Sky watchers have spotted a new jewel in the crown of northern lights that shimmer over the top of the world.
The new kind of spectacle is a rare, faint phenomenon dubbed the “dunes.” Unlike other auroras that hang in the sky like luminous curtains, the dunes appear as green bands running parallel to the ground and pointing toward the equator, researchers report online January 28 in AGU Advances.
Using photographs snapped from different locations across Finland in October 2018, researchers triangulated the position of a set of the dunes stretching from western Sweden to western Finland, and hovering about 100 kilometers above the ground.
“Aurorae are like fingerprints in the sky,” says study coauthor Minna Palmroth, a space physicist at the University of Helsinki. Broadly speaking, auroras — often called northern lights or southern lights — appear when electrons from the magnetic bubble, or magnetosphere, surrounding Earth rain into the atmosphere and set oxygen and nitrogen gas aglow (SN: 7/25/14). But the particulars of those particle interactions give each type of aurora its unique flare.
Palmroth and colleagues suspect that the unusual stripes of the dunes aurora arise from undulations of gas in the atmosphere, or atmospheric waves. The crests of those waves are regions of higher air density, where there should be more oxygen for cascading electrons to excite into glowing green. While many atmospheric waves jumble each other up, rare waves that are buffered on either side by slightly colder air can spread over long distances without getting washed out.
Such atmospheric waves could cause the broad, uniform structure of the dunes, agrees physicist Gerard Fasel of Pepperdine University in Malibu, who was not involved in the work. Collecting more observations of this type of aurora’s features, and trying to replicate those features in computer simulations, could help confirm what gives the dunes their special architecture, he says.
The dunes are only the latest in a long line of auroral oddities, which collectively are considered one of the natural wonders of the world. Researchers and citizen scientists are revealing a rich diversity in these vibrant displays.
Check out some of the more obscure auroras of the northern and southern hemispheres.
These auroral lights are blinking patches of sky, up to hundreds of kilometers across, that rhythmically brighten and dim. “They can actually happen most nights. It’s just that often times … you can see them with cameras, but you can’t see them very well just with your eyes,” says Allison Jaynes, a space physicist at the University of Iowa in Iowa City.
Pulsating auroras also “tend to happen after midnight, and then extend all the way into the morning hours,” Jaynes says. So many people aren’t awake to see them.
The flashes of light in these auroras are caused by ripples in Earth’s magnetosphere called chorus waves (SN: 12/5/12). These chorus waves affect electrons in the magnetosphere sort of like ocean waves that periodically deposit foam on a beach — intermittently pushing bunches of electrons down into the atmosphere to create a flickering auroral glow.
Unlike most well-known auroras, a cusp aurora is visible midday — that is, if you’re far enough north that it’s dark around noon. The Norwegian archipelago of Svalbard “is one of the few land masses where you can see cusp aurorae,” says space physicist Elizabeth MacDonald at NASA’s Goddard Space Flight Center in Greenbelt, Md., who founded the aurora-tracking citizen science project Aurorasaurus (SN: 4/3/15).
These auroras are named for the polar regions where Earth’s magnetic field lines bend inward, creating funnel-shaped holes in the magnetosphere. Whereas nighttime auroras are generated by electrons raining down on the atmosphere from inside the magnetosphere, cusp auroras are formed by solar wind particles funneled through the cusp directly into the atmosphere from outside Earth’s magnetic shield.
Solar wind particles that drift down to the atmosphere through the polar cusp generally aren’t as energetic as the electrons that cascade from inside the magnetosphere. So the solar wind particles that produce cusp auroras can only reach and excite oxygen atoms at very high altitudes to glow red — unlike lower-altitude oxygen molecules that glow green.
Like the dunes, the unusual airglow STEVE was named by citizen scientists. This light show shimmers farther south than typical auroras, and appears as a mauve smear from east to west, sometimes accompanied by a row of vertical green stripes called the picket fence (SN: 3/15/18).
The green stripes are caused by oxygen excited by a downpour of electrons, but the mauve streak is harder to account for. Scientists think it’s the signature of a plasma stream, which heats atmospheric particles up through friction to make them glow (SN: 4/30/19). But the kinds of particles responsible remain a mystery.
Spectral analyses of the light in STEVE’s mauve streak reveal a hodgepodge of different wavelengths. “That’s puzzling, because to produce such a spectrum, you need something that’s more complex than an atom,” says space physicist Bea Gallardo-Lacourt of NASA Goddard. But scientists don’t yet know of any molecules at STEVE’s altitude in the atmosphere that could be producing the observed spectrum.
A black aurora is a kind of anti-aurora, appearing as inky patches among the colorful glow of auroral ribbons — which can be difficult to discern from the backdrop of the night sky. While cascading electrons create an aurora’s bright features, other electrons surge upward due to electric fields in the atmosphere, Palmroth says. The ascending electrons don’t rise quickly enough to excite nitrogen and oxygen, so “instead of auroral light, one can see black stripes within the aurora,” she says. “These are the paths of the upwelling electrons, where no light comes.”