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Into thin air


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Gregory Earle

Gregory Earle

Traveling 100 km (approximately 62 miles) along the ground makes little difference to the physics around you, but traveling that same 100 km into the atmosphere changes everything. Not only are there fewer particles (less than 1/1 million) than at ground level, but also those particles are not the friendly oxygen and nitrogen we breathe. Instead, the particles are corrosive, unstable ions and electrons that behave nothing like their ground-level siblings.

This is the region that ECE professor Gregory Earle studies: from 100 km to about 600 km above Earth’s surface. He wants to understand the different behavior — the physics — of the upper atmosphere, so he builds devices and spacecraft to measure the various particles there.

“In the lower atmosphere, there’s a lot of chemistry going on, but in the upper regions the particles are much sparser, so the physics is what really matters there,” says Earle. In that region, there are neutral particles, ions, and electrons interacting.

“We do some computer analysis and models, but what I really like to do is build the things that make the measurements and see how they compare to the models and radar observations. I make in situ measurements that we can compare to radar data,” he explains.

Latitude matters

The physics of the upper atmosphere isn’t affected only by altitude. Particles also behave differently in each latitudinal sector of the earth. Along the magnetic equator, the magnetic field lines are parallel to the earth’s surface. So if an ion gets pushed north by the neutral particles, it will move north.

In the auroral region (around the poles), the field lines are perpendicular to the earth’s surface. So if an ion gets pushed, it won’t move. At middle latitudes the field lines are at 20 to 60 degree angles, so pushing a particle northward can actually move it upward in the Southern Hemisphere, or downward in the Northern Hemisphere.

Earle has done research in each of these regions, but is currently focusing on the equatorial latitudes. One of the unique difficulties he is studying is equatorial spread-F distortion. Spread-F typically occurs only at night, but it can distort communication signals, including GPS. Severe spread-F interference can totally disrupt GPS signals or give gross errors, Earle explains. This signal distortion is of particular concern to the military, he says.

Measuring the ultimate light breeze

Neutral particles move when the atmosphere changes temperature, like wind on the earth’s surface. “Those same winds exist in space,” Earle explains, “but there aren’t many particles, so there isn’t enough force for the wind to push very hard. You would not be able to feel a wind in space.”

Measuring winds in space presents a very difficult challenge. The particles are moving fast, but there are very few of them. “We’re basically trying to measure the wind in a vacuum,” says Earle. “On the ground, we measure winds by how much they displace a physical part of the anemometer, but in space, these technologies don’t work. You have to come up with different ways to do it. Now imagine you’re trying to do it from a satellite moving at roughly 8 km/s. It’s about the most complicated thing I know of to try to measure.”

Earle is developing a new kind of transducer to measure pressures in space. The instrument is basically a parallel plate capacitor in which one plate is a flexible silicon membrane with a thin metal coating: the other plate is a rigid grid. “If you change the distance between the plates, the capacitance changes. So the amount that the flexible plate deflects changes the capacitance, and converts the physical phenomenon into a measurable electrical signal.”

pictures leading up to the launch

Top: Final ground-based testing of an Air Force satellite designed to study thermospheric winds.

Center: A rocket used in Earle’s research rests on the launchpad at sunset.

Bottom: Ryan Davidson, who worked with Earle as a graduate student at the University of Texas at Dallas and is now a postdoctoral researcher at Virginia Tech, tests instruments inside a vacuum chamber.

According to Earle, the last time wind measurements were made in situ was in the mid 1980s. Since then, scientists have tried remote sensing techniques, but remote sensing measures the integrated effect along a line of sight. “In situ,” says Earle, “you’re measuring a point, and you know where that point is.”

Earle currently has two instruments on the Communication/Navigation Outage Forecast System (C/NOFS) military satellite. “These are the first in-situ wind instruments that we’ve tried to fly on satellites since the 1980s, though I’ve flown a couple on rockets,” says Earle. “The neutral winds in space are probably the biggest hole in our understanding of what goes on in this altitude range.”

Satellites move at approximately 8 km/s. “That’s fast,” says Earle. “They don’t spend much time in any particular portion of the earth’s atmosphere.” This isn’t a problem for measuring on a large scale, according to Earle, “but if you want to look at small scale phenomena, you have to sample very quickly.”

One part of the solution would be more, but smaller, satellites. With more satellites, scientists can gather more data points. “You don’t need such a precise measurement when you have a lot of data points,” says Earle. This is the approach the weather forecasters use: they have a lot of weather stations around the world, and even if some don’t give good data there are enough of them that the average can compensate. According to Earle, the same approach may apply in space. “It’s the next wave of how we can improve our forecasting ability for space weather,” he says.

Shrinking the satellites

This can only happen if satellites become smaller and cheaper, he says. Traditionally, satellites have been very large: “they ranged from the size of a train car to the size of a file cabinet.” Earle is a proponent of smaller satellites. “Within the last few years, we have been working on microsats (up to 100 kg), nanosats (up to 10 kg), and picosats (up to 1 kg),” he says. CubeSats, which are about half the size of a shoebox, fit into the nanosat category. “Not only are we building instruments to measure things that are hard to measure, now we’re trying to do it on a much smaller scale,” Earle explains.

The C/NOFS satellite that Earle’s instruments are currently flying on is testing the idea that space weather can be predicted. “We’re just at the very early stages of figuring out if it will really work,” Earle explains.

“The experiment team is taking real-time data from the satellite and feeding it to the computer model to make predictions. If the predictions are accurate, we will put 10 or 11 similar satellites in space and see how much improvement there is from the greater number.”

Smaller satellites, however, mean more challenges for the instruments. “Converting instruments to fly on a CubeSat instead of a larger satellite is not easy,” he says. “Big satellites have enough power and volume to deal with problems stemming from the harsh environment of space, but the little ones are more constrained. Earle’s research group recently received NASA funding to develop an innovative low-power transducer for use in CubeSat-sized wind instruments. They were also approved to launch two instruments on a CubeSat mission in 2014, in collaboration with NASA and the University of Illinois.