Some 50 miles up, where Earth’s atmosphere blends into space, the air itself hums with an electric current. Scientists call it the atmospheric dynamo, an Earth-sized electric generator. It’s taken hundreds of years for scientists to lay the groundwork to understand it, but the principles that keep it running are only just now being revealed in detail.
Following up on its
predecessor’s 2013 flight, the Dynamos, Winds, and Electric Fields in the
Daytime Lower Ionosphere-2, or Dynamo-2, sounding rocket mission will soon
pierce the atmospheric winds thought to keep the dynamo churning. With the
sounding rocket’s launch timed as NASA’s Ionospheric Connection Explorer
satellite passes nearby, these two space missions will combine their
perspectives to advance our understanding of the giant electric circuit in the
sky. See below for information on how to stream the launch and where it will be
visible in person.
The Dynamo mission
The atmospheric dynamo
is a pattern of electrical current swirling in continent-sized circuits high
above our heads. Driven by the Sun, it migrates across the planet, centered
wherever the Sun is directly overhead. It comes alive in Earth’s ionosphere, a
layer of the atmosphere where the Sun’s intense radiation separates electrons
from their atoms, allowing electricity to flow.
A map of the ionospheric currents at the time of Dynamo 1’s launch on July 4, 2013. Currents – whose intensity is marked by red and blue coloring – travel in opposite directions on either side of the magnetic equator, marked with a pink line. The yellow dots are magnetometer readings from the ground. Credits: NASA/JAXA/R. Pfaff et al
Most measurements of the dynamo come from magnetometers on the ground,
which monitor how that current disturbs Earth’s magnetic field (think of them
as souped-up compasses). Ground-based measurements have their advantages – they
can monitor one location for long periods of time, for instance. But to really
see what’s going on in detail, you have to make measurements from inside the
ionosphere, right where the electric current flows.
“It’s a really tricky part of space to get measurements, because the air is
much too thin for an aircraft, and yet it's still too dense to fly most
spacecraft,” said Scott England, space physicist at Virginia Tech in Blacksburg
and collaborator for the upcoming Dynamo-2 campaign. “So one way of making
these measurements is to fly a rocket through it.”
Sounding rockets, named for the nautical term “to sound,” meaning to
measure, launch to make brief measurements in space before falling back to
Earth a few minutes later. They excel at reaching hard-to-access regions of
space that are too low for satellites to measure and too high to reach with
scientific balloons – and they’re ideal for comparing wind speeds at different
altitudes, since they slice through the atmosphere near-vertically.
A picture of the rocket plumes shortly after the launch of both Dynamo rockets from Wallops Flight Facility on July 4, 2013. Credits: NASA/JAXA/R. Pfaff et al/Ken Kramer
“While ground-based methods can provide large-scale, integrated
measurements, sounding rockets give us local, fine-scale data on the
ionospheric current,” said Takumi Abe, space physicist at the Japan Aerospace
Exploration Agency, or JAXA, and collaborator for the Dynamo missions. “That's
when we use sounding rockets – when we'd like to see the small-scale physics.”
The first Dynamo mission – comprising scientists from NASA, JAXA, and
several U.S. universities – launched their rockets on the 4th of
July, 2013, from NASA’s Wallops Flight Facility on Wallops Island, Virginia.
The team divided their instruments between two rockets, the first measuring
electric fields while the second, launched just 15 seconds later, traced the
winds, leaving behind a cloudy plume that glistened red in the sunlight similar
to those observed in firework shows.
Observing from the ground and from a NASA aircraft, the team watched the
crimson clouds morph in the wind as simultaneous electric field measurements
were beamed back to the ground.
The vapor trail teased about in the wind, twisting and curling into a
spiraling zig-zag. The telltale shape meant the winds were changing direction
along the rocket’s flight path.
“They moved first to the east, and then a few miles above, they're all
moving to the west, and a few miles above, they're all moving back to the
east,” England said.
The zig-zag confirmed one aspect of the theory of atmospheric tides, which
create high-altitude winds thought to drive the atmospheric dynamo. Heat from
the ground below radiates up in waves, forcing parts of the atmosphere to move
back and forth like the ebb and flow of ocean waves as they hit the beach.
“The zig-zag is the signature of this huge wave moving through this
region,” England added.
Though the winds were expected by theory, their strength was not.
Based on magnetometer readings from the ground at the time, the team
expected a weak current and mild winds above. Indeed, things were calm below
the ionosphere’s base. But right where the reddish cloud trail pierced the
lower parts of the ionosphere, where the dynamo is strongest, it was rapidly
smeared across the sky.
“Just in the dynamo region, the wind suddenly takes off and gets very fast,
over 150 meters per second (335 miles per hour),” said Rob Pfaff, space
physicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and
principal investigator for both Dynamo missions. “It’s much stronger than
what's predicted.”
