The most enigmatic phenomena that light up the night sky are the mauve and
white streaks known as Steve and their frequent partner, a blazing green
"picket fence." The auroras, or northern and southern lights, are noted for
their shimmering green, red, and purple curtains.
Though Steve (a playful nod to the innocuous name given to a terrifying
hedge in a 2006 children's movie) and its picket fence were first identified
in 2018 as different from the typical auroras, they were believed to be
created by the same basic processes. However, the mechanism generating these
luminous emissions baffled experts.
A physics doctoral student at the University of California, Berkeley named
Claire Gasque has recently put up a scientific theory for these occurrences
that is entirely distinct from the mechanisms underlying the well-known
auroras. In order to determine whether she is right, she has joined up with
scientists at the Space Sciences Laboratory (SSL) on campus to suggest that
NASA send a rocket into the center of the aurora.
As the sun moves into the active phase of its 11-year cycle, bright auroras
and illuminating phenomena like Steve and the picket fence become more
frequent. November proved to be a strong month for Steve sightings in the
northern latitudes. Steve and the picket fence are good examples of unusual
events that may be studied during the approaching solar maximum since all of
these fleeting light phenomena are caused by solar storms and coronal mass
ejections from the sun.
Gasque will
explain
the findings in an invited session at the American Geophysical Union meeting
in San Francisco on December 14th. Gasque detailed the physics underlying
the picket fence in a study that was published last month in the journal
Geophysical Research Letters.
She estimated that the color spectrum of the picket fence may be produced
by electric fields parallel to Earth's magnetic field in a region of the
upper atmosphere that is farther south than the aurora formation area. If
this novel method is accurate, it will affect the way physicists see the
flow of energy between the ionosphere at the edge of space and the
magnetosphere that envelops and shields Earth from the solar wind.
"This would upend our modeling of what creates light and the energy in the
aurora in some cases," Gasque stated.
"What makes Claire's study so intriguing is that, as we've known for a few
years, the Steve spectrum indicates that some really unusual physics is
occurring. As an SSL assistant research physicist and co-author of the
report,
Brian Harding
stated, "We simply didn't know what it was." "Claire's paper showed that
parallel electric fields are capable of explaining this exotic
spectrum."
The work was a side project from Gasque's doctoral dissertation, which
focuses on the relationship between Earthly occurrences such as volcanoes
and phenomena occurring in the ionosphere, which is located at least 100
kilometers above our planet.
However, she couldn't help but investigate the physics underlying Steve and
the picket fence after learning about it at a conference in 2022 (Steve is
now an abbreviation for Strong Thermal Emission Velocity Enhancement).
"It's pretty awesome," she said. "It's one of the biggest mysteries in
space physics right now."
Steve and picket fence physics
When solar wind energizes particles in Earth's magnetosphere, frequently at
elevations greater than 1,000 kilometers above the surface, common auroras
are created. These charged particles travel in a spiral around the lines of
Earth's magnetic field, heading for the poles, where they collide and
energize molecules of nitrogen and oxygen in the upper atmosphere. Oxygen
releases certain green and red light frequencies as the molecules relax,
while nitrogen mostly produces a blue emission line with some red
light.
The resultant curtains of color and shimmering light can stretch hundreds
of kilometers across the northern or southern latitudes.
Steve, on the other hand, exhibits a wide variety of frequencies focused on
purple or mauve rather than separate emission lines. Nor does the picket
fence, nor Steve, emit blue light, which is produced when the most energetic
particles strike and ionize nitrogen, unlike auroras. In addition to the
aurora, Steve and the picket fence also occur at lower latitudes—possibly as
low as the equator.
Although there isn't a widely accepted scientific explanation for how
subauroral ion drift, or SAID, may produce the multicolored emissions, some
researchers have hypothesized that Steve is created by ion fluxes in the
high atmosphere.
Suggestions that low-altitude electric fields parallel to Earth's magnetic
field could be the source of the picket fence's emissions piqued Gasque's
interest. This was previously believed to be impossible because any electric
field aligned with the magnetic field should quickly short out and
disappear.
