Posted: August 29, 2008
ESA’s formation-flying Cluster satellites have discovered the physical mechanism that is driving the constant leak of oxygen out of Earth’s atmosphere and into space: the Earth’s own magnetic field.
Electrically charged oxygen atoms, known as ions, have been known to be accelerating from the Earth’s upper atmosphere and out into space for several decades, just as it has been known that the solar wind can connect to the magnetic field of the Earth over the poles, opening up an escape route for the oxygen ions. Measurements taken by satellites during the 1980s and 1990s showed that the escaping ions were traveling faster the higher they were observed, implying that something was causing them to accelerate, but just what the culprit was remained an unsolved mystery, until now.
This illustration shows electrically charged oxygen, hydrogen and helium ions leaking into space from the Earth's atmosphere. Cluster has found that this leak is driven by changes in direction of the Earth's own magnetic field. Image: NASA/ESA.
From 2001 to 2003, the quartet of Cluster satellites amassed information about the beams of oxygen ions flowing outwards from the polar regions into space, documenting the strength and direction of the Earth's magnetic field whenever the beams were present to see how it changes direction with time.
“The beams are at least very common,” lead scientist Dr Hans Nilsson of the Swedish Institute of Space Physics tells Astronomy Now. “We observe them in about two-thirds of the orbits which cover the dayside polar cap region and the outflow typically shows transient enhancements on a time scale of 5-10 minutes.”
This is the same typical time scale as auroral events seen in the dayside auroral oval. Aurorae are caused by the merging of the solar wind and Earth's magnetic fields at the Earth's magnetopause, channeling energetic particles from the solar wind along the magnetic field lines. When these particles strike the atmosphere of the Earth, they can produce aurorae over the poles. The same interactions provide the oxygen ions with enough energy to accelerate out of the atmosphere. In other words, the changing direction of the Earth’s magnetic field lines acts as a giant sling-shot, flinging the oxygen ions out into space.
Polar Orbiting Environmental Satellites (POES) keep an eye on auroral activity 24/7. These images show the size and position of the auroral ovals over the northern (left) and southern (right) poles earlier today. Image: NOAA.
“The changes of the magnetic field may also lead to deceleration,” says Nilsson. “In the region we observe, the spatial change of the magnetic field direction in relation to the flow of the particles is such that we typically have acceleration. In other parts of near Earth space we may have predominantly deceleration and we plan to extend our study to other parts of near-Earth space.”
At present, the escape of oxygen is nothing to worry about. Compared to the Earth's reservoir of the life-supporting gas, the amount escaping is negligible. But in the far future when the Sun begins to heat up at the end of its life, the balance might change and the oxygen escape may become significant. "We can only predict these future changes if we understand the mechanisms involved," says Nilsson.