These oppositely directed, high-speed winds were too fine-grained to be
detected from ground-based measurements.
“It might look from the ground like the wind is going east at a very low
speed,” said England. “But it turns out that's a very high speed to the east
and a slightly lower speed to the west, averaged together.”
A satellite and rocket
tag-team
Though the 2013 observations from the Dynamo rockets were surprising, they
jibe with newer measurements from NASA’s Ionospheric Connection Explorer, or
ICON, satellite.
ICON, a satellite mission launched in October 2019, flies at an altitude of
about 360 miles, looking down on the same ionospheric winds that Dynamo rockets
measured from within. Lately, ICON had also observed much faster winds than
expected by theory, and the team didn’t know what to make of them.
“Having the verification by these rocket results that what we're seeing
with ICON is real – it's even sharper than what we can see,” said England, who
is the project scientist for the ICON mission.
ICON’s wind measurements aren’t as high resolution as the Dynamo rockets’
were, but it can see much broader swaths of space, and can repeat those
observations on each orbit. The Dynamo-2 mission campaign will combine their
strengths.
“We are going to time it so that ICON is flying past around the same time
that rocket is launching,” England said. “That way we can really combine all
the amazing strengths in the data that's highlighted in this paper with the
larger picture view from ICON.”
Illustration of NASA’s Ionospheric Connection Explorer, or ICON. ICON explores Earth’s upper atmosphere and ionosphere, a region influenced by both terrestrial weather and changes in near-Earth space. Credits: NASA's Goddard Space Flight Center Conceptual Image Lab
The first Dynamo rockets launched together around noon, when the current
was flowing from east to west. This time, the Dynamo-2 rockets will likely
launch at different times, in the morning and afternoon, to capture the current
when it is flowing in different directions.
“We're going to take measurements in the morning and in the afternoon to
complete the circle, so to speak, and see how all this comes together in one
big picture,” Pfaff said.
However, Pfaff may instead launch one rocket during geomagnetically “quiet”
times and one during “disturbed” times, when the ionosphere’s activity is
especially complex, which would provide equally valuable insight. Which plan
they follow will depend on how solar activity and the dynamo currents
themselves are looking in real time once the launch window opens.
The Dynamo-2 rockets will also use a novel instrument developed by
co-investigator Jim Clemmons at the University of New Hampshire in Durham. The
instrument measures winds by monitoring pressure gradients in the air around
the rockets instead of releasing clouds that must be tracked from the ground or
sky.
“And the beauty of that is we don't have to rely on clear skies and we
don't have to get an airplane in the air – we can just do it,” Pfaff said.
Pfaff hopes the new results will help the team understand what’s driving
the unduly fast winds, and what the consequences are for understanding the
atmospheric dynamo.
Discovering the dynamo
in the sky
The atmospheric dynamo is so named because it operates with the same
principles as the electric dynamo, a kind of electric generator. The first
dynamo was not found in nature but rather constructed in a lab.
In the early 1800s, on the cusp of the Victorian era in Britain,
fascination with electricity was reaching a fever pitch as reports of
fundamental discoveries arrived from across Europe. The invention of the
battery, the discovery of electrical current, and several puzzling effects
relating electricity to magnetism were related on a nearly monthly basis.
Michael Faraday – a bookbinder’s apprentice turned self-taught
experimentalist – was toiling in his London lab, working on a strange new
device that, though he didn’t know it, would eventually change the world.
Faraday’s sketch of his first dynamo machine. Credits: © The Royal Society
It consisted of a copper disc, mounted like a bicycle wheel so as to spin
between two magnets. He connected the disc to an instrument that measured
electric current, invented just 10 years earlier.
Faraday rotated the disc and the needle on his instrument wiggled – a small
electric current was beginning to flow. Historians would later identify this
moment – October 28, 1831, according to his diary – as the first time humans
turned motion into electricity. Faraday had discovered electrical induction,
and as a bonus, built the first dynamo, or electric generator. It was the
prototype of a technology that today keeps our lights on, our computers
running, and the entire modern economy afloat.
What made Faraday’s device work were three key ingredients: a magnetic
field (created by the two magnets), a conductor (the copper disc), and motion.
Combining those three, he had discovered that moving a conductive material
within a stationary magnetic field – or moving a magnetic field around a
stationary conductor – will start an electric current flowing.
Eventually, scientists discovered each of those three ingredients operating
on Earth at a much larger scale.
The atmospheric dynamo,
one piece at a time
Of the three components of the atmospheric dynamo – a magnetic field, a
conductor, and motion – Earth’s magnetic field was discovered first.
