphasize the importance of the significant impact that humans are having on the moon, not only in the past but ongoing and in the future—before it’s too late.

At the same time, scientists also hope to call attention to the vulnerability of lunar sites with anthropological and historical value, which currently have no legal or policy protections against disturbance. This field of “space heritage” would aim to preserve or catalog items such as the flags, golf balls, rovers and footprints on the moon’s surface.

In a way, say the scientists, the footprints on the moon can be seen as an extension of humanity’s journey out of Africa, a pivotal milestone in our species’ history. They believe these imprints are intertwined with the overarching narrative of human evolution. It’s within this framework that they seek to capture the interest of not only planetary scientists but also archaeologists and anthropologists, who typically otherwise might not be engaged.


No matter where we journey, we need to be respectful nature travelers, learning about other lands and realms without harming them as we pass through.

Thoughtful travelers

It seems in this season of night-sky light shows and visitable moons, we have much to learn. Future space missions will undoubtedly provide us with knowledge about the universe we live in, which is a worthy goal, but we must also consider the deleterious effects we can have on lunar and other extra-Earth environments.

As with “unowned” Antarctica and the unfathomed oceans, in the skies we have another chance to enter new realms as respectful nature travelers, learning about new habitats without harming them as we pass through.

Here’s to finding your true places and natural habitats,