Plasma Patch Atlas
Listening for Ionized Islands in the Sky
Posted September 4, 2019
High above the surface of the Earth, flow giant, invisible clouds of charged gas that can degrade radio transmissions, disrupt GPS Signals and play havoc with other communications and navigation systems.
Photo Credit: Mike Lucibella
Located at McMurdo Station on the far side of Observation Hill, Chartier's transmitters broadcast their radio signals towards the South Pole.
But they're not always showing up when scientists predicted. This year geophysicist Alex Chartier traveled across Antarctica to figure out what's going on in Earth's upper atmosphere.
"I'm trying to build a radio station in Antarctica to measure the near space environment, the ionosphere," said Chartier, of Johns Hopkins University and principal investigator on the project.
By listening to the echoes from his radio transmitter, he's looking to solve a perplexing mystery about these clouds of dense plasma moving across the southern sky.
Chartier's research is supported by the National Science Foundation, which manages the U.S. Antarctic Program.
The ionosphere is the electrically-charged, or "ionized," upper segment of the Earth's atmosphere. It's caused by the constant bombardment of ultraviolet solar radiation, stripping the electrons from the layer's sparse atoms.
Within this layer during particular times of year, clumps of a kind of highly ionized gas known as a "plasma" form high over Earth's mid latitudes and migrate along the planet's magnetic field to the poles.
These plasma patches move across the sky at speeds of up to several kilometers per second and can be quite large - up to 1,000 kilometers across.
"It looks like an island of plasma that's flowing to the polar cap," Chartier said.
Scientists have been using ground-based instruments to monitor Northern Hemisphere plasma patches for decades, and up until recently thought they had a good handle on how they function.
However recent satellite data has left researchers scratching their heads.
Photo Credit: Mike Lucibella
On the side of Observation Hill at McMurdo Station, Alex Chartier installs an anchor into the ground to help secure one of his instruments.
"From what we can see from satellites, the picture in Antarctica looks completely different than what we expected," Chartier said.
North of the equator, scientists observed most of these patches during the winter, particularly in months of December and January. With virtually no ground-based observing stations south of the equator, data for the southern hemisphere was difficult to come by.
Once satellites started collecting data, researchers expected to observe the same pattern of heightened activity during the southern hemisphere's winter months, June and July, and lulls in the summer. But that's not what they saw.
"When you look in satellite data, you can see both hemispheres, and in the southern hemisphere you actually have lots of patches in the middle of summer and you don't have any patches in winter, and so that totally contradicts the theory," Chartier said.
Previously researchers had theorized that the Sun's direct rays in the summer elevated the overall charge of the Earth's ionosphere, effectively washing out the plasma patches, which is why they were rarely observed in summer. But now with these contradictory satellite observations, they're looking for a new explanation as to why the prevalence of these patches aren't seasonal, and happen on both ends of the planet in December and January.
"The first thing to do is to confirm," Chartier said. "This experiment is to confirm or to test that observational statement that there are more patches in summer and not many in winter in Antarctica."
Chartier traveled to Antarctica to set up instruments to measure how frequently these patches pass over a particular region of the continent by analyzing its effects on radio signals. He installed a radio transmitter, known as an ionosonde, on Observation Hill adjacent to NSF's McMurdo Station. At the South Pole, about 800 miles away, he installed a radio receiver to listen for the radio signals coming from McMurdo.
Photo Credit: Alex Chartier
At the South Pole, Chartier's receiver listens for the highest frequency signals that reflect down from the ionosphere.
"For a lower frequency, the signal is going to go up and hit the ionosphere, bounce off and come back down to Earth," Chartier said. "But when you get above the critical frequency, which is dictated by the density of the ionosphere, then the signal will go through and into space and you won't hear it anymore."
Chartier is using this effect to measure these plasma patches as they pass over the continent. Because they’re essentially dense spots of the ionosphere, they exacerbate that reflection, and can bounce higher frequencies back than would otherwise disappear into the emptiness of space.
Every minute the radio transmitter Chartier installed at McMurdo broadcasts a signal that starts out in the low frequencies then progresses into higher and higher frequencies. At the South Pole his receiver is listening for the highest-frequency signal it can hear bounced off the underside of the ionosphere. When a dense plasma patch passes over, it'll be able to hear higher frequencies than it would otherwise.
"We just scan through those frequencies and find the highest one that comes back," Chartier said. "If we can do that consistently over a period of months, then we can determine when we see these sporadic enhancements in the ionosphere."
Chartier wants to better understand the physics of these dense plasma patches because they can seriously affect certain kinds of radio signals. When the density of the ionosphere increases, it makes it difficult for radio signals to pass through them.
"It's a bit like if you had bubbles in a piece of glass… it scatters the light and you can't see clearly through it anymore," Chartier said. "You can get service outages, so you can no longer receive signals from GPS satellites for example, or other communications and navigation systems."
The ionosphere extends to about 1,000 kilometers above the surface of the planet, so it can even affect satellites in low Earth orbit. One constellation of European Space Agency satellites called Swarm has been shown to have been significantly affected by these patches.
"There was a recent study showing the signal loss statistics from the Swarm satellites," Chartier said. "If you look at the statistics of that, they match the plasma density enhancements in the ionosphere."
Chartier hopes that his work to better understand the dynamics of these plasma patches will help engineers and technicians who design communication and navigation systems develop ways to protect against outages.
"We want to predict and understand these technological effects," Chartier said. "That's one reason we need to know the ionosphere."
NSF-funded research in this story: Alex Chartier, Johns Hopkins University, Award No. 1643773.