By the early 1100s, Chinese seafarers were already using magnetic compasses
to navigate on cloudy, starless nights, though the reason for their reliable
alignment wasn’t known. William Gilbert’s De Magnete, published
in London in 1600, was the first to explain this behavior with the idea that
the Earth itself was a giant magnet.
Astronomers began mapping Earth’s magnetic field, and by 1701, English
astronomer Edmond Halley, charting the Atlantic with his compass, produced the
first map of Earth’s magnetic field.
First map of Earth’s magnetic field based on compass readings, by Edmond Halley, after sailing the Atlantic Ocean on the Paramore. Since we now know Earth’s magnetic pole shifts over time, these lines are not stable – scientists update the World Magnetic Model every five years. As of 2019, the magnetic north is moving towards Siberia at a rate of about 34 miles (55 km) per year. Credits: E. Halley/Princeton Library Historic Maps Collection
As compasses gained wider use for scientific purposes, some observers
noticed an irregularity: compass readings seemed to flicker on a daily
schedule.
“Ever since the 19th century, people would observe, particularly near noon,
this little wiggle on these really big compasses,” said Pfaff.
The wiggling compass needles fit well with new findings on the relationship
between electricity and magnetism. In 1820, Danish scientist Hans Christian
Ørsted had observed that running an electric current through a conductive wire
deflected the needle of a nearby compass, effectively “wiggling” the magnetic
field it sensed. Faraday’s dynamo machine, constructed 11 years later, showed
how a wiggling magnetic field could induce a current. Magnetic fields, motion,
and electricity – the three went together. If that was right, then the wiggling
compass needles on Earth might mean that somehow, an electric current was
running overhead. But where that current was coming from, and the conductor it
was traveling through, was far from clear.
In 1882, English scientist Balfour Stewart penned an Encyclopedia
Brittanica entry that correctly identified the source, though it was
conjecture at the time. A part of the upper atmosphere itself, he wrote, might
be conductive – the air above us could become electrified.
That conductive part of the atmosphere was eventually discovered
through practical experience. As World War I created a need for long-distance
radio communication, experimenters discovered that radio signals could travel
between continents – around the curvature of the Earth – by somehow bouncing
off of the sky. The only viable explanation for their success was a reflective
– that is, conductive – layer of the atmosphere.
Figure 1 from Appleton’s Nobel Prize lecture in 1947, demonstrating how a radio wave can travel long distances by reflecting off an ionized layer of the atmosphere. Credits: ©The Nobel Foundation
In 1927, English physicist Edward Appleton studied those radio signals to
confirm that there was indeed an electrically conductive layer of the
atmosphere. (He called it the “E-layer,” for
“electrically conductive”.) Over the following decades, several more
sublayers of what became known as the ionosphere – where Earth’s atmosphere
contains substantial populations of charged particles, ions and electrons –
would be discovered and characterized. The second component of Earth’s
atmospheric dynamo, the conductive ionosphere, had been found.
Still, the current didn’t seem to be flowing constantly. The wiggling
compass needles only twitched occasionally, most strongly at noon. Something
must be moving the ionosphere strongest when the Sun was right overhead.
The discovery of the final component of the atmospheric dynamo, the source
of motion, would have to wait for the space age, when rockets, balloons, and
early satellites could measure atmospheric winds. In 1970, systematizing two
decades of data, space physicists Sydney Chapman and Richard Lindzen developed
the theory of atmospheric tides, the key to the ionosphere’s pulsing currents.
The idea was that as the Sun beats down on Earth, its heat radiates back
upwards. In response, the entire atmosphere expands. A high-flying observer
would experience this expansion as strong gusts of wind.
When those winds reach the base of the ionosphere, where the Sun’s
radiation separates neutral particles into electrically charged ions and
electrons, they push them along too. As a result, the ionosphere – a conductor
– moves against Earth’s magnetic field, swishing to and fro with the wind.
“With these key ingredients together, the force of the wind pushing on
those ions and electrons in the presence of Earth's magnetic field, we can get
a current flowing in the Earth's upper atmosphere,” said England. “That's what
we call the dynamo.”
“We’ve come a long way since Faraday’s time,” Pfaff said. “After two
centuries of research, it is exciting to journey into space and observe
dynamos that are part of our natural environment.”
The Dynamo-2 rockets will launch from NASA’s Wallops Flight Facility on
Wallops Island, Virginia between July 6-20. The two rockets will not be
launched on the same day. The launch window on July 6 runs from 12:15 p.m.
to 2 p.m. EDT. On July 7-13, the launch window runs from 10 a.m. to 2
p.m. EDT and from 8 a.m. to noon EDT on July 14-20. Live coverage of the
launches will begin 20 minutes before the opening of the launch window on
the Wallops YouTube site. The NASA Visitor
Center at Wallops will not be open for this mission. The launches may be
visible in the mid-Atlantic region.
By Miles
Hatfield
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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