The boundary between Earth and space is complicated and constantly changing. Unlike the rest of the atmosphere, the upper atmosphere near the edge of space has a mix of both neutral particles similar to the air we breathe, as well as electrically charged particles called ions. Changes in this region are unpredictable, but they can affect satellites and garble signals, like GPS, that pass through this region. That’s why we’re launching ICON (the Ionospheric Connection Explorer) to get our first-ever comprehensive look at our interface to space.
About 60 miles above Earth’s surface, Earth’s atmosphere gives way to space. The change is gradual: The gases of the atmosphere get steadily thinner the higher you go. On the edge of space, the Sun’s radiation cooks some of those thin gases until they lose an electron (or two or three), creating a population of electrically charged particles swarming alongside the neutral particles. These charged particles make up the ionosphere.
Because the particles of the ionosphere are electrically charged, they respond uniquely to electric and magnetic fields. Dynamic conditions in space — including shifting fields and surges of charged particles, collectively called space weather — induce shifts in the ionosphere that can have far-reaching effects. The ionosphere is where space weather manifests on Earth, and it’s inextricably connected with the neutral upper atmosphere — so distortions in one part affect the other.
Changes in the ionosphere and upper atmosphere — including sudden shifts in composition, density, temperature, and conductivity — can affect satellites, building up electric charge that has the potential to disrupt instruments, and garble signals like those used by GPS satellites. Predicting these variances is hard, because the causes are so complex: They’re driven not only by space weather — usually a product of solar activity — but also by regular weather down near Earth’s surface.
Differences in pressure caused by events like hurricanes, or even something as simple as a sustained wind over a mountain range, can ripple upwards until they reach this region and trigger fluctuations. Weather’s influence on the upper atmosphere was only discovered in the past ten years or so — and ICON is the first mission designed specifically to look at that interaction.
ICON carries four types of instruments to study the ionosphere and upper atmosphere. Three of them rely on taking far-away pictures of something called airglow, a faint, global glow produced by reactive compounds in the upper atmosphere. The fourth type collects and analyzes particles directly.
- MIGHTI (the Michelson Interferometer for Global High-resolution Thermospheric Imaging) uses Doppler shift — the same effect that makes a siren change pitch as an ambulance passes you — to precisely track the speed and direction of upper-atmosphere winds.
- FUV (the Far Ultraviolet instrument) measures airglow produced by certain types of oxygen and nitrogen molecules on Earth’s day side, as well as oxygen ions on Earth’s night side.
- EUV (the Extreme Ultraviolet instrument) measures shorter wavelengths of light than FUV. Airglow measured by EUV is produced by oxygen ions on Earth’s day side, which make up the lion’s share of Earth’s daytime ionosphere.
- The two identical IVMs (Ion Velocity Meters) make very precise measurements of the angle at which ionized gas enters the instruments, helping us build up a picture of how this ionized gas around the spacecraft is moving.
We’re launching ICON on June 14 Eastern Time on an Orbital ATK Pegasus XL rocket from Kwajalein Atoll in the Marshall Islands, which will deploy from Orbital’s L-1011 Stargazer aircraft. NASA TV will cover the launch — stay tuned to nasa.gov/live for updates and follow the mission on Twitter and Facebook.
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