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November 30, 2021
by Lina Tran, Goddard Space Flight Center, NASA
What happens on earth does not stay on earth.
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Using observations from NASA’s ICON mission, scientists presented the first direct measurements of the long theorized Earth’s dynamo at the edge of space: a wind-powered electric generator that spans the globe more than 60 miles above our heads. The dynamo rotates in the ionosphere, the electrically charged boundary between earth and space. It is propelled by tidal winds in the upper atmosphere, faster than most hurricanes and rising from the lower atmosphere, creating an electrical environment that can affect satellites and technology on Earth.
The new job that is done today published in Nature Geoscience improves our understanding of the ionosphere, which helps scientists better predict space weather and protect our technology from its effects.
ICON, short for Ionospheric Connection Explorer, was launched in 2019 and is a mission to unravel how the weather on earth interacts with the weather in space. Radio and GPS signals cross the ionosphere, which is home to auroras and the International Space Station. Empty pockets or dense swellings of electrically charged particles can interfere with these signals.
Scientists studying the atmosphere and space weather have long included the Earth’s dynamo in their models because they knew it had important effects. But with little information, they had to make some assumptions about how it worked. ICON data is the first concrete observation of winds that drive the dynamo and eventually affect space weather to feed into these models.
« ICON’s first year in space has shown that predicting these winds is key to Enhancing our ability to predict what is happening in the ionosphere, said Thomas Immel, lead ICON researcher at the University of California, Berkeley and lead author of the new study.
The ionosphere is like a sloshing sea of electrically charged particles that generated by the sun and mixed with the neutral upper atmosphere. Nestled between earth and space, the ionosphere reacts to changes from both the sun above and the earth below. How much influence is coming from both sides is what the researchers want to find out. The researchers studied the ICON data for a year and found that a large part of the observed changes occurred in the lower atmosphere.
Generators work by repeatedly moving a current-carrying conductor – like a copper wire – through a magnetic field. Filled with electrically charged gases called plasma, the ionosphere behaves like a wire, or rather, a tangle of wires: electricity flows through it. Like the dynamo in the earth’s core, the dynamo in the atmosphere generates electromagnetic fields from movement.
Strong winds in the thermosphere, a layer of the upper atmosphere known for its high temperatures, push current-carrying plasma in the ionosphere over invisible magnetic field lines that are like orbiting an onion around the earth. The wind tends to push bulky, positively charged particles more strongly than small, negatively charged electrons. « Plus points move differently than minus points, » says co-author Brian Harding, a physicist at the University of California at Berkeley. “It’s an electric current.”
In most generators, these components are tightly bound so that they stay in place and act predictably. But the ionosphere can move freely as it likes. « The electricity creates its own magnetic field that combats the earth’s magnetic field as it passes through, » said Immel. “So you end up with a cable trying to get away from you. It’s a messy generator. ”
Following the whims of the ionosphere is key to predicting the potential effects of space weather. Depending on the direction from which the wind is blowing, plasma shoots into space in the ionosphere or plunges towards Earth. This behavior results from the tug-of-war between the ionosphere and the electromagnetic fields of the earth.
The dynamo, which lies at the lower end of the ionosphere, remained a mystery for a long time because it is difficult to observe. Too high for scientific balloons and too low for satellites, it has eluded many of the tools researchers have to study near-earth space. ICON is uniquely equipped to examine this part of the ionosphere from above, using the natural glow of the upper atmosphere to detect the movement of the plasma.
ICON observes high winds and wandering plasma at the same time. « This was the first time that we could determine without any assumptions how much the wind contributes to the behavior of the ionosphere, » said Astrid Maute, another co-author of the study and ICON scientist at the National Center for Atmospheric Research in Boulder, Colorado.
It was only in the past decade, said Immel, that scientists have realized how much these ascending winds vary. « The upper atmosphere was not expected to change quickly, » he said. « But it does, day in and day out. We find that all of this is due to changes that were driven up from the lower atmosphere. »
The winds that sweep the surface of the earth are known from gentle breezes to to strong gusts that blow in one direction and the other.
High-altitude winds are a different animal. From 60 to 95 miles above the ground, in the lower thermosphere, winds can blow in the same direction at the same speed – about 250 miles per hour – for a few hours before suddenly reversing direction. (In comparison, the winds of the strongest Category 5 hurricanes break at 157 miles per hour or more.) These dramatic shifts are the result of air waves called tides that appear on the surface of the earth when the lower atmosphere heats up during the day and cools down at night. They flow through the sky every day and carry changes from below with them.
The further the atmosphere moves away from the surface, the thinner it becomes and the less turbulence disturbs these movements. This means that small tides that are created near the surface can get much larger when they reach the upper atmosphere. « Changes in the winds up there are mainly driven by what happens below, » said Harding.
ICON’s new wind measurements are helping scientists understand these tidal patterns that span the globe and their effects.
The tides ripple through the sky, grow stronger, and grow before flowing through the ionosphere. The electrodynamo hums in response.The scientists analyzed the first year of the ICON data and found that high-altitude winds strongly influence the ionosphere. « We followed the ionosphere’s pattern of movement and there was a clear wave-like structure, » said Harding. Wind changes, he explained, corresponded directly to the dance of the plasma 370 miles above the surface of the earth.
« Half of the movement of the plasma can be attributed to the winds that we observe right there on the same magnetic field line, » said Immel. « This tells you that it is an important observation to make in predicting what plasma is doing. »
ICON’s first year of observation coincided with solar minimum, the quiet phase of the Sun’s 11-year cycle of activity. During this time the sun’s behavior was a faint, constant hum. « We know the sun doesn’t do a lot, but we’ve seen a lot of variability from below and then noticeable changes in the ionosphere, » said Immel. That told the researchers that they were able to rule out the sun as a major influence.
As the sun reaches its active phase, scientists will be able to study more complex changes and interactions between space and the earth’s atmosphere.
Immel said he was pleased with this confirmation of longstanding ionospheric theories. « We found half of what causes the ionosphere to behave the way it does right there in the data, » he said. « We wanted to know. »Nevertheless, Maute said: « This leaves room to explore what else contributes to the behavior of the ionosphere. »
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