watchers have spotted a new jewel in the crown of northern lights that shimmer over
the top of the world.
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
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
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
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.
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.
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
out some of the more obscure auroras of the northern and southern hemispheres.
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.
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.
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.
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
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.
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.
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).
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.
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
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.”
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