Gasque then demonstrated, using a widely used physical model of the
ionosphere, that electrons could be accelerated to an energy that would
excite nitrogen and oxygen and produce the light spectrum seen from the
picket fence at a height of approximately 110 km by a moderate parallel
electric field of about 100 millivolts per meter. Unusual circumstances in
that region, including a higher concentration of neutral oxygen and nitrogen
atoms and a lower density of charged plasma, may serve as insulation to
prevent the electric field from shorting out.
The picket fence's spectrum reveals a lot more green than one may
anticipate. Furthermore, none of the blue that results from nitrogen
ionization is present, according to Gasque. "What that's telling us is that
there's only a specific energy range of electrons that can create those
colors, and they can't be coming from way out in space down into the
atmosphere, because those particles have too much energy."
Instead, according to her, "the light from the picket fence is being
created by particles that have to be energized right there in space by a
parallel electric field, which is a completely different mechanism than any
of the aurora that we've studied or known before."
Harding and her believe that comparable mechanisms may also generate Steve.
The kind of UV emissions that would result from this process are also
predicted by their calculations, and they may be verified in order to
validate the novel theory on the picket fence.
The on-off glow that gives the phenomena its picket fence-like appearance
is probably caused by wave-like changes in the electric field, according to
Gasque, even if her calculations don't specifically address it.
Additionally, solar storms' disruption of the atmosphere is likely what
causes Steve and the picket fence, as well as the typical aurora, even if
the particles driven by the electric field are most likely not solar in
origin.
Picket fence-like light is shown by enhanced auroras.
The next stage, according to Harding, is to use these phenomena to launch a
rocket from Alaska and gauge the direction and intensity of the magnetic and
electric fields. That is what SSL scientists specialize in creating and
developing equipment for. Numerous of these equipment are now on spacecraft
that circle the sun and Earth.
The initial aim would be an enhanced aurora, which is a regular aurora with
emissions resembling picket fences imbedded in it.
"This brilliant layer that is included into the regular aurora is
essentially the enhanced aurora. The colors are comparable to the picket
fence in that they have more green from oxygen and more red from nitrogen,
with less blue. Though they are far more prevalent than picket fences, it is
hypothesized that they are likewise produced by parallel electric fields,"
Gasque stated.
According to her, the idea is not just to "fly a rocket through that
enhanced layer to actually measure those parallel electric fields for the
first time," but also to launch a second rocket higher to measure the
particles in order "to distinguish the conditions from those that cause the
auroras." She eventually dreams of a rocket that will pass through Steve and
the picket fence exactly.
This fall, Harding, Gasque, and associates presented NASA with a compelling
rocket campaign, and they anticipate hearing back from the agency about its
selection in the first half of 2024. Gasque and Harding saw the experiment
as a proposal in accordance with NASA's Low Cost Access to Space (LCAS)
program, which supports such initiatives, and as a significant step toward
understanding the chemistry and physics of the upper atmosphere, the
ionosphere, and the Earth's magnetosphere.
"It's fair to say that there's going to be a lot of study in the future
about how those electric fields got there, what waves they are or aren't
associated with, and what that means for the larger energy transfer between
Earth's atmosphere and space," said Harding. It's really unknown to us. The
first link in the chain of that knowledge is Claire's paper."
Gasque conveyed gratitude for the assistance she received in deciphering
the answer from experts in the fields of stratospheric and intermediate
ionosphere research.
"With this collaboration, we were able to make some really cool progress in
this field," she stated. "Honestly, it was just following our nose and being
excited about it."
Harding is co-authored with three other people: D. Megan Gillies from the
University of Calgary in Alberta, Canada; Justin Yonker from the Applied
Physics Laboratory at Johns Hopkins University in Laurel, Maryland; and Reza
Janalizadeh from Pennsylvania State University in University Park.
Provided by
University of California - Berkeley
