The new observations show that the polar wind, a very dilute wind of hydrogen and oxygen, flows unimpeded to very high altitudes. The outflow has previously only been possible to study from satellites at low altitudes, so it was not clear if it actually continued far into space or if it spread out and soon returned to Earth. It is now clear that the particles are actually lost from Earth, as the Cluster measurements have followed them flow to an altitude of almost ten times the diameter of the Earth.
“The polar wind is no threat to the atmosphere”, says study leader Erik Engwall, pointing out that an outflow of the magnitude observed will not make any dramatic change to our atmosphere even during the full expected lifetime of the solar system. But similar phenomena may be more important for other celestial bodies. “To understand how our atmosphere evolves is also important for understanding other atmospheres that can harbour life”, adds Erik Engwall.
The discovery was made when the scientists tried to understand why the Cluster instrument provided by the Uppsala team seemed to give unrealistic results in space above the Earth’s polar regions. “In regions of space where we expected to find very weak electric fields, we were surprised to find very strong fields in a direction that was just plain impossible”, says Anders Eriksson, a scientist operating the Electric Fields and Waves instrument. By computer simulation, Erik Engwall could show that the unexpected results were due to the spacecraft encountering a supersonic wind of charged particles flowing away into space from the Earth’s polar regions. The team could thus transform the apparent “measurement error” into a new method of observing the polar wind at unprecedented distances from the Earth. “In this region, the outflow was completely invisible to satellites until revealed by our new method”, says Erik Engwall.
The paper “Earth’s ionospheric outflow dominated by hidden cold plasma” by Erik Engwall, Anders Eriksson, Chris Cully, Mats André, Roy Torbert and Hans Vaith will appear in the January 2009 issue of Nature Geoscience, and was published online on 14 December. The results will also be presented at the fall meeting of the American Geophysical Union in San Francisco on Wednesday 17 December.
Cluster is a cornerstone project of the European Space Agency (ESA) and consists of four satellites, which have flown, in formation around the Earth since the summer of 2000. The Swedish Institute of Space Physics and the University of New Hampshire are responsible for two instruments measuring electric fields by very different methods on each of the satellites, and it was by comparing the two sets of results that the discovery was made. The Swedish group is financed by the Swedish National Space Board and the American group by NASA.